## Les activités de l'INTRIQ

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nov. 11, 2019

Organizers :
Éva Dupont Ferrier, Université de Sherbrooke
Dave Touchette, Université de Sherbrooke

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 11th from 9h30 to 10h30

Meeting program

November 11th

10h30 - 10h55  Registration

10h55 - 11h00  Opening remarks (Salon A)

11h00 - 11h45  Pr Frédéric Dupuis, Université de Montréal

11h45 - 12h05  Marco David, Student, McGill University

12h05 - 13h30  Lunch  (Dining room - 4 Canards)

13h30 - 14h15  Dr Louis Gaudreau, National Research Council (NRC) - Ottawa
Entanglement distribution via coherent photon-to-spin conversion in semiconductor quantum dot circuits

14h15 - 14h45  Dr Joel Griesmar, Université de Sherbrooke

14h45 - 15h15  Coffee break  (Salon B)

15h15 - 15h45  Dr Stephane Virally, Polytechnique Montréal

15h45 - 16h25  Industry & Startups in quantum technologies
)
Dr David  Roy-Guay, SB Quantum (www.sbquantum.com)
Pr David Poulin, Microsoft (0pen positions at Microsoft)

16h25 - 17h00  Equity, diversity & inclusion (minutes and photos of the workshop)

17h00 -             Poster session with refreshments (Salon B)

19h30 -             INTRIQ dinner (Knowlton room)

November 12th

8h30 -  9h00  Pr Anne Broadbent, Université d'Ottawa

9h00 - 10h00  Dr Tomas Jochym-O'Connor, IBM - Yorktown Heights, New York

10h00 - 10h30  Coffee break (Salon B)

10h30 - 11h30 Pr Signe Seidelin, Institut NEEL CNRS/UGA
Rare-Earth Doped Crystals for strain-coupled optomechanics

11h30 - 12h00  Dr Erika Janitz, McGill University

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

13h30 - 14h00  Pr Jérôme Bourassa, Cégep de Granby
Quantum illumination : exploiting quantum correlations when entanglement is lost

14h00 - 14h30  Dr Thomas Baker, Université de Sherbrooke
Modeling superconducting circuits with a tensor network

14h30 - 15h00  Dr Anirban Chowdhury, Université de Sherbrooke
Simulating thermal physics on quantum computers

15h00 - 15h30  Questions and answers

15h30 - 15h40  Closing remarks

mai 29, 2019

Colloque scientifique au congrès de l'Acfas, mercredi le 29 mai 2019

L’information quantique : avancées fondamentales et possibilités technologiques

Organisateurs :
Professeur Stéphane Kéna-Cohen, Polytechnique Montréal

janv. 8, 2019

CONFETI (CONFérence ÉTudiante de l'INTRIQ) is a yearly student conference sponsored by the INTRIQ. It attracts graduate students and post-docs in the fields of physics, mathematics, computer science and engineering working on quantum computing related projects.

Where and when
The conference will take place on January 8-10, 2019 at the Hôtel Château Bromont in Bromont, Québec.

Schedule

Registration

# Institut Transdisciplinaire d'Information Quantique (INTRIQ)

nov. 5, 2013
Posté par : Marc Leclair

### Fall 2013 INTRIQ meeting, November 5th & 6th 2013

At the Hotel of the Château Bromont

Organizer: Lilian Childress, McGill University

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

## Directions

(Note : the venue is at the Hôtel, not the Auberge)

Meeting program

Tuesday, November 5th

6h30 - 8h30    Breakfast

8h30 - 9h00   Registration
9h00 - 9h15   Opening remarks

9h15 - 10h15  Matteo Mariantoni, Professor at University of Waterloo
Tutorial: Towards a Fault-Tolerant Quantum Computer with the Surface Code

10h15 - 10h45 Coffee break

10h45 - 11h10 Kevin Lalumière, Student at Université de Sherbrooke
Interactions between superconducting qubits mediated by travelling photons
11h10 - 11h35 Farzad Qassemi, Postdoc at Université de Sherbrooke
Spin readout of a single electron in a double quantum dot
11h35 - 12h00 Iyer Pavithran, Student at Université de Sherbrooke
Hardness of decoding stabilizer codes on the independent X-Z channel

12h00 - 13h30 Lunch (Dining room)

