# Institut Transdisciplinaire d'Information Quantique (INTRIQ)

### Spring 2017 INTRIQ meeting

**When** : Thursday, May 11th, 2017

**Where** : Pavillon Lassonde, Polytechnique Montréal

**Organizer** : Professor Sébastien Francoeur, Polytechnique Montréal

For registration, click HERE

*INTRIQ Meeting program*

8h30 - 9h00 : Registration

9h00 - 9h10 : Opening remarks

9h10 - 10h10 : *Edo Waks, University of Maryland Quantum nanophotonics: controlling light with a single quantum dot*

10h10 - 10h40 : Coffee break

10h40 - 11h25 : *Glen Evenbly, Université de Sherbrooke Tensor networks methods for quantum many-body systems*

11h25 - 12h10 : *Denis Seletskiy, Polytechnique Montréal*

*Quantum Electrodynamics in Space-Time *

12h10 - 14h00 : Lunch

14h00 - 14h45 : *Stéphane Kéna-Cohen, Polytechnique Montréal*

** Quantum optics with light-matter particles**

14h45 - 15h15 : Coffee break

15h15 - 16h00 : *Yves Bérubé-Lauzière, Université de Sherbrooke Superpositions of cavity Fock states with active measurement-based quantum feedback*

16h00 -16h10 : Closing remarks

16h15 - : Business meeting (INTRIQ members)

**INVITED SPEAKERS**

*Professor Edo Waks*

University of Maryland**Quantum nanophotonics: controlling light with a single quantum dot**

Interactions between light and matter lie at the heart of optical communication and information technology. Nanophotonic devices enhance light-matter interactions by confining photons to small mode volumes, enabling optical information processing at low energies. In the strong coupling regime, these interactions are sufficiently large that a single photon creates a nonlinear response in a single atomic system. Such single-photon nonlinearities are highly desirable for quantum information processing applications where atoms serve as quantum memories and photons act as carriers of quantum information. In this talk I will discuss our effort to develop and coherently control strongly coupled nanophotonic devices using quantum dots coupled to photonic crystals. Quantum dots are semiconductor “artificial atoms” that can act as efficient photon emitters and stable quantum memories. By embedding them in a photonic crystal cavity that spatially confines light to less than a cubic wavelength we can attain the strong coupling regime. This device platform provides a pathway towards compact integrated quantum devices on a semiconductor chip that could serve as basic components of quantum networks and distributed quantum computers. I will discuss our demonstration of a quantum transistor, the fundamental building block for quantum computers and quantum networks, using a single electron spin in a quantum dot. I will then describe a realization of a new cavity QED approach to measure the state of a spin all-optically. This technique enables efficient spin readout even when the spin has a poor cycling transition. Finally, I will discuss our recent effort to extend our results into the telecommunication wavelengths, and to improve the efficiency and scalability of the structure in order to attain integrated multi-dot devices on a single chip.

*Professor Glen Evenbly*

Université de Sherbrooke**Tensor networks methods for quantum many-body systems**

Quantum many-body systems are hard to study because the associated Hilbert space, containing all possible many-body states, grows exponentially in the system size. However, in recent years progress in understanding quantum entanglement has revealed that only a small region of this huge Hilbert space is actually relevant to the study of quantum many-body systems. Tensor network states have been introduced to efficiently describe quantum states in this small, physically relevant region of the many-body Hilbert space. In this talk I will offer an introduction to tensor network methods and their applications towards the study of quantum many-body systems, and discuss some recent progress in the development of tensor networks as models of the AdS/CFT correspondence.

*Professor Denis Seletskiy*

Polytechnique Montréal**Quantum Electrodynamics in Space-Time**One of the greatest achievements of ultrafast science is to enable access to the elementary dynamics of the fundamental degrees of freedom of matter. The ability to trace the temporal evolution of quasiparticles, collective modes and their correlations in the condensed phases has fueled a leap in our understanding of microscopic many-body interactions. Moreover, recently developed methods of sampling the instantaneous electric field amplitude (in the 1 – 150 THz frequency range) provide us with direct information on ultrafast dynamics which is imprinted on the subcycle structure of the probe field and therefore contains the evolution of the complex response function of the studied system.

It can be argued that the next revolution in quantum physics would involve quantum spectroscopy of light and matter fields. I will review our progress toward the development of the building blocks for detection of quantum fields in space-time. First, I will present our results on first direct probing of vacuum fluctuations of the electric field using the technique of electro-optic sampling. Next, I will show how we produce and detect modified quantum states on the example of a strongly-squeezed phase-stable vacuum, exhibiting a time-domain manifestation of the Heisenberg’s uncertainty principle. Finally, I will outline a path toward a time-domain quantum tomography and conclude with a perspective for the emerging themes out of a personal vantage point: from time-resolved quantum spectroscopy to probing evolution of nonclassical fields in dynamic space-time -- the future of subcycle quantum electrodynamics is bright !

*Professor Stéphane Kéna-Cohen*

Polytechnique Montréal**Quantum optics with light-matter particles**

We will describe recent quantum optical experiments with hybrid light-matter particles called polaritons. In the first part of the talk, we will describe the fascinating physics of organic exciton-polaritons, quasiparticles that can form in optical microcavities. We will highlight how they can be used as low-threshold sources of coherent light and describe our recent experiments on polariton condensates, highlighting the spontaneous formation of quasi long-range order and the presence of nonlinear instabilities. Finally, we will show how the nonlinear properties of these quasiparticles allow for the first observation of room-temperature superfluidity. In the second part of the talk, we will describe traditional quantum optical experiments performed on-chip using nanoscale waveguides supporting surface plasmon-polaritons. In particular we will highlight how single quanta of surface plasmons can be generated and studied and finally we will show our results on the quantum interference of individual surface plasmon-polaritons‹a solid-state analog to the Hong-Ou-Mandel experiment.

*Professor Yves Bérubé-Lauzière*

Université de Sherbrooke**Superpositions of cavity Fock states with active measurement-based quantum feedback**The measurement-based quantum feedback scheme developed and implemented by Haroche and collaborators [Dotsenko et al., Phys. Rev. A 80, 013805 (2009) and Sayrin et al., Nature 477, 73-77 (2011)] to actively prepare and stabilize specific photon number states in cavity quantum electrodynamics (CQED) is a milestone achievement in actively protecting quantum states from decoherence. This feat was achieved by injecting, after each weak dispersive measurement of the cavity state via Rydberg atoms serving as cavity sensors, a low average number classical field (coherent state) to steer the cavity towards the targeted number state. This talk will present the generalization of the theory developed for targeting number states in order to prepare and stabilize desired superpositions of two cavity photon number states. A new distance measure will be introduced to quantify how close a quantum state superposition is to a targeted state and at the same time to more deeply discriminate different states. Results from realistic simulations taking into account decoherence and imperfections in a CQED set-up will be presented. These demonstrate the validity of the generalized theory and points to the experimental feasibility of preparing and stabilizing such superpositions. This is a further step towards the active protection of more complex quantum states than number states. This work, cast in the context of CQED, is also almost readily applicable to circuit QED.