Rencontres INTRIQ
Spring 2015 INTRIQ meeting, April 28th and 29th
At the Hotel of the Château Bromont
Organizers:
Professor Aashish Clerk, McGill University
Professor Thomas Szkopek, McGill University
90, rue Stanstead, Bromont QC J2L 1K6
Téléphone : 1 800 304 3433 Directions

Notes
 The INTRIQ Business meeting will be held in the room Les Cantons on April 28th from 9h30 to 10h30
 A bus shuttle from BerriUQAM is organised for students in the morning of April 28^{th} and returning on the 29th Bus shuttle BerriBromontBerri
Meeting program
Tuesday, April 28th
10h30  11h00 Registration
11h00  12h00 Professor Lorenza Viola, Dartmouth College (Salon A)
Dissipative quantum state preparation with quasilocal resources (pdf)
12h00  13h30 Lunch (Dining room)
13h30  15h15 Professor Sankar Das Sarma, University of Maryland (Salon A)
Majorana fermions for solid state topological quantum computation
15h15  15h45 Coffee break (Salon B)
15h45  16h15 Stéphane Virally, Postdoc, Université de Sherbrooke (Salon A)
Discrete photon statistics from continuous measurements (pdf)
16h15  17h00 Competition : My thesis in 180 seconds
 Christian Kraglund Andersen, Doctorate
 Pavithran Iyer, Doctorate
 MarcAntoine Lemonde, Doctorate
 Zackary Flansberry, Master
 Benjamin Levitan, Master
 Baptiste Royer Master
17h00  19h00 Poster session with refreshments (Salon B)
19h30  Diner (Dining room)
Wednesday, April 29th
6h30  8h30 Breakfast (Dining room)
8h00  8h55 Check out
9h00  9h30 Shruti Puri, Postdoc, Université de Sherbrooke (Salon A)
HighFidelity ResonatorInduced Phase Gate with Squeezing (pdf)
9h30  10h00 Anja Metelmann, Postdoc, McGill University (Salon A)
Nonreciprocal photon transmission and amplification via reservoir engineering
10h00  11h00 Coffee break and poster session (Salon B)
11h00  12h00 Professor Aephraim M. Steinberg, University of Toronto (Salon A)
Two novel applications of entanglement: compressing 3 qubits into 2, and making 1 photon act like
100
12h00  13h30 Lunch (Salon C)
13h30  14h15 Professor Gilles Brassard, Université de Montréal (Salon A)
Exact Classical Simulation of the GHZ Distribution
14h15  14h45 Benjamin Schmidt, Doctorate, McGill University (Salon A)
Timeresolved measurement of the electronphonon scattering rate in a 2DEG
14h45  15h15 Guillaume Dauphinais, Doctorate, Université de Sherbrooke (Salon A)
An introduction to anyons in the context of quantum information
15h15  15h20 Closing remarks and departure
INVITED SPEAKERS
Professor Sankar Das Sarma
University of Maryland
Majorana fermions for solid state topological quantum computation
Professor Aephraim M. Steinberg
University of Toronto
Two novel applications of entanglement: compressing 3 qubits into 2, and making 1 photon act like 100
I will present two recent experiments making use of photon entanglement for two very different tasks. In one, we show that all the information about an ensemble of N identically prepared qubits (such as one might use in tomography) may be compressed into log(N+1) qubits. Given a limitedsize quantum memory, this would enable one to more efficiently use the initial ensemble to make predictions about future measurements, relative to simply carrying out tomography and storing the resulting classical information. In the second experiment, we demonstrate a coldatombased optical nonlinearity which weakly entangles a singlephotonlevel beam with a coherentstate probe. We observe the phase shift due to single postselected photons, and show that "weakvalue amplification" can lead to effective photon numbers much larger than 1, as manifested in measured phase shifts much larger than the single photon value.
