October 9, 2020 9:00 AM

October 10, 2020 5:00 PM
October 9, 2020 9:00 AM

October 10, 2020 5:00 PM
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Virtuel sur Zoom et Remo
Virtuel sur Zoom et Remo
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.
Les partenaires de l'événement sont
Des tables de discussion seront animées par
Prenez le temps de visionner les vidéos de : Prompt, COPL, Quantino & Quantino virtual tour, Ki3 Photonics
9:00  9:10 Mot d'ouverture (sur zoom)
9:10  9:45 Professor Nicolas Godbout, Polytechnique Montréal, Canada
Québec Quantum overview
9:45  10:30 Professeur Jack Sankey, McGill University, Canada (sur zoom)
Sensing Stray Fields from SpinHallControlled Magnetic Nanowires with NitrogenVacancy Centers in Diamond
10:30  11:00 Joël Griesmar, Université de Sherbrooke, Canada
Towards a Microwave Single Photon Detector Using Inelastic Cooper Pair Tunneling
11:00  11:30 Pause café (sur Remo)
Participez aux tables de discussions
11:30  12:00 Présentations de regroupement stratégique du FRQNT (sur Remo)
Professor Mounir Boukadoum, ReSMiQ director
Professor Sophie Larochelle, COPL director, Presentation Présentation
12:00  13:00 Dîner (sur Remo)
Participez aux tables de discussions
13:00  14:00 Professeure Christine Silberhorn, Paderborn University, Gemany (sur zoom)
Nonlinear integrated quantum optics with pulsed light
14:15  15:00 Présentation de nos partenaires (sur Remo)
Optonique
Prompt & Prima
Jérome Cabana, Mitacs  Mitacs supporting collaborative research
15:00  15:30 Pause café (sur Remo)
Participez aux tables de discussions
15:30  16:00 Professeur Raymond Laflamme, IQC  Waterloo University, Canada (sur zoom)
Algorithmic cooling
16:00  16:30 Professeur Professor Kenneth R. Brown, Duke University, USA
Advantages of Subsystem Codes
16:30  17:00 Nicolas Delfosse, Ph.D. Microsoft, USA (sur zoom)
Decoding Hardware Requirements for FaultTolerant Quantum Computation
17:00  17:30 Steven T. Flammia, Ph.D. AWS Center for Quantum Computing, USA (sur zoom)
The XZZX Surface Code
9:00  10:00 Professeur Achim Kempf, Perimeter Institute, Canada (sur zoom)
Quantum Information Theoretic Methods in Inflationary Cosmology
10:00  11:00 Pause café (sur Remo)
Visitez les affiches ou participez aux tables de discussions
11:00  12:00 Professor Anand Natarajan, Massachusetts Institute of Technology (MIT), USA (sur zoom)
MIP* = RE
12:00  13:00 Dîner (sur Remo)
Visitez les affiches ou participez aux tables de discussions
13:00  14:00 Professeur Omar Fawzi, École normale supérieure de Lyon, France (Cancelled)
Constant overhead faulttolerance using quantum expander codes
Vouy êtes invités à vous joindre à nous sur Remo
14:00  14:45 Professeur Kartiek Agarwal, McGill University, Canada (sur zoom)
'Draiding' majoranas for quantum computing and Hamiltonian engineering
14:45  15:30 Pause café (sur Remo)
Visitez les affiches ou participez aux tables de discussions
15:30  16:30 Professeur Oussama Moutanabbir, Polytechnique Montréal, Canada (sur zoom)
An AllGroup IV Platform for Photonics and Quantum Engineering
16:30  17:00 Supartha Podder, Ph.D.Ottawa University, Canada (On zoom)
Symmetries, graph properties, and quantum speedups
17:00  Annonce des lauréats de la compétition d'affiche et mot de clôture (sur zoom)
Duke University, USA
Advantages of Subsystem Codes
Quantum error correction allows us to make highfidelity encoded qubits from noisy qubits. David Poulin pioneered subsystem quantum error correction where not all degrees of freedom are corrected. This gauge freedom can lead to improved decoding efficiency and remarkably better performance for errors outside the random Pauli error model. I will describe our theoretical work on leakage errors in the context of subsystem codes (arXiv:1903.03937) and our experimental implementation of the BaconShor subsystem code with the University of Maryland (arXiv:2009.11482).
Microsoft, USA
Decoding Hardware Requirements for FaultTolerant Quantum Computation
A major challenge for quantum error correction is to design a decoder capable of identifying errors faster than they accumulate. In this talk, I will describe a decoder microarchitecture for surface code error correction which can be executed fast enough to correct errors in a large scale quantum computer. I will discuss different computer architecture techniques which provides simultaneously a hardware acceleration of the decoder and a significant reduction of the bandwidth and decoding hardware requirements.
Based on joint work with Poulami Das, Chris Pattison, Bobbie Manne, Doug Carmean, Krysta Svore, Moin Qureshi
https://arxiv.org/abs/2001.06598
https://arxiv.org/abs/2001.11427
AWS Center for Quantum Computing, Pasadena, USA
The XZZX Surface Code
We show that a variant of the surface codethe XZZX codeoffers remarkable performance for faulttolerant quantum computation. The error threshold of this code matches what can be achieved with random codes (hashing) for every singlequbit Pauli noise channel; it is the first explicit code shown to have this universal property. We present numerical evidence that the threshold even exceeds this hashing bound for an experimentally relevant range of noise parameters. Focusing on the common situation where qubit dephasing is the dominant noise, we show that this code has a practical, highperformance decoder and surpasses all previously known thresholds in the realistic setting where syndrome measurements are unreliable. We go on to demonstrate the favorable subthreshold resource scaling that can be obtained by specializing a code to exploit structure in the noise. We show that it is possible to maintain all of these advantages when we perform faulttolerant quantum computation. We finally suggest some smallscale experiments that could exploit noise bias to reduce qubit overhead in twodimensional architectures. Joint work with J. P. Bonilla Ataides, D. K. Tuckett, S. D. Bartlett, and B. J. Brown, preprint: arXiv:2009.07851.
