A conference in honour of Gilles Brassard

For Professor Gilles Bassard’s 60th birthday, INTRIQ and the Centre de recherches mathématiques (CRM) are proud to recognize three decades of research in quantum cryptographyand two decades in quantum teleportation as well as the outstanding contributions of Professor Gilles Brassard for the emergence of the field of quantum information science with a series of scientific conferences held at Domaine Château-Bromont and a banquet in Montréal.

‍Biography

Gilles Brassard received a PhD in computer science in 1979 from Cornell University, specifically in cryptography. Since then, he has been a professor at the Université de Montréal and has held the Canada Research Chair in quantum information science since 2001. His Chair has recently been renewed until the end of 2021.

Professor Brassard is interested in all aspects of quantum information science, a field that is at the crossroads of computer science and quantum mechanics. In the late 1970's, he was one of the first researchers to apply quantum mechanical principles to information processing, giving birth to the field of quantum information science at a time when it seemed more like science fiction than science.

His invention of quantum cryptography was hailed in the February 2003 issue of Technology Review, a journal published by the Massachusetts Institute of Technology, as one of the « 10 emerging technologies that will change the world ». This makes it possible to communicate in perfect secrecy under the nose of an eavesdropper who has unlimited computational power and whose technology is restricted only by the known laws of physics. This technology, which was initially considered a fringe pursuit at best, has moved from proof-of-principle prototypes to sophisticated apparatus, to commercial venture. Gilles Brassard is also one of the inventors of quantum teleportation, which is universally recognized as a fundamental cornerstone of quantum information science. In 2012, Thomson Reuters predicted that he would one day be awarded the Nobel Prize in Physics for this discovery.

In addition to being the former editor-in-chief of the Journal of Cryptology, Professor Brassard has authored three books that have been translated into eight languages and, according to Google Scholar, his scientific articles and books have been cited over 36,000 times. He has been awarded many honours and accolades: most notably, he was the first Canadian to be made a Fellow of the International Association for Cryptologic Research (IACR). He has also won both the Gerhard Herzberg Canada Gold Medal for Science and Engineering and the Killam Prize for Natural Sciences, generally considered to be the two most prestigious science prizes in Canada. Professor Gilles Brassard was described as « one of Canada's science superstars » by British astronomer David Darling. He is a Fellow of the Royal Society of London and holder of honorary doctorates from the ETH (Eidgenössische Technische Hochschule) of Zurich, the University of Ottawa and the Università della Svizzera italiana, Lugano. He was recently appointed Officer of the Order of Canada.

November 2nd

10h30 - 10h55  Registration

10h55 - 11h00  Opening remarks

11h00 - 12h00  Charles Bennett, IBM Thomas J. Watson Research Cente (Salon A)
                           Is there such a thing as private information?

12h00 - 13h30  Lunch (Dining room)

13h30 - 14h30  Christopher Fuchs, University of Massachusetts Boston (Salon A)
                           What Huangjun Zhu would say if he were disguised as Chris Fuchs

14h30 - 15h00  Coffee break (Salon B)

15h00 - 16h00  Harry Buhrman, CWI - University of Amsterdam (Salon A)
                           Quantum position-based cryptography

16h00 - 16h45  Nicolas Quesada, Université de Sherbrooke (Salon A)
                           Efficiency limits in quantum frequency conversion

16h45 - 17h00  Alexandre Blais, Université de Sherbrooke (Salon A)
‍                           From quantum science to quantum technologies

16h45 - 19h00  Poster session with refreshments (Salon B)

19h30 -             Dinner (Dining room)

November 3rd (at Domaine Château-Bromont)

6h30 - 8h30   Breakfast (Dining room)
8h00 - 8h55   Check out

9h00 - 10h00  Nicolas Gisin, Université de Genève (Salon A)
                          Quantum Nonlocality with Arbitrary Limited Detection Efficiency and
                          Measurement Dependence

10h00 - 10h30 Paul Raymond-Robichaud, Université de Montréal (Salon A)
                          The tale of the Orouboros

