CQT Colloquium by Mikhail Baranov, Institute for Quantum Optics and Quantum Information, Austrian Academy of Sciences

Title: Subwavelength optical barriers for cold atoms: creation and potential application
Date/Time: 22-Nov, 04:00PM
Venue: CQT Level 3 Seminar Room, S15-03-15

Abstract: The generation of subwavelength optical barriers on the scale of tens of nanometers, as conservative optical potentials for cold atoms, is discussed both theoretically and experimentally. In the proposed scheme they originate from nonadiabatic corrections to Born-Oppenheimer potentials for position-dependent “dark states” in atomic Λ configurations. The subwavelength optical barriers represent a “Kronig-Penney” potential, and I discuss the corresponding band structure including the effects of spontaneous emission and atom loss due to “bright” channels. Inclusion of an interparticle dipole-dipole interaction leads to formation of “domain wall molecules” and to unconventional Hubbard models with modulated in space interparticle interactions. As a brief discussion of potential applications, the subwavelength barrier can be used as a “splitter” to create a double-wire (or double-layer) with subwavelength spacing and, therefore, with substantially increased couplings as compared to ordinary optical lattices. As another application, specially designed subwavelength atomic internal-state spatial structures can be used for building an atomic scanning microscope with subwavelength resolution.

CQT Talk by Tobias Vogl, The Australian National University

Title: Next-generation single-photon sources for satellite-based quantum communication
Date/Time: 27-Nov, 04:00PM
Venue: CQT Level 3 Seminar Room, S15-03-15

Abstract: Color centers in solid state crystals have become a frequently used system for single-photon generation, advancing the development of integrated photonic devices for quantum optics and quantum communication applications. Recently, defects hosted by two-dimensional (2D) hexagonal boron nitride (hBN) attracted the attention of many researchers around the world, due to its chemical and thermal robustness as well as high single-photon luminosity at room temperature. Unlike for NV centers in diamond and other solid-state quantum emitters in 3D systems, the 2D crystal lattice of hBN allows for an intrinsically ideal extraction efficiency, as none of the emitters are embedded in any high refractive index material and are consequently not affected by Fresnel or total internal reflection. In addition, atomically-thin crystals can be integrated into photonic circuits easily. Here, we present recent advances in coupling this new type of emitter with nano-photonic structures in the Purcell regime. An increased collection efficiency and quantum yield, combined with off-resonant noise suppression and improvement of photophysics allow to use this single-photon source for realistic quantum key distribution experiments and other quantum information protocols. The device outperformes state-of-the-art decoy state QKD protocols. We also assess the usage on Nanosatellites due of the source's high temperature robustness as well as its SWaP (Size, Weight and Power) requirements and show results on our implementation on a 1U CubeSat. In addition to the single-photon source, the CubeSat is also equipped with a single-photon interferometer with which, if launched into space, we can test certain quantum gravity theories and physics beyond the standard model.

11th Annual Symposium of the Centre for Quantum Technologies (CQT), Singapore

Date/Time: 24-Jan - 24-Jan
Venue: SFAH Auditorium, Level 2, Shaw Foundation Alumni House 11 Kent Ridge Drive, Singapore 119244

CQT’s annual symposia provide an opportunity to discuss hot topics aligned with the Centre’s research. These research directions span atomic, molecular and optical physics, quantum information, quantum foundations and computer science. Each year’s symposium will include different themes. This year’s programme has an emphasis on quantum computing and algorithms.

Join us on 24 January 2019 for a one-day programme that features talks and a panel discussion across a range of experimental and theoretical topics. Registration is free.

Website: https://cqt11.quantumlah.org/

CQT Talk by Luis Correa, University of Nottingham

Title: Recent progress on the theory of quantum thermometry
Date/Time: 04-Dec, 04:00PM
Venue: CQT Level 3 Seminar Room, S15-03-15

Abstract: Controlling and measuring the temperature in different devices and platforms that operate in the quantum regime is, without any doubt, essential for any potential application. While current quantum thermometric methods differ greatly depending on the experimental platform, the achievable precision, and the temperature range of interest, the theory of quantum thermometry is built under a unifying framework at the crossroads of quantum metrology, open quantum systems, and quantum many-body physics. At a fundamental level, theoretical quantum thermometry is concerned with finding the ultimate bounds and scaling laws that limit the precision of temperature estimation for systems in and out-of-thermal equilibrium. At a more practical level, it provides tools to formulate precise, yet feasible, thermometric protocols for relevant experimental architectures. In this talk I will review some of the recent theoretical progress in this area, with a special emphasis on local quantum thermometry; that is, the problem of inferring temperature from incomplete information extracted by probing a small fraction of a large ultra-cold quantum system in thermal equilibrium.

CQT Talk - Quantum Machine Learning Journal Club Talk by Patrick Rebentrost, CQT

Title: Quantum-inspired algorithms for machine learning
Date/Time: 23-Nov, 04:00PM
Venue: CQT Level 3 Seminar Room, S15-03-15

Abstract: Recent works present quantum-inspired classical algorithms for linear algebra and machine learning, in particular for principal component analysis and the pseudo-inverse of low-rank matrices. I review these works and the underlying classical sampling techniques.

CQT PhD Thesis Defense by Thi Ha Kyaw

Title: Towards large-scale quantum computing platform in ultrastrong coupling regime
Date/Time: 23-Nov, 02:00PM
Venue: CQT Level 3 Conference Room, S15-03-17

Abstract: Superconducting circuits architecture is one of the promising quantum computing platforms to date, due to its good scalability and outstanding controllability. Besides these amazing attributes, the circuit architecture allows us to explore physics beyond the usual quantum optics experiments. In particular, the light-matter coupling strength g/ω in the superconducting circuit experiment has reached to 1.34, while in standard quantum optics experiment, the coupling strength is on the order of 10^{-6}. In this so-called ultrastrong coupling (USC) regime, the Jaynes-Cummings model in rotating wave approximation, obtained from the minimal coupling Hamiltonian between a two-level atom and a quantized electromagnetic field, is not valid. More general quantum Rabi model (QRM) is required, which was proposed by Isidor Rabi 80 years ago. Moreover, the study of USC light-matter interaction and QRM is expedient due to the recent analytical integrability and experimental realizations in the superconducting circuit, metamaterial THz cavities, carbon nanotube microcavity exciton-polaritons, etc.

This thesis studies superconducting circuits in the USC regime, while we are inspired to attain quantum random access memory in the circuit for quantum machine learning algorithms. It has three core components. The first part concerns high fidelity subnanosecond two-qubit quantum gates with which construction of simple quantum error correction codes is made possible. The second part proposes to use the built-in Z_2 parity symmetry of the QRM to achieve quantum memory cell. In the third and final part, we discuss quantum Rabi system mediated qubit pairs coupling, and extension to two-dimensional circuit configuration from typical one-dimensional superconducting circuit platform.