CQT's Tenth Anniversary Events

Join us for events commemorating ten years of CQT

CQT celebrated its tenth birthday in December 2017, and we are planning a series of events in the run-up. Please mark your diaries for a conference on our anniverary, 7–8 December. We also invite you to join us for a special series of colloquia to be given by CQT Principal Investigators throughout the year. We will be updating this webpage with further details as information is available. Check back or sign up for CQT's mailing lists to get updates.

CQT10 Conference

We welcome all to join us for a conference 7-8 December to mark our tenth anniversary. This event will be in the spirit of the scientific symposium we have held in previous years on our birthday, extended into a two-day event.

Guild Hall, Level 1, NUSS Guild House
9 Kent Ridge Drive, Singapore 119241

Download Program in PDF

7 December, Thursday
9.00am Registration
9.15am Welcome Speech by CQT Director, Artur Ekert

Invited Talk by Ignacio Cirac, Max-Planck-Institut für Quantenoptik (MPQ)

Title: Quantum simulation with classical and quantum computers
Abstract: Simulating the dynamics and equilibrium properties of many body quantum systems is very hard, due to the exponential scaling of the resources required to do that with the number of particles, or volume. In this talk I will summarize some of the efforts we have pursued during the last year in three different fronts: (i) introducing simple quantum algorithms for small-size quantum computers; (ii) emulating the dynamics with atoms in optical lattices; (iii) employing tensor network techniques in classical computers.

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10.30am Coffee/Tea Break

Invited Talk by Umesh Vazirani, UC Berkeley

Title: Rigorous RG: a provably efficient algorithm for simulating 1D quantum systems
Abstract: One of the mysteries in computational condensed matter physics is the remarkable practical success of the Density Matrix Renormalization Group (DMRG) algorithm, since its invention a quarter century ago, for finding low energy states of 1D quantum systems (like the similarly successful simplex method for linear programming, DMRG takes exponential time in the worst case). From a computational complexity viewpoint, low energy states are simply near optimal solutions to (quantum) constraint satisfaction problems. Mathematically, the problem is specified by a succinctly described Hamiltonian - an exponentially large matrix, and the challenge is to find the eigenstates with small eigenvalues. Since the eigenstates live in an exponential dimensional space, it is a priori not even clear whether they can be succinctly described, let alone computed efficiently. In this talk I will describe novel combinatorial arguments showing that low energy states for systems of particles with nearest neighbor interactions in 1D can be described succinctly, leading to a provably efficient classical algorithm for computing them. A recent implementation of our algorithm shows promise for outperforming DMRG in certain hard cases. One of the cornerstones in physics is the Renormalization Group (RG) formalism, which provides a sweeping approach towards managing complexity in the quantum world. Our algorithm may be viewed as a rigorously justified RG-like procedure, and provides a new perspective on the subject. 
Based on joint work with Itai Arad, Zeph Landau and Thomas Vidick

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12.00pm Lunch

Invited Talk by Christopher Monroe, JQI, QuICS, and University of Maryland

Title: Quantum Computing with Trapped Atomic Ions
Abstract: Individual atoms are standards for quantum information science, acting as qubits that have unsurpassed levels of quantum coherence, can be replicated and scaled with the atomic clock accuracy, and allow near-perfect measurement. Quantum gate operations between atomic ions are mediated with control laser beams, allowing the qubit connectivity graph to be reconfigured and optimally adapted to a given algorithm or mode of computing. Existing work has shown >99.9% fidelity operations, fully-connected control with up to about 10 qubits, and quantum simulations with limited control with more than 50 qubits – all with the same atomic architecture. I will speculate on combining all of this into a single universal quantum computing device that can be co-designed with future applications.

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Contributed talk by Cord Mueller, University of Konstanz and Bavarian State Office of Weights and Measures, Germany

Title: Weak localization of magnons in disordered chiral magnets
Abstract: The elementary excitations of magnetic materials, spin waves or magnons, have been identified as promising carriers of (quantum) information in functional solid-state devices that permit to store and process information via stable magnetic structures such as skyrmions. We have studied the impact of quenched disorder, present in every real-world material, on magnon transport in two-dimensional ferromagnets with a chiral Dzyaloshinskii-Moriya interaction (DMI). By numerical simulations of an atomistic spin model, we find that the coherent backscattering signal appears with perfect contrast, but shifted in k-space. Measuring this shift of the backscattering peak thus offers an avenue of determining sign and magnitude of the DMI, together with proving the relevance of phase-coherent transport in the rapidly developing field of magnonics.