13h30 - 14h15 Thomas Szkopek, Professor at McGill University
Would Error Correction Provide a Benefit in Classical Computers?
14h15 - 15h00 Bertrand Reulet, Professor at Université de Sherbrooke
Electron shot noise is quantum light

15h00 - 15h15 Break

15h15 - 16h15 Daniel Lidar, Professor at University of Southern California
Quantum annealing and error correction with lots of qubits

16h15 - 17h00 (all professors)  2 slides presentation of current projects
17h00 - 19h30 Poster session with refreshments

19h30 Dinner (Dining room)

Wednesday, November 6th

6h30 - 8h30   Breakfast
8h30 - 9h00   Check-out

9h00 - 9h45  Wolfgang Tittel, Professor at University of Calgary
How to build a quantum repeater
9h45 - 10h30 Sébastien Francoeur, Professor at École Polytechnique de Montréal
Isoelectronic centers as building blocks for quantum information processing

10h30 - 12h00 Poster session

12h00 - 13h30 Lunch (Dining room)

13h35 -14h00  Hichem Eleuch, Invited researcher at McGill
Theoretical study of beam splitting and entanglement generation with two particular states:
excited coherent states and squeezed coherent states of Morse potential
14h00 - 14h25 Abdulrahman Al-lahham, Student at Université de Montréal
An introduction to quantum walk based search algorithms
14h25 - 14h50 Dave Touchette, Student at Université de Montréal
Noisy Interactive Quantum Communication
14h50 - 15h10 Michael Hilke, Professor at McGill University
History of INTRIQ

15h10 - 15h20 Closing remarks and departure

Note : Business meeting from 15h30 to 17h00

INVITED SPEAKERS

Sébastien Francoeur
École Polytechnique de Montréal

Isoelectronic centers are semiconductor nanostructures the size of a single atom or a small molecule. Shrinking semiconductor nanostructures to atomic sizes provide a number of key advantages: the uniformity and predictability of atomic defects, the size and composition tunability of conventional quantum dots, a perfectly defined symmetry, and a confinement of electrons and holes to volumes of atomic dimension.

In this presentation, I will describe what isoelectronic centers are and I will discuss our recent result on the quantum control of exciton qubits.  We found that this little-known nanostructure provides a relatively high dipole moment, which could prove to be an essential ingredient for the large scale integration of two-qubit gates and the realization of quantum networks. Furthermore, excitation induced dephasing mechanisms found in other semiconductor nanostructures are suppressed, allowing high-fidelity qubit-state preparation and control on shorter time scale.

Daniel Lidar
University of Southern California

Quantum annealing and error correction with lots of qubits
In October 2011 USC and Lockheed-Martin jointly founded a quantum computing center housing D-Wave’s 128-qubit Rainier processor, which implements programmable quantum annealing using superconducting flux qubits. In March this year the processor was upgraded to the 512-qubit Vesuvius model. These are special-purpose processors designed to find the ground state of a broad class of 2D classical Ising models with as many spins as qubits. This talk will describe some of the work we have done to benchmark the processors against highly optimized classical algorithms, to test for quantum effects, and to perform error correction.
References:
"Experimental signature of programmable quantum annealing", Nature Comm. 4, 2067 (2013)
"Quantum annealing with more than one hundred qubits",arXiv:1304.4595
"MAX 2-SAT with up to 108 qubits", arXiv:1307.3931
"Error corrected quantum annealing with hundreds of qubits", arXiv:1307.8190

Matteo Mariantoni

The ultimate goal of quantum information is to build a fault-tolerant quantum computer. Fault-tolerance means that errors occurring on the quantum bits (qubits) constituting the quantum computer can be detected and corrected in real time. In the past decade, a number of quantum error correcting algorithms has been developed. To date, the so-called surface code is the most promising candidate for a practical implementation of quantum error correction [1].
In this tutorial, I will introduce the main concepts about the surface code. In particular, I will explain how stabilizer measurements allow the detection of quantum errors. I will show how logical qubits are realized and present a unified error model for the surface code. I will then demonstrate how logical operations can be performed between logical qubits. Finally, I will outline a possible realization of the surface code based on superconducting quantum circuits.
[1] A.G. Fowler, M. Mariantoni, J.M. Martinis, and A.N. Cleland, Phys. Rev. A 86, 032324 (2012)

