- Keynote Speakers
- Claude Cohen-Tannoudli (Collège de France and Laboratoire Kastler-Brossel): Manipulating Atoms with Photons and Photons with Atoms
It is possible to use the basic conservation laws in atom-photon interactions for manipulating atoms with photons. It will be shown in this lecture how it is possible in this way to polarize atoms, to cool them to very low temperatures, in the microkelvin, and even in the nanokelvin range. A review will be given of recent developments in this field, including ultra-precise atomic clocks, with errors less than one second in one billion years, the realization of new states of matter such as Bose-Einstein condensates, matter waves and atom lasers. New perspectives opened by these results will be briefly discussed. It is also possible to manipulate photons with atoms. By sending atoms in a cavity containing photons, one can prepare mesoscopic linear superpositions of photon states and study how they are destroyed by the decoherence processes resulting from the coupling with the environment. By analysing the state of the atoms when they exit from the cavity, one can also detect the presence of a photon in the cavity without destroying it.
- Tony Leggett (Urbana-Champaign): What can we do with a quantum liquid?
Quantum liquids are physical systems which display the effects not only of quantum _mechanics_ but also those of quantum _statistics_,that is of the characteristic _indistinguishability_ of elementary particles.The most spectacular manifestations of quantum statistics are the phenomenon of Bose- Einstein condensation and the closely related one of Cooper pairing;in both cases a finite fraction of all the particles in the system are forced to all do exactly the same thing at the same time,and as a result effects which would normally be obscured by thermal noise may become visible,sometimes spectacularly so.I will review some examples of such behavior in degenerate alkali gases,superconductors and superfluid helium-3.
- Ronald de Wolf (CWI and University of Amsterdam): Quantum property testing: A survey and one new result
"Property testers" are algorithms that can efficiently handle very large amounts of data: given a large object that either has a certain property or is somehow “far” from having that property, a tester should efficiently distinguish between these two cases. In this talk we describe recent results obtained for quantum property testing. This area naturally falls into three parts. First, we may consider quantum testers for properties of classical objects. We survey the main examples known where quantum testers can be much more efficient than classical testers. We also describe one new result: a quantum algorithm for testing whether a given n-bit Boolean function f is a k-junta (i.e., depends on only k of the n input bits) using roughly sqrtk queries to f, which is quadratically faster than the best classical testers. Second, we may consider classical testers of quantum objects. This is the situation that arises for instance when one is trying to determine if untrusted quantum states or operations are what they are supposed to be, based only on classical input-output behavior. Finally, we may also consider quantum testers for properties of quantum objects, such as whether two states or unitaries are equal, whether a state is separable, etc. This is based on joint work with Ashley Montanaro (survey arXiv:1310.2035) and with Andris Ambainis, Aleksanders Belovs, and Oded Regev (k-junta testing).
- David Quéré (PHHM and Ecole polytechnique): Water pearls
We show how special materials found in Nature shape water in quasi-spherical pearls. We focus on the functions generated by these materials, such as water (and even oil) repellency, self-cleaning effect, anti-fogging abilities or super-aerophilicity. We also emphasize the very unusual dynamical behaviors of liquids generated in such situations, and discuss in particular the nature of the residual friction associated with these non-wetting states.
- Claude Cohen-Tannoudli (Collège de France and Laboratoire Kastler-Brossel): Manipulating Atoms with Photons and Photons with Atoms
- Computer Science
- Hartmut Klauck (CQT, NTU): Communication complexity bounds over distributions with limited information
The Yao-principle states that randomized complexity is equal to distributional deterministic complexity in nonuniform models of computation like communication complexity or query complexity. In communication complexity it is often convenient to study distributional complexity over product distributions. However, it is known that the bounds obtainable this way can be far from optimal. In this work we study distributional communication complexity for distributions that have limited information between Alice’s and Bob’s input. We investigate how the complexity of the Disjointness problem in the randomized and quantum case changes in terms of the information parameter. We also give an example of a function f, where the optimal lower bound is achieved only when the information in the input distribution is much larger than the communication complexity of f itself.