Professor Lorenza Viola
Dartmouth College
Dissipative quantum state preparation with quasilocal resources
Techniques for quantum reservoir and dissipation engineering are playing an increasingly important role in controlling open quantum systems. Implications range from dissipative quantum state preparation and quantum computation, to nonequilibrium quantum phases of matter and quantum thermodynamics. In this talk, I will describe progress toward developing a general controltheoretic framework for the analysis and synthesis of quasilocal opensystem dynamics that admits a desired quantum state as its unique asymptotically stable state, with focus on continuoustime Markovian dynamics and entangled target states. In particular, after reviewing existing necessary and sufficient conditions for quasilocal stabilization of a pure state, I will discuss the additional challenges that the stabilization problem entails for a general mixed target state, and present recent rigorous results for a natural class of quasilocal frustrationfree Lindblad dynamics.
INTRIQ SPEAKERS
Professor Gilles Brassard
Université de Montréal
Exact Classical Simulation of the GHZ Distribution
John Bell has shown that the correlations entailed by quantum mechanics cannot be reproduced by aclassical process involving noncommunicating parties. But can they be simulated with the help of bounded communication? This problem has been studied for more than two decades and it is now well understood in the case of bipartite entanglement. However, the issue was still widely open for multipartite entanglement, even for the simplest case, which is the tripartite GreenbergerHorneZeilinger (GHZ) state. We give an exact simulation of arbitrary independent von Neumann measurements on general npartite GHZ states. Our protocol requires O(n^2) bits of expected communication between the parties, and O(n log n) expected time is sufficient to carry it out in parallel. Furthermore, we need only an expectation of O(n) independent unbiased random bits, with no need for the generation of continuous real random variables nor prior shared random variables. In the case of equatorial measurements, we improve on the prior art with a protocol that needs only O(n log n) bits of communication and O(log^2 n) parallel time. At the cost of a slight increase in the number of bits communicated, these tasks can be accomplished with a constant expected number of rounds.
Guillaume Dauphinais
Doctorate, Université de Sherbrooke
Director: David Poulin
An introduction to anyons in the context of quantum information
In this talk, I will briefly present various concepts of the algebraic theory of anyons. The focus will be put on the use of anyons in the context of quantum information, namely on the various ways to encode quantum information in such systems and how braiding operations can be used to apply gates. Simple analytical models giving rise to anyonic quasiparticles will be presented to concretely illustrate the concepts introduced in the algebraic theory of anyons. If time permits, I will also briefly discuss error corretion and faulttolerance in the context of quantum memories built with systems of nonabelian anyons.
Anja Metelmann
Postdoc, McGill University
Director: Aashish Clerk
Nonreciprocal photon transmission and amplification via reservoir engineering
The general desire to break reciprocity in engineered photonic structures has garnered an immense amount of recent interest. For example nonreciprocal microwavefrequency devices are crucial to efforts at quantuminformation processing with superconducting circuits. We discuss a general method for constructing nonreciprocal, cavitybased photonic devices, based on matching a given coherent interaction with its corresponding dissipative counterpart; it generalizes the basic structure used in the theory of cascaded quantum systems. In contrast to interferencebased schemes, our approach allows directional behavior over a wide bandwidth. We show how it can be used to devise isolators and directional, quantumlimited ampliﬁers; of particular interest is a directional phasesensitive ampliﬁer which is not limited by any fundamental gainbandwidth constraint. Our approach is particularly wellsuited to implementations using superconducting microwave circuits.
Shruti Puri
Postdoc, Université de Sherbrooke
Director: Alexandre Blais
HighFidelity ResonatorInduced Phase Gate with Squeezing
With the recent developments in superconducting parametric amplifiers, it is now possible to generate squeezed microwave fields and to couple this radiation to a resonator. Here we propose to use a squeezing to increase the fidelity of twoqubit gates in circuit QED. We focus on the resonatorinduced phase gate where a drive, offresonant with both the qubit and the resonator, leads to a controlledphase gate. This gate has the advantage of working with fixed frequency qubits that have longer coherence times than their frequencytunable counterparts. The gate fidelity is limited by ''whichpath information'' due to photons leaking from the resonator. We show that there exists an optimal squeezing angle and strength to erase whichpath information. Analytic estimate of the gate fidelity is compared to numerical simulations. With realistic parameters, we find that it is possible to achieve a twoqubit controlledZ gate of average fidelity >99.9 % within ~ 200 ns.