Wolfgang Tittel
University of Calgary

How to build a quantum repeater
Quantum communication, in particular quantum key distribution, holds great promise for the future of ICT. In my talk I will introduce the concept of a quantum repeater (first proposed by Briegel et al. in 1998 [1]), which combines the distribution of entangled states of light, entanglement swapping, and quantum memory to overcome the current distance barrier of around 200 km. Furthermore, I will present our recent work on frequency-multiplexed quantum memories [2], and discuss why (and how) it may allow building a quantum repeater within the next few years.
[1] H. Briegel, W. Dür, J. I. Cirac, and P. Zoller, Phys. Rev. Lett. 81, 5932 (1998)
[2] N. Sinclair et al, arXiv:1309.3202

INTRIQ SPEAKERS

Abdulrahman Al-lahham
Student, Université de Montréal
Director : Gilles Brassard

An introduction to quantum walk based search algorithms
In this talk, an intuitive treatment of quantum walks and its application in solving search problems will be  presented. We will start by reviewing the amplitude amplification algorithm. The classical Markov chain will be
introduced along with its quantization. By combining these two ingredients, we get the MNRS algorithm: A quantum walk based search method. A quick application of this algorithm to element distinctness problem will be given as a conclusion.

Postdoc, Université de Sherbrooke

Spin readout of a single electron in a double quantum dot
In this talk, I present my work on single spin readout in double quantum dot. In laterally confined double quantum dots, electrons are controlled through gate voltages applied locally to each dot. The interdot tunneling,  which is due to electrons wave functions overlap at two quantum dots, is generally spin independent. Hence, for reading out spin state in quantum transport, a scheme for spin-to-charge conversion is required. In the usual singlet-triplet quantum dots where initial and final spin states are orthogonal,  the current is blocked by spin selection rules,  leading to a phenomenon called Pauli spin blockade. However, interactions between the electron spin and the environment (e.g. nuclear spins in III-V semiconductors) lift this spin selection rule. Here, I present a new idea to measure a single spin in the first dot using the second dot as a spin filter.

Hichem Eleuch
Invited researcher, McGill University

Theoretical study of beam splitting and entanglement generation with two particular states: excited coherent states and squeezed coherent states of Morse potential
In this talk we explore the entanglement generation through beam splitter for two particular states, the excited coherent states and the coherent states of Morse potential. We analyze the dynamical evolution of the output field entanglement.
[1] K. Berrada, S. Abdel-Khalek, H. Eleuch and Y. Hassouni, Quantum Inf. Process. 12, 69 (2013).
[2] A. Hertz, V. Hussin, H. Eleuch, paper in preparation.

Michael Hilke
Professor, McGill University

Realizing that most students and more than half of the PIs were not there at the beginnings almost a decade ago, I will give an illustrated anecdotal account of INTRIQ's history and subjective future.

Kevin Lalumière
Student, Université de Sherbrooke
Director : Alexandre Blais

Superconducting qubits are promising candidates for quantum information processing because of their relatively long coherence time combined with short operation time scales. Moreover, several of these qubits can be made to interact at long distances by coupling them to the standing waves of the electromagnetic field in a transmission line resonator. This setup, referred to as circuit QED, is analogous to cavity QED where single photons can be stored for long times in a high quality cavity. In this work, we study the consequences of letting qubits interact via electromagnetic waves free to travel in a open one-dimentional transmission line. We show how interaction between the qubits mediated by this open environment can be revealed in reflection and transmission experiments. Theoretical predictions are compared to experimental results from the ETH Zurich group.

Iyer Pavithran
Student, Université de Sherbrooke
Director : David Poulin

I will address the computational hardness of a decoding problem, pertaining to quantum stabilizer codes considering independent X and Z errors on each qubit. Much like classical linear codes, errors are detected by measuring certain check operators which yield an error syndrome, and the decoding problem consists of determining the most likely recovery given the syndrome. The corresponding classical problem is known to be NP-Complete, and a similar decoding problem for quantum codes is known to be NP-Complete too. However, this decoding strategy is not optimal in the quantum setting as it does not take into account error degeneracy, which causes distinct errors to have the same effect on the code. Here, we show that optimal decoding of stabilizer codes is computationally much harder than optimal decoding of classical linear codes, it is #P-Complete.

Bertrand Reulet
Professor, Université de Sherbrooke

Electrons in conductors have a disordered motion, a phenomenon commonly referred to as "noise". In classical physics, this noise (more precisely, the variance of current fluctuations) is proportional to the temperature and the conductance of the conductor. When we consider a tiny device placed at very low temperature, things change: one can no longer consider the electrons as classical particles, but quantum mechanics dictates their behavior. We will describe some concepts and experiments related to quantum noise in conductors. In particular we will show very recent experiments in which we observe that the noise emitted by such a conductor may consist of correlated photons and that it can be squeezed just like light in quantum optics.

