# Introduction à l'information quantique

### Fall 2019 INTRIQ meeting, November 11th & 12th

**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** Purely quantum polar codes *

*11h45 - 12h05 Marco David, Student, McGill University** QED. The Quest to Formally Verify Mathematics*

*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** A mesoscopic spectrometer based on the Josephson effect*

*14h45 - 15h15 Coffee break (Salon B)*

*15h15 - 15h45 Dr Stephane Virally, Polytechnique Montréal** Quantum optics in the time domain *

*15h45 - 16h25 Industry & Startups in quantum technologies** Dr Félix Beaudoin, Les Technologies Nanoacademic Inc **(www.nanoacademic.com**)*

*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** Quantum encryption with certified deletion*

* 9h00 - 10h00 Dr Tomas Jochym-O'Connor, IBM - Yorktown Heights, New York** Disjointness in stabilizer codes*

*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** Cavity-Enhanced Photon Emission from a Single Germanium-Vacancy Center in a Diamond Membrane*

*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*

**Invited speakers **

*Pr Signe Seidelin**Institut NEEL CNRS/UGA*

**Rare-Earth Doped Crystals for strain-coupled optomechanics**A challenge of modern physics is to investigate the quantum behavior of a bulk material object - for instance a mechanical oscillator - placed in a non-classical state. One major difficulty relies in interacting with the mechanical object without perturbing with its quantum behavior. An approach consists of exploiting a hybrid quantum system consisting of a mechanical oscillator coupled to an atom-like object, and interact via the atom-like object. A particularly appealing coupling mechanism between resonator and “atom” is based on material strain. Here, the oscillator is a bulk object containing an embedded artificial atom (dopant, quantum dot, ...) which is sensitive to mechanical strain of the surrounding material. Vibrations of the oscillator result in a time-varying strain field that modulates the energy levels of the embedded structure. We have suggested to use rare-earth doped crystals for strain-coupled systems [1] and proposed a mechanism to cool down the resonator [2]. In this talk, I will report on our progress towards realizing experimentally these protocols. We are using an yttrium silicate (Y

_{2}SiO

_{5}) crystal containing triply charged europium ions (Eu

^{3+}), which are optically active. The reason behind this choice stems from the extraordinary coherence properties of this dopant, combined with its high strain-sensitivity: the Eu

^{3+}in an Y

_{2}SiO

_{5}matrix has an optical transition with the narrowest linewidth known for a solid-state emitter, and the transition is directly sensitive to strain. We have successfully fabricated mechanical resonators, designed and set up the experiment, and achieved a signal-to-noise ratio compatible with the planned measurements, as well as measured the strain sensitivity of europium ions in bulk Y

_{2}SiO

_{5}crystals.

[1] K. Mølmer, Y. Le Coq and S. Seidelin, Dispersive coupling between light and a rare-earth ion doped mechanical resonator, Phys. Rev. A 94, 053804 (2016)

[2] S. Seidelin, Y. Le Coq and K. Mølmer, Rapid cooling of a strain-coupled oscillator by an optical phase-shift measurement, Phys. Rev. A 100, 013828 (2019)

*Dr Tomas Jochym-O'Connor*

*IBM - Yorktown Heights, New York*

**The disjointness for stabilizer codes in quantum error correction is an algebraic quantity tied to the structure of the stabilizer generators of a code. It can be used to characterize the properties of different classes of logical gates, placing bounds on the level of the Clifford hierarchy attainable by constant depth circuits. This talk will introduce the notion of disjointness, highlight its usefulness with several examples, and posit open questions for which the disjointness may be useful in addressing.**

*Disjointness in stabilizer codes*

* Pr Frédéric Dupuis**Université de Montréal**Purely quantum polar codes*

We provide a purely quantum version of polar codes, achieving the coherent information of any quantum channel. Our scheme relies on a recursive channel combining and splitting construction, where random two-qubit Clifford gates are used to combine two single-qubit channels. The inputs to the synthesized bad channels are frozen by sharing EPR pairs between the sender and the receiver, so our scheme is entanglement assisted. We further show that a Pauli channel polarizes if and only if a specific classical channel over four symbol input set polarizes. We exploit this equivalence to prove fast polarization for Pauli channels, and to devise an efficient successive cancellation based decoding algorithm for such channels. This is joint work with Ashutosh Goswami, Mehdi Mhalla and Valentin Savin.