10h30 - 11h30  Coffe break and Poster session (Salon B)

11h30 - 12h00  Yuval Elias, Université de Montréal (Salon A)
                           Algorithmic Cooling in Liquid State NMR

12h00 - 13h30 Lunch (Dining room)

13h30 - 14h15   Michael Hilke, McGill University (Salon A)
                           Probing localization with a qubit

14h15 - 14h45  Shruti Puri, Université de Sherbrooke (Salon A)
                          Fighting Kerr-Induced Non-Gaussianity with Squeezing

14h45 - 15h15  Dany Lachance-Quirion, Université de Sherbrooke (Salon A)
                          Observation of magnon number states in
                            a superconducting qubit spectrum

15h15 - 15h20  Closing remarks

15h40 -             Departure of the chartered bus to Le Cercle restaurant at HEC

Evening of November 3rd

17h45 - 18h15  Meet and Greet at Restaurant Le Cercle, HEC Montréal

18h15 -             Banquet

INVITED SPEAKERS

Dr Charles H. Bennett

‍Thomas J. Watson Research Center, Yorktown Heights, NY USA
Is there such a thing as private information?
Quantum information theory originated in the practical use of quantum laws to keep classical information private, but soon grew to encompass the whole classical theory, generalizing it as powerfully as the complex numbers generalize the reals. Conceptually, private classical information sits on a silppery slope between quantum information and public information. Indeed decoherence theory suggests that the slope may be so slippery that private classical information doesn't exist: once an ebit is measured by Alice and Bob to generate a bit of classical key, the news necessarily escapes into the environment, rendering the key no longer private.

Professor Christopher Fuchs

‍University of Massachusetts Boston
What Huangjun Zhu would say if he were disguised as Chris Fuchs

Professor Nicolas Gisin

‍Université de Genève
Quantum Nonlocality with Arbitrary Limited Detection Efficiency and Measurement Dependence

Professor Harry Buhrman

‍CWI - University of Amsterdam
Quantum position-based cryptography
On 20 July 1969, millions of people held their breath as they watched, live on television, Neil Armstrong set foot on the Moon. Yet Fox Television has reported that a staggering 20% of Americans have had doubts about the Apollo 11 mission. Could it have been a hoax staged by Hollywood studios here on Earth?  Position based cryptography may offer a solution. This kind of cryptography  uses the geographic position of a party as its sole credential. Normally digital keys or biometric features are used.
A central building block in position-based cryptography is that of position-verification. The goal is to prove to a set of verifier that one is at a certain geographical location. Protocols typically assume that messages can not travel faster than the speed of light. By responding to a verifier in a timely manner one can guarantee that one is within a certain distance of that verifier.  Quite recently it was shown that position-verification protocols only based on this relativistic principle can be broken by  attackers who simulate being at a the claimed position while physically residing elsewhere in space.
Because of the no-cloning property of quantum information (qubits) it was believed that with the use of quantum messages one could devise protocols that were resistant to such collaborative attacks. Several schemes were proposed that later turned out to be insecure. Finally it was shown that also in the quantum case no unconditionally secure scheme is possible. We will review the field of position-based quantum cryptography and highlight some of the research currently going on  in order to develop, using reasonable assumptions on the capabilities of the attackers, protocols that are secure in practice.