Thomas Szkopek
Professor, McGill University

High-end classical computers suffer a mean time between faults of about one week, a modest improvement from the two days between faults experienced by ENIAC users in 1946. Can error correction at the hardware level address this problem? The error rate in complementary transistor circuits at the heart of modern classical computers is suppressed exponentially in electron number, arising from an intrinsic physical implementation of fault-tolerant error correction. Contrariwise, the explicit assembly of gates into the most efficient known classical fault-tolerant architecture is characterized by a subexponential suppression of error rate with electron number, and incurs significant overhead in wiring and complexity. Classical computation thus appears to abide by the adage ‘‘a stitch in time saves nine,’’ where it is much more efficient to prevent logical errors with physical fault tolerance than to correct logical errors with fault-tolerant architecture. Possible implications for quantum computing will be discussed.

Dave Touchette
Student, Université de Montréal
Directors : Alain Tapp & Gilles Brassard

We study the problem of simulating protocols in a quantum communication setting over noisy channels. This problem falls at the intersection of quantum information theory and quantum communication complexity, and is of particular importance for real-world applications of interactive quantum protocols, which can be proved to have exponentially lower communication costs than their classical counterparts for some problems. Under random noise, our simulation strategy has a communication rate proportional to the capacity of the channel used. In contrast, a naive strategy that individually encodes each particular round of communication to achieve comparable success would have asymptotic rate going to 0.
Under adversarial noise, our strategy can withstand error rates up to 1/2, which we prove to be optimal. In contrast, the naive strategy described above would not work for any constant fraction of errors. Our results hold in particular in the quantum communication complexity setting of Yao's and Cleve-Buhrman's models.

This is joint work with Gilles Brassard, Ashwin Nayak, Alain Tapp, and Falk Unger.
Full version at:  http://arxiv.org/abs/1309.2643

KIOSK
Dolores Martinez
Director Scientific Affairs

Christian Sarra-Bournet
Coordinator of the Quantum Materials and Devices Infrastructure (IMDQ)
Département de Physique, Université de Sherbrooke

Quebec’s Nanotechnology Infrastructure – Quantum Materials and Devices Infrastructure (IMDQ)
Under the banner of NanoQuébec, Quebec's Nanotechnology Infrastructure (QNI) brings together the human and technical nanotechnology expertise found in Quebec’s universities. Its mission is to structure and consolidate this expertise, giving academic and industrial researchers optimal access to a full range of state-of-the-art nanotechnology equipment and expertise. Being an open infrastructure with more than \$300 M in equipment and 180 highly qualified personnel, the QNI can provide users with a full range of services including training and support, use of equipment, as well as contractual services and development of major R&D projects.

The Quantum Materials and Devices Infrastructure, located at the Physics Department at Université de Sherbrooke, is part of the QNI and is composed of clean rooms and state-of-the-art equipment for the fabrication and characterization of magnetic, supraconductor and semiconductor materials and devices. Researchers are thus able to develop their own quantum devices and characterize those in cryogenic temperature environment and in presence of intense magnetic fields. The expertise and know-how of our infrastructure is unique in Québec and is able to answer the current quantum information nano/microfabrication challenges.

POSTER SESSION

Jonas Anderson
Postdoc, Université de Sherbrooke
Director : David Poulin

Decoding Quantum LDPC Codes
I will discuss the advantages of Quantum Low-Density Parity-Check (QLDPC) codes for use in future quantum computing architectures as well as the challenges involved in their decoding.

Félix Beaudoin

Student, McGill University

Director : Bill Coish

Enhanced hyperfine-induced spin dephasing in a magnetic-field gradient
Magnetic-field gradients are important for single-site addressability and electric-dipole spin resonance of spin qubits in semiconductor devices. We show that these advantages are offset by a potential reduction in coherence time due to the non-uniformity of the magnetic field experienced by a nuclear-spin bath interacting with the spin qubit. We theoretically study spins confined to quantum dots or at single donor impurities, considering both free-induction and spin-echo decay. For quantum dots in GaAs, we find that, in a realistic setting, a magnetic-field gradient can reduce the Hahn-echo coherence time by almost an order of magnitude. This problem can, however, be resolved by applying a moderate external magnetic field to enter a motional averaging regime. For quantum dots in silicon, we predict a cross-over from non-Markovian to Markovian behavior that is unique to these devices. Finally, for very small systems such as single phosphorus donors in silicon, we predict a breakdown of the common Gaussian approximation due to finite-size effects.