- Dima Gavinsky (Czech Academy of Sciences): Equality, revisited
The Equality function is, probably, one of the most fundamental problems studied in the field of Communication Complexity (and as old as the field itself, both having been introduced in 1979 by A. Yao).
We propose a new method for analysing the communication complexity of Equality in the Simultaneous Message Passing model, which
(i) gives a number of new tight lower bounds, and
(ii) offers a unified way of treating Equality in virtually all imaginable variations of the original model: quantum, classical, mixed (quantum-classical), with Merlin added, etc.
- Iordanis Kerenidis (CNRS): Privacy in Quantum Communication Complexity
In two-party quantum communication complexity, Alice and Bob receive some classical inputs and wish to compute some function that depends on both these inputs, while minimizing the communication. This model has found numerous applications in many areas of computer science. One question that has received a lot of attention recently is whether it is possible to perform such protocols in a private way. We show that defining privacy for quantum protocols is not so straightforward and it depends on whether we assume that the registers where Alice and Bob receive their classical inputs are in fact classical registers (and hence unentangled with the rest of the protocol) or quantum registers (and hence can be entangled with the rest of the protocol or the environment). We provide new quantum protocols for the Inner Product function and for Private Information Retrieval, and show that the privacy assuming classical input registers can be exponentially smaller than the privacy assuming quantum input registers. We also argue that the right notion of privacy of a communication protocol is the one assuming classical input registers, since otherwise the players can deviate considerably from the protocol.
- Antonios Varvitsiotis (CQT, NTU): Determining the existence of perfect quantum strategies for nonlocal games
It is a well known fact that there exist Bell inequalities for which the maximally entangled state fails to achieve the maximum violation. This was a surprising result as the maximally entangled state has proven to be extremely useful for various quantum information tasks. On the other hand, for a nonlocal game that can be won perfectly using shared entanglement, it is not known whether there also exists a perfect strategy that uses maximally entangled state. In this work we address this question and we identify a natural class of games for which this is indeed the case. Our main contribution lies in identifying two necessary and sufficient conditions for a game to have a perfect projective strategy using a maximally entangled state. These conditions imply that this problem can be "reduced" to deciding whether the so-called independence number game admits a perfect strategy. Our approach is graph theoretical: To any nonlocal game we associate an undirected graph, whose properties we relate to the (non)existence of perfect strategies for the game.
- Robin Kotari (MIT): Quantum algorithms for simulating sparse Hamiltonians
Simulating physical systems is a major potential application of quantum computers, since no efficient classical algorithm is known for this problem. A significant fraction of the world’s computing power today is spent in simulating quantum systems that arise in chemistry, materials science, physics, etc. I will survey known quantum algorithms for this task and describe some of the ideas behind them.
- Troy Lee (CQT, NTU): Quantum query complexity in expectation
We study the query complexity of computing a nonnegative function in expectation. This requires the algorithm on input x to output a nonnegative random variable whose expectation equals f(x), using as few queries to the input x as possible. We exactly characterize both the randomized and the quantum query complexity by two polynomial degrees, the nonnegative literal degree and the sum-of-squares degree, respectively
These query complexities relate to (and are motivated by) the extension complexity of polytopes. The linear extension complexity of a polytope is characterized by the randomized communication complexity of computing its slack matrix in expectation, and the semidefinite (psd) extension complexity is characterized by the analogous quantum model. Since query complexity can be used to upper bound communication complexity of related functions, we can derive some upper bounds on psd extension complexity by constructing efficient quantum query algorithms. As an example we give an exponentially-close entrywise approximation of the slack matrix of the perfect matching polytope with psd-rank only 2^(n^(1/2+\eps)). Finally, we show there is a precise sense in which randomized/quantum query complexity in expectation corresponds to the Sherali-Adams and Lasserre hierarchies, respectively.
