### Colloquia

October 19, 20174.00 pm: Thermodynamics of Quantum Devices By Ronnie Kosloff, Hebrew University |

October 26, 20174.00 pm: Quantum optics with Rydberg atoms By Wenhui Li, Centre for Quantum Technologies, NUS |

November 2, 20174.00 pm: Quantum communication and fundamental quantum physics By Nobuyuki Imoto, Osaka University |

November 23, 20174.00 pm: Foundations of Lattice-based Cryptography By Divesh Aggarwal, Centre for Quantum Technologies, NUS |

→ expand colloquia list and access videos...

**Date:** 13 January 2017, 4pm

**Venue:** CQT Seminar Room, S15-03-15

**Speaker:** James P. Crutchfield, University of California at Davis

**Demon Dynamics: Deterministic Chaos, the Szilard Map, and the Intelligence of Thermodynamic Systems**

**Abstract:**

We introduce a deterministic chaotic system—the Szilard Map—that encapsulates the measurement, control, and erasure protocol by which Maxwellian Demons extract work from a heat reservoir. Implementing the Demon's control function in a dynamical embodiment, our construction symmetrizes Demon and thermodynamic system, allowing one to explore their functionality and recover the fundamental trade-off between the thermodynamic costs of dissipation due to measurement and due to erasure. The map's degree of chaos—captured by the Kolmogorov-Sinai entropy—is the rate of energy extraction from the heat bath. Moreover, an engine's statistical complexity quantifies the minimum necessary system memory for it to function. In this way, dynamical instability in the control protocol plays an essential and constructive role in intelligent thermodynamic systems.

**Date:** 2 February 2017, 4pm

**Venue:** CQT Seminar Room, S15-03-15

**Speaker:** Fernando Pastawski, Institute for Quantum Information and Matter (IQIM)

**Holographic quantum error-correcting codes**

**Abstract:**

In this talk, I will explore the recent connection between two profound ideas, quantum error correction and holography. The first, represents the realization that reliable quantum information processing could be achieved from imperfect physical components. The second, is a duality between two physical systems on different spatial dimensions which may be identified leading to the exact same predictions. Notably, only one of the two systems explicit includes gravitational features. Recently, quantum information has emerged as a natural tool to relate these two descriptions. As such, concepts familiar to quantum information scientists such as entanglement, compression and quantum error correction are playing important roles in understanding this duality. Conversely, the holographic duality is proposing a new lens through which to explore aspects of quantum error correction. In this talk, I will introduce some of the properties imposed by holography on corresponding quantum error-correcting codes, describe explicit tensor network codes which exhibit some of these properties and explore the implications of holographic predictions from a code-theoretic perspective.

**Date:** 23 March 2017, 4pm

**Venue:** CQT Seminar Room, S15-03-15

**Speaker:** Vlatko Vedral, CQT, NUS

**Quantum Physics: A Possible Theory of the World as a Whole**

**Abstract:**

Quantum mechanics is commonly said to be a theory of microscopic things: molecules, atoms, subatomic particles. Most physicists, though, think it applies to everything, no matter what the size. The reason its distinctive features tend to be hidden is not a simple matter of scale. Over the past few years experimentalists have seen quantum effects in a growing number of macroscopic systems. The quintessential quantum effect, entanglement, can even occur in large systems as well as warm ones - including living organisms - even though molecular jiggling might be expected to disrupt entanglement.

I will discuss how techniques from information theory, quantum and statistical physics, can all be combined to elucidate the physics of macroscopic objects. Can it be that part of the macroscopic world is quantum, while the rest is, in some sense, classical? This question is also of fundamental importance to the development of future quantum technologies, whose behavior takes place invariably in the macroscopic non-equilibrium quantum regime.

I will discuss the concept of quantum macroscopicity and argue that it should be quantified in terms of coherence based on a set of conditions that should be satisfied by any measure of macroscopic coherence. I will show that this enables a rigorous justification of a previously proposed measure of macroscopicity based on the quantum Fisher information. This might shed new light on the standard Schrödinger cat type interference experiment that is meant to demonstrate the existence of macroscopic superpositions and entanglement.