INTRIQ SPEAKERS

Alexandre Blais

Professor, Université de Sherbrooke
From quantum science to quantum technologies

Yuval Elias

Postdoc, Université de Montréal
Director: Gilles Brassard
Algorithmic Cooling in Liquid State NMR
Heat-bath algorithmic cooling (HBAC) employs thermalization to purify quantum systems that interact with a heat bath, such as ensembles of nuclear spins, or cold atoms in an optical lattice. When applied to spins, HBAC produces ensembles of highly polarized spins, which enhance the signal strength in nuclear magnetic resonance (NMR). According to this cooling approach, spin-half nuclei in a constant magnetic field are considered as bits, or more precisely quantum bits, in a known probability distribution. Algorithmic steps on these bits are then translated into NMR pulse sequences using common NMR quantum computation tools. The algorithmic cooling of spins is achieved by alternately combining reversible, entropy-preserving manipulations (borrowed from data compression algorithms) with selective transfer of entropy from spins to the environment.HBAC, and multi-cycle HBAC, were previously achieved with nuclear spins using solid-state nuclear magnetic resonance (NMR), where spin diffusion allowed some spins to relax much more rapidly than others. In the liquid state, a partial variant of HBAC, without entropy compression, was applied to the three qubits of 13C-enriched trichloroethylene, resulting in modest cooling of the spin system. Recently, we utilized gradient ascent pulse engineering (GRAPE), an optimal control algorithm, to generate pulses with high fidelity for the compression and polarization exchange (SWAP) gates. Here, we combine those pulses to implement HBAC on this 3-spin system. Various cooling algorithms were applied, cooling the system beyond Shannon’s entropy bound in several different ways. In particular, in one experiment a carbon qubit was cooled by a factor of 4.61.This work is a step towards potentially integrating HBAC and other tools of NMR quantum computing into in vivo magnetic resonance spectroscopy.

Michael Hilke

‍Professor, McGill University
Probing localization with a qubit
We look at the dynamics of a qubit that is coupled to a disordered quantum wire. We show that we can use the time evolution of the qubit to measure the static localization properties of a disordered system. The disordered system can be an electronic gate or lead with impurities, or equivalently remote spins. We also evaluate the decoherence rate of the qubit as a function of the amount of disorder in the gate and lead.
This work is a collaboration with Mackenzie and Eleuch.

‍Dany Lachance-Quirion

Doctorat, Université de Sherbrooke
Director: Michel Pioro-Ladrière
Observation of magnon number states in a superconducting qubit spectrum
A magnon is a collective excitation in an ordered spin system such as a ferromagnet. Interacting with both microwave and optical light, magnons in the ferrimagnetic insulator yttrium iron garnet (YIG) are candidates for transducing quantum information between those two frequency domains. Coherent interaction between a single-magnon excitation in a YIG sphere and a superconducting qubit has recently been demonstrated through their interaction with a common microwave cavity mode.
The dispersive limit of the magnon-qubit interaction is discussed in this presentation. Magnon number states are experimentally resolved in the superconducting qubit spectrum when creating a coherent state in the magnetostatic mode. The probability distribution of magnon number states recovered from these measurements is found to agree well with the Poisson distribution expected for a coherent state.
Resolving magnon number states is a necessary ingredient for implementing a protocol in which quantum information of the qubit is encoded in the magnetostatic mode. This encoding would be the first step toward the realization of a quantum transducer between microwave and optical domains.
This work has been done in the group of Pr. Yasunobu Nakamura at the Research Center for Advanced Science and Technology of the University of Tokyo.

Paul Raymond-Robichaud

Doctorate, Université de Montréal
Director: Gilles Brassard
The tale of the Orouboros
Beyond "it from bit", John Archibal Wheeler, expressed his desire for a primeval ophidian self-reflexive recursive structure able to give birth to itself.   What can the autological serpent impart on Boltzmann brains, the nature of information, and of causality? Is our ontology leading to an epistemology leading to the very same ontology leading to the very same epistemology?  Should Jǫrmungandr fail to eat it's tail, knell Ragnarøkkr, the heterological darkness!

Nicolas Quesada

Postdoc, Unviersité de Sherbrooke
Director: Alexandre Blais
Efficiency limits in quantum frequency conversion
Frequency conversion is an enabling process in many quantum information protocols. In this talk we study fundamental limits to high efficiency frequency conversion imposed by time ordering corrections. Using the Magnus expansion, we argue that these corrections, which are usually considered detrimental, can be used to increase the efficiency of conversion under certain circumstances. The corrections induce a nonlinear behaviour in the probability of upconversion as a function of the pump intensity, significantly modifying the sinusoidal Rabi oscillations that are otherwise expected. Finally, by using a simple scaling argument, we explain why cascaded frequency conversion devices attenuate time ordering corrections, allowing the construction of near ideal quantum pulse gates.

