October 17, 2023 11:00 AM

October 18, 2023 4:30 PM
October 17, 2023 11:00 AM

October 18, 2023 4:30 PM
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Hôtel Château Bromont
Hôtel Château Bromont
Opening Remarks
Pr Mathieu Juan, Institut quantique  Université de Sherbrooke
Clasical and quantum computations as tensor networks
Pr Stefanos Kourtis, Institut quantique  Université de Sherbrooke
Classical and quantum computations as tensor networks
Break
Event organized in collaboration with the RQMP and animated by Mrs. Chloé Freslon, founder of URelles
Falisha Karpati, Ph.D.
Think Differently Together: Strengthening research and innovation by embracing cognitive diversity
LouisPhilippe Lamoureux (Slides / Présentation)
Thierry Debuischert, Thales  France (postponed to Monday at 13:15 / reporté à lundi 13h15)
Closing remarks of the day
Opening remark of the day
Thierry Debuischert, Thales  France
Professor Tami PeregBarnea, McGill University
Dynamic topology  quantized conductance and Majoranas on wires
Professor Philippe StJean, Université de Montréal
Topological physics with light and matter: new horizons
Break
Louis Gaudreau, National Research Council Canada (Ottawa)
Entanglement distribution via coherent photontospin conversion in semiconductor quantum dot circuits
Philippe Lamontagne, National Research Council Canada (Montréal)
BlackBox Impossibility in the Common Reference Quantum State Model
Olivier GagnonGordillo, Québec quantique lead
Presentation of the Québec Quantum ecosystem
Institut quantique  Université de Sherbrooke
Classical and quantum computations as tensor networks
Tensor networks are multilinearalgebra data structures that are finding application in diverse fields of science, from quantum manybody physics to artificial intelligence. I will introduce tensor networks and illustrate how they can be used to represent classical and quantum computations. I will then motivate tensor network algorithms that perform or simulate computations in practice and demonstrate their performance on benchmarks of current interest, such as model counting and quantum circuit simulation. I will close with an outline of ongoing work and an outlook on future directions.
Institut quantique  Université de Sherbrooke
Optomechanics with a nonlinear cavity
The possibility to operate massive mechanical oscillators close to or in the quantum regime has become central in fundamental sciences. LIGO is a prime example where quantum states of light are now used to further improve the sensitivity. Concretely, optomechanics relies on the use of photons to control the mechanical motion of a resonator, providing a path toward quantum states of massive objects and for the development of quantum sensors. In order to improve this control many approaches have been explored, some more complicated than others. In particular, in order to cool the mechanical motion a cavity can be used to realise sideband cooling. In general, linear cavities are favoured to allow for large photon number providing stronger cooling. I will show that, surprisingly, nonlinear cavities can be used to achieve very efficient cooling at low powers. Indeed, even in the bad cavity limit, we have been able to cool a mechanical resonator from 4000 thermal phonons down 11 phonons. Currently limited by flux noise, this approach opens promising opportunities to achieve quantum control of massive resonators, an avenue to study foundational questions.
McGill University
Dynamic topology  quantized conductance and Majoranas on wires
This talk will address the issue of outofequilibrium topological systems. While many materials and devices produced in labs today are topological at equilibrium, it is desirable to have a knob to tune or induce topological properties. For example, if we could dynamically turn a superconductor into a topological superconductor we may create the sought after Majorana fermions which are potential building blocks of quantum bits.
In this context we will explore the possibility of perturbing quantum systems using timeperiodic fields (i.e., radiation) and use the Floquet theory to characterize the driven states. We find that in topological systems, beyond the expected splitting of the spectrum into side bands, a change in the topology may occur. In the case of a topological superconductor, the driven system may develop new Majorana modes which do not exist at equilibrium and can be exchanged on a single wire. A protocol for exchanging Majoranas will be presented.
Université de Montréal
Topological physics with light and matter: new horizons
Topology is a branch of mathematics interested in geometric properties that are invariant under continuous deformation, e.g. the number of holes in an object. In the early 1980s it was demonstrated that similar topological properties can be defined for solids presenting appropriate symmetry elements. The discovery of these topological phases of matter has profoundly impacted our understanding of condensed matter, its influence ranging from better explaining the universality of the conductivity plateaus in the quantum Hall effect to developing new platforms for faulttolerant quantum computation[i]. In the late 2000s, Duncan Haldane (colaureate of the Nobel Prize in physics for the discovery of topological phases of matter) demonstrated that this topological physics is not restricted to condensed matter but can also emerge in artificial systems like photonic crystals through a careful engineering of their symmetry properties[ii]. Since then, these photonics platforms have proven to be an amazing resource for pushing the exploration of topological matter beyond what is physically reachable in the solidstate, leading to the emergence of a blooming field called topological photonics[iii].
In this presentation, I will describe recent experimental works based on excitonpolaritons, a hybrid lightmatter quasiparticle, which have opened new horizons in topological photonics[iv]. The main advantages of polaritonic systems arise from their dual nature: their photonic part allows for tailoring welldefined topological properties in lattices of coupled microcavities and makes them inherently nonhermitian; on the other hand, their matter part gives rise to a strong Kerrlike nonlinearity and to lasing[v]. I will then discuss in more details a recent work in which we took profit of these assets to experimentally extract topological invariants  a fundamental quantity in topology  in a polaritonic analog of graphene[vi]. Importantly, this has allowed us to directly probe the topological phase transition occurring in a critically strained lattice  i.e. where Dirac cones have merged  a condition impossible to reach in the solidstate. I will conclude this presentation by discussing how topological protection can provide a powerful asset for generating and stabilizing manybody quantum states of light and matter. Such mesoscopic quantum objects are highly desirable as they would provide an extended playground for quantum simulation, sensing applications or for generating exotic states of light such as manybody entangled states[vii].
[i] M. Z. Hasan and C. L. Kane. Rev. Mod. Phys. 82, 3045 (2010)
[ii] F. D. M. Haldane and S. Raghu. Phys. Rev. Lett. 100, 013904 (2008)
[iii] T. Ozawa et al. Rev. Mod. Phys. 91, 015006 (2019)
[iv] D. D. Solnyshkov, G. Malpuech, P. StJean et al. Opt. Mat. Express 11, 1119 (2021)
[v] I. Carusotto and C. Ciuti. Rev. Mod. Phys. 85, 299 (2013)
[vi] P. StJean et al. Phys. Rev. Lett. 126, 127403 (2021)
[vii] P. Lodahl et al. Nature 541, 473 (2017)
Think Differently Together: Strengthening research and innovation by embracing cognitive diversity
This talk will cover:
Biography
Falisha Karpati, PhD is a neuroscientist turned inclusion consultant. Falisha’s work focuses on using neuroscience to build inclusive environments in academic, research, and scientific organizations. Her approach to inclusion centres on the interconnectedness of cognitive, demographic, and experiential diversity. Prior to starting her consultancy practice, she worked as the Training and Equity Advisor for Healthy Brains, Healthy Lives at McGill University.
