# Institut Transdisciplinaire d'Information Quantique (INTRIQ)

### Fall 2018 INTRIQ meeting, November 13 & 14th

Organizer :

Pr Guillaume Gervais, McGill University

**90, rue Stanstead, Bromont QC J2L 1K6****Téléphone : 1 800 304 3433**

**Note : The INTRIQ Business meeting (reserved for members) will be held in room "Salle des cantons" on November 13th from 9h30 to 10h30**

*Meeting program*

*November 13th*

10h30 - 10h55 Registration

10h55 - 11h00 Opening remarks (Salon A)

11h00 - 12h00 *Dr. Avishay Tal, Simons Institute & Stanford University* (Salon A)

*Oracle Separation of Quantum Polynomial Time and the Polynomial Hierarchy*

12h00 - 13h30 Lunch (Dining room - 4 Canards)

13h30 - 14h00 *Dr. Gustav Kalbe, Head of Unit, High Performance Computing & Quantum Technologies, European Commission* (Salon A)

*The Quantum Technologies FET Flagship Initiative of the European Union*

14h00 - 15h00 *Dr. Dominique Laroche, Delft University of Technology* (Salon A)

*Probing the building blocks of topological qubits in superconducting InAs nanowires*

15h00 - 15h30 Coffee break (Salon B)

15h30 - 16h00 *Dr. Yehua Liu, Postdoc, Université de Sherbrooke* (Salon A)

*Neural Belief-Propagation Decoders for Quantum Error-Correcting Codes*

16h00 – 16h20 *Marc-Olivier Proulx, Master, Université d'Ottawa* (Salon A)

*A limit on quantum nonlocality from an information processing principle*

16h20 - 17h00 Thomas Szkopek, Professor, McGill University (Salon A)

*An account of the commercialization of an idea*

17h00 - 17h10 *Maxime Tremblay & Alexandre Choquette-Poitevin, Université de Sherbrooke* (Salon A)

*Quantum Quarter*

17h00 - Poster session with refreshments (Salon B)

19h30 - INTRIQ dinner (Knowlton room)

*November 14th*

9h00 - 10h00 Dr. Kartiek Agarwal, *Princeton University* (Salon A)

**Spatio-temporal quenches for fast preparation of ground states of critical models**

10h00 - 10h45 *Alexander Maloney, Professor, McGill University* (Salon A)

*Quantum Information Theory, Black Holes and Space-time*

10h45 - 11h15 *Dr. Jonathan Gross, Postdoc, Université de Sherbrooke* (Salon A)

*Systems illuminated by squeezed wave-packet modes*

11h15 - 12h00 Coffee break (Salon B)

12h00 - 13h30 Lunch (Dining room - 4 Canards)

13h30 - 14h00 Yves Bérubé-Lauzière, Professor, Université de Sherbrooke (Salon A)

*QSciTech NSERC-CREATE Training Program - Bridging the Gap between Quantum Science and Quantum Technologies*

14h00 - 15h00 *Johannes Pollanen, Professor, Michigan State University* (Salon A)

*Hybrid quantum information systems with electrons on helium*

15h00 - 15h30 *Dr. Clément Godfrin, Postdoc, Université de Sherbrooke* (Salon A)

**Coherent manipulation of single nuclear spin**

15h30 - 15h40 Closing remarks

*INVITED SPEAKERS*

*Dr. Kartiek Agarwal**Princeton University**Spatio-temporal quenches for fast preparation of ground states of critical models*

The difficulty of preparing highly entangled quantum states poses an important challenge in engineering artificial quantum systems for the purposes of computation and simulation. Adiabatic methods which slowly evolve unentangled states to entangled states are typically slow and particularly fail at criticality where the gap between eigenstates vanishes. The search for novel non-adiabatic methods for quantum state preparation is a topic of current research interest, and immense experimental relevance. I will describe the state of the art in the field and discuss our proposal(s) using spatio-temporal quenches to efficiently prepare the ground states of arbitrary interacting critical theories in one dimension and beyond.