Samuel Boutin
Student, Université de Sherbrooke
Director : Alexandre Blais

Numerical optimization of quantum gates and measurement in cQED
In circuit quantum electrodynamics (cQED), superconducting qubits based on Josephson junctions are coupled to a resonator. In this architecture, both qubit control and readout are based on sending microwave pulses to the resonator. An important challenge of the field is to improve the fidelity of logical gates and state measurement in order to reach the quantum error-correction threshold. An avenue to diminish the errors of those operations is to use optimal control optimization to find numerically the best microwave protocols (pulse shape, frequency,… ) to realize those experiments.
In this work, a version of the gradient ascent pulse engineering algorithm for open quantum systems (open GRAPE) is implemented and adapted to work with big Hilbert space in order to optimize the measurement of a superconducting qubit in the cQED architecture. Also, suggestions from the literature are added to the algorithm in order to accelerate the convergence and to take into account the constraints of the standard laboratory equipment used in that type of experiment. In this talk, the cQED architecture is briefly introduced. The numerical methods at the core of this work are then discussed. Finally, some preliminary results on the optimization of the measurement protocol are presented.

Winton Brown
Postdoc, Université de Sherbrooke
Director : David Poulin

Thermalization and Quantum Information Theory
The simplest way to think of the approach to thermal equilibrium is that states that are originally distinguishable with respect to a restricted set of measurements become indistinguishable from an equilibrium state after the system has evolved for a sufficient amount of time. On the other hand, the condition for recovery of an encoded quantum memory is equivalent to the indistinguishability of the different quantum states of the code space with respect to the information the environment can obtain through interacting with the memory and thereby inducing noise. Thus, there is a direct analogy between a good encoding circuit for a quantum error correcting code and the process of thermalization under coherent unitary dynamics. Exploiting this analogy, it follows that the capacity of the noise channel induced by a set of potential measurements puts an upper bound on the size of the space of initial states that may thermalize. I show that such a bound, corresponding to the entanglement assisted capacity of the erasure channel, can be achieved by typical local dynamics in an amount of time that is only slightly larger than the minimum amount of time required for the dynamics to propagate a signal across the system.

Julien Camirand-Lemyre
Student, Université de Sherbrooke

Measuring the spin state of a single electron in a double quantum dot
Measuring the spin state of a single electron in quantum dots requires spin to charge conversion. In actual approaches an additional electron is needed to perform such a task via Pauli spin blockade. In our experiment a single electron is trapped in a double quantum dot.
In presence of a magnetic field gradient between the two dots the dipolar moment of the electron couples to it's spin degree of freedom allowing for spin to charge conversion. I will present the first results of the experiment using the magnetic field gradient produced by the nuclear spins and
explain why micromagnet need to be added to the device.

Christopher Chamberland
Student, McGill University
Directors : Aashish Clerk and Bill Coish

Geometric dephasing for adiabatic state transfer
We consider the quantum state transfer of a qubit state via laser couplings of a four level system. The state transfer process is performed in the adiabatic regime where the Hamiltonian performs a closed loop in parameter space giving rise to a Berry's phase. We study the environment-induced Berry's phase and the dephasing arising from noisy laser phase fluctuations.

Benjamin D'Anjou
Student, McGill University
Director : Bill Coish

Anomalous magnetotransport through reflection-symmetric molecules
We calculate magnetotransport oscillations in current through a triple-quantum-dot molecule, accounting for higher harmonics (having flux period h/ne, with n an integer). For a reflection-symmetric triple quantum dot, we find that harmonics with n odd can dominate over those with n even. This is opposite to the behavior theoretically predicted due to dark-state' localization, but has been observed in recent experiments [L. Gaudreau et al., Phys. Rev. B, 80, 075415 (2009)], albeit in a triple-dot that may not exhibit reflection symmetry. This feature arises from a more general result: In the weak-coupling limit, we find that the current is flux-independent for an arbitrary reflection-symmetric Aharonov-Bohm network. We further show that these effects are observable in nanoscale systems even in the presence of typical dephasing sources.

