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: Classical simulation of quantum-coherent thermal machines.
Date/Time: 04-Dec, 04:00PM
Venue: CQT Level 3 Seminar Room, S15-03-15

Abstract: The performance enhancements observed in various models of continuous quantum thermal machines have been linked to the buildup of coherences in a preferred basis. But, is this connection always an evidence of'quantum-thermodynamic supremacy'? By force of example, we show that this is not the case. In particular, we compare a power-driven three-level quantum refrigerator with a four-level combined cycle, partly driven by power and partly by heat. We focus on the weak driving regime and find the four-level model to be superior since it can operate in parameter regimes in which the three-level model cannot, it may exhibit a larger cooling rate, and, simultaneously, a better coefficient of performance. Furthermore, we find that the improvement in the cooling rate matches the increase in the stationary quantum coherences exactly. Crucially, though, we also show that the thermodynamic variables for both models follow from a classical representation based on graph theory. This implies that we can build incoherent stochastic-thermodynamic models with the same steady-state operation or, equivalently, that both coherent refrigerators can be simulated classically. More generally, we prove this for any N-level weakly driven device with a 'cyclic' pattern of transitions. Therefore, even if coherence is present in a thermal machine, it is often unnecessary for the underlying energy conversion process.

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 Talk by Prabha Mandayam, IIT Madras

Title: Impact of local dynamics on the entangling power of unitary operators
Date/Time: 20-Nov, 04:00PM
Venue: CQT Level 3 Seminar Room, S15-03-15

Abstract: We study the evolution of the entangling power of nonlocal unitary operators, when successive applications of such nonlocal operators are interlaced with local unitary dynamics. This is a commonly encountered scenario in physical systems where the time-evolution involves both local one-body operators as well as nonlocal interactions. In this context we investigate two complementary measures: (a) the well known ‘entangling power’ which quantifies the amount of entanglement generated on average with a single application of a nonlocal operator on a product state , and (b) a new quantity called ‘gate-typicality’ which can distinguish between distinct classes of gates with zero entangling power. Analysing multiple applications of any fixed entangling operator U, interlaced with random local gates, we prove that both quantities approach their asymptotic values in simple exponential form. These asymptotic values coincide with the averages for the nonlocal operators on the full bipartite space, and could be larger the values attained by repeated application of U without the local gates. We illustrate our results for specific classes of gates such as diagonal unitary gates and controlled-unitary gates.

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.