*Dr. Gustav Kalbe**Head of Unit**High Performance Computing & Quantum Technologies**European Commission The Quantum Technologies FET Flagship Initiative of the European Union*The Quantum Technologies Flagship is a Future and Emerging Technologies (FET) research initiative that was launched by the EU. This large-scale, long-term Flagship joins both academic and industry researchers to make scientific discoveries in the quantum field, using such innovations to help grow the economy so that Europe can resolve its current societal challenges. As a pan-European programme with support from each of its Member States, the Quantum Technologies Flagship will continue to develop using previous European funding from the last 20 years and allow Europe to match the research implemented by its competitors on the world stage.

*Pr Alexander MaloneyMcGill University Quantum Information Theory, Black Holes and Space-time*I will describe recent progress on the relationship between quantum information theory, quantum field theory and quantum gravity. I will review the theoretical evidence that space-time emerges from the entanglement of more fundamental quantum mechanical degrees of freedom. This evidence comes from recent advances in our understanding of quantum black holes and the holographic (AdS/CFT) correspondence. The relationship between quantum information theory, field theory and gravity has already led to new results, including new positive energy theorems in general relativity and a unified theory of renormalization group flows. If time permits, I will explain the notion that space-time can be regarded as a (quantum) error-correcting code. This will be an expository talk; no advanced background aside from some elements of quantum information theory will be assumed.

*Dr. Dominique Laroche**Delft University of Technology* *Probing the building blocks of topological qubits in superconducting InAs nanowires*

Utilizing the exotic properties of non-Abelian quasiparticles, topological qubits offer an approach towards quantum computing where the information is stored non-locally, making this an architecture resilient against most decoherence sources. Majorana bound states (MBS) arising in proximity-induced superconducting nanowires are currently the prime candidate for the implementation of such topological quantum bits. Thus far, MBS signatures chiefly relied on single electron tunnelling measurements, which lead to decoherence of the quantum information stored in the MBS through quasi-particle poisoning. In this talk, I will present a novel experimental platform where proximitized nanowire devices are coupled on-chip to microwave detectors and spectrometers, allowing for measurements of the building blocks of topological qubits in a parity conserving manner. Utilizing this platform, we directly observed a transition from a 2π- to a 4π-periodic Josephson radiation in InAs nanowire as a function of both magnetic field and chemical potential. This transition is consistent with the onset of MBS in the nanowires. We also performed microwave spectroscopy of the fundamental unit of a prospective topological qubit, a nanowire-based Cooper pair transistor. In addition to confirm the large tunability of this system, we were able to directly measure the population of the odd and of the even parity sector in these devices.

*Pr Johannes Pollanen**Michigan State University Hybrid quantum information systems with electrons on helium*

Electrons floating on the surface of liquid helium at low temperature were one of the first platforms proposed for quantum computation [1]. In this hybrid quantum system the surface of the liquid helium functions as a fantastically pristine substrate without the defects and imperfections that are unavoidable in almost all other material systems. Electrons placed near this liquid substrate are bound to it and float (in vacuum) about 10 nanometers above the surface. The motion of these electrons relative to the surface of the helium, as well as their spin, are quantum mechanical and form the basis for potentially new types of qubits. These qubits, if they can be realized, are predicted to be shielded from decoherence by the isolation provided by liquid helium substrate. I will describe the state of the art in the field, experimental milestones demonstrating the fantastic level of control that can be achieved in this system, and how the time is now ripe for using the hardware tool kit of circuit quantum electrodynamics for developing novel qubits from electrons on helium.

[1] P.M. Platzman and M.I. Dykman, Quantum Computing with Electrons Floating on Liquid Heilum, Science 284, 1967 (1999).

*Dr. Avishay Tal**Simons Institute & Stanford University***Oracle Separation of Quantum Polynomial Time and the Polynomial Hierarchy**

In their seminal paper, Bennett, Bernstein, Brassard and Vazirani [SICOMP, 1997] showed that relative to an oracle, quantum algorithms are unable to solve NP-complete problems in sub-exponential time (i.e., that Grover's search is optimal in this setting).

In this work, we show a strong converse to their result. Namely, we show that, relative to an oracle, there exist computational tasks that can be solved efficiently by a quantum algorithm, but require exponential time for any algorithm in the polynomial hierarchy. (The polynomial hierarchy is a hierarchy of complexity classes that captures P, NP, coNP, and their generalizations.)