- Frédéric Magniez (CNRS): Optimal parallel quantum query algorithms
We study the complexity of quantum query algorithms that make up to p queries in parallel in each timestep. We show tight bounds for a number of problems, including element distinctness and k-sum. Our upper bounds are obtained by parallelized quantum walk algorithms, and our lower bounds are based on the adversary lower bound method, combined with recent results on learning graphs. We also prove some general bounds, in particular that quantum and classical p-parallel complexity are polynomially related for all total functions f when p is small compared to f’s block sensitivity.
- Stacey Jeffery (Caltech): Approximate span programs
Span programs are a model of computation that completely characterize quantum query complexity, and have also been used in some cases to get upper bounds on quantum time complexity. Any span program can be converted to a quantum algorithm that, given an input x, decides whether x is "accepted" by the span program, or "rejected" by the span program.
We consider more general ways of using this model to design quantum algorithms, by relaxing the notion of which inputs are accepted and which inputs rejected by a particular span program. We describe two new types of algorithms that can be constructed from any span program. The first algorithm "approximately" evaluates the span program: given an input to the span program, it decides if the input is close to being accepted or far from being accepted. The second algorithm estimates the span program witness size of an input.
This talk is based on joint work with Tsuyoshi Ito.
- Zhaohui Wei (CQT, NTU): The square root rank of the correlation polytope is exponential
The square root rank of a nonnegative matrix $A$ is the minimum rank of a matrix $B$ such that $A=B \circ B$, where $\circ$ denotes entrywise product. We show that the square root rank of the slack matrix of the correlation polytope is exponential. Our main technique is a way to lower bound the rank of certain matrices under arbitrary sign changes of the entries using properties of the roots of polynomials in number fields. The square root rank is an upper bound on the positive semidefinite rank of a matrix, and corresponds the special case where all matrices in the factorization are rank-one.
- Miklos Santha (CNRS, CQT): Hidden subgroups and hidden polynomials
Efficient solutions for the abelian hidden subgroup problem (HSP) constitute probably the greatest success of quantum computing. As a sequel to these results, various generalizations of the abelian HSP have been investigated, such as the non abelian HSP, hidden symmetry problems, and hidden structures of higher degree. In this talk we review an interesting connection between the HSP in some semi direct product groups and some hidden polynomial graph problems, and we present an efficient quantum solution for both, based on some new results for solving systems of polynomial equations in finite fields.
Joint work with Gabor Ivanyos.
- Jamie Sikora (CQT): QMA with subset state witnesses
The class QMA plays a fundamental role in quantum complexity theory and it has found surprising connections to condensed matter physics and in particular in the study of the minimum energy of quantum systems. In this talk, we further investigate the class QMA and its related class QCMA by asking what makes quantum witnesses potentially more powerful than classical ones. We provide a definition of a new class, SQMA, where we restrict the possible quantum witnesses to the "simpler" subset states, i.e. a uniform superposition over the elements of a subset of n-bit strings. Surprisingly, we prove that this class is equal to QMA, hence providing a new characterization of the class QMA. We also prove the analogous result for QMA(2) and describe a new complete problem for QMA and a stronger lower bound for the class QMA_1.
- Thomas Vidick (Caltech): A multiprover interactive proof system for the local Hamiltonian problem
The local Hamiltonian problem is the central problem in Hamiltonian complexity: given a description of a local Hamiltonian, how hard is it to estimate the Hamiltonian’s ground state energy. In this talk, motivated in part by the quantum PCP conjecture (which asks about the hardness of obtaining constant-factor approximations), I will present a quantum interactive proof system for the local Hamiltonian problem in which (i) the verifier has a single round of interaction with five entangled provers, (ii) the verifier sends a classical message on O(log n) bits to each prover, who replies with a constant number of qubits, and (iii) completeness and soundness are separated by an inverse polynomial in n. As the same class of proof systems, without entanglement between the provers, is included in QCMA, this result provides the first indication that quantum multiprover interactive proof systems with entangled provers may be strictly more powerful than unentangled-prover interactive proof systems. A distinguishing feature of our protocol is that the completeness property requires honest provers to share a large entangled state, obtained as the encoding of the ground state of the local Hamiltonian via an error-correcting code. This result can be interpreted as a first step towards a multiprover variant of the quantum PCP conjecture, which I will briefly discuss.