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Contributed talk by Martial Ducloy, Laboratoire de Physique des Lasers, Université Paris 13 & NTU, Singapore

Title: Spectral monitoring of Cs vapour-surface interactions
Abstract: Selective reflection spectroscopy at dielectric-vapour interfaces allows one to probe the optical response of atoms (molecules) at a distance range ~ λ/2π from the surface. It thus gives access to Casimir-Polder atom-surface interactions [1]. These interactions can be controlled via resonant coupling with surface polaritons: either by thermal excitation of surface polariton modes (Cesium vapour with hot sapphire surfaces [2]), or by frequency-tunable surface plasmon polaritons in adequate metallic metasurface (hybrid atom-metamaterial devices [3]).
[1] M. Ducloy & M. Fichet, J. Phys. II (Paris) 1, 1429 (1991)
[2] A. Laliotis et al, Nature Communications 4, 4364 (2014) ; M. P. Gorza & M. Ducloy, Eur. Phys. J. D, 40, 343 (2006)
[3] S. A. Aljunid et al, NanoLetters 16, 3137 (2016); E. A. Chan et al, Science Advances, submitted

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4.30pm Lab Visits @ CQT, NUS, Block S15 (registered guest only)

8 December, Friday

Invited Talk by Charles Clark, National Institute of Standards and Technology and Joint Quantum Institute

Title: The neutron as a quantum particle and wave
Abstract:I give a simple overview of the quantum properties of the neutron, emphasizing their similarities to, and differences from, those of light. Neutron interferometry enables one to realize the quantum limit of the Young double slit experiment, when only one neutron is ever present in the interferometer. How can only one neutron go through both slits? We have used neutron interferometry and holography to address some of the questions of structured quantum waves that have recently been studied with photons, electrons and atoms.

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12.00pm Lunch

Invited Talk by Michael Brooks, Freelance Writer

Title: Jerome Cardano: The Quantum Astrologer
Jerome Cardano was a Milanese polymath of the 16th century. He was a physician and astrologer to kings, popes, archbishops and emperors, but also invented probability theory and was the first to recognise the validity of imaginary numbers, setting up the twin pillars on which quantum theory has been built. He constructed a complex cosmology and throughout his life attempted to make sense of the universe around him, even teaching the general public how to do astronomy for themselves. Gottfried Leibniz said Cardano was “a great man, with all his faults; without them he would have been incomparable”, and Tycho Brahe cited Cardano’s writings with great respect. So how did he become so unknown? In this talk, Michael Brooks will explore the rise and fall of one of the Renaissance’s forgotten heroes.

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Contributed talk by Kavan Modi, Monash University

Title: Quantum causal modelling and stochastic processes
Abstract: In science we often want to characterise the processes undergone by a system of interest; this allows us to both identify the underlying physics driving the process and to predict what will happen to the system the next time the process occurs. If the state of the system at any time depends only on the state of the system at the previous time-step and some predetermined rule then these dynamics are characterised with relative ease. For instance, the dynamics of quantum mechanical systems in isolation is described in this way. But, when a quantum system repeatedly interact with an environment, the environment often ’remembers’ information about the system's past. This leads to non-Markovian processes, which depend nontrivially on the state of the system at all times during its evolution and they are not, in general, be easily characterised using conventional techniques. Since the early days of quantum mechanics it has been a challenge to describe non-Markovian processes. Here we will show that using operational tools from quantum information theory we can fully characterise any non-Markovian process. In general the full characterisation is not efficient, as it requires exponentially large number of experiments. To overcome this obstacle we map the full process to a many-body state. We show that this can be achieved by using linear, in the number of time steps, amount of bipartite entanglement. Next, the state can be measured to any desired precision, thus the process can be characterised to any desired precision. Finally, we define a natural measure for the degree of non-Markovianity.

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Contributed talk by Ritayan Roy, University of Sussex

Title: Quantum technology using magnetic conveyor belt and two-photon transition
Abstract: Adiabatic transport and precise positioning of cold atoms are important for quantum sensors and information processing. A minimalistic and optimised approach for adiabatic transportation of cold atoms using a magnetic conveyor belt for “atom chips” would be presented in the first part of my talk1. Later, I will describe a two-photon transition of rubidium (Rb) atoms from the ground state (5S1/2) to the excited state (4D5/2), using a home-built ytterbium (Yb)-doped fiber amplifier at 1033 nm. This is the first demonstration of an atomic frequency reference at 1033 nm as well as of a onecolour two-photon transition for the above energy levels2. I will conclude with an outline how these techniques could be used in quantum technology, e.g., background-free high-resolution quantum microscopy imaging and in quantum networking (amongmany others).