*Dr Louis Gaudreau**National Research Council (NRC) - Ottawa*** Entanglement distribution via coherent photon-to-spin conversion in semiconductor quantum dot circuits**In this talk, I will present our proposed long distance entanglement distribution scheme that aims to overcome fundamental limitations present in current optical schemes. By using direct band gap semiconductor quantum dots, efficiency and heralding advantages can be exploited through photon-to-spin conversion. For this reason, materials such as GaAs are superior to Si in this type of applications. I will review current schemes to transfer polarization or time-bin encoded photonic qubits to electron spin qubits and will describe adaptations to employ heavy holes which have a number of attractive properties including g-factor tunability. Finally, I will show preliminary results on quantum dot devices using Van der Waals heterostructures which present several potential advantages such as higher confinement energies due to their atomically thin geometry, easier combination with different substrates and the possibility of encoding information in their valley degree of freedom.

*INTRIQ speakers*

*Dr Thomas Baker**Université de Sherbrooke**Modeling superconducting circuits with a tensor network*

We introduce a tensor network method tailored for the simulation of large-scale superconducting circuits. We leverage information unavailable to other state-of-the-art simulation methods to produce first-principles coherence-time estimates for the fluxonium qubit. In particular, we study the charge dispersion of this qubit due to coherent quantum phase slip processes as a function of the array-junction impedance, and we provide direct numerical evidence of the validity of an effective theory introduced in [V. E. Manucharyan et. al, PRB 85, 024521 (2012)]. We can analyze circuits in terms of entanglement, which can signal how protected quantum information is in circuits and motivate better ways of encoding it. Measurements of the energy participation ratio of circuit components indicates how the qubit transitions are affected by dissipation channels and circuit-element disorder.

*Pr Jérôme Bourassa**Cégep de Granby**Quantum illumination : exploiting quantum correlations when entanglement is lost*

sTwo-mode squeezed states quantum states of light where photons in a pair of distinct beams are entangled together. These EPR-states of light are an excellent resource for quantum sensing and quantum communications protocols. Excess noise in a lossy transmission channel however breaks the entanglement and prohibits secure quantum communication between parties. In spite of this, the quantum correlations in the final state are resilient to environment noise and remain larger than what is classically allowed. Consequently, illuminating a distant object with quantum entangled light, or quantum illumination, has been shown to lead to quantum enhancements in remote sensing detection capabilities and resilience to adversary noise. Quantum illumination could provide invaluable performance enhancements of observations made in noisy environments, from biological samples in the optical range to satellite surveillance in the microwaves. In this talk, I will introduce the concept of quantum illumination, present an overview of different detection schemes and discuss their capabilities and limitations. With a particular focus on microwave technology, I will present the requirements and progress towards building microwave quantum-enhanced radar system.

*Pr Anne Broadbent**Université d'Ottawa**Quantum encryption with certified deletion*

Given a ciphertext, is it possible to prove the deletion of the underlying plaintext? Since classical ciphertexts can be copied, clearly, such feat is impossible using classical information alone. In stark contrast to this, we show that quantum encodings allow such certified deletion. More precisely, we show that it is possible to encrypt classical data into a quantum ciphertext such that the recipient of the ciphertext can produce a classical string which proves to the originator that the recipient has relinquished any chance of recovering the plaintext, should the decryption key be revealed. Our scheme is feasible with current quantum technology: the honest parties only require quantum devices for single-qubit preparation and measurements, and is robust against noise in these devices. Furthermore, we provide an analysis that is suitable in the finite-key regime.

https://arxiv.org/abs/1910.03551

*Dr Anirban Chowdhury**Université de Sherbrooke**Simulating thermal physics on quantum computers*

Simulating systems in thermal equilibrium and computing their properties are important problems in physics. These are also however known to be computationally hard problems, especially for quantum systems. I will talk about quantum algorithms that can, under certain conditions, provide advantages in sampling from thermal Gibbs states and computing partition functions of quantum Hamiltonians. A key idea in our algorithms is to approximate an imaginary-time evolution as a linear combination of (unitary) real-time evolutions. This linear combination can then be easily implemented on a usual gate-model quantum computer. I will also discuss how this approximation can be used to compute the partition function on a restricted model of computation that uses only one pure qubit.