Head of Applied Quantum Physics
Thales Research & Technology
Researcher
National Research Council Canada (Ottawa)
In this talk, I will present our proposed long distance entanglement distribution scheme that aims to overcome fundamental limitations present in current optical schemes. By using direct band gap semiconductor quantum dots, efficiency and heralding advantages can be exploited through photontospin conversion. For this reason, materials such as GaAs are superior to Si in this type of applications. I will review current schemes to transfer polarization or timebin encoded photonic qubits to electron spin qubits and will describe adaptations to employ heavy holes which have a number of attractive properties including gfactor tunability. Finally, I will show preliminary results on quantum dot devices using Van der Waals heterostructures which present several potential advantages such as higher confinement energies due to their atomically thin geometry, easier combination with different substrates and the possibility of encoding information in their valley degree of freedom.
Biography
Louis Gaudreau studied physics at Sherbrooke University, followed by a masters and PhD in cosupervision with Andrew Sachrajda at NRC and Alexandre Blais at Sherbrooke. During his graduate studies, Louis studied electrostatic quantum dots and realized for the first time a coupled triple quantum dot system leading to the investigation of the first exchangeonly qubit. During this period he was invited to perform quantum dot experiments in Stefans Ludwig’s group at LMU in Munich. After his PhD, Louis changed fields and studied lightmatter interactions by combining quantum emitters and graphene to create different hybrid systems. These experiments were done during his postdoc at ICFO in Barcelona in the nanooptoelectronics group with Frank Koppens where he was awarded the prestigious MarieCurie fellowship. Finally, since 2015, Louis has worked as research officer at the NRC where he investigates different technologies linked to quantum information.
Researcher
National Research Council Canada (Montréal)
BlackBox Impossibility in the Common Reference Quantum State Model
We explore the cryptographic power endowed by arbitrary shared physical resources. We introduce the Common Reference Quantum State (CRQS) model, where the parties involved share a fresh entangled state at the outset of each protocol execution. This model is a natural generalization of the wellknown Common Reference String (CRS) model but appears to be more powerful. In the twoparty setting, a CRQS can sometimes exhibit properties associated with a Random Oracle queried once. We formalize this notion as a Weak OneTime Random Oracle (W1TRO), where we only ask of the output to have some randomness when conditioned on the input is still beyond the reach of the CRQS model. We prove that the security of W1TRO cannot be blackbox reduced to any assumption that can be framed as a cryptographic game. Our impossibility result employs the simulation paradigm formalized by Wichs (ITCS ’13) and has implications for other cryptographic tasks.
 There is no universal implementation of the FiatShamir transform whose security can be blackbox reduced to a cryptographic game assumption. This extends the impossibility result of Bitansky et al. (TCC ’13) to the CRQS model.
 We impose severe limitations on constructions of quantum lightning (Zhandry, Eurocrypt ’19). If a scheme allows n lightning states’ serial numbers (of length m such that n > m) to be combined in such a way that the outcome has entropy, then it implies W1TRO, and thus cannot be blackbox reduced to a cryptographic game assumption.
Senior Product Manager
Aspen Technology
Biography
Montrealbased quantum physicist, senior product manager, and full stack developer with strong experience building awardwinning hardware and software products. Currently Senior Product Manager at Aspen Technology leading connectivity and AI inference at the Edge. Prior to Aspen Technology, I worked at MachineToMachine Intelligence (M2Mi) a leader in IoT Security and Management located at NASA Ames research center in the heart of Silicon Valley.
Prior to M2Mi, built SQR Technologies a belgian quantum based, hardware security startup that pioneered distributed quantum key generation. Acquired by IDQ (Switzerland). Awarded a Ph.D. in Physics (Quantum Cryptography) from the University of Brussels. Research interests include: quantum cloning, experimental quantum cryptography, quantum noise reduction, and quantum random number generation.
10h55 Mot d'ouverture (Salon A)
11h00 Quoc Huy Vu, Léonard de Vinci Pôle Universitaire (Salon A)
PublicKey Encryption with Quantum Keys
12h00 Lunch (Salle Knowlton)
13h30 Philippe Lamontagne, CNRC Montréal (Salon A)
Recent Advances in the Bounded Quantum Storage Model
14h15 Denis Rochette, Université d'Ottawa (Salon A)
Monogamy of highly symmetric states
14h40 Ashutosh Marwah, Université de Montréal (Salon A)
Source correlations for QKD
15h00 Pause café (Salon C)
15h25 Kai Wang, Université McGill (Salon A)
NonHermitian topological photonics in synthetic dimensions
16h10 Rigel Zifkin, Université McGill (Salon A)
Cavity Enhancement of Diamond Defect Centers
16h30 Présentations de l'écosystème quantique
17h15 Session d'affiches avec rafraichissements (Salon C)
19h30 Souper INTRIQ (Salle Knowlton)
9h00 – Denis Seletskiy, Polytechnique Montréal (Salon A)
Nonclassical pulsed light for nonlinear quantum optics
9h45 – Baptiste Royer, Université de Sherbrooke (Salon A)
Encoding qubits in harmonic oscillators with quantum error correction
10h30 – Pause
11h00 – Mark Friesen, University of WisconsinMadison (Salon A)
Strategies for enhancing the valley splitting in Si/SiGe quantum dot qubits
12h00 – Lunch
13h30 – Val Zwiller. KTH Royal Institute of Technology (Salon A)
Subject to be annouced
14h30 – Pause
15h00 – Cristóbal Lledó, Université de Sherbrooke (Salon A)
Dispersive readout in circuit QED
15h45  Benjamin D’Anjou, Université de Sherbrooke (Salon A)
Predicting the onset of nonQND effects in superconducting qubit readout
16h10  Mot de clôture
University of WisconsinMadison
Strategies for enhancing the valley splitting in Si/SiGe quantum dot qubits
For quantum dot spin qubits, we want the first excited state of the dot to be a spin excitation. However, in Si/SiGe quantum wells, the first excited state is often a conductionband valley excitation, with an energy splitting so small that it competes with the spin to form qubits. To overcome this challenge, special heterostructures have been engineered, including sharp interfaces and narrow quantum wells. In this talk, I show that such strategies do not solve the problem of low valley splittings. Using theoretical methods and numerical simulations, we study interface disorder, including atomic steps and SiGe random alloys. Comparing our results to experiments, we show that the latter are responsible for the wide range of valley splittings observed in experiments, even among nominally identical samples. We propose to leverage this variability by electrically controlling the dot’s position, to tune the valley splitting.
Léonard de Vinci Pôle Universitaire, Research Center, France
PublicKey Encryption with Quantum Keys
In the framework of Impagliazzo's five worlds, a distinction is often made between two worlds, one where publickey encryption exists (Cryptomania), and one in which only oneway functions exist (MiniCrypt). However, the boundaries between these worlds can change when quantum information is taken into account. Recent work has shown that quantum variants of oblivious transfer and multiparty computation, both primitives that are classically in Cryptomania, can be constructed from oneway functions, placing them in the realm of quantum MiniCrypt (the socalled MiniQCrypt). This naturally raises the following question: Is it possible to construct a quantum variant of publickey encryption, which is at the heart of Cryptomania, from oneway functions or potentially weaker assumptions?
In this talk, I will present new notions of quantum publickey encryption (qPKE), i.e., publickey encryption where keys are allowed to be quantum states. I will then discuss the (im)possibility of constructing quantum PKE from different assumptions.
This is based on joint work with Khashayar Barooti, Alex B. Grilo, Loïs HugueninDumittan, Giulio Malavolta, Or Sattath and Michael Walter.