The tasks that exhibit this "quantum advantage" arise from a pseudo-randomness approach initiated by Aaronson [STOC, 2010]. Our core technical result is constructing a distribution over Boolean strings that "look random" to constant-depth circuits of quasi-polynomial size, but can be distinguished from the uniform distribution by very efficient quantum algorithms.

(joint work with Ran Raz)

*INTRIQ SPEAKERS*

*Yves Bérubé-Lauzière**Professor, Université de Sherbrooke**QSciTech NSERC-CREATE Training Program - Bridging the Gap between Quantum Science and Quantum Technologies*

The QSciTech NSERC-CREATE program aims at training the next generation of quantum scientists, engineers and entrepreneurs. The program will provide integrative and targeted training to graduate students (PhD's and MSc's) in the field of quantum technologies, one of Canada's high-tech economic sector of priority. The goal is to train job-ready candidates so that they get the view of the whole chain of quantum technology development, encompassing basic quantum science, engineering methods, and professional skills. The training will provide engineering skills to quantum physics students and quantum awareness to engineering and computer science students. An overall view will be given of this innovative training program. The prerequisites and selection process of students, funding structure, training phases, and the targets that are expected to be reached for the coming 6 years of this new program will be presented.

*Clément Godfrin**Postdoc, Université de Sherbrooke*Director : Eva Dupont-Ferrier

**Coherent manipulation of single nuclear spin**Advances in experimental techniques offer physicists the opportunity to implement simple systems worth of the "gedanken-experiments" imagined by the founders of quantum theory. During the presentation, I propose to study one of these toy model systems, namely a single 3/2 nuclear spin. The presentation will start by investigating the read-out process and the coherent manipulation of the 4 nuclear spin states using a single molecular magnet transistor [1,2]. These preliminary results demonstrate that we have a fully controlled 4-level quantum system, a qudit, on which we recently implemented a quantum algorithm. With their state space of large dimension, qudits open fascinating experimental prospects. Protocols based on a generalization of the Ramsey interferometry to a multi-level system enable to measure, among others, the accumulation of geometric phases and of quantum gate phase [3]. As an outlook, I will display how, using a larger single nuclear spin, we could apply quantum error correction protocol [4], to obtain a self-corrected qubit.

1 Thiele S. et al. Science 344, 1135 (2014)

2 Godfrin C. et al. Phys. Rev. Lett. 119, 187702 (2017)

3 Godfrin C. et al. accepeted to Nature partener journal quantum information.

4 Pirandola S. et al. Phys. Rev. A 77, 032309 (2008)

*Jonathan Gross**Postdoc, Université de Sherbrooke*Director : Alexandre Blais

*Systems illuminated by squeezed wave-packet modes*White-noise theory is incapable of describing photon-counting measurements in the presence of thermal and squeezed noise. We accommodate such scenarios by considering an environment that includes traveling wave packets that are squeezed, deriving a hierarchy of equations similar to those used to describe traveling wave packets with fixed photon number. Squeezing introduces qualitatively different effects, however, complicating numerical solution of these hierarchies. We provide preliminary numerical analysis of the formalism and showcase its utility by calculating the resonance fluorescence of a two-level atom in squeezed vacuum with squeezing bandwidth narrower than the atomic linewidth, a regime inaccessible to previous techniques.

*Yehua Liu**Postdoc, Université de Sherbrooke**Director : David Poulin**Neural Belief-Propagation Decoders for Quantum Error-Correcting Codes*

Belief-propagation (BP) decoders play a vital role in modern coding theory. However, the classical design impairs their performance in quantum information processing. Inspired by an exact mapping between BP and deep neural networks, we train neural BP decoders for quantum low-density parity-check codes, with a loss function tailored for the quantum setting. Training substantially improves the performance of the original BP decoders. The flexibility and adaptability of the neural BP decoders make them suitable for low-overhead error correction in near-term quantum devices.