*Marco David, Undergrad studentDirector : Bill CoishMcGill University*Formal correctness and formal truth are of great value. Through formal verification, one can ensure that the autopilot software of commercial airplanes does not endanger its passengers, and correctness proofs are already standard practice in the industry. Typically, this is done through a precise mathematical formulation of the algorithm and deductive reasoning about its properties, which is then machine-checked. Yet, mathematicians themselves have not developed any interest in carrying out formal verifications. Although mathematics prides itself as the most fundamental of sciences, almost all proofs are conventionally written with many abstractions and omissions. This leads to an astonishingly high number of significant errors in published mathematical literature. Building on the deep dilemma between what a proof is and what it should be, this talk explores the field of formal verification and interactive theorem proving with particular emphasis on mathematics, and also gives an outlook over applications to quantum algorithms.

**QED. The Quest to Formally Verify Mathematics**

*Dr Joel Griesmar**Université de Sherbrooke**A mesoscopic spectrometer based on the Josephson effect*

A key element of mesoscopic topological systems, such as hybrid semiconductor-superconductor circuits, are Andreev Bound States, single quasiparticles localized at superconducting weak links. The characteristic transition energy of these states is twice the superconducting gap (90 GHz in aluminum). Conventional microwave techniques allow probing these states but only in a limited bandwidth. We propose a new broadband spectrometer operating at frequencies up to 180 GHz based on the Josephson effect which converts a DC voltage to microwave oscillations at a frequency proportional to this voltage. Using a symmetrical SQUID biased at half a flux quantum allows decoupling the spectrometer from environmental modes. In addition, careful design of the biasing circuit reduces the number of remaining modes and damps them. The fabricated mesoscopic spectrometer has a linewidth of 2 MHz, a bandwidth of 180 GHz and a minimal theoretical sensitivity of 5 kHz.

*Dr Erika Janitz**McGill University**Cavity-Enhanced Photon Emission from a Single Germanium-Vacancy Center in a Diamond Membrane*

Atomic-like defects in diamond provide a promising light-matter interface for quantum information applications. Looking at alternatives to the well-studied nitrogen-vacancy (NV) center, we examine interactions between a defect with superior optical properties, the germanium-vacancy (GeV), and an open microcavity. We demonstrate room-temperature coupling between a single emitter in a diamond membrane and a finesse 11,000 fiber cavity, evidenced by a 8-fold increase in the spectral density of emission. The experimental advancements in this work set the stage for future cryogenic experiments, where for the existing system we predict a factor of 7 increase in the GeV spontaneous emission rate.

*Dr Stéphane Virally**Polytechnique Montréal**Quantum optics in the time domain*

Quantization of the electromagnetic (EM) field is carried in two steps: first quantization selects discrete modes, and second quantization promotes the classical amplitude of these modes to quantum operators. In quantum optics textbooks, the first step is always carried in the frequency domain (i.e. monochromatic modes are selected). This is very useful in many situations where the signal is indeed quasi-monochromatic. However, the description is inadequate for the few-cycle, broadband signals that can be found e.g. in ultrafast optics and in many experiments in the microwave domain. We thus show how it is possible to carry a full quantization of the EM with the first step in time rather than frequency [1]. This procedure yields interesting consequences such as the existence of a dual of the Hamiltonian in the time domain and of a non-causal relationship between the EM field transporting the probability amplitude for the energy and the “photonics field”, which transports the probability amplitude for photon clicks on a broadband detector.