Sujet à venir
KTH Royal Institute of Technology
Sujet à venir
Professional, Université de Sherbrooke
Directeur: Alexandre Blais
Predicting the onset of nonQND effects insuperconducting qubit readout
It is generally believed that a scalable quantum processor will necessitate error correction. Many of the most promising quantum error correction protocols require that processor qubits be continually read out to identify logical errors. To be useful for error correction, it is desirable that the readout have the following properties. First, it must be performed faster than the decoherence rate of the data qubits. Second, it must leave the syndromereadout qubits in a known state for further processing. This last condition is naturally met if the readout is quantum nondemolition (QND). Superconducting qubits coupled to microwave readout resonators constitute one of the most promising platforms to realize such a fast QND readout. However, reducing the readout time is generally achieved by driving the resonator with a highpower microwave tone. This can induce unwanted qubit transitions that leave the system in an unknown state. Predicting the onset of these unwanted transitions has been a long standing issue in the study of superconducting qubits. In this work, we show that the onset of nonQND transitions in a driven transmon qubit can be accurately predicted from a purely classical model. More precisely, we show that chaotic motion induced by the transmon nonlinearity at high drive powers shrinks the phase space area required to sustain stable qubit states [1]. We use this observation to define a "chaosassisted critical photon number" which we find to be in surprising agreement with recent experimental results over a large range of parameters [2]. Finally, we perform fully quantum and semiclassical simulations to refine our predictions, and we propose avenues for mitigation of nonQND effects.
[1] Cohen et al., PRX Quantum 4, 020312 (2023)
[2] Khezri et al., arXiv:2212.05097 (2022)
Chercheur, CNRC Montréal
Recent Advances in the Bounded Quantum Storage Model
The bounded quantum storage (BQS) model is the cryptographic assumption that quantum adversaries have imited quantum storage capabilities; usually as a fraction of the number of qubits transmitted during a protocol. For a variety of tasks, the BQS assumption can replace computational assumptions based on the existence of hard problems. The BQS model is motivated by the difficulty of storing transmitted qubits for more than a few seconds with high fidelity and with the improved security properties (secrets are unconditionally hidden after the protocol ends, instead of being hard to compute).
Most research into the BQS model took place 1015 years ago and provided an alternative to computational assumptions for generic multiparty cryptographic tasks. Recently, the BQS assumption was revisited as a way to achieve tasks that would otherwise be impossible, even using computational assumptions. This is made possible by the fact that protocols for the BQS model typically require fewer rounds of interaction than their classical counterparts. In this talk, we will present some of these new results.
Postdoc, Université de Sherbrooke
Directeur: Alexandre Blais
Dispersive readout in circuit QED
Introduction to the dispersive regime and dispersive readout in circuit QED. If time allows, we will discuss recent methods for improving readout over the standard approach.
Doctorant, Université de Montréal
Directeur: Frédéric Dupuis
Source correlations for QKD
Protocols in quantum cryptography often require an honest party to produce multiple independent quantum states. For example, quantum key distribution (QKD) protocols require the honest participant, Alice to produce an independently chosen quantum state from a set of states in every round of the protocol. The security proofs for these protocol rely on the fact that the quantum state produced in each round of the protocol is independent of the other rounds. However, this is a difficult property to enforce practically. All physical devices have an internal memory, which is difficult to characterise and control. This memory can cause the quantum states produced in different rounds to be correlated with one another. In this talk, we will demonstrate how a simple source preparation test can be used to bound these correlations and incorporate them in the security proof for QKD protocols.
Postdoc, Université d'Ottawa
Directrice: Anne Broadbent
Monogamy of highly symmetric states
We study the question of how highly entangled two particles can be when also entangled with other particles. In order to do so we solve optimization problems motivated by manybody physics, computational complexity and quantum cryptography. In particular, we determine the exact maximum values of the projection to the maximally entangled and antisymmetric Werner state possible for a system of size $n$. We find these optimal values by use of SDP duality and representation theory of the symmetric and orthogonal groups, and the Brauer algebra, directly relating our work to the study of entanglement in the context of Werner and isotropic states, and quantum de Finetti theorems.
Professeur, Université de Sherbrooke
Encoding qubits in harmonic oscillators with quantum error correction
Although quantum at its fundamental level, nature appears classical on a macroscopic scale. Indeed, large objects entangle quickly with the surrounding environment, resulting in the rapid loss of individual quantum coherences. The goal of building a quantum computer with a large number of qubits is at odds with this fundamental phenomenon. Fortunately, quantum error correction (QEC) allows us, in principle, to preserve the information indefinitely in a quantum computer at the cost of added hardware and operations, a price sometimes referred to as control overhead. In the attempts to engineer such a QEC process so far, the control overhead has created a surplus of errors that overwhelm the error correcting ability of the process itself.
In this talk, I will introduce a promising approach to quantum computers: using harmonic oscillator modes to encode qubits. Due to their large number of quantum levels and to the fact that they can be made longlived, harmonic oscillators such as microwave cavities constitute a hardware efficient approach to QEC. I will discuss recent experimental results that demonstrate for the first time a feature at the heart of QEC: it is possible to overcome the control overhead and use a composite system to extend the lifetime of quantum information beyond the coherence time of the individual subsystems. I will also discuss different advances that allowed to achieve this milestone: improved QEC protocols, new control techniques and the use of modern reinforcement learning.
Professeur, Polytechnique Montréal
Nonclassical pulsed light for nonlinear quantum optics
Professeur, McGill University
NonHermitian topological photonics in synthetic dimensions
The nontrivial topological features in the energy bands of nonHermitian systems provide promising pathways to achieve robust physical behaviors in classical or quantum open systems. Recent advances in synthesizing dimensions beyond the spatial degree of freedom, especially in photonics, have provided unprecedented flexibility in realizing lattice Hamiltonians. A syntheticdimension approach to nonHermitian topology can enable new opportunities for observing nonHermitian topological effects that are difficult to achieve in other means.
In this talk, I will summarize some of our results in the experimental exploration of nonHermitian eigenvalue topology enabled by the concept of synthetic dimensions. Specifically, a key topological feature of nonHermitian systems is the nontrivial winding of one energy band in the complex energy plane. I will show our experimental demonstrations of the topological winding of nonHermitian band energies, achieved by implementing nonHermitian lattice Hamiltonians along a frequency synthetic dimension formed in a ring resonator undergoing simultaneous phase and amplitude modulations. Furthermore, with two or more nonHermitian bands, the system can be topologically classified by nontrivial braid groups. By generalizing the experiment to two modulated ring resonators, we performed the experimental demonstration of such braidgroup topology with two energy bands braiding around each other, forming nontrivial knots or links. Our experiments also show that the topological winding or braiding can be controlled by changing the modulation waveform.
PhD student, McGill University
Director: Lilian Childress
Cavity Enhancement of Diamond Defect Centers
Defect centers in diamond offer a promissing spinphoton interface platform due to their atomlikespindependent optical transitions. In addition, their coupling to nuclear spins allows for a multitude of quantum information applications. Achieving high coupling to such emitters presents a long standing challenge to realizing a highcooperativity cavity QED system and limits stateoftheart realizations of entanglement protocols. By incorporating defect centers into lowloss, micronscale optical cavities, the Purcell effect can greatly improve the emission and collection rate of coherent single photons; however, this introduces numerous experimental challenges. I will present our recent results of achieving significant excitedstate lifetime reductions in a germaniumvacancy defect system coupled to a cryogenic microcavity.