- Hartmut Klauck (CQT, NTU): Communication complexity bounds over distributions with limited information
- Soft matter
- David Quéré:On the Leidenfrost phenomenon (and beyond)
We discuss in this talk the Leidenfrost effect, as it happens when a volatile liquid is brought in contact with a hot solid. We first discuss historical facts and recent developments (solid Leidenfrost, bubble production, inverse Leidenfrost, cold Leidenfrost). Then we concentrate on dynamical aspects such as friction, self-propulsion (how vapor production can be exploited to induce controlled motion), and dynamical Leidenfrost-like phenomena (induces by air blowing, or by a motion of the substrate).
Contributors to this talk: Guillaume Dupeux, Dan Soto, Anaïs Gauthier, Hélène de Maleprade, Christophe Clanet, Philippe Bourrianne
- Tomas Paterek (NTU):
Quantum foundations, quantum information and quantum biology and the main topics studied in our group. I will briefly describe our recent results as well as plans for the future.
- Claus-Dieter Ohl (NTU): Nanobubbles, do they exist ?
Nanobubbles are gaseous objects in liquids found on the solid surface. They have been postulated, found, misinterpreted as contamination and reconfirmed. In a search of their dynamic properties I’ll summarise our group’s trial & errors.
- Tony Leggett (Urbana-Champaign) : The marriage of condensed matter theory and quantum information : some potential honeymoon problem
Over the last 15 years or so,considerable interest has developed in the possibility of using strongly interacting condensed matter systems for quantum-information purposes;a particularly striking example is proposals for topologically protected quantum computing,where the strongly entangled nature of these systems plays an essential role.Almost universally, theoretical analyses of such proposals have relied on the "tried and true" prescriptions of traditional condensed matter theory such as the mean-field method in the theory of superconductivity.I will use simple examples to argue that in the more sophisticated context of quantum information these prescriptions are dangerous and may even lead to qualitatively incorrect results.
- Ivan Shelykh (NTU):Polariton Spin Transport
In our talk we will give an overview of the current state of art in the theory of nonlinear polariton transport. The main focus will be on the interplay between nonlinearities coming from polariton- polariton interactions and polarization dynamics. In particular, we will consider the following topics:
1. We will consider the role of nonlinearities in the Optical Spin Hall Effect (OSHE). We show that the spin domains, generated in the linear optical spin Hall effect by the analog of spin-orbit interaction for exciton polarities, are associated with the formation of a Skyrmion lattice. In the nonlinear regime, the spin anisotropy of the polariton-polariton interactions results in a spatial compression of the domains and in a transmutation of the Skyrmions into oblique half-solitons. This phase transition is associated with both the focusing of the spin currents and the emergence of a strongly anisotropic emission pattern (See Figure).
2. We will consider the ways to account for the dissipative processes in the theoretical description of the dynamics of cavity polaritons in real space and time analyzing in detail the role of polariton-polariton and polariton-phonon interactions.
3. We will consider the role of polariton- polariton interactions in resonantly driven 2D polariton systems and polariton wires, focusing on the interplay between the phenomenon of polarization multistability and relaxation dynamics.
- Fabrice Laussy: What happens to photon correlations when retaining the frequency information?