Benjamin Schmidt
Doctorate, McGill University
Director: Guillaume Gervais
Timeresolved measurement of the electronphonon scattering rate in a 2DEG
A number of groups have studied the electronphonon coupling in GaAs/AlGaAs twodimensional electron gases; however existing estimates of the electronphonon scattering rate have been calculated by combining measurements of DC power dissipation with a simple model of the 2DEG specific heat at zero magnetic field. We have measured the scattering rate directly, using a timeresolved technique, in an ultrahigh mobility Corbino sample below 100 mK. By combining these results with standard DC power dissipation measurements, we can infer the heat capacity of the 2DEG. We propose to use this technique to study exotic phases in the fractional quantum Hall regime, including the possibly nonAbelian 5/2.
Stéphane Virally
Postdoc, Université de Sherbrooke
Director: Bertrand Reulet
Discrete photon statistics from continuous measurements
In a recent paper [J.C. Forgues et al., PRL 114, 130403 (2015)], a twophoton squeezed state was observed in the shot noise of a tunnel junction. Squeezed states are associated with the generation of photon pairs (in this case, microwave photons).There are no good photon detectors in the microwave regime to directly detect these pairs.
However, we show that it is possible to extract discrete photon statistics from the cumulants of the continuous distribution of measured voltages at the output of an electrical circuit. Since we want to measure very small signals, we use a parametric amplifier as the quantumlimited first stage in the amplification chain, keeping the number of noise photons in the low single digits. We can thus reconstruct photon statistics for signals with less than one photon on average. We apply this technique to an attenuated coherent state (sine wave) and to the case of the squeezed state generated by a tunnel junction. We also place limits on the expected behavior of classical signals.
POSTER SESSION
Félix Beaudoin
Doctorate, McGill University
Director: Bill Coish
Microscopic models for chargenoiseinduced dephasing of solidstate qubits
Several experiments have shown qubit coherence decay of the form exp[(t/T2)^alpha] due to environmental chargenoise fluctuations. We present a microscopic description for temperature dependences of the parameters T2 and alpha. Our description is appropriate to qubits in semiconductors interacting with spurious twolevel charge fluctuators coupled to a thermal bath. We find distinct powerlaw dependences of T2 and alpha on temperature depending on the nature of the interaction of the fluctuators with the associated bath. We consider fluctuator dynamics induced by first and secondorder tunneling with a continuum of delocalized electron states. We also study one and twophonon processes for fluctuators in either GaAs or Si. These results can be used to identify dominant chargedephasing mechanisms and suppress them.
Salil Bedkihal
Postdoc, McGill University
Director: Bill Coish
Magnetotransport in AharonovBohm interferometers: Numerically exact path integrals The linear conductance of two terminal AharonovBohm interferometer is an even function of magnetic flux, as dictated by OnsagerCasimir symmetry. Away from linear response this symmetry is broken when many body  interactions are in effect. In this work we study quantum dynamics of various models of doubledot AharonovBohm interferometer with genuine many body interactions using numerically exact influence functional path integral method (INFPI).
In modelI we consider spinless two level double dot interferometer with interdot Coulomb interaction. In modelII interferometer is interacting with a dissipative environment that may be driven out of equilibrium. In both these models depending on vertical and horizontal mirror symmetries of setup, nonlinear transport coefficients obey certain symmetry relations. Numerically exact results are compared to phenomenological methods.
Simon Bernard
Master, McGill University
Director: Jack Sankey
Toward Stronger Optomechanical Coupling: Highreflectivity Photonic Crystal Membranes
A central goal in the field of optomechanics is to use the forces exerted by laser light as a new means of control over the motion of microelectromechanical system (MEMS). Radiation forces can be used to tune a solid object's mechanical frequency or dissipation, laser cool its motion to the quantum mechanical ground state, or levitate it. To maximize the impact of each photon, it is desirable to create MEMS that are highly reflective, lightweight, and mechanically compliant. These requirements can be simultaneously met by fabricating a 2D photonic crystal (i.e. an appropriatelyspaced lattice of holes) into a thin (~ 50  100 nm) freestanding dielectric membrane. The process flow for fabrication as well as the measurement and simulation of the photonic crystal's transmission will be reported.