**Date:** 27 April 2017, 4pm

**Venue:** CQT Seminar Room, S15-03-15

**Speaker:** Valerio Scarani, CQT, NUS

**The applied side of Bell nonlocality**

**Abstract:**

Since its formulation in 1964, Bell's theorem has been classified under "foundations of physics". Ekert's 1991 attempt to relate it to an applied task, quantum cryptography, was quenched by an approach that relied on a different basis and was allegedly equivalent.
Ekert's intuition was finally vindicated with the discovery of "device-independent certification" of quantum devices. In this colloquium, I shall revisit the tortuous history of that discovery and mention some of the subsequent results.

Some references that review this topic:

V. Scarani, Acta Physica Slovaca 62, 347 (2012) [https://arxiv.org/abs/1303.3081]

N. Brunner et al., Rev. Mod. Phys. 86, 419 (2014) [https://arxiv.org/abs/1303.2849]

S. Pironio et al., New J. Phys. 18, 100202 (2016) [http://iopscience.iop.org/1367-2630/focus/Focus-on-Device-Independent-Quantum-Information]

**Date:** 18 May 2017, 4pm

**Venue:** CQT Seminar Room, S15-03-15

**Speaker:** Joe Fitzsimons, CQT, NUS & SUTD

**Secure quantum computation**

**Abstract:**

The realisation that conventional information theory and models of computation do not account for the full generality of states and operations described by quantum mechanics has led to the burgeoning field of quantum information processing. By harnessing quantum phenomena it is possible to produce stronger forms of cryptography and more efficient algorithms than could exist in a purely classical world. Computer security lies at the intersection of computation and cryptography, and has become an increasingly important topic in recent years. Since quantum information processing leads to advantages in cryptography and computation separately, it is natural to ask whether it may also enhance computer security. In this talk I will argue that the answer to this question is a resounding “yes”, and discuss recent developments in the field.

**Date:** 27 Jul 2017, 4pm

**Venue:** CQT Seminar Room, S15-03-15

**Speaker:** Dimitris Angelakis, CQT, NUS

**Quantum simulations with strongly interacting photons: Merging condensed matter with quantum optics for quantum technologies**

**Abstract:**

Classical computers require enormous computing power and memory to simulate even the most modest quantum systems. That makes it difficult to model, for example, why certain materials are insulators and others are conductors or even superconductors. R. Feynman had grasped this since the 1980s and suggested to use instead another more controllable and perhaps artificial quantum system as a "quantum computer" or specifically in this case a "quantum simulator".

Working examples of quantum simulators today include extremely cold atoms trapped with lasers and magnetic fields and ions in electromagnetic traps. Photons and polaritons in light-matter systems have also recently emerged as a promising avenue especially for simulating out of equilibrium many-body phenomena in a natural driven-dissipative setting.

I will briefly review in non-specialist terms the main results in this area including the early ideas on realizing Mott insulators, Fractional Hall states and Luttinger liquids with photons [1,2,3]. After that I will present in more detail a recent experiment in many-body localization physics using interacting photons in the latest superconducting quantum chip of Google [4]. A simple method to study the energy-levels-and their statistics - of many-body quantum systems as they go through the ergodic to many-body localized (MBL) transition, was proposed and implemented. The formation of a mobility edge of an energy band was observed and its shrinkage with disorder toward the center of the bands was measured, a direct observation of a canonical condensed matter concept perhaps for the first time.

Beyond the applications in understanding fundamental physics, the potential impact of this field in different areas of quantum and nano technology and material science will be touched upon.