One of the great insights of modern science was Glauber’s realization [1] that the concept which properly defines coherence of an optical field is that of photon correlations, rather than monochromaticity, as had been thought since Huygens in the 17th century. This conceptual breakthrough led to the field of quantum optics, with the so-called second order coherence function g(2)(τ)—measuring the tendency of photons to be detected with time delay τ —as the fundamental observable. This quantity takes the values 0, 1 and 2 for light that is thereby classified as, respectively, quantum, coherent and thermal. The coherent light is thus, in the modern understanding, that which presents no photon correlations, being therefore the more wavelike as can be in a quantum setting. In this talk, we restore frequency as the central component to define quantum coherence and show how the Glauber theory is the particular case of a full picture of photon correlations that is revealed when considering not only correlations in time, τ , but also in frequency, ω (or, equivalently, energy, hω)[2]. Only by retaining the frequency information can one make sense of all the types of correlated emission. First, we present a formalism that allows to compute frequency-resolved photon correlations in a wide class of quantum emitters with no approximation [3]. This allows us to introduce the concept of a two-photon spectrum (2PS) g(2) (τ, ω1, ω2 ) [4, 5] by spanning correlations over all frequencies. Such 2PS reveal new classes of photon correlations, such as “leapfrog processes” that are mediated by virtual states and are strongly quantum correlated, violating classical inequalities [6] and opening new perspectives for optimizing quantum devices by frequency filtering [7] or even creating a new paradigm of light altogether [8, 9]. Glauber’s theory is recovered as a lossy averaging of these structures that subsides an intricate landscape into a single number. Some of these theoretical predictions have been recently confirmed experimentally [10].
[1] Glauber, R. J. Photon correlations. Phys. Rev. Lett. 10, 84 (1963).
[2] Silva, B. et al. Measuring photon correlations simultaneously in time and frequency. arXiv:1406.0964 (2014)
[3] del Valle, E., Gonzalez-Tudela, A., Laussy, F. P., Tejedor, C. & Hartmann, M. J. Theory of frequency-filtered and time-resolved n-photon correlations. Phys. Rev. Lett. 109, 183601 (2012).
[4] Gonzalez-Tudela, A., Laussy, F. P., Tejedor, C., Hartmann, M. J. & del Valle, E. Two-photon spectra of quantum emitters. New J. Phys. 15, 033036 (2013).
[5] del Valle, E. Distilling one, two and entangled pairs of photons from a quantum dot with cavity QED effects and spectral filtering. New J.Phys. 15, 025019 (2013).
[6] Sanchez Munoz, C., del Valle, E., Tejedor, C. & Laussy, F. Violation of classical inequalities by photon frequency filtering. Phys. Rev. A 90, 052111 (2014).
[7] Gonzalez-Tudela, A., del Valle, E. & Laussy, F. P. Optimization of photon correlations by frequency filtering. arXiv:1501.01799 (2015).
[8] Sanchez Munoz, C. et al. Emitters of N -photon bundles. Nat. Photon. 8, 550 (2014).
[9] Strekalov, D. V. Cavity quantum electrodynamics: A bundle of photons, please. Nat. Photon. 8, 500 (2014).
[10] Peiris, M. et al. Two-color photon correlations of the light scattered by a quantum dot. arXiv:1501.00898 (2015).
- Christophe Couteau:Towards integrated single photon sources
The integration of single photon emitters with optical buses has recently attracted great interests in the realm of photonic integration, as the generation and transfer of single photons become essential in quantum information processing (QIP) [1]. Many platforms, such as photonic crystals and cavities involving complex fabrications, have been proposed towards the collection of light from single nanoscale emitters [2]. Herein we report different strategies based on quantum nanodevices to tackle the issue of efficient integrated single photon sources. In particular, we realised the localised excitation of a single photon source made of a CdSe/CdS nanocrystal using a nanowaveguide made of a single ZnO nanowire, which acts as a passive or an active sub-wavelength nanowaveguide to excite the single photon source, depending on whether we use above or below bandgap energy to couple light into this nanowire waveguide. The efficient excitation of the single photon source and the waveguiding behaviour within the nanowire in active and passive cases are characterised using a photoluminescence set-up corroborated by FDTD simulations, as well as using a Hanbury-Brown and Twiss interferometer for photon correlation measurements [3]. Combined with the intriguing properties of semiconductor nanowires, such integration can be extended to various applications, for instance electrically driven single photon emitter, efficient single photon detection and quantum information interconnect between nodes.