‍Shruti Puri

Postdoc, Unviersité de Sherbrooke
Director: Alexandre Blais
Fighting Kerr-Induced Non-Gaussianity with Squeezing
Effort is being directed towards using the large Hilbert space of microwave resonators to encode quantum information. Universal quantum gates can be performed on this encoded information by using the dispersive interaction between resonators and off-resonant qubits. The dispersive interaction is however valid only for small resonator photon numbers and a  Kerr-type interaction emerges at moderately large powers. This leads to unwanted evolution and distortion of the resonator state and thereby logical errors. We propose to eliminate the effect of this Kerr nonlinearity by using an input squeezed vacuum on the resonator. By choosing the appropriate angle and strength of squeezing, we show that the Kerr-induced distortions in the resonator state can be removed, thereby increasing the fidelity of operations, such as creation of cat states, rotation of coherent state in phase space, QND measurement of qubit state, etc.

POSTER SESSION

Alessandro Ricottone

Doctorate, McGill University
Director: Bill Coish
Dynamic Nuclear Polarization and Nuclear­Spin Superradiance in 1D systems
The hyperfine coupling between an electron and many nuclear spins confined in a quantum dot can be a pathway to reach and measure high spin polarization of the nuclear spin system. Furthermore, the presence of long ­range coherence in the nuclear spin system could lead to the observation of nuclear­ spin superradiance, in which the spin ­flip rate is drastically enhanced.

Félix Beaudoin

Doctorate, McGill University
Director: Bill Coish
Hamiltonian simulation for improved state transfer and readout in cavity QED
Quantum state transfer into a memory, state shuttling over long distances via a quantum bus, and high-fidelity readout are important tasks for quantum technology. Generating the Hamiltonians that realize these tasks is often challenging; inhomogeneous broadening leads to dephasing during state transfer and an insufficiently strong interaction between a qubit and a cavity used for measurement may lead to an imperfect readout. Here, we use average Hamiltonian theory to design the desired Hamiltonians in cavity QED. In particular, we present a protocol for state transfer between a qubit and a cavity. This protocol gives a high fidelity even for inhomogeneous broadening that is larger than the qubit-cavity coupling. In addition, we design a time-averaged interaction that allows for a fast quantum nondemolition readout that is Purcell insensitive. These ideas can be applied directly to novel systems coupling single spins [Viennot et al, Science 349, 408 (2015)] or spin ensembles to a microwave cavity, or magnon modes to a transmon qubit [Tabuchi et al, Science 349 405 (2015)].

Julien Camirand Lemyre

Doctorate, Université de Sherbrooke
Director: Michel Pioro-Ladrière
Integration of nano-magnets for spin rotation in quantum dots

Guillaume Dauphinais

Doctorat, Université de Sherbrooke
Director: David Poulin
Error Correction for Systems with Non-Abelian Anyons
Topologically ordered systems giving rise to excitations which are non-abelian are of prime interest in terms of quantum computation, as these systems are thought to be naturally resilient against noise. There is nevertheless growing evidence that error correction will still be needed if one wishes to use such a system as a building block for a quantum computer. Difficulties in term of error correction arise from the non-abelian nature of the excitations, and very little work has been done on this topic. We present both numerical and analytical work on a fault-tolerant error correction scheme for systems that give rise to non-abelian anyonic excitations.

Benjamin D'Anjou

Doctorate, McGill University
Director: Bill Coish
Maximal adaptive-decision speedups in quantum-state readout
The average time T required for high-fidelity readout of quantum states can be significantly reduced via a real-time adaptive decision rule. An adaptive decision rule stops the readout as soon as a desired level of confidence has been achieved, as opposed to setting a fixed readout time tf. The performance of the adaptive decision is characterized by the "adaptive-decision speedup," tf/T. In this work, we reformulate this readout problem in terms of the first-passage time of a particle undergoing stochastic motion. This formalism allows us to theoretically establish the maximum achievable adaptive-decision speedups for several physical two-state readout implementations. We show that for two common readout schemes (the Gaussian latching readout and a readout relying on state-dependent decay), the speedup is bounded by 4 and 2, respectively, in the limit of high single-shot readout fidelity. We experimentally study the achievable speedup in a real-world scenario by applying the adaptive decision rule to a readout of the nitrogen-vacancy-center (NV-center) charge state. We find a speedup of ≈2 with our experimental parameters. In addition, we propose a simple readout scheme for which the speedup can in principle be increased without bound as the fidelity is increased. Our results should lead to immediate improvements in nano-scale magnetometry based on spin-to-charge conversion of the NV-center spin, and provide a theoretical framework for further optimization of the bandwidth of quantum measurements.