Institute for quantum computing (IQC)  Waterloo Unversity, Canada
Algorithmic cooling
Algorithmic cooling has been shown to purify qubits by redistribution of entropy and multiple contact with a heatbath. In this talk, I will give an overview of
the field and mention recent results where an implicit restriction assumed in all previous work about the interaction with the bath has been removed. I will show that more efficient algorithmic cooling can be achieved. I will then turn to another application of algorithmic cooling, distinguishing state preparation and measurment errors in quantum computers.
Perimeter Institute  Waterloo Unversity, Canada
Quantum Information Theoretic Methods in Inflationary Cosmology
At the Planck scale, and in the very early universe, the very notions of spacetime and of matter are expected to reach the limit of their range of applicability. The use of quantum informationtheoretic tools could be useful in these circumstances because even if the phenomena at the Planck scale are far removed from our intuition, it should always make sense to quantify how much information processing is involved. I will first review related informationtheoretic concepts such as covariant bandlimitation. I will then focus on the prospect that informationtheoretic features of Planck scale physics have left an imprint in inflation and, therefore, possibly observably, in the cosmic microwave background.
École normale supérieure de Lyon, France
Constant overhead faulttolerance using quantum expander codes
We prove that quantum expander codes can be combined with quantum faulttolerance techniques to achieve constant overhead: the ratio between the total number of physical qubits required for a quantum computation with faulty hardware and the number of logical qubits involved in the ideal computation is asymptotically constant, and can even be taken arbitrarily close to 1 in the limit of small physical error rate. This improves on the polylogarithmic overhead promised by the standard threshold theorem. To achieve this, we exploit a framework introduced by Gottesman together with a family of constant rate quantum codes, quantum expander codes. Our main technical contribution is to analyze an efficient decoding algorithm for these codes and prove that it remains robust in the presence of noisy syndrome measurements, a property which is crucial for faulttolerant circuits. We also establish two additional features of the decoding algorithm that make it attractive for quantum computation: it can be parallelized to run in logarithmic depth, and is singleshot, meaning that it only requires a single round of noisy syndrome measurement.
Based on joint work with Antoine Grospellier and Anthony Leverrier
available https://arxiv.org/abs/1808.03821.
Polytechnique Montréal, Canada
An AllGroup IV Platform for Photonics and Quantum Engineering
Compound semiconductors have been successfully used for precise and simultaneous control of lattice parameters and bandgap directness bringing to existence a variety of functional heterostructures and lowdimensional systems. Extending this paradigm to group IV semiconductors will be a true breakthrough that will pave the way to create an entirely new class of scalable, siliconintegrated optoelectronic, photonic, and quantum devices. In this presentation, I will share with the INTRQ community strategies and new material systems to independently engineer lattice dynamics and band structure in group IV device structures. The materials to be discussed are grown epitaxially using chemical vapor deposition system equipped with isotopically enriched precursors with purity higher than 99.9%: 28SiH4, 29SiH4, 30SiH4, 74GeH4, and 76GeH4 in addition to Sn precursors. I will address (Si)GeSn semiconductors and their use in Sibased midinfrared photonics and optoelectronics. I will also describe our progress in materials and metrology to control stable isotopes as an additional degree of freedom the tailor the basic behavior of semiconductor quantum structures.
Massachusetts Institute of Technology (MIT), USA
MIP* = RE
Interactive proof systems are a classic idea in theoretical computer science, and have led to fundamental advances in complexity theory and cryptography. Remarkably, in quantum information, interactive proof systems with multiple provers have become an important tool for studying quantum entanglement and nonlocality, extending the pioneering work of Bell in the 1960s and Tsirelson in the 80s. In this talk I will discuss our recent work characterizing the power of the complexity class MIP* of such proof systems where the provers share entanglement. On the complexity side, we show that MIP* is equal to the class of recursively enumerable languages, a class including the halting problem. As consequence we show a separation between two models of quantum nonlocality: the tensor product and the commuting operator model. This answers Tsirelson’s problem, and also implies a negative answer to the Connes embedding problem in operator algebras. At the heart of this work are new protocols that use classical PCP techniques together with the rules of quantum mechanics to let a classical client precisely control an untrusted quantum server.
Based on https://arxiv.org/abs/2001.04383, joint with Zhengfeng Ji, Thomas Vidick, John Wright, and Henry Yuen
Integrated Quantum Optics, Department Physics, Universität Paderborn, Germany
Nonlinear integrated quantum optics with pulsed light
Quantum technologies promise a change of paradigm for many fields of application, for example in communication systems, in highperformance computing and simulation of quantum systems, as well as in sensor technology. They can shift the boundaries of today’s systems and devices beyond classical limits and seemingly fundamental limitations. Photonic systems, which comprise multiple optical modes as well as many nonclassical light quantum states of light, have been investigated intensively in various theoretical proposals over the last decades. However, their implementation requires advanced setups of high complexity, which poses a considerable challenge on the experimental side. The successful realization of controlled quantum network structures is key for many applications in quantum optics and quantum information science.