*Marc-Olivier Proulx**Master, Université d'Ottawa*Director : Anne Broadbent

*A limit on quantum nonlocality from an information processing principle*In their most common formulation, the axioms of quantum mechanics provide the mathematical description of quantum states, measurements and time evolution. The mathematical nature of the axioms enables us to make very precise predictions but provides little physical intuition on the quantum theory. For instance, the absence of consensus on the interpretation of the measurement process is a symptom of the lack of physical intuition in the mathematical axioms the quantum theory builds on. Nonlocality is another feature of quantum mechanics that arises from the mathematical axioms but that lacks an intuitive understanding. Indeed, quantum entanglement is known to give rise to nonlocal correlations that are not possible in a classical theory. Even though quantum correlations are stronger than classical correlations, they are still limited by the mathematical structure of quantum mechanics. Since physical limits usually emerge from physical principles, multiple principles were suggested in order to give a more physical explanation of the quantum limit on nonlocal correlations. None of these principles were able to completely rule out all super-quantum correlations. In this work, we study the principle of non-trivial communication complexity (NTCC), that sets a limit on what can be done in a particular information processing setting. Nonlocal correlations that violate this principle are believed to be impossible in nature. In this work, we expand the set of super-quantum correlations that are known to be ruled out by the NTCC principle, thus providing an explanation for their impossibility in quantum mechanics.

*Thomas SzkopekProfessor, McGill University*Through a number of serendipitous events, an idea that emerged in my laboratory - the application of graphene materials to acoustic transduction - is on the path to commercialization via the start-up ORA Graphene Audio. My talk will give a first hand account of the simple physics behind the idea, and how students were able to build a hardware start-up based upon it. I will highlight lessons learned through successes and failures encountered along the way.

**An account of the commercialization of an idea**

*Maxime Tremblay & Alexandre Choquette-Poitevin**Doctorate & Master students, Université de Sherbrooke*Directors: David Poulin & Alexandre Blais

*Quantum Quarter*In this talk, we will present the Q2 project. This student project aims to bridge the gap between the quantum industries and universities by creating new career opportunities for physics and engineering students.

**POSTER SESSION**

*Thomas Baker**Postdoc, Université de Sherbrooke**Director : Glen Evenbly**Role of canting and depleted-triplet minima in superconducting spin valve structures*

The trilayer and pentalayer spin valve structures are revisited to determine the behavior of pair correlations and Josephson current when the magnetic layers are canted at arbitrary angle. The two systems display markedly different behaviors in the center magnetic layer. While the trilayer generates a triplet component that is weakly affected by canting, the pentalayer tunes in singlet pair correlations depending heavily on canting. We also show that a minimum with depleted m=+/-1 triplet components, rather than a 0-π transition, may be observed in the current profile Ic(dF) of a trilayer spin valve. The depleted-triplet minimum (DTM) is directly attributable to a decrease of m=+/-1 triplet correlations with increased thickness of the central ferromagnet, accompanied by a hidden, simultaneous sign change of the Gor'kov functions contributed from the left and right superconductors. We introduce a toy model for superconducting-magnetic proximity systems to better illuminate the behavior of individual components of the Gor'kov function and compare with a full numerical calculation.

*Simon Bernard**Doctorate, McGill University**Director : Jack Sankey**Progress toward optical control of mechanical geometry*

We report experimental progress toward achieving our group's recent proposal [1] to use the field inside a Fabry-Perot cavity to apply a spatially confined optical spring to a single lattice site of a phononic crystal. This perturbation of the otherwise pristine phononic crystal will allow an unprecedented level of optomechanical control over the shape and mass of a mechanical mode, enabling the smooth localization of the spatial distribution of oscillating mass from the centimeter scale to the micron scale. Such control over shape and effective mass has never been demonstrated, and represents an entirely unexplored avenue in the field.

Thus far, we have reliably fabricated the requisite (delicate!) mechanical structures, and observed the characteristic "phononic bandgap" (a necessary ingredient for localization). We present here the design of a rigid, vibration-isolated apparatus to measure these effects in ultrahigh vacuum. Furthermore, we show preliminary attempts at decreasing the finesse of a fiber-mirror in a controllable manner by etching away some of the layers forming the high-reflectivity Bragg stack. Achieving a specific finesse value for our fiber-mirrors is necessary to assemble a fiber cavity that will generate a strong optical spring whilst displaying a sufficiently large decay rate, therefore minimizing anti-damping of the phononic crystal, and facilitating cavity locking.