[1] Virally, S. and Reulet, B., “Unidimensional time domain quantum optics”, PRA 100, 023833 (2019)

*Poster session*

*Simon BernardDoctorate, McGill University*

*Director : Jack Sankey*

**We report experimental progress toward achieving our group's recent proposal [1] to gain an unprecedented level of optomechanical control over the shape and mass of a mechanical mode, using an optical spring to perturb a single lattice site of a phononic crystal, thereby smoothly localizing the spatial distribution of oscillating mass from the centimeter scale to the micron scale. Such control over shape and mass has never been demonstrated, and represents an entirely unexplored avenue in the field. Thus far, we have reliably fabricated the requisite (delicate!) mechanical structures, and observed the characteristic "phononic bandgap" (a necessary ingredient for localization). We have assembled a rigid, vibration-isolated fiber-fiber cavity with a phononic crystal in the middle to measure these effects. We show preliminary attempts at locking this cavity on resonance and obtaining the crystal Brownian motion from the error signal. Finally, we present a technique to decrease the finesse of a fiber-mirror in a controllable manner by smoothly etching away the bilayers forming the distributed Bragg reflector. We etch six bilayers, and subsequently measure finesse after each bilayer is removed. We find that the measured finesse agrees remarkably well with a 1D transfer matrix model. Achieving a specific finesse value for our fiber-mirrors is necessary so that our cavity will generate a strong optical spring whilst displaying a sufficiently large decay rate, therefore minimizing anti-damping of the phononic crystal and facilitating cavity locking.**

Progress toward optical control of mechanical geometry

Progress toward optical control of mechanical geometry

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

*Dr. Youcef A. Bioud**Research professionnal**Institut Quantique*** Suspended NbN superconducting resonator for reducing intrinsic losses**Superconducting coplanar waveguide (CPW) microwave resonators are crucial elements in Photon detectors, Quantum-limited parametric amplifiers, Narrow-band filters, Read-out, interconnect in quantum processors and Hybrid devices, connecting solid-state spins with superconducting circuits [1-3]. In the quantum regime, the dominant loss mechanism for high-Q superconducting resonators can be attributed to parasitic two-level systems (TLSs) in the dielectrics. Interface TLSs are common by products of the fabrication process, often introduced by impurities associated with Si surfaces [3]. To reduce intrinsic losses, we employ isotropic deep reactive-ion etching (DRIE) of Si substrate to create suspended NbN superconducting resonators (SSR).

In this study, thin films of niobium (Nb) and niobium nitride (NbN) are deposited on Si substrate by a DC magnetron sputtering system. The influence of the N2/Ar gas ratio, the deposition current, the substrate bias potential on the superconducting critical temperature of the films are investigated. Plasma etching of Nb and NbN thin films in a SF6 and Cl2-BCl3 gas plasma is studied using inductively coupled plasma (icp) reactor. Parametric studies on the effects of total gas flow rate and chamber pressure on the edge angles and etch rates are reported. Finally, suspended NbN superconducting resonator is fabricated and will be tested. This could be applied to the fabrication of superconducting qubits in integrated circuits, offering a path towards longer qubit coherence times.

[1] Landig et al. Coherent spin–photon coupling using a resonant exchange qubit. Nature 560, 179–184 (2018)

[2] Tosi et al. Silicon quantum processor with robust long-distance qubit couplings. Nat Commun 8, 450 (2017)

[3] Bruno et al. Reducing intrinsic loss in superconducting resonators by surface treatment and deep etching of silicon substrates, Appl. Phys. Lett. 106, 182601 (2015)

*Alexandre Choquette**Master, Université de Sherbrooke**Director : Alexandre Blais**Variational Quantum Algorithms for the Fermi-Hubbard model*

Noisy intermediate-scale quantum computation has the potential to be useful for the simulation of quantum materials. A prominent simulation approach is using variational quantum algorithms (VQAs), which have some resilience to noise and can handle limited qubit connectivity. Here, we aim to simulate the Fermi-Hubbard-model ground state by means of a VQA. We investigate a number of possibilities for engineering the variational state-preparation circuit, and benchmark these in presence of realistic noise. We find that Hamiltonian-inspired variational forms have better performance over a hardware-efficient approach. This work is progress towards the simulation of high-Tc superconductivity on a quantum device.