McGill University
'Draiding' majoranas for quantum computing and Hamiltonian engineering
We propose and analyze a family of periodic braiding protocols in systems with multiple localized Majorana modes (majoranas) for the purposes of Hamiltonian engineering. The protocols rely on double braids – draids – which flip the signs of both majoranas, as one is taken all the way around the other. Rapid draiding can be used to dynamically suppresses some or all intermajorana couplings. Suppressing all couplings can drastically reduce residual majorana dynamics, producing a more robust computational subspace. Nontrivial topological models can be obtained by selectively applying draids to some of the overlapping (imperfect) majoranas. Remarkably, draids can be implemented without having to physically braid majoranas or performing projective measurements. For instance, we show that draids can be performed by periodically modulating the coupling between a quantum dot and a topological superconducting wire to dynamically suppress the hybridization of majoranas in the quantum wire. In current experimental setups, this could lead to suppression of this coupling by a few orders of magnitude. The robustness of this protocol can be shown to parallel the topological robustness of physically braided majoranas. We propose an architecture that implements draids between distant majorana modes within quantum register using a setup with multiple quantum dots and also discuss measurementbased ways of implementing the same.
Postdoctoral fellow at Université de Sherbrooke
Supervisor : Max Hofheinz
Towards a Microwave Single Photon Detector Using Inelastic Cooper Pair Tunneling
The detection of single photons is a fundamental quantum measurement, complementary to linear amplification. However, in the microwave domain this is a difficult task due to the low energy of the photons. We present here a photomultiplier using the energy of a Cooper pair tunneling across a voltagebiased Josephson junction to convert one microwave photon into several photons at a different frequency. This process relies on the strong nonlinearity provided by the interaction between a Josephson junction and its highimpedance electromagnetic environment. We have fabricated and measured a device composed of a low critical current SQUID galvanically coupled to two highimpedance resonators. It showed conversion from one to two photons with an efficiency of 80% and also exhibited conversion from one to three photons. By cascading two of these multiplication stages and adding a quantum limited amplifier, it should be possible to discriminate itinerant single photon states from vacuum without dead time.
McGill University
Sensing Stray Fields from SpinHallControlled Magnetic Nanowires with NitrogenVacancy Centers in Diamond
The individual spins associated with nitrogenvacancy (NV) centers in diamond can serve as exquisite nanoscale magnetic field sensors, enabling (among other things) the measurement of stray fields near YIG [1] and Py [2] films influenced by spin transfer torques. This presentation provides an overview of our latest efforts to (i) reliably fabricate highquality Py/Pt nanowires on a singlecrystal diamond substrate while maintaining the performance of NVs implanted 10's of nanometers below the surface, (ii) develop a Bayesian protocol for optimal estimation of NV spin relaxation, and (iii) estimate thermal time scales in these (and other) nanocircuits while calibrating the applied microwave current. Time permitting, I will also discuss preliminary measurements (microwave transport and NV magnetometry) of driven and thermally populated spin wave modes controlled by spin Hall torques.
Postdoctoral fellow at Ottawa University
Supervisor : Anne Broadbent
Symmetries, graph properties, and quantum speedups
Aaronson and Ambainis (2009) and Chailloux (2018) showed that fully symmetric (partial) functions do not admit exponential quantum query speedups. This raises a natural question: how symmetric must a function be before it cannot exhibit a large quantum speedup? In this work, we prove that hypergraph symmetries in the adjacency matrix model allow at most a polynomial separation between randomized and quantum query complexities. We also show that, remarkably, permutation groups constructed out of these symmetries are essentially the only permutation groups that prevent superpolynomial quantum speedups. We prove this by fully characterizing the primitive permutation groups that allow superpolynomial quantum speedups. In contrast, in the adjacency list model for boundeddegree graphs (where graph symmetry is manifested differently), we exhibit a property testing problem that shows an exponential quantum speedup. These results resolve open questions posed by Ambainis, Childs, and Liu (2010) and Montanaro and de Wolf (2013).
arxiv:2006.12760
(reference : poster #1) See the Poster and see the Video
PhD student at Université de Montréal
Supervisor : Gilles Brassard
From PseudoTelepathy Games to KochenSpecker Sets
It is known that, for a pseudotelepathy (PT) game with 2 players, the minimal input cardinality is 3 x 3, the minimal output cardinality is 2 x 3 and the minimal dimension of the shared entangled state is 3 x 3. Some PT games are known such that 2 out of these 3 minimal conditions are met (e.g. the impossible coloring game), but no game is known such that all 3 conditions are met simultaneously. Is there such a "minimal" PT game? In fact, it falls down to asking the following question: what is the size of the smallest KochenSpecker (KS) set in 3 dimensions? These intriguing sets of vectors show the incompatibility between realism and noncontextuality in quantum mechanics. Wolf and Renner showed that there is an interesting link between PT games and KS sets. I will explore this relation and explain how it could be used to prove the existence (or not) of a "minimal" PT game.
(reference : poster #2) See the Poster and see the Vidéo
PhD student at Polytechnique Montréal
Supervisor : Denis Seletskiy
Conditioned thermal states for subcycle sampling of quantum fields
Timedomain quantum electrodynamics is coming of age with recent demonstrations of direct probing of femtosecond quantum fields. In current experimental implementations, the achievable signaltonoise ratio of these weak measurements is ultimately limited by the onset of quantum backaction. Here, we propose to get around this issue by harnessing nonclassical states of light from the postselection of bright entangled sources, dramatically improving detection capabilities for fieldresolved metrology.
