### Colloquia

July 27, 20174.00 pm: Quantum simulations with strongly interacting photons: Merging condensed matter with quantum optics for quantum technologies By Dimitris Angelakis, Centre for Quantum Technologies, NUS |

August 31, 20174.00 pm: What do the data tell us? By Berge Englert, Centre for Quantum Technologies, NUS |

September 07, 20174.00 pm: Evolution and Perspective of Planar Waveguide Devices By Katsunari Okamoto, Okamoto Laboratory, Japan |

→ 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.

### Forthcoming Talks

CQT Talk by Xiaoting Wang, University of Electronic Science and Technology of China

Title: From quantum control to quantum computing --How control and optimization design reduces quantum errors

Date/Time: 22 Aug, 04:00 PM

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

Abstract:

Quantum information processing(QIP) has been identified as one of the key future technologies that are crucial for communication, cryptography, computing, complex-system simulation, metrology, artificial intelligence and national security. Quantum control, on the other hand, provides a powerful tool to analyze and improve the physical performances of different QIP devices. In this talk, I will introduce three different methods that can be readily implemented in experiment to reduce quantum noise, the big obstacle towards large-scale quantum computing. These methods are (1) optimal cooling, (2) minimal noise subsystems, and (3) quantum brachistochrone control. Finally, as an application, I present a work in collaboration with the experimentalists, implementing the brachistochrone design to reduce errors on spin qubits in solid.

CQT CS Talk by Thomas Vidick, Caltech

Title: Entanglement Tests from Group Representations.

Date/Time: 23 Aug, 02:00 PM

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

Abstract: I will present a novel perspective on entanglement tests, including the CHSH inequality, the Magic Square game, and their n-qubit extensions, based on the theory of (approximate) representations of non-abelian groups.

I will show how this powerful approach, whose origins lie in works of Slofstra and Gowers-Hatami, can in particular be used to give a simple, improved analysis of:

(i) The first family of self-tests (or, two-player entangled games) for arbitrarily high-dimensional entanglement that is robust to a constant fraction of noise. (Joint work with Natarajan,

arXiv:1610.03574.)

(ii) The first finite self-test such that for any eps>0, entangled states of dimension poly(1/eps) are both necessary and sufficient to achieve success probability 1-eps. (Joint work with Slofstra, in

preparation.)

CQT Talk by Alastair Kay, Royal Holloway, University of London

Title: Quantum State Transfer in the Presence of Errors

Date/Time: 24 Aug, 04:00 PM

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

Abstract: One of the biggest challenges for the practical realisation of quantum computers is how to deal with errors. Certainly, we have well-developed theories of error correction, which are ideally suited to the gate model of quantum computation, and Pauli errors. What if your quantum computer doesn't look like that? What if it's based on Hamiltonian evolution, like an analogue quantum simulator? Even if errors start off like Pauli errors, they quickly evolve into very different entities. In this talk, I will discuss strategies for dealing with these errors for a specific form of Hamiltonian, in the hope that some of the ideas can be used more broadly in the future. Ultimately, we arrive at true error correction strategies in this evolving Hamiltonian scenario.

CQT PhD QE II Presentation by Srijita Kunda

Title: Composition theorems for query and communication complexity

Date/Time: 25 Aug, 11:00 AM

Venue: CQT Level 6 Seminar Room, S15-06-22

Abstract: A composition theorem expresses a complexity measure of a composed function $f \circ g^n$ in terms of complexity measures of $f$ and $g$. Using such a theorem, one may get a lower bound for the complexity of the composed function in terms of the complexities of the composing functions -- in particular this can help "lift" a lower bound from a simpler computation model to a more complex one. In this work, we study composition theorems for randomized query and communication complexity. In the first part, we show that for two query functions $f$ and $g$, the constant error randomized query complexity of $f \circ g^n$ is at least the constant error randomized query complexity of $f$ times the randomized query complexity of g with $1/2 - 1/poly(n)$ error. In the second part, we show that for a query function $f$ and a communication function $g$, the constant error randomized communication complexity of the composed communication function $f \circ g^n$ is at least the constant error randomized query complexity of $f$ times $(\log(1/disc(g)) - O(\log n))$, where $disc(g)$ is another communication complexity measure. Our results directly imply many previously known lower bounds, and may be useful in producing new lower bounds as well.

### Workshops & Conferences

4-8 September 2017 :
17th Asian Quantum Information Science Conference |