*Jordan Ducatel**Intern, National Research Council (NRC) - Ottawa**Director : Andrew Sachrajda**Experimental Study of single hole EDSR with strong spin-orbit coupling in a gated GaAs/AlGaAs Double Quantum Dot*

*Genki Fukuda**Master, National Research Council (NRC) - Ottawa**Director : Andrew Sachrajda**Fabrication and irradiation effects of field-induced shallow two-dimensional electron gas in dopant-etched modulation-doped GaAs/AlGaAs heterostructures*

Collaboration with Takafumi Fujita, Yasushi Kanai, Kazuhiko Matsumoto, Yuji Sakai, Haruki Kiyama, Julian Ritzmann, Arne Ludwig, Andreas D. Wieck, and Akira Oiwa (Osaka University, Ruhr University Bochm).: Electron spin qubits based on GaAs/AlGaAs gate-defined quantum dots (QDs) formed in a two-dimensional electron gas (2DEG) are strong candidates for a single photon and single electron spin quantum interface. However, suppression of the irreversible response of photon irradiation in conventional doped GaAs/AlGaAs gate-defined QDs is indispensable for performing stable and continual manipulation of a single electron spin generated by a single photon. It is one of the most promising solution to utilize QDs formed in a field-induced 2DEG on undoped heterostructures since these problems are caused by Si dopants. On the other hand, it is necessary in conventional undoped structures to optimize complicated etching and deposition processes for fabrication of Ohmic contacts to avoid creating a discontinuous 2DEG and undesired leakage paths to the top gates. Here, we show the fabrication and irradiation effects of an improved field-induced 2DEG structure in dopant-etched modulation-doped GaAs/AlGaAs heterostructures taking advantage of a conventional doped structure to simplify ohmic contact fabrication. We successfully induce a 2DEG and evaluate the effects of the light irradiation on the transport properties while an electric current decays with time. This work may provide an additional way to realize undoped GaAs gate-defined QDs for robust quantum interface against photon irradiation.

*Ioanna Kriekouki**Doctorate, Université de Sherbrooke**Director : Michel Pioro-Ladrière**28nm UTBB FD-SOI technology for Silicon-based quantum dots and cryo-CMOS electronics*

Although remarkable progress has been made over the last few years in the field of Silicon-based spin qubits hosted in small scale conventional CMOS technologies, coupling several qubits together for the needs of large-scale logical operations remains a challenge. Utilizing state of the art mass production process methods from the field of microelectronics seems to be a very promising candidate to solve this problem. Another problem that needs to be tackled concerns the control electronics for the qubits. Indeed, in the case where more than a thousand qubits will need to be controlled, the heat and noise produced by standard electronics will rapidly be a major issue.

Our goal towards a compact quantum processor is to develop a Silicon-based electron or hole spin qubit architecture fabricated via industrial techniques, with on-chip embedded control achieved by co-integrating classical electronics at cryogenic temperatures. In this poster, we present our progress on both the qubit architecture and the cryogenic electronics. Our qubit device is designed and fabricated using exclusively CMOS industrial manufacturing techniques, based on STMicroelectronics’ 28nm fully depleted silicon-on-insulator (FD-SOI) planar ultra-thin body and BOX (UTBB) technology. We report our progress on both single and multiple quantum dot systems in 1D (array) and 2D (matrix) architectures.We also propose a design for cryogenic CMOS electronics for manipulation and read-out of many qubits, in order to perform logical operations for the needs of large-scale quantum computing. FD-SOI is a promising technology for cryoCMOS implementation, as it has already been demonstrated to operate down to 4K. We studied the performance of ring oscillators, consisting of 28nm FD-SOI NMOS and PMOS transistors, designed to generate 6 to 10 GHz microwave signals.Our ring oscillator is coupled to a frequency divider, enabling on-chip down-conversion, allowing readout with conventional electronics measurement setup in the MHz regime.