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3.50pm Coffee/Tea Break

Contributed talk by Marcelo Santos, Universidade Federal do Rio de Janeiro – UFRJ

Title: Photonic Cooper pairs
Abstract: Photons are the elementary particles of light. Contrary to most particles, photons do not interact directly with each other in vacuum. However, when propagating in a material, e.g. water, photon pairs may interact through the medium. In the Raman effect, for example, it is possible that a photon creates or absorbs a vibrational excitation of the material. In this work, we demonstrate theoretically and experimentally that photon pairs may interact via a virtual vibration, meaning that the energy exchanged in the process does not correspond to a quantum of vibrational energy. The same process occurs in a metal at very low temperatures, where virtual vibrations of the medium create an effective attractive interaction between electrons, forming the so-called Cooper pairs. This phenomenon changes a normal metal into a superconductor. We have shown theoretically and experimentally the analogue of this phenomenon with light, namely an effective photon-photon interaction mediated by a virtual vibration, i.e, a photonic Cooper pair. An important next step is to test how far the analogy with superconductivity extends.

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Contributed talk by Yong Siah Teo, Seoul National University

Title: Adaptive compressed sensing without a priori information
Abstract: If one knows that a given unknown state is at most of rank r, then the standard technique of compressed sensing (CS) allows for a strictly informationally complete (strictly-IC) characterization of the state using a specialized rank-r CS measurement with quantum positivity constraints enforced and few measurement settings. The a priori knowledge of the value of r however requires experimental verification, and a target state is usually needed to verify the reconstruction. I shall propose a novel and feasible scheme of adaptive compressed sensing (ACS) with which the required strictly-IC measurement settings are adaptively found based only on a posteriori information encoded in the measurement data, and a simple state-space inspection technique that relies on the properties of convex functions. This scheme thus achieves CS without the need for any a priori information about the unknown state, and the reconstructed state is validated without a target state. The ACS scheme can also flexibly work for any sort of measurements that are restrictively implemented in any realistic experimental situation that still enables CS tomography.

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Contributed talk by Wu Xingyao, Joint Center for Quantum Information and Computer Science, University of Maryland

Title: Super-polynomial separations for quantum-enhanced reinforcement learning
Abstract: Recent work on quantum machine learning has demonstrated that quantum computers can offer dramatic improvements over classical devices for data mining, prediction and classification. However, less is known about the advantages using quantum computers may bring in the more general setting of reinforcement learning, where learning is achieved via interaction with a task environment that provides occasional rewards. Reinforcement learning can incorporate data-analysis-oriented learning settings as special cases, but also includes more complex situations where, e.g., reinforcing feedback is delayed. In a few recent works, Grover-type amplification has been utilized to construct quantum agents that achieve up-to quadratic improvements in learning efficiency. These encouraging results have left open the key question of whether super-polynomial improvements in learning times are possible for genuine reinforcement learning problems, that is problems that go beyond the other more restricted learning paradigms. In this work, we provide a family of such genuine reinforcement learning tasks, and we construct quantum-enhanced learners which learn super-polynomially faster than any classical reinforcement learning model.

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Contributed talk by Priyam Das, Indian Institute of Technology Delhi

Title: Collectively induced many-vortices topology via rotatory Dicke quantum phase transition
We examine the superradiance of a Bose-Einstein condensate pumped with a Laguerre-Gaussian laser of high winding number, e.g., 7 2    . The laser beam transfers its orbital angular momentum (OAM) to the condensate at once due to the collectivity of the superradiance. An  -fold rotational symmetric structure emerges with the rotatory superradiance. Consequently, we observe that  number of single-charge vortices appear at the edges of this structure. Even though the pump and the condensate profiles initially have cylindrical symmetry, we find that it is broken to  -fold rotational symmetry at the superradiance. Breaking of the cylindrical symmetry into the  -fold symmetry and OAM transfer to the condensate becomes significant after a critical pump strength. Reorganization of the condensate resembles the rotational analog of the ordering in the experiment by Esslinger and colleagues [Nature 264, 1301 (2010)]. We verify through our simulation that the critical point for the onset of the reorganization, as well as the properties of the emitted pulse, conform to the characteristics of superradiant quantum phase transition.

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6.00pm Poster Session with cocktail reception
List of posters

Register to attend: https://www.quantumlah.org/events/other/registration.php


Since the Centre started, CQT has run an active schedule of scientific talks. A highlight of the programme is the monthly colloquia, typically given by a distinguished visiting scientist. Colloquia present a broad view of a hot topic at a level accessible to people working in other domains. As CQT turns ten, we have invited our own Principal Investigators to give this treatment to their own areas of research.