Doctorant, Université de Sherbrooke
Directeur: Bertrand Reulet
Sujet à venir
Étudiante à la maîtrise, Polytechnique Montréal
Directeur: Oussama Moutanabbir
Sujet à venir
Postdoc, CNRC Ottawa
Directeur: Louis Gaudreau
Sujet à venir
Doctorant, Université McGill
Directeur: Bill Coish
Simulating a Topological Quantum Walk in Sythetic Dimensions
The framework of synthetic dimensions offers a new way to engineer physical systems. In this work, we present theoretical progress toward simulating a topological quantum walk in an open system using classical light in a system of coupled fibre loops. The model is interesting, as it features an observable tied to its topological phase taking on universal values if the structure of the environment fulfils certain requirements. The experimental platform which we propose is low cost and highly tunable. We collaborate closely with the group of Philippe StJean at the Université de Montréal, which works on the experimental implementation.
Doctorant, Polytechnique Montréal
Directeur: Nicolas Quesada
Sujet à venir
Étudiant à la maîtrise, Polytechnique Montréal
Directeur: Nicolas Quesada
Sujet à venir
Étudiant à la maîtrise, Université de Sherbrooke
Directeur: Stéfanos Kourtis
Sujet à venir
Étudiante et étudiant à la maîtrise, Université de Sherbrooke
Directeur: Alexandre Blais
Predicting the onset of nonQND effects in the readout of superconducting qubits
Dispersive readout of the transmon can be nonQND due to leakage of the qubit to higher excited states. Full dynamics, analysis of dressed eigenstates, Floquet theory and signature of classical chaos were used to predict onset of nonQNDness. In this work we show that these different approaches lead to the same predictions, which we compare to experiments.
Doctorant, Université de Sherbrooke
Directeur : Bertrand Reulet
Violation of detailed balance in microwave circuits
We investigate the violation of detailed balance, both in frequency and timedomain of circuits driven out of equilibrium by the presence of two noise sources and probed by two microwave amplifiers. In the frequency domain we consider the heat, the crosspower and the angular momentum in the voltage plane as global metrics. We show how to relate these quantities to the scattering matrix of the circuits as well as measurements of those metrics at room temperature in circuits with various spatial and timereversal symmetries operating between 4 and 8 GHz. In the time domain, we provide a direct probe of the violation of the detailed balance by imaging the probability current density in the voltage plane.
Étudiante à la maîtrise, Université de Sherbrooke
Directeur: Mathieu Juan
Quantum dots in undoped GaAs : Improving fabrication and measurements
Quantum dot devices provide excellent coherence and measurement efficiency as well as small dimensions, thus offering a promising approach for quantum computing. Nevertheless, integrating a large number of quantum dots remains challenging . This project addresses this issue by simplifying the architecture of quantum dots. Concretely, using a double quantum dot device^{1} coupled to a superconducting resonator on undoped GaAs and by sending light at a chosen wavelength to create electron hole pairs, we can remove many of the components usually present in standard gate defined quantum dots. Nevertheless , some issues arise in the process, such as the difficulty to predict the behaviour of the device before and after creating charges, the efficiency of the gate’s geometry, the need for a fast and accurate readout, among others. In this work , we are exploring different solutions to some of the current limitations of quantum dot devices . Solving Poisson’s and Schrödinger’s equations for the device and studying the light matter interaction gives us a better understanding of its physical properties and the effects of the gate’s geometry on the wave function. Concerning the readout, the use of a custom made, low cost, and modular up/down conversion circuit allows a fast and accurate measurement of charge population in the device. The resonator’s quality factor will also be investigated using different fabrication methods and geometries.
1 Pierre Lefloïc and Steve Lamoureux’s poster will present in more detail the operation of our current devices.
Étudiant à la maîtrise, Université de Sherbrooke
Directeur: Bertrand Reulet
Sujet à venir
Doctorant, Université de Sherbrooke
Directeur: Alexandre Blais
Combining machine learning characterization and quantum optimal control to improve superconducting qubit operations
Quantum optimal control theory offers a powerful toolbox to design pulse shapes that can realize, in numerical simulations, desired quantum operations with extremely high fidelity. When implementing these pulses in practice, however, the benefit of using optimal pulses over simple analytical forms is often greatly reduced. A significant part of this discrepancy can be attributed to failures of the numerical model to precisely capture the complete quantum dynamics generated by the control electronics. Here, we address this issue directly by building a framework where we break down the problem of realizing highfidelity quantum operations into two parts. First, we use physicsinspired machine learning to infer an accurate model of the dynamics from experimental data. We investigate a range of trainable models from blackbox neural networks to physically informative Lindblad master equation solvers. Second, we use such a trained numerical model in combination with stateoftheart quantum optimal control to find pulse shapes that realize quantum gates with maximal accuracy given our experimental constraints. Using numerical simulations, we show the feasibility of learning from realistically available data to accurately characterize qubit dynamics and to discover highfidelity arbitrary singlequbit gates. We then demonstrate our framework in an experimental setting by optimizing the Clifford gate set of a superconducting transmon qubit.
Doctorant, Polytechnique Montréal
Directeur: Nicolas Quesada
Quantum circuit simulation of linear optics using fermion to qubit encoding
We propose a digital simulation protocol for the linear scattering process of distinguishable bosons based on the effectively bosonic states ofmultifermions. By combining this phenomenon with the transformation method of fermions and qubits such as the JordanWigner transformation, we build a protocol to simulate linear scatttering process of N photons in M modes with digital quantum computers. Our method also includes a simple and intuitive way to control the distinguishablity of bosons. We designed a quantum circuit for the simulation of HongOuMandel dip of bosons with a discrete internal degree of freedom, which is verified by IBMQ and IonQ.
Doctorants, Université de Sherbrooke
Directeur: Alexandre Blais
Toolbox for Nonreciprocal dispersive models in circuit QED
To address the growing complexity of quantum processors, systematic approaches have been developed to characterize the effective lowenergy quantum Hamiltonian of superconducting circuits, which consider the multimode nature of the distributed circuit, while agnostic on the specific circuit design. Here, we expand the existing toolbox to incorporate nonreciprocal elements and correlated decay rates. Our method provides effective dispersive Lindblad master equations for weaklyanharmonic superconducting qubits coupled by a generic dissipationless nonreciprocal linear system, with effective coupling parameters and decay rates written in terms of the immittance parameters characterizing the coupler. This method can be used for the design of complex superconducting quantum processors with nontrivial routing of quantum information, as well as analog quantum simulators of condensed matter systems.
Doctorant, CNRC Ottawa
Directeur: Louis Gaudreau
Electronic transport measurements of electrostatically defined Hall bar devices in monolayer WSe2
The quest of always reaching for smaller and thinner materials has led researchers to attain the ultimate frontier when, in 2004, the first report of monolayer graphene was published. This discovery led to the creation of a new area of research, the study of twodimensional (2D) materials and their heterostructures. These materials are composed of layers that can be isolated due to weak interlayer van der Waals forces. One of the most promising categories of 2D materials is the semiconducting transition metal dichalcogenides (TMDs). TMDs have the particularity of changing from indirect band gap semiconductors to direct band gap ones, as they are thinned down from bulk crystal to monolayer i.e. one layer of atoms, leading to a change in their optical and electrical characteristics.