Pierre-François Duc

Doctorate, McGill University
Director: Guillaume Gervais
Critical Flow and Dissipation in a Quasi-One-Dimensional Superfluid
In one of the most celebrated examples of the theory of universal critical phenomena, the phase transition to the super fluid state of 4He belongs to the same three dimensional O(2) universality class as the onset of ferromagnetism in a lattice of classical spins with XY symmetry. Below the transition, the super fluid density Rho and super fluid velocity V increase as powerlaws of temperature described by a universal critical exponent constrained to be equal by scale invariance. As the dimensionality is reduced towards one dimension (1D), it is expected that enhanced thermal and quantum fluctuations preclude long-range order, thereby inhibiting super fluidity. We have measured the flow rate of liquid helium and deduced its superfluid velocity in a capillary flow experiment occurring in single ~30nm long nanopores with radii ranging down from ~20nm to ~3nm. As the pore size is reduced towards the 1D limit, we observe:  i)  a suppression of the pressure dependence of the superfluid velocity; ii) a temperature dependence of V that surprisingly can be well-fitted by a powerlaw with a single exponent over a broad range of temperatures; and iii) decreasing critical velocities as a function of decreasing radius for channel sizes below ~20nm, in stark contrast with what is observed in micron sized channels. We interpret these deviations from bulk behavior as signaling the crossover to a quasi-1D state whereby the size of a critical topological defect is cut off by the channel radius.

Zackary Flansberry

Master, McGill University
Director: Lilian Childress
NV Centres to Study Magnetization Dynamics in STT Devices

Luke Govia

Postdoc, McGill University
Director: Aashish Clerk
Qubit-Cavity Entanglement in the Dispersive Regime: Non-Demolition Measurement and Initialization
The dispersive regime of the Jaynes-Cummings interaction between electromagnetic radiation in a cavity and a two-level qubit has proven to be useful in many quantum information processing tasks. For example, in contemporary circuit quantum electrodynamics (cQED) architectures, the dispersive regime can be used for qubit readout and for gate operations. As the dispersive regime is derived after a frame transformation, to connect to external experimental devices the inverse frame transformation back to the lab frame is required. In this work, we show that in the lab frame the system is best described by an entangled state known as the dressed coherent state. We discuss how this results in further qubit evolution depending on both the amplitude and phase of the cavity field. In addition, we show how standard dispersive qubit readout is quantum non-demolition (QND) only in the limit of infinite measurement time, as the formation of a dressed coherent state in the qubit-cavity system results in an effective qubit rotation. A protocol to correct for this, and improve qubit initialization, is also introduced.

Jonathan Graveline

Master, Université de Sherbrooke
Director: Bertrand Reulet
Cryogenic nanobolometer as a noise detector
Building a good quantum computer working with many qubits requieres a good understanding of the electronic noise. To understand fundamental concepts of the electronic transport, we need the right tools. A cryogenic nanobolometer is an exemple of tools able to detect electron noise without bandpass limitation. The goals of this project are to build a such device, caracterize it in a cryogenic system and optimize its use.