Guillaume Dauphinais
Student, Université de Sherbrooke
Director : David Poulin

Quantum Error Correction with Non-Abelian Anyons
Non-abelian anyons are interesting from the point of view of quantum information as one can use them in principle to encode quantum states, and perform gates which are topologically protected. Majorana fermions is an example of such a system that has recently attracted a lost of interest. In this project, a decoding algorithm for a phenomenological noise model applied to Majorana fermions on a torus is developped and its performances are studied.

Nicolas Delfosse
Postdoc, Université de Sherbrooke
Director : David Poulin

Decoding color codes by projection onto surface codes
Surface codes and color codes are two families of quantum error correcting codes which are particularly well suited to fault-tolerant quantum computing. They are defined by local constraints on qubits placed on a surface and they allow for efficient decoding.
We propose a new strategy to decode color codes, which is based on the projection of the error onto three surface codes. This provides a method to transform every decoding algorithm of surface codes into a decoding algorithm of color codes. These results are based on a chain complex interpretation of surface codes and color codes.

Nicolas Didier

Postdoc, McGill University
Directors : Aashish Clerk and Alexandre Blais

Perfect squeezing by damping modulation in circuit quantum electrodynamics
Dissipation-driven quantum state engineering uses the environment to steer the state of quantum systems and preserve quantum coherence in the steady state. We show that modulating the damping rate of a microwave resonator generates a new squeezing mechanism that creates a vacuum squeezed state of arbitrary squeezing strength, thereby allowing perfect squeezing. Given the recent experimental realizations in circuit QED of a microwave resonator with a tunable damping rate, superconducting circuits are an ideal playground to implement this technique. By dispersively coupling a qubit to the microwave resonator, it is possible to obtain qubit-state dependent squeezing.

Guillaume Duclos-Cianci
Student, Université de SherbrookeDirector : David Poulin`

A family of magic states and their distillation circuits
In this work we introduce a family of magic states to implement arbitrary one-qubit Pauli rotations of angle 2\pi / 2^k. We describe the states, give a circuit necessary to perform distillation on these and analyze how errors behave. Preliminary results show an improvement in resource consumption compared to existing techniques.

Gabriel Gasse
Student, Université de Sherbrooke
Director : Bertrand Reulet

Observation of Squeezing in the Electron Quantum Shot Noise of a Tunnel Junction
We report the measurement of the fluctuations of the two quadratures of the electromagnetic field generated by a quantum conductor, a dc- and ac-biased tunnel junction placed at very low temperature.We observe that the variance of the fluctuations on one quadrature can go below that of vacuum, i.e., that the radiated field is squeezed. This demonstrates the quantum nature of the radiated electromagnetic field.

Erika Janitz
Student, McGill University
Director : Lilian Childress

Cavity Assisted Quantum Processes with NV Centers
The peculiar properties of quantum mechanics provide tools to explore novel types of information processing that could achieve unprecedented computational speed and security. Realizing such technologies requires precise control over a quantum system, and a method of joining individual elements into a scalable network. Spins in solids have been proposed as quantum bits, and recently it has been shown that individual spins in diamond can be addressed via an optically-active defect called the nitrogen-vacancy center. The central goal of this project is to develop an efficient quantum interface between these defects and light, enabling optically mediated interactions between distant spins. Our approach will involve placing these defects in a highly tunable microscopic cavity formed by optical fibers, thereby enhancing coherent coupling between light and a single nitrogen-vacancy center. This flexible platform will directly impact quantum information applications, and will also open new avenues for exploring fundamental quantum optics with condensed matter systems.

Saeed Khan
Student, McGill University
Director : Aashish Clerk

Large gain quantum-limited qubit state measurement using a two mode nonlinear cavity
A single nonlinear cavity dispersively coupled to a qubit functions as a large gain detector near a bifurcation, but also has an unavoidable large backaction that prevents QND measurement at weak couplings [1]. We show theoretically that a modified setup involving two cavities (one linear, one nonlinear) and a dispersively coupled qubit allows for a far more optimal measurement. In particular, operating near a point of bifurcation, one is able to both achieve a large gain as well as a near quantum-limited backaction. We present analytic results for the gain and noise of this detector and a heuristic understanding of the physics, thus presenting a complete description of this new way of performing weak qubit state measurements. The setup we describe can easily be realised in experiments with superconducting circuits involving Josephson junctions [2, 3].
[1] C. LaFlamme, A.A. Clerk, Phys. Rev. A 83, 033803 (2011)
[2] F.R. Ong et al., Phys. Rev. Lett. 106, 167002 (2011)
[3] M. Hatridge et al., Phys. Rev. B 83, 134501 (2011)