*Justin LiMaster, McGill UniversityDirector : Claude Crépeau*

*General Strategy for Bit Commitment Scheme from Pseudo Telepathy Games*Quantum information processing contributed greatly to the rapid advancement of modern cryptography with it being the intersection of mathematics, physics and computer science. Communication tasks that are previously impossible to realize classically can be accomplished with the use of quantum entanglement. Pseudo-telepathy game is an application of this where a game exhibits pseudo-telepathy with the existence of a quantum winning strategy and the lack of its classical counterpart. After the examination of known pseudo-telepathy games, a general strategy for devising a bit commitment scheme using pseudo-telepathy games is presented. Due to the nature of the game, the bit commitment schemes derived from the games are unconditionally secure classically, but can be cheated by quantum players.

*Jiaxing Ma**Director : Jack Sankey**Master : McGill University** Mechanical Motion Driven by Quantum Radiation Pressure Noise*We present progress towards observing mechanical motion of a trampoline driven by quantum radiation pressure noise (QRPN). Lasers have quantum fluctuations which are caused by fluctuations in photons emission, also called shot noise, and our goal is to measure the effect of theses fluctuations "kicking" a trampoline mechanical resonator. To do so, we will place the trampoline inside a short (~10 microns) fiber-fiber cavity in a vacuum chamber at room temperature and shine laser-light in it. The trampoline will be driven by radiation pressure caused by the shot noise of the laser inside the cavity. The trampoline possesses great mechanical properties (high quality factor and low force noise) and the short cavity allows high decay rate without compromising the finesse, which enables us to have the trampoline's motion dominated by QRPN instead of thermal noise. We currently have characterized the mechanical properties of the trampoline, which has high mechanical quality factor (> 20 million), and we have proven that we are able to get shot noise predominated laser by squashing its classical noise. The future work will be focused on increasing the bandwidth of the shot noise predominated frequency range of the laser, and assembling the chamber and cavity.

*Tomohiro Nakagawa**Doctorate, National Research Council (NRC) - Ottawa**Director : Andrew Sachrajda***Formation of quantum dots on (110) GaAs substrate**

Collaboration with Rio Fukai, Yuji Sakai, Takafumi Fujita, Haruki Kiyama, Takashi Nakajima, Julian Ritzmann, Arne Ludwig, Andreas D. Wieck, Seigo Tarucha, and Akira Oiwa (Osaka University, Ruhr University Bochm, RIKEN, and University of Tokyo).: Gate-defined semiconductor devices provide a platform which enable us to perform various quantum transport experiments. Electron spins in gate-defined GaAs quantum dots (QDs) have been extensively studied for integrated and stable quantum bits. In those experiments, epitaxial layers grown on the (001) plane are generally used. In a (110) GaAs quantum well (QW), we expect more efficient photon-electron spin quantum state conversion by exciting heavy-hole Zeeman levels under in-plane magnetic field. However, experimental works on the electrically-defined low dimensional systems on the (110) substrate have been hardly done. From the Hall bar measurements, the mobility and electron density of a QW grown on (110) substrate were estimated as 7×10^{4} cm^{2}/Vs and 9×10^{10} cm^{-2}, respectively at a temperature of 1.5 K. Subsequently, we fabricated gate-defined lateral QDs and measured them using a dilution refrigerator. We clearly observe Coulomb oscillations in the few-electron regime by monitoring a nearby charge sensor current. We expect that the (110) GaAs QDs will provide a new platform for studying photon-spin coupling.

*Aviv Padawer-Blatt**Intern, National Research Council (NRC) - Ottawa**Director : Andrew Sachrajda**Single Hole Energy Spectrum in a GaAs/AlGaAs DQD with Strong Spin-Orbit Interaction and Dot-Specific g-factors*

*Anne-Laurence Phaneuf**Doctorate, Polytechnique Montréal**Director : Sébastien Francoeur**Exciton-Polaritons as a tool to control the emission characteristics of excitons and trions bound to Te _{2} in ZnSe*

Te

_{2}molecules in ZnSe form a quantum defect that offers advantageous characteristics, including a high optical uniformity due to its atomic nature and a strong optical dipole moment matching those from semiconductor nanostructures, for the development of efficient spin-photon interfaces for applications in quantum optics, communications and networks.