[1] O’Brien, J. L.; Furusawa, A.; & Vučković, J. Photonic Quantum Technologies. Nat. Photonics 2009, 3, 687–695.
[2] Northup, T. E.; & Blatt, R. Quantum Information Transfer Using Photons. Nat. Photonics 2014, 8, 356–363.
[3] Geng, W.; Manceau, M.; Rahbany, N.; Sallet, V.; De Vittorio, M.; Carbone, L.; Glorieux, Q.; Bramati, A.; & Couteau, C. Localised excitation of a single photon source by a nanowaveguide. Submitted to Sci. Rep.
- Sun Handong (NTU): Microlasers : Flexible fabrication and applications
We present the preparation and characterization of microlasers with variable configurations. Application of detecting refractive index is also demonstrated for bio- and chemical sensing.
- Pinaki Sengupta (NTU): The Haldane phase – an example of a topological insulator
Ever since their theoretical prediction, followed by the experimental discovery, topological insulators have attracted great interest. The Haldane phase of integer spin Heisenberg model remains one of the best understood of all topological phases. In this talk I’ll introduce the Haldane phase and explain its topological character. A key ingredient in the study of topological phases is the identification of symmetries of the state and gapless edge states. I’ll discuss in detail the symmetries that characterize (protect) the Haldane phase and the gapless edge states in a pedagogic way. I’ll end the talk with our recent work in devising a new way of identifying edge states in an arbitrary quantum state.
- Gagik Gurzadyan : Ultrafast dynamics in molecules, thin films, organic crystals, graphene
I will present some results of femtosecond spectroscopic studies on TCNQ based charge transfer complex, heat transfer in thin films, singlet exciton fission in rubrene and relaxation dynamics in graphene.
- David Wilkowski (UMI-NTU-CQT): An atom/surface Metamaterial Hybrid device
We report recent studies on an atom/surface Metamaterial Hybrid device. The plasmonic resonance of the Metamaterial are designed to cover a transition of Caesium atom. We study the coupling properties of this system to EM radiation and find a Fano-like resonance behavior.
- David Quéré:On the Leidenfrost phenomenon (and beyond)
- Cold Atomic Gases
- Vincent Josse: Weak (and Anderson) localization of ultracold atoms
Ultracold atomic systems in presence of disorder have attracted a lot of interest over the past decade, in particular to study the physics of Anderson localization (AL) in a renewed perspective. Landmark experiments have been demonstrated, in 1D [1,2] and 3D [3,4,5,6] geometries. However many challenges remain and new ideas have emerged, as for instance the search for original signatures of Anderson localization in momentum space initiated in the group of C. Miniatura in Singapore [7]. Here I will describe our progresses along that line where a weak localization effect has been directly observed, i.e. the Coherent Backscattering (CBS) phenomenon [8]. In particular I will report on the recent observation of suppression and revival of CBS when a controlled dephasing kick is applied to the system [9]. This observation demonstrates a novel and general method, introduced by A. Altland and coworkers [10], to study probe phase coherence in disordered systems by manipulating time reversal symmetry of the experimental time sequence.
References
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[9] K. Müller et al., arXiv.1411.1671 (2014).
[10] T. Micklitz et al., arXiv.1406.6915 (2014).
- Cord Müller : The superfluid fraction of disordered Bose-condensates
A superfluid by definition should flow without dissipation around obstacles. On the other hand, disorder is known to reduce the superfluid fraction. So what is really the effect of spatial randomness on the superfluid fraction? In this talk, I will discuss some aspects of disordered superfluids. In particular, it is known that the Josephson sum rule relates the superfluid density to the condensate order parameter, via the infrared residue of the single-particle Green’s function. For disordered bosons, one can establish an effective Josephson relation for disordered condensates valid upon ensemble averaging. This relation has the merit to show explicitly how superfluidity links to the coherent density, or density of particles with zero momentum. Detailed agreement with inhomogeneous Bogoliubov perturbation theory is reached in the limit of weak disorder.