(reference : poster #3) See the Poster and see the Video
PhD student at McGill University
Supervisor : Bill Coish
Adiabatic conversion for qubit readout: Optimal pulse shapes and dephasing
The balance between 'adiabaticity' and dephasing of adiabatic conversion schemes can be optimized to improve performance. We give an explicit construction that allows for optimal state conversion in qubit readouts. Applying this scheme to the specific case of spin qubits in quantum dots, we show that a highfidelity (better than 99.9%) singleshot allelectrical readout is possible.
(reference : poster #4) See the Poster and see the Video
Undergraduate intern at McGill University
Supervisor : Kartiek Agarwal
Exploring the effect of noise on (polyfractal) driving to create multiple symmetries in manybody quantum systems
Symmetries (and their spontaneous rupturing) can be used to protect and engender novel quantum phases and lead to interesting collective phenomena. In [1], the authors described a general dynamical decoupling (“polyfractal”) protocol that can be used to dynamically engineer multiple discrete symmetries in manybody systems. The present work expands on [1] by studying the effect of noise on such a dynamical scheme. To make the analysis tractable, and numerical simulations efficient, we insert errors in the pattern of a Fibonacci replacement sequence. We find generically that the scheme yields symmetries that are protected up to exponentially long times in the inverse error rate. We also discuss how such symmetries can be engineered to protect quantum information and the affect of noise using this scheme. 1. K. Agarwal, I. Martin, Dynamical enhancement of symmetries in manybody systems, Phys Rev. Lett. 125, 080602 (2020)
(reference : poster #5) See the Poster and see the Video
PhD student at McGill University
Supervisor : Bill Coish
NonMarkovian qubit spectroscopy in cavity QED
Markovian models of qubit dynamics break down for charge qubits coupled to 1/f noise and for spin qubits coupled to slow nuclearspin baths. For spin/charge qubits also coupled to a cavity, it can be difficult to directly extract timedomain coherence dynamics because the AC control fields used to prepare and measure these qubits have the potential to excite the cavity mode. We present a way of extracting the coherence dynamics of a qubit coupled to a cavity purely from frequencydependent measurements of the cavity response in cavity quantum electrodynamics (QED). In contrast to a more standard equationofmotion approach, we make neither a Markov approximation nor a weakcoupling approximation for the qubitbath dynamics. Using this approach, we calculate the spectroscopic response of a spin qubit coupled to nuclear spins. This response shows pronounced nonLorentzian features, indicative of nonMarkovian dynamics, arising from a manyspin collective mode. We also consider the case of a qubit coupled to a single bosonic mode corresponding to, e.g., a mechanical degree of freedom, phonon, or cavity mode. In this instance, strong coupling gives rise to higher harmonics in the qubit coherence spectrum.
(reference : poster #6) See the Poster and see the Video
Master student at McGill University
Supervisor : Kartiek Agarwal
Reducing Majorana Hybridization via Periodic Driving
It is an ongoing challenge to engineer setups where Majorana zero modes at the ends of onedimensional topological superconductors are well isolated which is the essence of topological protection. Recent developments have indicated that periodic driving of a system can dynamically induce symmetries that its static counterpart does not possess [1]. We further develop the original protocol [2] where this idea [1] is applied to a system of a quantum dot (QD) coupled to a Kitaev chain hosting imperfect (overlapping) Majoranas. We numerically simulate a dynamical protocol in which an electron hops back and forth from the QD and the chain by LandauZener transition driven via time periodic local potential on the QD. We demonstrate that the current protocol reduces a nonzero hybridization energy that manifests from imperfect Majoranas by orders of magnitude.[1] K. Agarwal and I. Martin, Phys. Rev. Lett. 125, 080602 (2020)[2] I. Martin and K. Agarwal, arXiv preprint arXiv:2004.11385 (2020).
(reference : poster #7) See the Poster and see the Video
Undergraduate intern at McGill University
Supervisor : Bill Coish
The Use of Restricted Boltzmann Machines for Modeling a Manybody Quantum System
Manybody quantum systems are computationally challenging to simulate, since the number of amplitudes required to describe a general quantum state grows exponentially with the number of particles. For some specific manybody problems, machinelearning algorithms have been effective in reducing this complexity. We are investigating the use of a restricted Boltzmann machine, to more efficiently simulate the centralspin model. This model describes the interaction between a central spin and multiple environmental spins. It is used in describing the hyperfine interaction between an electron spin in a quantum dot and surrounding nuclear spins. In addition, the centralspin model can be mapped to the BCS model of superconductivity. If more efficient modeling methods are found for the centralspin model, then they could be additionally applied to problems in nonequilibrium superconductivity.
(reference : poster #8) See the Poster and see the Video
PhD student at McGill University
Supervisor : Lilian Childress
Sensing Stray Fields From Magnetic Nanocircuits With NitrogenVacancy Defects
We discuss our latest efforts toward using a single, optically active nitrogenvacancy (NV) spin sensor (implanted in a singlecrystal diamond substrate) to measure magnetic thermal noise modified by spin Hall torques [1] in a Py/Pt nanowire [2]. In this poster, we first present our subtractive method for fabricating magnetic nanocircuits on diamond, and initial characterization of working Py(5 nm)/Pt(5 nm) nanowires. We reliably achieve contacts with fewohms of resistance and 200 nm of overlap at each end of the 2µmlong, 400nmwide wire, and we observe an anisotropic magnetoresistance of 0.4%. Importantly, we observe that the subtractive patterning process (masked ion milling) used to define the devices reduces the NV spin resonance contrast by a factor of ~5, but that subsequent exposure to an oxygen plasma returned the contrast to its nominal level without affecting the nanowire behavior. Finally, we present progress toward using transport measurements to estimate thermal time scales in these systems (while calibrating the RF current in the nanowire), as well as preliminary NV readout of spintransfercontrolled magnetic thermal noise.