In this work, we demonstrate that excitons-polaritons generated in the ZnSe host material can be used to deterministically control the emission characteristics of excitons and trions bound to a single Te

_{2}molecule. In particular, the emission efficiency is increased by two orders of magnitude, indicating a very efficient coupling between free excitons and Te2 bound states. Scanning the free-exciton band with a narrow-frequency tunable laser over the free exciton spectral region reveals strong in-phase oscillations with a period of about 1 meV in the bound exciton emission intensity, emission energy, and emission linewidth. These modulations are explained by the strong coupling naturally occurring in ZnSe between photons and free-excitons, or exciton-polaritons.

This type of cooperative process whereby a host excitation is used to control the behavior of a single emitter has never been reported before. It allows deterministically controlling the emission properties and enables the development of new coherent control schemes.

*Mathias Pont**Master, Polytechnique Montréal**Director : Sébastien Francoeur**Optical stabilization of the spectral wandering of an exciton bound to a Te isoelectronic center in ZnSe*

We report on the stabilization of the electrostatic environment of a single emitter bound to a Te isoelectronic center in ZnSe resulting in an increased coherence time T2. Nonresonant excitation is used to reduce charge fluctuations in the vicinity of the emitter. Ultra-weak nonresonant excitation powers allow for radiative recombination of holes trapped in the vicinity of the emitter thus lowering the red-shift of the resonance energy induced by DC Stark effect. The PLE spectrum is the result of the superposition of non-resolved Lorentzian lines each corresponding to a certain charge distribution. With higher nonresonant excitation powers we reach saturation and populate the trap states with excitons, thus inducing another red-shift through the Heitler-London interaction.

*Cesar Daniel Rodríguez Rosenblueth**Master, McGill University*

*Director : Lilian Childress*

**Towards Coupling Color Centers in Diamond to Fiber Microcavities**Color centers in diamond are attractive spin-photon interfaces for future quantum technologies. The “flagship” nitrogen vacancy center (NV-) is a proven platform but suffers from a low emission probability (~3%) of photons into the zero-phonon line (ZPL), which is necessary for remote entanglement protocols [1,2]. One potential workaround leverages resonant coupling to an optical cavity to enhance the emission efficiency [3]. In addition, germanium vacancy (GeV) centers can be used for their higher photon emission rate into the ZPL and lower inhomogenous broadening [4]. We present our on-going effort to couple NV- and GeV centers to a fiber-based micro-cavity configured as a scanning microscope. The high cavity finesses (> 30k) obtained using ultralow-loss diamond membranes pave the road to the observation of Purcell enhancement of individual spin transitions but also set stringent requirements regarding cavity stabilisation.

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

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

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

[4] P. Siyushev el a., PRB 96 (2017)

*Claude Rohrbacher**Doctorate, Université de Sherbrooke**Director : Eva Dupont-Ferrier**High-resolution cryogenic capacitance measurements of FD-SOI structures using a capacitance bridge *

As CMOS structures are envisioned to host silicon spin qubits and for co-integrating quantum systems with control electronics, the cryogenic behaviour of such structures must be investigated. Capacitance-voltage (CV) measurements are widely used to determine semiconductor parameters. Performing high-resolution CV measurements at cryogenic temperatures is necessary to characterize CMOS structures and can set a path towards charge readout of quantum dots based on capacitance change. However, in nanoscale devices the capacitance to be measured is often reduced to hundreds of attofarads. Moreover, the wiring of dilution fridges introduces lengths of cables that have a parasitic capacitance on the order of nanofarads, hindering such measurements. Here we present a highly sensitive capacitance bridge circuit designed to perform measurements inside a dilution refrigerator with attofarad resolution. We demonstrate the utility of our circuit by performing split C-V measurements on 28 nm Fully Depleted Silicon on Insulator (FD-SOI) nanostructures at 20 mK. Preliminary results also show that Coulomb blockade is observable using this method; a first step towards charge readout using a capacitance bridge.*Marc-Antoine Roux**Master, Université de Sherbrooke**Director : Michel Pioro-Ladrière**Fast tuning of quantum dots using FPGAs*

Spin qubits are a promising architecture for quantum computers due to their long coherence time and compatibility with industrial fabrication techniques. However, scalability issues arise when trying to create a system with many qubits. The more qubits there are, the more time it takes to initialize the system in the desired configuration and the more equipment is needed to control each quantum dot. Therefore, a scalable qubit control system addressing both these issues is presented. By using a Field Programmable Gate Array (FPGA) based system, it is possible to greatly accelerate device characterization while being relatively compact. The FPGA system can be several orders of magnitude faster than typical apparatus allowing for real-time measurements.