References

1. D.G. Angelakis and C. Noh “Many-body physics and quantum simulations with light” Report of Progress in Physics, 80 016401 (2016)

2. "Quantum Simulations with Photons and Polaritons: Merging Quantum Optics with Condensed Matter Physics" edited by D.G. Angelakis, Quantum Science and Technology Series, Springer International Publishing, 2017, ISBN 978-3-319-52023-0, DOI 10.1007/978-3-319-52025-4

3. Keil, Noh, Rai, Stutzer, Nolte, Angelakis, A. Szameit "Optical simulation of charge conservation violation and Majorana dynamics", Optica 2, 454 (2015)

4. P. Roushan, C. Neill, J. Tangpanitanon,V.M. Bastidas,, …, D.G. Angelakis, J. Martinis. “Spectral signatures of many-body localization of interacting photons”, under review

**Date:** 31 August 2017, 4pm

**Venue:** CQT Seminar Room, S15-03-15

**Speaker:** Berge Englert, CQT, NUS

**What do the data tell us?**

**Abstract:**

We gather information about physical systems by observation. In the realm of quantum physics, the experiments give us probabilistic data with natural statistical fluctuations that cannot be reduced by better instrumentation. What do such data tell us about the quantum system under study? A systematic and reliable answer can be given with the methods of quantum state estimation and quantum parameter estimation. I will report on recent developments.

**Date:** 7 Sept 2017, 4pm

**Venue:** CQT Seminar Room, S15-03-15

**Speaker:** Katsunari Okamoto, Okamoto Laboratory, Japan

**Evolution and Perspective of Planar Waveguide Devices**

**Abstract:**

The talk will review progress and future prospects of planar waveguide devices. Silica-based PLCs (planar lightwave circuits) and InP PICs (photonic integrated circuits) are widely used in the current WDM and FTTH systems. The success of silica PLCs and InP PICs strongly depends on their well controlled core geometries and refractive-index uniformities. On the other hand silicon photonics is widely regarded as a promising technology to meet the requirements of rapid bandwidth growth and energy-efficient communications while reducing cost per bit. One of the most prominent advantages of photonics interconnection over metallic interconnects is higher bandwidth and signal routing functionality using WDM technology. Expectations on Si photonics and technical challenges for silicon photonics will be described.

**Date:** 19 October 2017, 4pm

**Venue:** CQT Seminar Room, S15-03-15

**Speaker:** Ronnie Kosloff, Hebrew University

**Thermodynamics of Quantum Devices**

**Abstract:**

Quantum thermodynamics addresses the emergence of thermodynamical laws from quantum mechanics. The viewpoint advocated is based on the intimate connection of quantum thermodynamics with the theory of open quantum systems. Quantum mechanics inserts dynamics into thermodynamics giving a sound foundation to finite-time-thermodynamics. The emergence of the 0-law I-law II-law and III-law of thermodynamics from quantum considerations will be presented through examples. I will show that the 3-level laser is equivalent to Carnot engine. I will reverse the engine and obtain a quantum refrigerator. Different models of quantum refrigerators and their optimization will be discussed. A heat-driven refrigerator (absorption refrigerator) is compared to a power-driven refrigerator related to laser cooling. This will lead to a dynamical version of the III-law of thermodynamics limiting the rate of cooling when the absolute zero is approached. The thermodynamically equivalence of quantum engines in the quantum limit of small action will be discussed. I will address the question why we find heat exchangers and flywheels in quantum engines. I will present a molecular model of a heat rectifier and a heat pump in a non-Markovian and strong coupling regime.

**Date:** 26 October 2017, 4pm

**Venue:** CQT Seminar Room, S15-03-15

**Speaker:** Wenhui Li, Centre for Quantum Technologies, NUS

**Quantum optics with Rydberg atoms**

**Abstract:**

There have been growing research activities involving Rydberg atoms in different directions of quantum optics, both fundamental and applied. In this talk, I first briefly review a few systems and examples, which exploit the exotic properties of Rydberg atoms for new phenomena and applications. I then discuss some of our experimental efforts, including electromagnetically induced transparency and microwave-optical conversion using Rydberg atoms.