[1] A. Z. Barasheed et al., Phys. Rev. A 93, 053811 (2016)

*Sophie Berthelette**Master, Université de Montréal**Director : Gilles Brassard**A wonderful combination of the parallel repetition theorem, Kolmogorov complexity and the magic square game*

The classical magic square game consists of a 3x3 grid that needs to be filled with bits (0 or 1), respecting an even number of ones in each row and an odd number of ones in each column. In a 2-players' version of this game, Alice and Bob each receive a trit (1, 2 or 3) and are asked to fill the corresponding row (containing an even number of ones) and column (containing an odd number of ones) respectively such that the intersection of their row and column matches, that is, it contains the same bit. With a classical strategy, they can win at most with probability 8/9. The parallel repetition theorem by Ran Raz says that if they play n repetitions in parallel of a game, the probability that they win all of the n repetitions decreases exponentially with n. This theorem can be used to prove some interesting results on the Kolmogorov complexity of Alice and Bob's answers to the magic square game. More precisely, we can show that the answer (row or column) must be uncomputable even given the respective question (the trit indicating which row or column to fill).

*Yinan Fang**Postdoc, McGill University**Director : Bill Coish**Collective nuclear spins coherent effects in a quantum electron shuttle*

In a spin-blockade quantum dot with coherent electron-nuclear spin interaction, initially polarized nuclear spins may lead to a transient boost in leakage-current owning to the dynamically built correlations among the nuclear spins - a reminiscent of fluorescence emission in the optical Dicke superradiance effect. We study how this correlation could manifest in an electron shuttle device where a small ”island” of nuclear spins was embedded along the shuttling path. By incorporating hyperfine flip-flop effect exactly into the dissipator under a "box-model" setting, we qualitatively showed that how a three-steps protocol might be used to modify the nuclear spins distribution and reveal their collective coherent dynamical features.

*Yannik Fontana & Cesar Daniel Rodriguez Rosenblueth**Postdoc & Master, McGill University**Director : Lilian Childress**Coupling fiber microcavities and color centers in diamond*

Color centers in diamond are attractive spin-photon interfaces for future quantum technologies. The nitrogen vacancy center (NV-) spin is a robust qubit potentially giving access to multi-qubit registers [1,2]. However, only 3% of all photons emitted by an NV- can be used in remote entanglement protocols.We present our on-going effort to couple color centers embedded in ultralow-loss diamond membranes to a fully tunable optical micro-cavity directly interfaced with an optical fiber. The required passive and active cavity stabilisation mechanisms will be discussed, as well as our state-of-the-art results for the loaded cavity finesse. At cryogenic temperatures both NV- and GeV decay rates and fraction of “useful” photons will be enhanced, scaling with the cavity finesse [3]. In addition, GeV centers could also be used to improve the “Indistinguishability x Rate” figure of merit for room temperature single photon sources.

[1] E. Hogan et al., Nature 466 (2010)

[2] N. Kalb et al., Science 356 (2017)

[3] D. Riedel et al., PRX 7 (2017)

*Alexandru Petrescu**Postdoc, Université de Sherbrooke**Director : Alexandre Blais**Frequency and relaxation rate renormalization of driven weakly anharmonic superconducting qubits: The readout problem*

Recent experiments in circuit QED with a transmon qubit have found a strong dependence of the qubit relaxation rate on read-out power [Mundhada, Shankar, Narla, Zalys-Geller,Girvin,Devoret, APS 2016]. We discuss a plausible mechanism that is consistent with available experimental data up to intermediate cavity occupation n_c < 5.

*Jason Phoenix**Master, National Research Council Canada**Director : Andrew Sachrajda**Investigation of Single Photon Emitters Based on InAsP Quantum Dots Imbedded in Site-Controlled InP Nanowires*

Various quantum technologies, such as quantum cryptography and quantum communication, require sources of indistinguishable single photons. InAsP quantum dots imbedded in InP nanowires are proven sources of such high-purity single photons [1]. By encapsulating the nanowires in InP tapered waveguides, the emitted photons have a Gaussian profile ideal for coupling to optical fibres [2]. Our goal is to develop a fibre-only system to collect indistinguishable single photons emitted by these quantum dots. A lensed fiber is used to map out site-selectively grown nanowires, and to perform PL experiments. This study is performed at millikelvin temperatures and in magnetic fields up to 6 T. The phonon sidebands of the emission lines are measured at different temperatures to investigate the role phonons play in photon indistinguishability. Hanbury Brown-Twiss auto-correlation measurements reveal single photon emission from these dots. Magnetic field spectroscopy allows us to identify the different exciton complexes (e.g. X, X*, XX) responsible for specific emission lines in the PL spectrum and to measure their associated electron g-factors.