Benjamin D'Anjou
Student, McGill University
Director: Bill Coish
Soft decoding of a qubit readout apparatus
Qubit readout is commonly performed by thresholding a collection of analog detector signals to obtain a sequence of singleshot bit values. The intrinsic irreversibility of the mapping from analog to digital signals discards soft information associated with an "a posteriori" confidence that can be assigned to each bit value when a detector is well characterized. Accounting for soft information, we show significant improvements in enhanced state detection with the quantum repetition code as well as quantum state or parameter estimation. These advantages persist in spite of nonGaussian features of realistic readout models, experimentally relevant small numbers of qubits, and finite encoding errors. These results show useful and achievable advantages for a wide range of current experiments on quantum state tomography, parameter estimation, and qubit readout.
Mark Dimock
Master, McGill University
Director: Lilian Childress
Toward cavity assisted processes with NV centers
Nicolas Didier
Postdoc, Université de Sherbrooke
Director : Alexandre Blais
In collaboration with : Jérôme Bourassa, Professor at Cégep de Granby
Fast quantum nondemolition readout from longitudinal qubitoscillator interaction
We show how to realize highfidelity quantum nondemolition qubit readout using longitudinal qubitoscillator interaction. This is realized by modulating the longitudinal coupling at the cavity frequency. The qubitoscillator interaction then acts as a qubitstate dependent drive on the cavity, a situation that is fundamentally different from the standard dispersive case. Singlemode squeezing can be exploited to exponentially increase the signaltonoise ratio of this readout protocol. We present an implementation of this idea in circuit quantum electrodynamics and a possible multi qubit architecture.
Zackary Flansberry
Master, McGill University
Director: Lilian Childress
NV Centres to Study the Magnetization Dynamics of STT Devices
In spite of its tremendous potential for the development of a new generation of random access memory, the SpinTransfer Torque (STT) effect continues to be difficult to use efficiently; in particular, the writing of prototypical memory bits requires current densities that are still well above the theoretical expectations. In order to gain fundamental insight onto the origins of this discrepancy (among others), we use NV centres as atomicscale probes to study the local magnetization dynamics of STTdriven devices using the Spin Hall Effect. By virtue of a ground state manifold that exhibits the Zeeman splitting, this defect centre in the crystal structure of diamond can resolve magnetic excitations of the gigahertz domain on spatial scales below 10 nanometers by using versatile and easily accessible optical methods.
This presentation shall overview the design and fabrication of the studied magnetic nanocircuits onto diamond substrates previously embedded with NV centres; it will also discuss ferromagnetic resonance experiments performed on those devices, the response of the defect centres to such resonant excitations as well as the applicability of this system to other magnetic configurations.
Julia Hildmann
Postdoc, McGill University
Director: Bill Coish & Aashish Clerk
Optimal dynamical decoupling from colored noise with finitebandwidth pulses
Patrick Hofer
Doctorate, McGill University
Director: Aashish Clerk
Squeezed light from a tunnel junction
We theoretically investigate the creation of squeezed light by a tunnel junction biased with a time dependent voltage as observed recently [Gasse et al. PRL 111, 136601 (2013)]. We find that the 3db limit can be reached by biasing the tunnel junction with a train of Lorentzian voltage pulses. A comparable amount of squeezing can be obtained by a biharmonic voltage bias. We discuss the squeezing as resulting from a noisy force on the photonic mode provided by the tunneling electrons and point out the limitations of this scheme due to the accompanying heating effects.
Pavithran Iyer
Doctorate, Université de Sherbrooke
Director: David Poulin
Metrics to measure the strength of noise on quantum information
Every error correcting code can only protect quantum information against noise upto a certain threshold. It is very important to know this threshold, for any given error correcting code and a quantum channel. A considerable amount of work has been done to derive numerical estimates of the threshold of various error correcting codes, however, often over quantum channels where the noise is limited to the set of Pauli errors. On the other hand, this restriction to Pauli errors is an oversimplification of a realistic noise model. In a real experimental scenario, noise affecting a quantum system is never as simple as a Pauli error. There is an intrinsic difficulty with a noise model which is not Pauli — there is no general method to say than a channel is more noisy than the other. In this work, we are trying to characterize the strength of a generic noise model by some metrics and observing how the strength of noise changes after error correction.