[1] C. Du, T. van der Sar, et al., Science, 357, 195198 (2017)
[2] A. Solyom, Z. Flansberry, et al., Nano Lett., 18, 64946499 (2018)
Partners of the Fall 2020 INTRIQ events
Discussion tables
Take time to see the video of : Prompt, COPL, Quantino & Quantino virtual tour, Ki3 Photonics
9:00  9:10 Opening remarks (on Zoom)
9:10  9:45 Professor Nicolas Godbout, Polytechnique Montréal, Canada
Québec Quantum overview
9:45  10:30 Professor Jack Sankey, McGill University, Canada
Sensing Stray Fields from SpinHallControlled Magnetic Nanowires with NitrogenVacancy Centers in Diamond
10:30  11:00 Dr Joël Griesmar, Université de Sherbrooke, Canada
Towards a Microwave Single Photon Detector Using Inelastic Cooper Pair Tunneling
11:00  11:30 Coffee break (on Remo platform)
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11:30  12:00 FRQNT strategic networks presentations (on Remo platform)
Professor Mounir Boukadoum, ReSMiQ director
Professor Sophie Larochelle, COPL director, Presentation slides
12:00  13:00 Lunch (on Remo platform)
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13:00  14:00 Professor Christine Silberhorn, Paderborn University, Gemany (on Zoom)
Nonlinear integrated quantum optics with pulsed light
14:15  15:00 Partner's talk (on Remo platform)
Optonique
Prompt & Prima
Jérome Cabana, Mitacs  Mitacs supporting collaborative research
15:00  15:30 Coffee break (on Remo platform)
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15:30  16:00 Professor Raymond Laflamme, IQC  Waterloo University, Canada
Algorithmic cooling
16:00  16:30 Professor Professor Kenneth R. Brown, Duke University, USA
Advantages of Subsystem Codes
16:30  17:00 Dr Nicolas Delfosse, Microsoft, USA
Decoding Hardware Requirements for FaultTolerant Quantum Computation
17:00  17:30 Dr Steven T. Flammia, AWS Center for Quantum Computing, USA
The XZZX Surface Code
9:00  10:00 Professor Achim Kempf, Perimeter Institute, Canada (On zoom)
Quantum Information Theoretic Methods in Inflationary Cosmology
10:00  11:00 Coffee break (on Remo platform)
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11:00  12:00 Professor Anand Natarajan, Massachusetts Institute of Technology (MIT), USA (On zoom)
MIP* = RE
12:00  13:00 Lunch (on Remo platform)
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13:00  14:00 Professor Omar Fawzi, École normale supérieure de Lyon, France (Cancelled)
Constant overhead faulttolerance using quantum expander codes
Please joint us at the poster competition on Remo
14:00  14:45 Professor Kartiek Agarwal, McGill University, Canada (On zoom)
'Draiding' majoranas for quantum computing and Hamiltonian engineering
14:45  15:30 Coffee break (on Remo platform)
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15:30  16:30 Professor Oussama Moutanabbir, Polytechnique Montréal, Canada (On zoom)
An AllGroup IV Platform for Photonics and Quantum Engineering
16:30  17:00 Dr Supartha Podder, Ottawa University, Canada (On zoom)
Symmetries, graph properties, and quantum speedups
17:00  Poster awards announcement and closing remarks (On zoom)
Duke University, USA
Advantages of Subsystem Codes
Quantum error correction allows us to make highfidelity encoded qubits from noisy qubits. David Poulin pioneered subsystem quantum error correction where not all degrees of freedom are corrected. This gauge freedom can lead to improved decoding efficiency and remarkably better performance for errors outside the random Pauli error model. I will describe our theoretical work on leakage errors in the context of subsystem codes (arXiv:1903.03937) and our experimental implementation of the BaconShor subsystem code with the University of Maryland (arXiv:2009.11482).
Microsoft, USA
Decoding Hardware Requirements for FaultTolerant Quantum Computation
A major challenge for quantum error correction is to design a decoder capable of identifying errors faster than they accumulate. In this talk, I will describe a decoder microarchitecture for surface code error correction which can be executed fast enough to correct errors in a large scale quantum computer. I will discuss different computer architecture techniques which provides simultaneously a hardware acceleration of the decoder and a significant reduction of the bandwidth and decoding hardware requirements.
Based on joint work with Poulami Das, Chris Pattison, Bobbie Manne, Doug Carmean, Krysta Svore, Moin Qureshi
https://arxiv.org/abs/2001.06598
https://arxiv.org/abs/2001.11427
AWS Center for Quantum Computing, Pasadena, USA
The XZZX Surface Code
We show that a variant of the surface codethe XZZX codeoffers remarkable performance for faulttolerant quantum computation. The error threshold of this code matches what can be achieved with random codes (hashing) for every singlequbit Pauli noise channel; it is the first explicit code shown to have this universal property. We present numerical evidence that the threshold even exceeds this hashing bound for an experimentally relevant range of noise parameters. Focusing on the common situation where qubit dephasing is the dominant noise, we show that this code has a practical, highperformance decoder and surpasses all previously known thresholds in the realistic setting where syndrome measurements are unreliable. We go on to demonstrate the favorable subthreshold resource scaling that can be obtained by specializing a code to exploit structure in the noise. We show that it is possible to maintain all of these advantages when we perform faulttolerant quantum computation. We finally suggest some smallscale experiments that could exploit noise bias to reduce qubit overhead in twodimensional architectures. Joint work with J. P. Bonilla Ataides, D. K. Tuckett, S. D. Bartlett, and B. J. Brown, preprint: arXiv:2009.07851.