[1] D. Dalacu, et al. Ultraclean Emission from InAsP Quantum Dots in Defect-Free Wurtzite InP Nanowires, Nano Letters, Vol. 12 (11), 5919-5923. (2012)

[2] G. Bulgarini, et al. Nanowire Waveguides Launching Single Photons in a Gaussian Mode for Ideal Fiber Coupling, Nano Letters, Vol. 14 (7), 4102-4106. (2014)

*Jean Olivier Simoneau**Doctorate, Université de Sherbrooke**Director : Bertrand Reulet**Exploration of the Photon Statistics of a Josephson Paramp via continuous microwave measurements*

Authors: Jean Olivier Simoneau, Stéphane Virally, Christian Lupien & Bertrand Reulet

*Motoi Takahashi**Doctorate, National Research Council Canada**Director : Andrew Sachrajda**EDSR of a single heavy hole in a lateral GaAs/AlGaAs quantum dot qubit*

Single holes are attractive as spin qubits due to their advantageous properties which include a reduced hyperfine interaction, a strong spin-orbit coupling for sub-nanosecond spin rotations, and the absence of valley complications.

Here we report single hole electric dipole spin resonance (EDSR) measurements over the 20-50 GHz range taking advantage of the strong spin-orbit coupling. The experiment was performed in a GaAs double quantum dot device described in [1] tuned in such way that only one of the dots contained a single heavy hole with the Fermi level of the adjacent lead positioned in between Zeeman split spin states. In this situation one hole is initialized in the lowest spin level and the current is energy blockaded. A small microwave voltage is applied to a plunger gate to mediate EDSR rotating the hole spin from the lower to the upper spin level allowing the hole to tunnel to the lead. The spin resonance is detected as an increase in current when the resonant condition is fulfilled. The second dot is used as an auxiliary tool to tune the g-factor via a strong spin-dependent tunnel coupling [1]. We show that g-factor can be tuned in the range of 30% by a small change of the voltage applied to the auxiliary dot plunger gate.

[1] A. Bogan et al., Phys. Rev. Lett. 120, 207701 (2018).

*Alessandro Ricottone**Doctorate, McGill University**Director : Bill Coish**Optimized polarization control in a central-spin system*

In this work [1], we study the zero-temperature phase diagram and the dissipative dynamics of the central-spin system, where one “central” spin is homogeneously coupled with many “ancilla” spins. An archetypical example of this model is given by an electron spin coupled to nuclear spins in a quantum dot via hyperfine interactions. This same central-spin model has been shown to improve the efficiency of quantum-annealing protocols. We establish the zero-temperature phase diagram with phases characterized by the polarization of the ancilla spins relative to the central spin. By rapidly tuning a parameter in the Hamiltonian, the ancilla-spin polarization can be rapidly modified through a dissipative equilibration process mediated by the central spin.

Remarkably, we find that the dissipation rate can be optimized to minimize the time scale for polarization dynamics. These results may be important for protocols to quickly polarize nuclear spins in semiconductor quantum dots or to rapidly and efficiently equilibrate a quantum annealer.

[1] A. Ricottone, Y.N. Fang, S. Chesi and W.A. Coish, in preparation

*Marc-Antoine Roux**Master, Université de Sherbrooke**Director : Michel Pioro-Ladrière**Use of a guard ring as an ESD protection component for tunnel junctions*

Modern electronic fabrication processes allow to make nanoscale devices. However, the small size of those devices increases their sensitivity to electrostatic discharge (ESD) due to the bigger current density for the same applied voltage. In many cases, it becomes challenging to manipulate and characterize samples without damaging them especially when many preparation steps are required before the final experiment. Therefore, the use of a guard ring that shorts every connection on the samples can protect them when used with simple ESD precautions. Following sample fabrication, two guard ring removal processes, with diamond tip scribing and laser cutting, have been investigated. On Al/Co tunnel junctions isolated by a thin aluminum oxide layer, the diamond tip scribing lead to a 93% yield for junction integrity. However, removal of the guard ring with laser cutting doesn’t remove the electrical conductivity between connections.