Erika Janitz
Doctorate, McGill University
Director: Lilian Childress
Toward Cavity Assisted Processes with NV Centers
Benjamin Levitan
Master, McGill University
Director: Aashish Clerk
Dispersive qubit measurement using an integrated onchip parametric amplifier
Tina Müller
Postdoc, McGill University
Director: Jack Sankey
Enhanced optomechanical levitation of minimally supported dielectrics
Hugo Ribeiro
Postdoc McGill University
Director: Bill Coish
Coherent mechanicallymediated state transfer between a superconducting qubit and a cavity
Alessandro Ricottone
Doctorate, McGill University
Director: Bill Coish
Dynamic Nuclear Polarization and Nuclear Spin Superradiance in 1D systems
The hyperfine coupling between an electron and many nuclei spin 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 strong coherence between the nuclei could lead to the observation of nuclear superradiance, a phase in which the spin flip rate is drastically enhanced. We propose a new device for the observation of this collective phase.
AnneMarie Roy
Master, Université de Sherbrooke
Director: Michel PioroLadrière
Tunable radiofrequency charge sensor
Manipulating the spin of single electrons in quantum dots is a promising avenue for quantum information processing. As the readout of the spins is performed via spintocharge conversion, establishing a charge sensing technique that is fast and highly sensitive is crucial. For this reason, radiofrequency quantum point contact charge sensors have become widespread. Here we present a tunable quantum point contact charge sensor using a cryogenic variable capacitor, tunable from 2 to 12 pF. We obtain optimal impedance matching for different quantum dot devices over a frequency range from 125 to 210 MHz. The flexibility of our setup allows the integration of radiofrequency charge sensors to a variety of nanostructures.
Baptiste Royer
Master, Université de Sherbrooke
Director: Alexandre Blais
Lattice waveguide QED : manybody interactions by dissipation
In waveguide QED, superconducting qubits acting as artificial atoms are coupled to a 1D superconducting transmission line playing the role of common bath for the qubits. By controlling their effective separation, it is possible to engineer various types of dissipationinduced interactions between the qubits. We consider the situation where multiple superconducting qubits are coupled to a lattice of superconducting transmission lines. Depending on the choice of lattice, the qubits exhibit a rich variety of interactions. We present a Markovian master equation framework to describe these systems, and discuss results obtained for simple lattices.
Maximilian Ruf
Master, McGill University
Director: Jack Sankey
Toward cavity assisted processes with NV centers
Jean Olivier Simoneau
Master, 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 quantumlimited 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.
JeanRené Souquet
Postdoc, McGill University
Director: Aashish Clerk
Photonassisted tunnelling with nonclassical light
Vahid Tayari
Postdoc, McGill University
Director: Thomas Szkopek
TwoDimensional Magnetotransport in a Black Phosphorus Naked Quantum Well
Black phosphorus (bP) is the second known elemental allotrope with a layered crystal structure that can be mechanically exfoliated down to atomic layer thickness. We have fabricated bP naked quantum wells in a backgated field effect transistor geometry with bP thicknesses ranging from 6±1 nm to 47±1 nm. Using an encapsulating polymer superstrate, we have suppressed bP oxidation and have observed field effect mobilities up to 600 cm2/Vs and on/off current ratios exceeding 105. Importantly, Shubnikovde Haas (SdH) oscillations observed in magnetotransport measurements up to 35 T reveal the presence of a 2D hole gas with Schrodinger fermion character in an accumulation layer at the bP/oxide interface. Our work demonstrates that 2D electronic structure and 2D atomic structure are independent. 2D carrier confinement can be achieved in layered semiconducting materials without necessarily approaching atomic layer thickness, advantageous for materials that become increasingly reactive in the fewlayer limit such as bP.