One of the most promising applications of 2D semiconductors are quantum devices based on the confinement of charge carries in monolayer TMDs using electrostatic gates. Towards that goal, one of the key figures of merit of quantum devices using TMDs is the field effect mobility since it is required for efficient operation of the devices. Low mobility of 2D semiconductors is currently one of the main limiting factors to the wider use of these materials in practical applications.
In this poster, I will be presenting our latest efforts towards the investigation of the mobility as well as the quantum Hall effect in monolayer WSe2 based quantum devices that have Hall bar geometry, a standard of measurement in the field.
Stagiaire, Polytechnique Montréal
Directeur: Nicolas Quesada
Sujet à venir
Étudiant à la maîtrise, CNRC Ottawa
Directeur : Louis Gaudreau
Trapping photocarriers in undoped accumulationmode quantum dots using an onchip microwave resonator for charge readout
Scaling up gatedefined quantum dot systems is hampered by the rapid growth in the number of control gates. To tackle this challenge, we propose a novel scheme, in which the quantum dots are created from optically generated charges trapped beneath accumulation gates.
By shining an abovethegap laser light onto an undoped GaAs substrate, we demonstrate that it is possible to create and separate electronhole pairs to form quantum dots with one of the two polarities. By pairing this technique with a superconducting coplanar waveguide resonator for the charge readout, we achieve a working manycharge double quantum dot device with controllable interdot charge exchange. The device, comprised of only two plunger gates and one tunnel coupling gate, shows that populating the quantum dots does not require ohmic contact reservoirs, source/drain bias, or external atomic dopants. Therefore, the number of gates can be reduced and the fabrication process can be simplified. Moreover, this method can be applied to a wide range of semiconductor quantum dot systems.
Such a hybrid device is the first step towards a more scalable design for quantum dot arrays. It is also a good starting point for quantum transducing thanks to the optical–matter– microwave interaction.
Étudiant à la maîtrise, Unviersité de Sherbrooke
Directeur: Stéfanos Kourtis
Sujet à venir
Doctorant, Université McGill
Directeur: Bill Coish
Sujet à venir
Doctorant, Université de Sherbrooke
Directeur: Bertrand Reulet
Sujet à venir
Postdoc, Université McGill
Directeur: Kai Wang
Sujet à venir
Postdoc, Université de Sherbrooke
Directeur: Alexandre Blais
A quantuminspired approximation algorithm for MaxCut
It has been conjectured that nearterm quantum computers might offer a speedup in solving combinatorial optimization problems. Much work has been done exploring the extent to which this goal can be achieved via variational quantum algorithms, without very conclusive answers. A side product of this effort has been the construction of quantuminspired algorithms. Improved or novel classical algorithms which exploit special structures of the problems under study unveiled by their quantum counterparts. In this work we focus on the MaxCut problem and inspired by the solution unitaries obtained with certain adaptive variational algorithms, we introduce a new quantuminspired approximation algorithm for this problem. This algorithm is polynomial in the input both in time and space. We study its performance on different families of graphs and provide copious evidence that, for graphs up to 200 nodes, our algorithm produces a better approximated solution than the best classical algorithm, that of Goemans and Williamson.
Doctorant, Polytechnique Montréal
Directeur: Sébasteur Francoeur
Sujet à venir
Doctorant, Unviersité de Sherbrooke
Directeur: Max Hofheinz
Sujet à venir
Étudiante à la maîtrise, Université McGill
Directeur: Kai Wang
Sujet à venir
Doctorant, Université de Montréal
Directeur: Philippe StJean
Wavefunction tomography in topological dimer chains with longrange couplings
Doctorant, Université de Sherbrooke
Directeur: Yves BérubéLauzière
Sujet à venir
Étudiante à la maîtrise, Université McGill
Directeur: Bill Coish
Sujet à venir
Postdoc, Unviersité de Sherbrooke
Directeur: Stéfanos Kourtis
Stability of Majorana bound states in presence of nonMarkovian noise
We studied the dynamics of Majorana box qubits in the presence of nonMarkovian environment by performing analytical calculations of the decoherence parameter. We investigated the impact of longrange pairing interactions within the Kitaev pwave superconductors on this parameter. Our findings demonstrate that these interactions decrease the transition amplitude from zeroenergy level to excited states, leading to a reduction of decoherence parameter. This outcome proposes a practical approach for creating Majorana qubits that exhibit high resilience against nonMarkovian noise.
Étudiant à la maîtrise, Université de Sherbrooke
Directeur: Mathieu Juan
Sujet à venir
Doctorant, Université de Sherbrooke
Directeur: Stéfanos Kourtis
Sujet à venir
Doctorant, Université McGill
Directrice: Tami PeregBarnea
Sujet à venir
Stagiaire, Université McGill
Directeur: Bill Coish
Neural network quantum state for learning dynamics and state reconstruction
We present two of our recent works on using neural network quantum state ansatz to help learning properties of a quantum system. The first work studies the dynamics of the Gaudin magnet (“centralspin model”) using machinelearning methods. We use a neuralnetwork representation (restricted Boltzmann machine) to obtain accurate representations of the ground state and of the lowlying excited states of the Gaudinmagnet Hamiltonian through a variational Monte Carlo calculation. From the lowlying eigenstates, we find the nonperturbative dynamic transverse spin susceptibility, describing the linear response of a central spin to a timevarying transverse magnetic field in the presence of a spin bath. In the second work, we present “neural–shadow quantum state tomography” (NSQST)—an alternative neural networkbased QST protocol that uses infidelity as the loss function. The infidelity is estimated using the classical shadows of the target state. Infidelity is a natural choice for training loss, benefiting from the proven measurement sample efficiency of the classical shadow formalism. We numerically demonstrate the advantage of NSQST at learning the relative phases of three target quantum states of practical interest, as well as the advantage over direct shadow estimation.
10:55 Opening remarks (Salon A)
11:00 Quoc Huy Vu, Léonard de Vinci Pôle Universitaire (Salon A)
PublicKey Encryption with Quantum Keys
12:00 Lunch (Knowlton room)
13:30 Philippe Lamontagne, NRC Montréal (Salon A)
Recent Advances in the Bounded Quantum Storage Model
14:15 Denis Rochette, Ottawa University (Salon A)
Monogamy of highly symmetric states
14:40 Ashutosh Marwah, Université de Montréal (Salon A)
Source correlations for QKD
15:00 Coffee break (Salon C)
15:25 Kai Wang, McGill University (Salon A)
NonHermitian topological photonics in synthetic dimensions
16:10 Rigel Zifkin, McGill University (Salon A)
Cavity Enhancement of Diamond Defect Centers
16:30 Quantum ecosystem presentations
17:15 Poster session with refreshments (Salon C)
19:30 INTRIQ dinner (Knowlton room)
9:00 Denis Seletskiy, Polytechnique Montréal (Salon A)
Nonclassical pulsed light for nonlinear quantum optics
9:45 Baptiste Royer, Université de Sherbrooke (Salon A)
Encoding qubits in harmonic oscillators with quantum error correction
10:30 Coffee break (Salon C)
11:00 Mark Friesen, University of WisconsinMadison (Salon A)
Strategies for enhancing the valley splitting in Si/SiGe quantum dot qubits
12:00 Lunch (Knowlton room)
13:30 Val Zwiller. KTH Royal Institute of Technology (Salon A)
Subject to be annouced
14:30 Coffee break (Salon C)
15:00 Cristóbal Lledó, Université de Sherbrooke (Salon A)
Dispersive readout in circuit QED
15:45 Benjamin D’Anjou, Université de Sherbrooke (Salon A)
Predicting the onset of nonQND effects in superconducting qubit readout
16:10 Closing remarks (Salon A)
University of WisconsinMadison
Strategies for enhancing the valley splitting in Si/SiGe quantum dot qubits
For quantum dot spin qubits, we want the first excited state of the dot to be a spin excitation. However, in Si/SiGe quantum wells, the first excited state is often a conductionband valley excitation, with an energy splitting so small that it competes with the spin to form qubits. To overcome this challenge, special heterostructures have been engineered, including sharp interfaces and narrow quantum wells. In this talk, I show that such strategies do not solve the problem of low valley splittings. Using theoretical methods and numerical simulations, we study interface disorder, including atomic steps and SiGe random alloys. Comparing our results to experiments, we show that the latter are responsible for the wide range of valley splittings observed in experiments, even among nominally identical samples. We propose to leverage this variability by electrically controlling the dot’s position, to tune the valley splitting.