*Martin Schnee**Undergrade student, Université de Sherbrooke**Director : Eva Dupont-Ferrier Silicon Donor Spin-Photon Coupling*Thanks to their record coherence time and their ability to be controlled precisely, donor spin qubit in silicon are very promising candidates for long-lived quantum information storage in the solid state. In particular, it has been showed recently that spin states can be electrically controlled through electric dipole degrees of freedom: this new “all-electrical” architecture is called flip-flop qubit.

On the other hand, superconducting resonators are inherently low-dissipative systems making them ideal ingredient for long distance coupling of two qubits. In addition, the fact that they can be fabricated using integrated-circuit processing techniques makes them attractive for scaling to a large number of qubits.

Combining these two entities into a hybrid system would bring best of both worlds: a long lasting quantum memory encoded in the spin state and scalable platform for electrical manipulation and long distance coupling using superconducting circuits. The goal of this project is to study and optimize the coupling of a single spin in silicon and a microwave photon in an NbN superconducting 2D resonator. A special effort is made to keep the design as minimalist as possible to settle the quantum device scalability challenge.

*Adrian Solyom**Doctorate, McGill University**Director : Lilian Childress Probing a Spin Transfer Controlled Nanomagnet with a Single Nitrogen Vacancy in Diamond*We apply spin-transfer (ST) torques to magnetic nanowires to efficiently tune the damping of the ferromagnetic dynamics, which we measure via local stray-field readout using a single nitrogen vacancy center in diamond. By reducing the nanowire’s geometry, we can increase the frequency separation of the standing spin-wave modes until they are resolvable, which would allow for the detection of mode-dependent ST effects; however doing so requires new techniques for patterning these increasingly small devices onto diamond. In this poster, we present preliminary data characterizing an argon-milling subtractive process for patterning nanomagnetic structures onto diamond, and discuss the ramifications for NV readout.

*Suresh Vishnunarayanan**Doctorate, Université de Sherbrooke**Directors : Bertrand Reulet & Guillaume Gervais, McGill University**Quantized Microwave Faraday rotation*

We report the first quantitative measurement of microwave Faraday rotation originating from the cyclotron motion of electrons in a low-dimensional semiconductor hetero-structure. As with the Hall effect, a continuous classical Faraday effect is observed as well as a quantized Faraday effect.The high electron mobility of our GaAs/AlGaAs hetero structure enables a large single-pass Faraday rotation of θF≈45°( ≈0.8 rad) to be achieved at a modest magnetic field of B≈100 mT. In the quantum regime, the Faraday rotation θFis naturally quantized in units of the fine structure constant α≈ 1/137, giving a geometric prescription for the fine structure constant. Electromagnetic confinement leads to Faraday rotation that is quantized in units of an effective α*, whose value is on the order of its free space value

*Rigel Zifkin**Master, McGill University**Director : Lilian Childress**Towards Coupling Color Centers in Diamond to Fiber Microcavities*

Color centers in diamond are attractive spin-photon interfaces for future quantum technologies. The “flagship” nitrogen vacancy center (NV-) is a proven platform but suffers from a low emission probability (~3%) of photons into the zero-phonon line (ZPL), which is necessary for remote entanglement protocols [1,2]. One potential workaround leverages resonant coupling to an optical cavity to enhance the emission efficiency [3]. In addition, germanium vacancy (GeV) centers can be used for their higher photon emission rate into the ZPL and lower inhomogenous broadening [4]. We present our on-going effort to couple NV- and GeV centers to a fiber-based micro-cavity configured as a scanning microscope. The high cavity finesses (> 30k) obtained using ultralow-loss diamond membranes pave the road to the observation of Purcell enhancement of individual spin transitions but also set stringent requirements regarding cavity stabilisation.

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

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

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

[4] P. Siyushev el a., PRB 96 (2017)