# Conference program

*The emergence of quantum information.*

A conference in honour of Gilles Brassard

A conference in honour of Gilles Brassard

**Monday, November 2nd (at Domaine Château-Bromont)**

10h30 - 10h55 *Registration*

10h55 - 11h00 *Opening remarks*

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

12h00 - 13h30* Lunch (Dining room)*

13h30 - 14h30* Professor 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 *Professor Harry Buhrman, CWI - University of Amsterdam *

*(Salon A)*

*Quantum position-based cryptography*16h00 - 16h45* Nicolas Quesada, Postdoc, Université de Sherbrooke (Salon A) Efficiency limits in quantum frequency conversion*

16h45 - 17h00 *Professeur 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)*

**Tuesday, November 3rd (at Domaine Château-Bromont)**

6h30 - 8h30* Breakfast (Dining room)* 8h00 - 8h55

*Check out*

9h00 - 10h00* Professor Nicolas Gisin, Université de Genève (Salon A) Quantum Nonlocality with Arbitrary Limited Detection Efficiency and Measurement Dependence*10h00 - 10h30

*Paul Raymond-Robichaud, Doctorate, Université de Montréal*

*(Salon A)***The tale of the Orouboros**10h30 - 11h30 *Coffe break and Poster session (Salon B) *

11h30 - 12h00* Yuval Elias, Postdoc, Université de Montréal (Salon A)*

Algorithmic Cooling in Liquid State NMR

12h00 - 13h30 * Lunch (Dining room)*

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

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

14h45 - 15h15 Dany Lachance-Quirion, Doctorat, Université de Sherbrooke (Salon A)

*Observation of magnon number states in a superconducting qubit spectrum*

15h15 - 15h20* Closing remarks of the scientific program*

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

**Tuesday, evening of November 3rd (at Restaurant Le Cercle)**

17h45 - 18h15 Meet and Greet

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? (pdf)*

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 (pdf)*

*Professor Nicolas Gisin *

Université de Genève*Quantum Nonlocality with Arbitrary Limited Detection Efficiency and Measurement Dependence (pdf)*

*Professor Harry Buhrman *

CWI - University of Amsterdam**Quantum position-based cryptography (pdf)**

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 SherbrookeFrom quantum science to quantum technologies (pdf)*

*Yuval EliasPostdoc, Université de MontréalDirector: Gilles Brassard*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.

**Algorithmic Cooling in Liquid State NMR (pdf)**

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.

*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.*

Michael Hilke

Professor, McGill University

Michael Hilke

Professor, McGill University

**Probing localization with a qubit**This work is a collaboration with Mackenzie and Eleuch.

Dany Lachance-Quirion

Dany Lachance-Quirion

*Doctorat, Université de Sherbrooke*

Director: Michel Pioro-Ladrière

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 QuesadaPostdoc, Unviersité de SherbrookeDirector: Alexandre Blais*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.

**Efficiency limits in quantum frequency conversion**

Shruti Puri

Shruti Puri

*Postdoc, Unviersité de Sherbrooke*

*Director: Alexandre Blais*

*Fighting Kerr-Induced Non-Gaussianity with Squeezing*(pdf)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 RicottoneDoctorate, McGill UniversityDirector: Bill Coish*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.

**Dynamic Nuclear Polarization and NuclearSpin Superradiance in 1D systems**

*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 LemyreDoctorate, Université de SherbrookeDirector: Michel Pioro-Ladrière*

**Integration of nano-magnets for spin rotation in quantum dots**

*Guillaume DauphinaisDoctorat, Université de SherbrookeDirector: David Poulin*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.

**Error Correction for Systems with Non-Abelian Anyons**

*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 GoviaPostdoc, McGill UniversityDirector: *

*Aashish Clerk*

**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.**

Qubit-Cavity Entanglement in the Dispersive Regime: Non-Demolition Measurement and Initialization

Qubit-Cavity Entanglement in the Dispersive Regime: Non-Demolition Measurement and Initialization

*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*

**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.**

Hyperfine Interactions for Hole Spins

Hyperfine Interactions for Hole Spins

*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 TiwariDoctorate, McGill UniversityDirector: Bill Coish*

**Magnetotransport in topological Kondo insulators**