**Date:** 02 November 2017, 4pm

**Venue:** CQT Seminar Room, S15-03-15

**Speaker:** Nobuyuki Imoto, Osaka University

**Quantum communication and fundamental quantum physics**

**Abstract:**

If a high-quality entanglement is shared between two distant parties, there are a lot of innovative things we can do such as device-independent QKD, which is in fact initiated by Ekert [1]. For this, faithful quantum communication via a noisy and lossy channel is an important element, which is the main research item of my research group [2]. As we proceed this type of research, we encounter some fundamental research themes. For example, we developed a frequency converter of a single photon that maintains coherence and entanglement [3]. Our frequency converter acts as a beamsplitter in frequency domain, whose conversion efficiency is tunable. Using this frequency-domain beamsplitter, we performed Hong-Ou-Mandel interference [4], where two different-color input photons are converted into two same-color output photons whose color stochastically becomes either of the original two colors. We also performed Mach-Zehnder interference [5], which, in frequency domain, is more difficult than the HOM interference. The second example of fundamental research themes, other than the frequency converter, is cheat-sensitive type communication [6], where we can guess the result of the measurement performed by our partner regardless whether he/she chose from the linear and circular polarization measurements, which at first glance appears to be in conflict with the uncertainty principle. The key is that we not only prepare the initial state of the photon before the partner’s measurement but also measure its final state after the partner’s measurement. This concept is generalized to so-called weak value [7], and we are pursuing the meaning and usage of this new concept [8].

[1] A. K. Ekert, PRL67, 661 (1991).

[2] T. Yamamoto et al., Nature 421, 343-346 (2003); Nat. Photon. 2, 488 - 491 (2008).

[3] R. Ikuta et al., Nat. Commun. 2, 1544 (2011).

[4] T. Kobayashi et al., Nat. Photon. 10, 441–444 (2016).

[5] T. Kobayashi et al., Optics Express, 25, 012052_1_9 (2017).

[6] K. Shimizu et al., PRA84, 022308 (2011).

[7] Y. Aharonov et al., PRL, 60, 1351 (1988).

[8] K. Yokota et al., New J. Phys. 18, 123002 (2016).

**Date:** 23 November 2017, 4pm

**Venue:** CQT Seminar Room, S15-03-15

**Speaker:** Divesh Aggarwal, Centre for Quantum Technologies, NUS

**Foundations of Lattice-based Cryptography**

**Abstract:**

Lattice-based cryptosystems are perhaps the most promising candidates for post-quantum cryptography as they have strong security proofs based on worst-case hardness of computational lattice problems and are efficient to implement due to their parallelizable structure. Attempts to solve lattice problems by quantum algorithms have been made since Shor’s discovery of the quantum factoring algorithm in the mid-1990s, but have so far met with little success if any at all. The main difficulty is that the periodicity finding technique, which is used in Shor’s factoring algorithm and related quantum algorithms, does not seem to be applicable to lattice problems.

In this talk, I will survey some of the main developments in lattice cryptography over the last decade or so. The main focus will be on the Learning With Errors (LWE) and the Short Integer Solution (SIS) problems, their ring-based variants, their provable hardness under the intractability assumptions of lattice problems and their cryptographic applications.

### Forthcoming Talks

CQT Talk by Jun Suzuki, The University of Electro-Communications

Title: Ultimate precision bound in the quantum estimation theory:
Explicit formula for the Holevo bound for qubit-state estimation problems

Date/Time: 22 Feb, 04:00 PM

Venue: CQT Level 3 Seminar Room, S15-03-15

Abstract:

In this talk, we first review an ultimate precision bound, the Holevo bound, for the problem of parameter estimation about quantum states.

We discuss previously known cases where the Holevo bound is expressed explicitly in terms of a given model, i.e., quantum versions of Fisher information. The main contribution of this talk is to derive an explicit expression for the Holevo bound for estimating any two-parameter family of qubit mixed-states in terms of a given model.

The obtained formula depends solely on the symmetric logarithmic derivative (SLD), the right logarithmic derivative (RLD) Fisher information, and a given weight matrix.