Léonard de Vinci Pôle Universitaire, Research Center, France
PublicKey Encryption with Quantum Keys
In the framework of Impagliazzo's five worlds, a distinction is often made between two worlds, one where publickey encryption exists (Cryptomania), and one in which only oneway functions exist (MiniCrypt). However, the boundaries between these worlds can change when quantum information is taken into account. Recent work has shown that quantum variants of oblivious transfer and multiparty computation, both primitives that are classically in Cryptomania, can be constructed from oneway functions, placing them in the realm of quantum MiniCrypt (the socalled MiniQCrypt). This naturally raises the following question: Is it possible to construct a quantum variant of publickey encryption, which is at the heart of Cryptomania, from oneway functions or potentially weaker assumptions?
In this talk, I will present new notions of quantum publickey encryption (qPKE), i.e., publickey encryption where keys are allowed to be quantum states. I will then discuss the (im)possibility of constructing quantum PKE from different assumptions.
This is based on joint work with Khashayar Barooti, Alex B. Grilo, Loïs HugueninDumittan, Giulio Malavolta, Or Sattath and Michael Walter.
KTH Royal Institute of Technology
Subject to be annouced
Professional, Université de Sherbrooke
Director: Alexandre Blais
Predicting the onset of nonQND effects insuperconducting qubit readout
It is generally believed that a scalable quantum processor will necessitate error correction. Many of the most promising quantum error correction protocols require that processor qubits be continually read out to identify logical errors. To be useful for error correction, it is desirable that the readout have the following properties. First, it must be performed faster than the decoherence rate of the data qubits. Second, it must leave the syndromereadout qubits in a known state for further processing. This last condition is naturally met if the readout is quantum nondemolition (QND). Superconducting qubits coupled to microwave readout resonators constitute one of the most promising platforms to realize such a fast QND readout. However, reducing the readout time is generally achieved by driving the resonator with a highpower microwave tone. This can induce unwanted qubit transitions that leave the system in an unknown state. Predicting the onset of these unwanted transitions has been a long standing issue in the study of superconducting qubits. In this work, we show that the onset of nonQND transitions in a driven transmon qubit can be accurately predicted from a purely classical model. More precisely, we show that chaotic motion induced by the transmon nonlinearity at high drive powers shrinks the phase space area required to sustain stable qubit states [1]. We use this observation to define a "chaosassisted critical photon number" which we find to be in surprising agreement with recent experimental results over a large range of parameters [2]. Finally, we perform fully quantum and semiclassical simulations to refine our predictions, and we propose avenues for mitigation of nonQND effects.
[1] Cohen et al., PRX Quantum 4, 020312 (2023)
[2] Khezri et al., arXiv:2212.05097 (2022)
Researcher, NRC Montréal
Recent Advances in the Bounded Quantum Storage Model
The bounded quantum storage (BQS) model is the cryptographic assumption that quantum adversaries have imited quantum storage capabilities; usually as a fraction of the number of qubits transmitted during a protocol. For a variety of tasks, the BQS assumption can replace computational assumptions based on the existence of hard problems. The BQS model is motivated by the difficulty of storing transmitted qubits for more than a few seconds with high fidelity and with the improved security properties (secrets are unconditionally hidden after the protocol ends, instead of being hard to compute).
Most research into the BQS model took place 1015 years ago and provided an alternative to computational assumptions for generic multiparty cryptographic tasks. Recently, the BQS assumption was revisited as a way to achieve tasks that would otherwise be impossible, even using computational assumptions. This is made possible by the fact that protocols for the BQS model typically require fewer rounds of interaction than their classical counterparts. In this talk, we will present some of these new results.
Postdoc, Université de Sherbrooke
Director: Alexandre Blais
Dispersive readout in circuit QED
Introduction to the dispersive regime and dispersive readout in circuit QED. If time allows, we will discuss recent methods for improving readout over the standard approach.
PhD student, Université de Montréal
Director: Frédéric Dupuis
Source correlations for QKD
Protocols in quantum cryptography often require an honest party to produce multiple independent quantum states. For example, quantum key distribution (QKD) protocols require the honest participant, Alice to produce an independently chosen quantum state from a set of states in every round of the protocol. The security proofs for these protocol rely on the fact that the quantum state produced in each round of the protocol is independent of the other rounds. However, this is a difficult property to enforce practically. All physical devices have an internal memory, which is difficult to characterise and control. This memory can cause the quantum states produced in different rounds to be correlated with one another. In this talk, we will demonstrate how a simple source preparation test can be used to bound these correlations and incorporate them in the security proof for QKD protocols.
Postdoc, Ottawa University
Director: Anne Broadbent
Monogamy of highly symmetric states
We study the question of how highly entangled two particles can be when also entangled with other particles. In order to do so we solve optimization problems motivated by manybody physics, computational complexity and quantum cryptography. In particular, we determine the exact maximum values of the projection to the maximally entangled and antisymmetric Werner state possible for a system of size $n$. We find these optimal values by use of SDP duality and representation theory of the symmetric and orthogonal groups, and the Brauer algebra, directly relating our work to the study of entanglement in the context of Werner and isotropic states, and quantum de Finetti theorems.
Professeur, Université de Sherbrooke
Encoding qubits in harmonic oscillators with quantum error correction
Although quantum at its fundamental level, nature appears classical on a macroscopic scale. Indeed, large objects entangle quickly with the surrounding environment, resulting in the rapid loss of individual quantum coherences. The goal of building a quantum computer with a large number of qubits is at odds with this fundamental phenomenon. Fortunately, quantum error correction (QEC) allows us, in principle, to preserve the information indefinitely in a quantum computer at the cost of added hardware and operations, a price sometimes referred to as control overhead. In the attempts to engineer such a QEC process so far, the control overhead has created a surplus of errors that overwhelm the error correcting ability of the process itself.
In this talk, I will introduce a promising approach to quantum computers: using harmonic oscillator modes to encode qubits. Due to their large number of quantum levels and to the fact that they can be made longlived, harmonic oscillators such as microwave cavities constitute a hardware efficient approach to QEC. I will discuss recent experimental results that demonstrate for the first time a feature at the heart of QEC: it is possible to overcome the control overhead and use a composite system to extend the lifetime of quantum information beyond the coherence time of the individual subsystems. I will also discuss different advances that allowed to achieve this milestone: improved QEC protocols, new control techniques and the use of modern reinforcement learning.