Dany Lachance-Quirion
Student, Université de Sherbrooke

Coupling a single-electron spin qubit and a superconducting microwave resonator for hybrid quantum circuits
Superconducting qubits in the circuit quantum electrodynamics (cQED) architecture have proven to have all the requirements of a solid-state quantum processor. However their coherence times are currently limited to fractions of a millisecond, thus requiring a quantum memory to build a universal quantum computer. Spin qubits are one of the most promising candidates for this task because of their potentially very long coherence time. In this poster I will show how a spin qubit defined as a single-electron in a double quantum dot (DQD) with a large magnetic field anisotropy can be strongly coupled to a superconducting resonator. I will also show how this coupling can be manipulated electrically in a timescale shorter than the spin qubit coherence time, thus enabling the resonator to act as a quantum bus.

Pawel Mazurek
Student, McGill University
Director : Bill Coish

Sensitivity of the decay of entanglement of quantum dot spin qubits to the magnetic ﬁeld
We study the decay of entanglement of quantum dot electron-spin qubits under hyperﬁne interaction mediated decoherence. We show that two qubit entanglement of a single entangled initial state may exhibit decay characteristic of the two disentanglement regimes in a single sample, when the external magnetic ﬁeld is changed. The transition is manifested by the supression of time-dependent entanglement oscillations which are superimposed on the slowly varying entanglement decay related to phase decoherence (which result in oscillatory behaviour of entanglement sudden death time as a function of the magnetic ﬁeld). This unique behaviour allows us to propose the double quantum dot two-electron spin Bell state as a promising candidate for precise measurements of the magnetic ﬁeld.

Clemens Mueller
Postdoc, Université de Sherbrooke

Director : Alexandre Blais

T1-Fluctuations in Superconducting Circuits - The Revenge of the Two-Level Fluctuators
Spurious two-level systems (TLS) as sources of noise are ubiquitous in solid state systems, be it as the simplest possible theoretical model or as real physical entities observed in experiment. Here, I will show how ensembles of interacting TLS can be responsible for fluctuations of qubit decoherence rates, a phenomenon which has recently been observed in high-coherence superconducting 3D transmon samples.

Pericles Philippopoulos
Student, McGill University
Director : Bill Coish

Hyperfine Interaction in Materials with Strong Spin-Orbit Coupling:Group Theoretic Analysis
Due to the anisotropic nature of the hyperfine coupling for hole spins in semiconductor quantum dots, these systems may show significantly longer coherence times than electron spins given the correct quantum-dot geometry and magnetic field orientation. This advantage of hole spins relies on the hyperfine tensor taking-on an Ising-like form. This Ising-like form of the hyperfine coupling has been recently called into question with experiments that extrapolate their results suggesting a strong hybridization of p-like and d-like components for the valence band. However, this extrapolation relies on two assumptions: (1) That spin-orbit coupling is weak in these systems compared to the anisotropic crystal field, and (2) that higher-angular-momentum (f-like, g-like, ...) contributions are negligible. Assumption (1) is questionable in light of the fact that the spin-orbit energy is even larger than the principle gap in InAs, and assumption (2) is generally questionable in any crystal that breaks pure rotational symmetry. Using a generalization of the group-theoretic analysis in [1], we show here that relaxing either of these assumptions can restore the Ising-like nature of the hyperfine tensor, albeit for a particular choice of coupling constants. Future work will address the issue of a generic limit in which the Ising-like hyperfine coupling will be robust for hole spins.

David Racicot-Desloges
Student, Université de Sherbrooke
Director : David Poulin

Degenerate Decoding for Quantum Turbo Codes
Emily Pelchat and David Poulin presented, in "Degenerate Viterbi Deconding", an algorithm for quantum convolutional codes that finds the class of degenerate errors with the largest probability, conditioned on a given syndrome. Monte Carlo simulations strongly suggested that the algorithm gives better results than its non-degenerate counterpart, particularly at low error rates. Thus, a natural next step would be to use the degenerate decoder in turbo codes, where its benefits could be further amplified. I shall present this extension in my poster.

Sophie Rochette
Student, Université de Sherbrooke