Institute for quantum computing (IQC)  Waterloo Unversity, Canada
Algorithmic cooling
Algorithmic cooling has been shown to purify qubits by redistribution of entropy and multiple contact with a heatbath. In this talk, I will give an overview of
the field and mention recent results where an implicit restriction assumed in all previous work about the interaction with the bath has been removed. I will show that more efficient algorithmic cooling can be achieved. I will then turn to another application of algorithmic cooling, distinguishing state preparation and measurment errors in quantum computers.
Perimeter Institute  Waterloo Unversity, Canada
Quantum Information Theoretic Methods in Inflationary Cosmology
At the Planck scale, and in the very early universe, the very notions of spacetime and of matter are expected to reach the limit of their range of applicability. The use of quantum informationtheoretic tools could be useful in these circumstances because even if the phenomena at the Planck scale are far removed from our intuition, it should always make sense to quantify how much information processing is involved. I will first review related informationtheoretic concepts such as covariant bandlimitation. I will then focus on the prospect that informationtheoretic features of Planck scale physics have left an imprint in inflation and, therefore, possibly observably, in the cosmic microwave background.
École normale supérieure de Lyon, France
Constant overhead faulttolerance using quantum expander codes
We prove that quantum expander codes can be combined with quantum faulttolerance techniques to achieve constant overhead: the ratio between the total number of physical qubits required for a quantum computation with faulty hardware and the number of logical qubits involved in the ideal computation is asymptotically constant, and can even be taken arbitrarily close to 1 in the limit of small physical error rate. This improves on the polylogarithmic overhead promised by the standard threshold theorem. To achieve this, we exploit a framework introduced by Gottesman together with a family of constant rate quantum codes, quantum expander codes. Our main technical contribution is to analyze an efficient decoding algorithm for these codes and prove that it remains robust in the presence of noisy syndrome measurements, a property which is crucial for faulttolerant circuits. We also establish two additional features of the decoding algorithm that make it attractive for quantum computation: it can be parallelized to run in logarithmic depth, and is singleshot, meaning that it only requires a single round of noisy syndrome measurement.
Based on joint work with Antoine Grospellier and Anthony Leverrier
available https://arxiv.org/abs/1808.03821.
Polytechnique Montréal, Canada
An AllGroup IV Platform for Photonics and Quantum Engineering
Compound semiconductors have been successfully used for precise and simultaneous control of lattice parameters and bandgap directness bringing to existence a variety of functional heterostructures and lowdimensional systems. Extending this paradigm to group IV semiconductors will be a true breakthrough that will pave the way to create an entirely new class of scalable, siliconintegrated optoelectronic, photonic, and quantum devices. In this presentation, I will share with the INTRQ community strategies and new material systems to independently engineer lattice dynamics and band structure in group IV device structures. The materials to be discussed are grown epitaxially using chemical vapor deposition system equipped with isotopically enriched precursors with purity higher than 99.9%: 28SiH4, 29SiH4, 30SiH4, 74GeH4, and 76GeH4 in addition to Sn precursors. I will address (Si)GeSn semiconductors and their use in Sibased midinfrared photonics and optoelectronics. I will also describe our progress in materials and metrology to control stable isotopes as an additional degree of freedom the tailor the basic behavior of semiconductor quantum structures.
Massachusetts Institute of Technology (MIT), USA
MIP* = RE
Interactive proof systems are a classic idea in theoretical computer science, and have led to fundamental advances in complexity theory and cryptography. Remarkably, in quantum information, interactive proof systems with multiple provers have become an important tool for studying quantum entanglement and nonlocality, extending the pioneering work of Bell in the 1960s and Tsirelson in the 80s. In this talk I will discuss our recent work characterizing the power of the complexity class MIP* of such proof systems where the provers share entanglement. On the complexity side, we show that MIP* is equal to the class of recursively enumerable languages, a class including the halting problem. As consequence we show a separation between two models of quantum nonlocality: the tensor product and the commuting operator model. This answers Tsirelson’s problem, and also implies a negative answer to the Connes embedding problem in operator algebras. At the heart of this work are new protocols that use classical PCP techniques together with the rules of quantum mechanics to let a classical client precisely control an untrusted quantum server.
Based on https://arxiv.org/abs/2001.04383, joint with Zhengfeng Ji, Thomas Vidick, John Wright, and Henry Yuen
Integrated Quantum Optics, Department Physics, Universität Paderborn, Germany
Nonlinear integrated quantum optics with pulsed light
Quantum technologies promise a change of paradigm for many fields of application, for example in communication systems, in highperformance computing and simulation of quantum systems, as well as in sensor technology. They can shift the boundaries of today’s systems and devices beyond classical limits and seemingly fundamental limitations. Photonic systems, which comprise multiple optical modes as well as many nonclassical light quantum states of light, have been investigated intensively in various theoretical proposals over the last decades. However, their implementation requires advanced setups of high complexity, which poses a considerable challenge on the experimental side. The successful realization of controlled quantum network structures is key for many applications in quantum optics and quantum information science.