Professor, Polytechnique Montréal
Nonclassical pulsed light for nonlinear quantum optics
Professor, McGill University
NonHermitian topological photonics in synthetic dimensions
The nontrivial topological features in the energy bands of nonHermitian systems provide promising pathways to achieve robust physical behaviors in classical or quantum open systems. Recent advances in synthesizing dimensions beyond the spatial degree of freedom, especially in photonics, have provided unprecedented flexibility in realizing lattice Hamiltonians. A syntheticdimension approach to nonHermitian topology can enable new opportunities for observing nonHermitian topological effects that are difficult to achieve in other means.
In this talk, I will summarize some of our results in the experimental exploration of nonHermitian eigenvalue topology enabled by the concept of synthetic dimensions. Specifically, a key topological feature of nonHermitian systems is the nontrivial winding of one energy band in the complex energy plane. I will show our experimental demonstrations of the topological winding of nonHermitian band energies, achieved by implementing nonHermitian lattice Hamiltonians along a frequency synthetic dimension formed in a ring resonator undergoing simultaneous phase and amplitude modulations. Furthermore, with two or more nonHermitian bands, the system can be topologically classified by nontrivial braid groups. By generalizing the experiment to two modulated ring resonators, we performed the experimental demonstration of such braidgroup topology with two energy bands braiding around each other, forming nontrivial knots or links. Our experiments also show that the topological winding or braiding can be controlled by changing the modulation waveform.
PhD student, McGill University
Director: Lilian Childress
Cavity Enhancement of Diamond Defect Centers
Defect centers in diamond offer a promissing spinphoton interface platform due to their atomlikespindependent optical transitions. In addition, their coupling to nuclear spins allows for a multitude of quantum information applications. Achieving high coupling to such emitters presents a long standing challenge to realizing a highcooperativity cavity QED system and limits stateoftheart realizations of entanglement protocols. By incorporating defect centers into lowloss, micronscale optical cavities, the Purcell effect can greatly improve the emission and collection rate of coherent single photons; however, this introduces numerous experimental challenges. I will present our recent results of achieving significant excitedstate lifetime reductions in a germaniumvacancy defect system coupled to a cryogenic microcavity.
PhD student, Université de Sherbrooke
Director: Bertrand Reulet
Subject to be annouced
Master student, Polytechnique Montréal
Director: Oussama Moutanabbir
Subject to be annouced
Postdoc, NRC Ottawa
Director: Louis Gaudreau
Subject to be annouced
PhD student, McGill University
Director: Bill Coish
Simulating a Topological Quantum Walk in Sythetic Dimensions
The framework of synthetic dimensions offers a new way to engineer physical systems. In this work, we present theoretical progress toward simulating a topological quantum walk in an open system using classical light in a system of coupled fibre loops. The model is interesting, as it features an observable tied to its topological phase taking on universal values if the structure of the environment fulfils certain requirements. The experimental platform which we propose is low cost and highly tunable. We collaborate closely with the group of Philippe StJean at the Université de Montréal, which works on the experimental implementation.
PhD student, Polytechnique Montréal
Director: Nicolas Quesada
Suject to be announced
Master student, Polytechnique Montréal
Director: Nicolas Quesada
Suject to be announced
Master student, Université de Sherbrooke
Director: Stéfanos Kourtis
Suject to be announced
Master students, Université de Sherbrooke
Director: Alexandre Blais
Predicting the onset of nonQND effects in the readout of superconducting qubits
Dispersive readout of the transmon can be nonQND due to leakage of the qubit to higher excited states. Full dynamics, analysis of dressed eigenstates, Floquet theory and signature of classical chaos were used to predict onset of nonQNDness. In this work we show that these different approaches lead to the same predictions, which we compare to experiments.
PhD student, Université de Sherbrooke
Director: Bertrand Reulet
Violation of detailed balance in microwave circuits
We investigate the violation of detailed balance, both in frequency and timedomain of circuits driven out of equilibrium by the presence of two noise sources and probed by two microwave amplifiers. In the frequency domain we consider the heat, the crosspower and the angular momentum in the voltage plane as global metrics. We show how to relate these quantities to the scattering matrix of the circuits as well as measurements of those metrics at room temperature in circuits with various spatial and timereversal symmetries operating between 4 and 8 GHz. In the time domain, we provide a direct probe of the violation of the detailed balance by imaging the probability current density in the voltage plane.
Master student, Université de Sherbrooke
Director: Mathieu Juan
Quantum dots in undoped GaAs : Improving fabrication and measurements
Quantum dot devices provide excellent coherence and measurement efficiency as well as small dimensions, thus offering a promising approach for quantum computing. Nevertheless, integrating a large number of quantum dots remains challenging . This project addresses this issue by simplifying the architecture of quantum dots. Concretely, using a double quantum dot device^{1} coupled to a superconducting resonator on undoped GaAs and by sending light at a chosen wavelength to create electron hole pairs, we can remove many of the components usually present in standard gate defined quantum dots. Nevertheless , some issues arise in the process, such as the difficulty to predict the behaviour of the device before and after creating charges, the efficiency of the gate’s geometry, the need for a fast and accurate readout, among others. In this work , we are exploring different solutions to some of the current limitations of quantum dot devices . Solving Poisson’s and Schrödinger’s equations for the device and studying the light matter interaction gives us a better understanding of its physical properties and the effects of the gate’s geometry on the wave function. Concerning the readout, the use of a custom made, low cost, and modular up/down conversion circuit allows a fast and accurate measurement of charge population in the device. The resonator’s quality factor will also be investigated using different fabrication methods and geometries.
1 Pierre Lefloïc and Steve Lamoureux’s poster will present in more detail the operation of our current devices.
Master student, Université de Sherbrooke
Director: Bertrand Reulet
Subject to be annouced
PhD student, Université de Sherbrooke
Director: Alexandre Blais
Combining machine learning characterization and quantum optimal control to improve superconducting qubit operations
Quantum optimal control theory offers a powerful toolbox to design pulse shapes that can realize, in numerical simulations, desired quantum operations with extremely high fidelity. When implementing these pulses in practice, however, the benefit of using optimal pulses over simple analytical forms is often greatly reduced. A significant part of this discrepancy can be attributed to failures of the numerical model to precisely capture the complete quantum dynamics generated by the control electronics. Here, we address this issue directly by building a framework where we break down the problem of realizing highfidelity quantum operations into two parts. First, we use physicsinspired machine learning to infer an accurate model of the dynamics from experimental data. We investigate a range of trainable models from blackbox neural networks to physically informative Lindblad master equation solvers. Second, we use such a trained numerical model in combination with stateoftheart quantum optimal control to find pulse shapes that realize quantum gates with maximal accuracy given our experimental constraints. Using numerical simulations, we show the feasibility of learning from realistically available data to accurately characterize qubit dynamics and to discover highfidelity arbitrary singlequbit gates. We then demonstrate our framework in an experimental setting by optimizing the Clifford gate set of a superconducting transmon qubit.
PhD student, Polytechnique Montréal
Director: Nicolas Quesada
Quantum circuit simulation of linear optics using fermion to qubit encoding
We propose a digital simulation protocol for the linear scattering process of distinguishable bosons based on the effectively bosonic states ofmultifermions. By combining this phenomenon with the transformation method of fermions and qubits such as the JordanWigner transformation, we build a protocol to simulate linear scatttering process of N photons in M modes with digital quantum computers. Our method also includes a simple and intuitive way to control the distinguishablity of bosons. We designed a quantum circuit for the simulation of HongOuMandel dip of bosons with a discrete internal degree of freedom, which is verified by IBMQ and IonQ.