One of the important results is that a general model other than special cases exhibits an unexpected property:

The structure of the Holevo bound changes smoothly when the weight matrix varies.

CQT Talk by Dax Koh, Massachusetts Institute of Technology (MIT)

Title: Complexity Classification of Conjugated Clifford Circuits

Date/Time: 23 Feb, 03:00 PM

Venue: CQT Level 3 Seminar Room, S15-03-15

Abstract: Clifford circuits — i.e. circuits composed of only CNOT, Hadamard, and π/4 phase gates — play a central role in the study of quantum computation. However, their computational power is limited: a well-known result of Gottesman and Knill states that Clifford circuits are efficiently classically simulable. We show that in contrast, “conjugated Clifford circuits” (CCCs) — where one additionally conjugates every qubit by the same one-qubit gate U — can perform hard sampling tasks. In particular, we fully classify the computational power of CCCs by showing that essentially any non-Clifford conjugating unitary U can give rise to sampling tasks which cannot be simulated classically to constant multiplicative error, unless the polynomial hierarchy collapses. Furthermore, by standard techniques, this hardness result can be extended to allow for the more realistic model of constant additive error, under a plausible complexity-theoretic conjecture. This work can be seen as progress towards classifying the computational power of all restricted quantum gate sets.

Based on arXiv:1709.01805 (joint work with Adam Bouland and Joseph F. Fitzsimons).

CQT Colloquium by Kazimierz Rzazewski, Center for Theoretical Physics, Polish Academy of Sciences

Title: Cold dipolar bosons

Date/Time: 01 Mar, 04:00 PM

Venue: CQT Level 3 Seminar Room, S15-03-15

Abstract: I will review recent research on the role of long range forces in quantum gases. I will mention experimental efforts in condensing chromium, erbium and dysprosium. I will describe in some detail the properties of just two dipolar atoms in a trap comparing magnetic and electric dipolar systems. Then I will talk about dipolar dark solitons, ending with our construction of a multi-particle roton state.

CQT Talk by Nick Proukakis, Joint Quantum Centre (JQC) Durham-Newcastle, Newcastle University

Title: Non-Equilibrium Superfluid Mixtures

Date/Time: 02 Mar, 11:00 AM

Venue: CQT Level 3 Conference Room, S15-03-17

Abstract:

Mixtures of quantum fluids, or superfluids, are nowadays routinely generated in controlled environments of ultracold atomic gases: they can take the form of different internal states, different isotopes, or different elements, and can even be made up of systems obeying different quantum statistics.

A first important property regards the co-existence of such superfluids: in particular, what sets the extent of spatial overlap between two interacting quantum gases? I will review experimental evidence of phase profiles, present the “basic” criterion routinely used to interpret them, and then critically assess its limitations, supported by extensive mean-field simulations. As I will show, a quantitative analysis of such experiments requires either a new criterion for in-situ imaging, or careful consideration of dynamical processes within the mixture: specifically, I will show that this transition can be mapped out experimentally by measuring the damping rate and the frequency of the dipole oscillations [1].

Presenting an extended kinetic model with full collisional damping [2], and implementing it to study the coupled expansion dynamics at different temperatures I will show how the homogeneous phase-separation criterion actually emerges dynamically in mixtures during expansion as a result of mechanical equilibrium across the condensate interface region, with the presence of a non-negligible gravitational sag found to be advantageous for identifying the (broad) transition region [3].

The relevance of such findings to “dual (Bose-Fermi) superfluids”, and the role of growth dynamics in the establishment of the emerging phase profiles [4] will also be discussed.

[1] Lee et al., PRA, 94, 013602 (2016);

[2] Edmonds et al., PRA, 91, 011602(R) (2015); PRA 92, 063607 (2015); Lee et al., JPB, 49, 214003 (2016);

[3] Lee et al., arXiv:1712.07481 (2017);

[4] Liu et al., PRA 93, 023628 (2016).

### Workshops & Conferences

7-8 December 2017 :
CQT10 Conference |