McGill University
'Draiding' majoranas for quantum computing and Hamiltonian engineering
We propose and analyze a family of periodic braiding protocols in systems with multiple localized Majorana modes (majoranas) for the purposes of Hamiltonian engineering. The protocols rely on double braids – draids – which flip the signs of both majoranas, as one is taken all the way around the other. Rapid draiding can be used to dynamically suppresses some or all intermajorana couplings. Suppressing all couplings can drastically reduce residual majorana dynamics, producing a more robust computational subspace. Nontrivial topological models can be obtained by selectively applying draids to some of the overlapping (imperfect) majoranas. Remarkably, draids can be implemented without having to physically braid majoranas or performing projective measurements. For instance, we show that draids can be performed by periodically modulating the coupling between a quantum dot and a topological superconducting wire to dynamically suppress the hybridization of majoranas in the quantum wire. In current experimental setups, this could lead to suppression of this coupling by a few orders of magnitude. The robustness of this protocol can be shown to parallel the topological robustness of physically braided majoranas. We propose an architecture that implements draids between distant majorana modes within quantum register using a setup with multiple quantum dots and also discuss measurementbased ways of implementing the same.
Postdoctoral fellow at Université de Sherbrooke
Supervisor : Max Hofheinz
Towards a Microwave Single Photon Detector Using Inelastic Cooper Pair Tunneling
The detection of single photons is a fundamental quantum measurement, complementary to linear amplification. However, in the microwave domain this is a difficult task due to the low energy of the photons. We present here a photomultiplier using the energy of a Cooper pair tunneling across a voltagebiased Josephson junction to convert one microwave photon into several photons at a different frequency. This process relies on the strong nonlinearity provided by the interaction between a Josephson junction and its highimpedance electromagnetic environment. We have fabricated and measured a device composed of a low critical current SQUID galvanically coupled to two highimpedance resonators. It showed conversion from one to two photons with an efficiency of 80% and also exhibited conversion from one to three photons. By cascading two of these multiplication stages and adding a quantum limited amplifier, it should be possible to discriminate itinerant single photon states from vacuum without dead time.
McGill University
Sensing Stray Fields from SpinHallControlled Magnetic Nanowires with NitrogenVacancy Centers in Diamond
The individual spins associated with nitrogenvacancy (NV) centers in diamond can serve as exquisite nanoscale magnetic field sensors, enabling (among other things) the measurement of stray fields near YIG [1] and Py [2] films influenced by spin transfer torques. This presentation provides an overview of our latest efforts to (i) reliably fabricate highquality Py/Pt nanowires on a singlecrystal diamond substrate while maintaining the performance of NVs implanted 10's of nanometers below the surface, (ii) develop a Bayesian protocol for optimal estimation of NV spin relaxation, and (iii) estimate thermal time scales in these (and other) nanocircuits while calibrating the applied microwave current. Time permitting, I will also discuss preliminary measurements (microwave transport and NV magnetometry) of driven and thermally populated spin wave modes controlled by spin Hall torques.
Postdoctoral fellow at Ottawa University
Supervisor : Anne Broadbent
Symmetries, graph properties, and quantum speedups
Aaronson and Ambainis (2009) and Chailloux (2018) showed that fully symmetric (partial) functions do not admit exponential quantum query speedups. This raises a natural question: how symmetric must a function be before it cannot exhibit a large quantum speedup? In this work, we prove that hypergraph symmetries in the adjacency matrix model allow at most a polynomial separation between randomized and quantum query complexities. We also show that, remarkably, permutation groups constructed out of these symmetries are essentially the only permutation groups that prevent superpolynomial quantum speedups. We prove this by fully characterizing the primitive permutation groups that allow superpolynomial quantum speedups. In contrast, in the adjacency list model for boundeddegree graphs (where graph symmetry is manifested differently), we exhibit a property testing problem that shows an exponential quantum speedup. These results resolve open questions posed by Ambainis, Childs, and Liu (2010) and Montanaro and de Wolf (2013).
arxiv:2006.12760
(reference : poster #1) See the Poster and see the Video
PhD student at Université de Montréal
Supervisor : Gilles Brassard
From PseudoTelepathy Games to KochenSpecker Sets
It is known that, for a pseudotelepathy (PT) game with 2 players, the minimal input cardinality is 3 x 3, the minimal output cardinality is 2 x 3 and the minimal dimension of the shared entangled state is 3 x 3. Some PT games are known such that 2 out of these 3 minimal conditions are met (e.g. the impossible coloring game), but no game is known such that all 3 conditions are met simultaneously. Is there such a "minimal" PT game? In fact, it falls down to asking the following question: what is the size of the smallest KochenSpecker (KS) set in 3 dimensions? These intriguing sets of vectors show the incompatibility between realism and noncontextuality in quantum mechanics. Wolf and Renner showed that there is an interesting link between PT games and KS sets. I will explore this relation and explain how it could be used to prove the existence (or not) of a "minimal" PT game.
(reference : poster #2) See the Poster and see the Vidéo
PhD student at Polytechnique Montréal
Supervisor : Denis Seletskiy
Conditioned thermal states for subcycle sampling of quantum fields
Timedomain quantum electrodynamics is coming of age with recent demonstrations of direct probing of femtosecond quantum fields. In current experimental implementations, the achievable signaltonoise ratio of these weak measurements is ultimately limited by the onset of quantum backaction. Here, we propose to get around this issue by harnessing nonclassical states of light from the postselection of bright entangled sources, dramatically improving detection capabilities for fieldresolved metrology.
(reference : poster #3) See the Poster and see the Video
PhD student at McGill University
Supervisor : Bill Coish
Adiabatic conversion for qubit readout: Optimal pulse shapes and dephasing
The balance between 'adiabaticity' and dephasing of adiabatic conversion schemes can be optimized to improve performance. We give an explicit construction that allows for optimal state conversion in qubit readouts. Applying this scheme to the specific case of spin qubits in quantum dots, we show that a highfidelity (better than 99.9%) singleshot allelectrical readout is possible.