PhD students, Université de Sherbrooke
Director: Alexandre Blais
Toolbox for Nonreciprocal dispersive models in circuit QED
To address the growing complexity of quantum processors, systematic approaches have been developed to characterize the effective lowenergy quantum Hamiltonian of superconducting circuits, which consider the multimode nature of the distributed circuit, while agnostic on the specific circuit design. Here, we expand the existing toolbox to incorporate nonreciprocal elements and correlated decay rates. Our method provides effective dispersive Lindblad master equations for weaklyanharmonic superconducting qubits coupled by a generic dissipationless nonreciprocal linear system, with effective coupling parameters and decay rates written in terms of the immittance parameters characterizing the coupler. This method can be used for the design of complex superconducting quantum processors with nontrivial routing of quantum information, as well as analog quantum simulators of condensed matter systems.
PhD student, NRC Ottawa
Director: Louis Gaudreau
Electronic transport measurements of electrostatically defined Hall bar devices in monolayer WSe2
The quest of always reaching for smaller and thinner materials has led researchers to attain the ultimate frontier when, in 2004, the first report of monolayer graphene was published. This discovery led to the creation of a new area of research, the study of twodimensional (2D) materials and their heterostructures. These materials are composed of layers that can be isolated due to weak interlayer van der Waals forces. One of the most promising categories of 2D materials is the semiconducting transition metal dichalcogenides (TMDs). TMDs have the particularity of changing from indirect band gap semiconductors to direct band gap ones, as they are thinned down from bulk crystal to monolayer i.e. one layer of atoms, leading to a change in their optical and electrical characteristics.
One of the most promising applications of 2D semiconductors are quantum devices based on the confinement of charge carries in monolayer TMDs using electrostatic gates. Towards that goal, one of the key figures of merit of quantum devices using TMDs is the field effect mobility since it is required for efficient operation of the devices. Low mobility of 2D semiconductors is currently one of the main limiting factors to the wider use of these materials in practical applications.
In this poster, I will be presenting our latest efforts towards the investigation of the mobility as well as the quantum Hall effect in monolayer WSe2 based quantum devices that have Hall bar geometry, a standard of measurement in the field.
Undergraduate intern, Polytechnique Montréal
Director: Nicolas Quesada
Subject to be annouced
Master student, NRC Ottawa
Director: Louis Gaudreau
Trapping photocarriers in undoped accumulationmode quantum dots using an onchip microwave resonator for charge readout
Scaling up gatedefined quantum dot systems is hampered by the rapid growth in the number of control gates. To tackle this challenge, we propose a novel scheme, in which the quantum dots are created from optically generated charges trapped beneath accumulation gates.
By shining an abovethegap laser light onto an undoped GaAs substrate, we demonstrate that it is possible to create and separate electronhole pairs to form quantum dots with one of the two polarities. By pairing this technique with a superconducting coplanar waveguide resonator for the charge readout, we achieve a working manycharge double quantum dot device with controllable interdot charge exchange. The device, comprised of only two plunger gates and one tunnel coupling gate, shows that populating the quantum dots does not require ohmic contact reservoirs, source/drain bias, or external atomic dopants. Therefore, the number of gates can be reduced and the fabrication process can be simplified. Moreover, this method can be applied to a wide range of semiconductor quantum dot systems.
Such a hybrid device is the first step towards a more scalable design for quantum dot arrays. It is also a good starting point for quantum transducing thanks to the optical–matter– microwave interaction.
Master student, Unviersité de Sherbrooke
Director: Stéfanos Kourtis
Subject to be annouced
PhD student, McGill University
Director: Bill Coish
Subject to be annouced
PhD student, Université de Sherbrooke
Director: Bertrand Reulet
Subject to be annouced
Postdoc, McGill University
Director: Kai Wang
Subject to be annouced
Postdoc, Université de Sherbrooke
Director: Alexandre Blais
A quantuminspired approximation algorithm for MaxCut
It has been conjectured that nearterm quantum computers might offer a speedup in solving combinatorial optimization problems. Much work has been done exploring the extent to which this goal can be achieved via variational quantum algorithms, without very conclusive answers. A side product of this effort has been the construction of quantuminspired algorithms. Improved or novel classical algorithms which exploit special structures of the problems under study unveiled by their quantum counterparts. In this work we focus on the MaxCut problem and inspired by the solution unitaries obtained with certain adaptive variational algorithms, we introduce a new quantuminspired approximation algorithm for this problem. This algorithm is polynomial in the input both in time and space. We study its performance on different families of graphs and provide copious evidence that, for graphs up to 200 nodes, our algorithm produces a better approximated solution than the best classical algorithm, that of Goemans and Williamson.
PhD student, Polytechnique Montréal
Director: Sébasteur Francoeur
Subject to be annouced
PhD student, Unviersité de Sherbrooke
Director: Max Hofheinz
Subject to be annouced
Master student, McGill University
Director: Kai Wang
Subject to be annouced
PhD student, Université de Montréal
Director: Philippe StJean
Wavefunction tomography in topological dimer chains with longrange couplings
PhD student, Université de Sherbrooke
Director: Yves BérubéLauzière
Subject to be annouced
Master student, McGill University
Director: Bill Coish
Subject to be annouced
Postdoc, Unviersité de Sherbrooke
Director: Stéfanos Kourtis
Stability of Majorana bound states in presence of nonMarkovian noise
We studied the dynamics of Majorana box qubits in the presence of nonMarkovian environment by performing analytical calculations of the decoherence parameter. We investigated the impact of longrange pairing interactions within the Kitaev pwave superconductors on this parameter. Our findings demonstrate that these interactions decrease the transition amplitude from zeroenergy level to excited states, leading to a reduction of decoherence parameter. This outcome proposes a practical approach for creating Majorana qubits that exhibit high resilience against nonMarkovian noise.
Master student, Université de Sherbrooke
Director: Mathieu Juan
Subject to be annouced
PhD student, Université de Sherbrooke
Director: Stéfanos Kourtis
Subject to be annouced
PhD student, McGill University
Director: Tami PeregBarnea
Subject to be annouced
Under gratuate intern, McGill University
Director: Bill Coish
Neural network quantum state for learning dynamics and state reconstruction
We present two of our recent works on using neural network quantum state ansatz to help learning properties of a quantum system. The first work studies the dynamics of the Gaudin magnet (“centralspin model”) using machinelearning methods. We use a neuralnetwork representation (restricted Boltzmann machine) to obtain accurate representations of the ground state and of the lowlying excited states of the Gaudinmagnet Hamiltonian through a variational Monte Carlo calculation. From the lowlying eigenstates, we find the nonperturbative dynamic transverse spin susceptibility, describing the linear response of a central spin to a timevarying transverse magnetic field in the presence of a spin bath. In the second work, we present “neural–shadow quantum state tomography” (NSQST)—an alternative neural networkbased QST protocol that uses infidelity as the loss function. The infidelity is estimated using the classical shadows of the target state. Infidelity is a natural choice for training loss, benefiting from the proven measurement sample efficiency of the classical shadow formalism. We numerically demonstrate the advantage of NSQST at learning the relative phases of three target quantum states of practical interest, as well as the advantage over direct shadow estimation.