Pavithran Iyer

Doctorate, Université de Sherbrooke
Director: David Poulin
Learning the noise rate of a quantum channel
Noise is intrinsic to a quantum computation process. The theory of quantum error correction was developed to show that it is in fact possible to perform meaningful quantum computation in the presence of noise. However, it assumes a simplistic picture, where the noise on a quantum system can be modelled as the action of Pauli matrices. In such a case, the effect of the noise process on the quantum system can be encapsulated into a quantity often called the noise rate, whose value can help determine if an error correction procedure can save the computation result from the effect of noise.
An experimental scenerio is far from the simplistic picture of Pauli noise, one can imagine amplitude damping, leakage errors or even small rotations which cannot be captured by Pauli matrices. In this case, it is useful to provide a notion of a noise strength which can directly relate to the usefulness of an error correction procedure. Here, we try to come up with a measure of a noise strength for a noise process on a single qubit system. To do this, we apply machine learning techniques with a large database of quantum channels and we aim to arrive at a metric for the channel that can best describe the logical error on the quantum computation.

Pericles Philippopoulos

Doctorate, McGill University
Director: Bill Coish
Hyperfine Interactions for Hole Spins
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 form of the hyperfine coupling has been recently called into question with experiments [1] that have been interpreted to indicate a strong hybridization of p-like and d-like components in the valence band of III-V semiconductors. However, this interpretation 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 contributions are negligible. Assumption (1) may break down in light of the fact that the spin-orbit energy is even larger than the principal gap in InAs, and assumption (2) is difficult to justify in any crystal that breaks pure rotational symmetry. Using a generalization of the group-theoretic analysis in , 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. We propose a way in which to test for these d-like components using optical selection rules. Finally, we use density functional theory (D.F.T.) to estimate hyperfine couplings. In contrast to other works, we include off-diagonal elements of the spin-density operator.

Edouard Pinsolle

Postdoc, Université de Sherbrooke
Director: Bertrand Reulet
The Noise Thermal Impedance
We have developed a technique to probe heat relaxation times of the electron gaz. We applied it to study a metallic wire as a test. From those measurements we were able to separate two regimes where the heat relaxation is due to different phenomenons. We distinguished between an electron-phonon regime and a diffusion regime and extracted the associated relaxation times and there dependence on temperature.

Saeid Molladavoudi

Postdoc, Université d'Ottawa
Director: Anne Broadbent
Topological Invariants for Abelian Reduced Spaces of Multi-particle Quantum States
We present a mathematical framework to study geometry and topology of quotients for multi-particle quantum systems. In particular, we are interested in geometrical and topological properties of abelian symplectic reduced spaces of pure multipartite states, as complex projective spaces, which are acted upon in a Hamiltonian fashion by maximal tori of the semisimple compact Local Unitary Lie groups. We discuss that the existing geometrical methods equip us with a powerful set of tools to compute topological invariants for these reduced spaces. More precisely, given the components in the moment (Kirwan) polytope for multi-qubits, we utilize a recursive wall-crossing formula for the Poincar\'e polynomials and Euler characteristics of abelian symplectic quotients and as some examples we elaborate the procedure for quantum systems with two and three qubits in their pure states and propose an algorithm for a general r-qubits.

David Roy-Guay

Doctorate, Université de Sherbrooke
Directors: Denis Morris, Michel Pioro-Ladrière
Vectorial magnetometry with NV centers

Jean Olivier Simoneau

Doctorate, Université de Sherbrooke
Director: Bertrand Reulet
Photon statistics of shot noise measured using a Josephson parametric amplifier
Quantum measurements are very sensitive to external noise sources. Such measurements require careful amplification chain design so as not to overwhelm the signal with extraneous noise. A quantum-limited amplifier, like the Josephson parametric amplifier (paramp), is thus an ideal candidate for this purpose. We used a paramp to investigate the quantum noise of a tunnel junction. This measurement scheme allowed us to improve upon previous observations of shot noise by an order of magnitude in terms of noise temperature. With this setup, we have measured the second and fourth cumulants of current fluctuations generated by the tunnel junction within a 40 MHz bandwidth around 6 GHz. From theses measurements, we deduce the variance of the photon number fluctuations for various bias schemes of the junction. In particular, we investigate the regime where the junction emits pairs of photons.

Kunal Tiwari

Doctorate, McGill University
Director: Bill Coish
Magnetotransport in topological Kondo insulators

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