(reference : poster #4) See the Poster and see the Video
Undergraduate intern at McGill University
Supervisor : Kartiek Agarwal
Exploring the effect of noise on (polyfractal) driving to create multiple symmetries in manybody quantum systems
Symmetries (and their spontaneous rupturing) can be used to protect and engender novel quantum phases and lead to interesting collective phenomena. In [1], the authors described a general dynamical decoupling (“polyfractal”) protocol that can be used to dynamically engineer multiple discrete symmetries in manybody systems. The present work expands on [1] by studying the effect of noise on such a dynamical scheme. To make the analysis tractable, and numerical simulations efficient, we insert errors in the pattern of a Fibonacci replacement sequence. We find generically that the scheme yields symmetries that are protected up to exponentially long times in the inverse error rate. We also discuss how such symmetries can be engineered to protect quantum information and the affect of noise using this scheme. 1. K. Agarwal, I. Martin, Dynamical enhancement of symmetries in manybody systems, Phys Rev. Lett. 125, 080602 (2020)
(reference : poster #5) See the Poster and see the Video
PhD student at McGill University
Supervisor : Bill Coish
NonMarkovian qubit spectroscopy in cavity QED
Markovian models of qubit dynamics break down for charge qubits coupled to 1/f noise and for spin qubits coupled to slow nuclearspin baths. For spin/charge qubits also coupled to a cavity, it can be difficult to directly extract timedomain coherence dynamics because the AC control fields used to prepare and measure these qubits have the potential to excite the cavity mode. We present a way of extracting the coherence dynamics of a qubit coupled to a cavity purely from frequencydependent measurements of the cavity response in cavity quantum electrodynamics (QED). In contrast to a more standard equationofmotion approach, we make neither a Markov approximation nor a weakcoupling approximation for the qubitbath dynamics. Using this approach, we calculate the spectroscopic response of a spin qubit coupled to nuclear spins. This response shows pronounced nonLorentzian features, indicative of nonMarkovian dynamics, arising from a manyspin collective mode. We also consider the case of a qubit coupled to a single bosonic mode corresponding to, e.g., a mechanical degree of freedom, phonon, or cavity mode. In this instance, strong coupling gives rise to higher harmonics in the qubit coherence spectrum.
(reference : poster #6) See the Poster and see the Video
Master student at McGill University
Supervisor : Kartiek Agarwal
Reducing Majorana Hybridization via Periodic Driving
It is an ongoing challenge to engineer setups where Majorana zero modes at the ends of onedimensional topological superconductors are well isolated which is the essence of topological protection. Recent developments have indicated that periodic driving of a system can dynamically induce symmetries that its static counterpart does not possess [1]. We further develop the original protocol [2] where this idea [1] is applied to a system of a quantum dot (QD) coupled to a Kitaev chain hosting imperfect (overlapping) Majoranas. We numerically simulate a dynamical protocol in which an electron hops back and forth from the QD and the chain by LandauZener transition driven via time periodic local potential on the QD. We demonstrate that the current protocol reduces a nonzero hybridization energy that manifests from imperfect Majoranas by orders of magnitude.[1] K. Agarwal and I. Martin, Phys. Rev. Lett. 125, 080602 (2020)[2] I. Martin and K. Agarwal, arXiv preprint arXiv:2004.11385 (2020).
(reference : poster #7) See the Poster and see the Video
Undergraduate intern at McGill University
Supervisor : Bill Coish
The Use of Restricted Boltzmann Machines for Modeling a Manybody Quantum System
Manybody quantum systems are computationally challenging to simulate, since the number of amplitudes required to describe a general quantum state grows exponentially with the number of particles. For some specific manybody problems, machinelearning algorithms have been effective in reducing this complexity. We are investigating the use of a restricted Boltzmann machine, to more efficiently simulate the centralspin model. This model describes the interaction between a central spin and multiple environmental spins. It is used in describing the hyperfine interaction between an electron spin in a quantum dot and surrounding nuclear spins. In addition, the centralspin model can be mapped to the BCS model of superconductivity. If more efficient modeling methods are found for the centralspin model, then they could be additionally applied to problems in nonequilibrium superconductivity.
(reference : poster #8) See the Poster and see the Video
PhD student at McGill University
Supervisor : Lilian Childress
Sensing Stray Fields From Magnetic Nanocircuits With NitrogenVacancy Defects
We discuss our latest efforts toward using a single, optically active nitrogenvacancy (NV) spin sensor (implanted in a singlecrystal diamond substrate) to measure magnetic thermal noise modified by spin Hall torques [1] in a Py/Pt nanowire [2]. In this poster, we first present our subtractive method for fabricating magnetic nanocircuits on diamond, and initial characterization of working Py(5 nm)/Pt(5 nm) nanowires. We reliably achieve contacts with fewohms of resistance and 200 nm of overlap at each end of the 2µmlong, 400nmwide wire, and we observe an anisotropic magnetoresistance of 0.4%. Importantly, we observe that the subtractive patterning process (masked ion milling) used to define the devices reduces the NV spin resonance contrast by a factor of ~5, but that subsequent exposure to an oxygen plasma returned the contrast to its nominal level without affecting the nanowire behavior. Finally, we present progress toward using transport measurements to estimate thermal time scales in these systems (while calibrating the RF current in the nanowire), as well as preliminary NV readout of spintransfercontrolled magnetic thermal noise.
[1] C. Du, T. van der Sar, et al., Science, 357, 195198 (2017)
[2] A. Solyom, Z. Flansberry, et al., Nano Lett., 18, 64946499 (2018)