Reference: arXiv.1409.6987 (PRA accepted)
- Dario Poletti (SUTD): Emergent quantum many-body physics in driven Hamiltonians and in Dissipative systems
Recent experiments have fuelled the interest both in engineering effective Hamiltonians with periodic drivings and in dissipative systems. We will discuss the relaxation dynamics in many-body open quantum systems. We show that the interplay of kinetic energy, interactions and dissipation can give rise to non-trivial relaxation dynamics including power-decays, stretched exponentials and aging. We will then describe some recent works in which the generation of effective density dependent tunnelling gives rise to fractional Mott-insulators or parity and string-order.
- Rainer Dumke (NTU): Superconducting and Cryogenic Atom Chips
Recently superconducting atom chips have generated a lot of interest due to their attractive properties, such as the Meissner effect for type-I superconductors and vortices for type-II superconductors. Thermal and technical noise in proximity to superconducting surfaces have been shown both theoretically and experimentally to be significantly reduced compared to conventional atom chips. Superconducting atom chips have the potential to coherently interface atomic and molecular quantum systems with quantum solid state devices. I will present recent developments in our superconducting atom chip experiment. Trapping ultra cold atoms with vorticies induced in a type II superconducting micro structure and electric field measurements originating from absorbates on a cryogenic atomchip.
- David Wilkowski (UMI-CQT-NTU): Cooperative phenomena in optically thick scattering medium
We investigate the transient coherent transmission of light through an optically thick cold Strontium gas. We observe a coherent superflash just after an abrupt probe extinction, with peak intensity more than 3 times the incident one. We show that this coherent superflash is a direct signature of the cooperative forward emission of the atoms. By engineering fast transient phenomena on the incident field, we give a clear and simple picture of the physical mechanisms at play. Finally, taking advantage of the fast decay time of the cooperative emission, we create a pulse train of light with a repetition time shorter than the atomic lifetime.
- Mark Goerbig Two-dimensional materials beyond graphene — a playground for novel electronic properties
The study of truly two-dimensional (2D) atomic crystals has been one of the most active fields of research in condensed-matter physics during the last decade. Its probably most prominent representative is graphene, isolated in 2004, with its fascinating combined electronic and mechanical properties. From a fundamental-research point of view, the electronic properties stem from a semimetallic behaviour due to graphene electrons being a condensed-matter realisation of massless fermions that are theoretically described in terms of an (ultra)relativistic Dirac equation rather than the more conventional Schroedinger equation.
In parallel to the above-mentioned research, graphene has triggered investigations in other 2D crystals that can be obtained in a similar fashion as graphene. Indeed, already in 2005, the Manchester group showed that the exfoliation technique can be used to obtain single-layer boron nitride or transition-metal dichalcogenides, such as MoS2, WS2, MoSe2, WSe2,... and more recently black phosphorus, silicene, germanene, etc. In contrast to graphene, most of these materials are 2D semiconductors or insulators the electronic properties of which are highly debated at present. Do we need to build up a novel description of electrons in these materials, in terms of a massive Dirac equation similarly to graphene, or is the more conventional desription in terms of Schroedinger fermions is sufficient? Naturally this question is only pertinent if there are physical phenomena that allow one to distinguish between the two types of fermions.
The aim of this talk is to provide a short overview of 2D crystals beyond graphene. I will mainly concentrate on the relevance of a description of some 2D semiconductors in terms of massive Dirac (or "mixed-type") fermions. I will show how this character can be unveiled via a magnetic field applied perpendicular to the 2D system, both in magneto-optical spectroscopy [1] and with the help of magneto-transport [2].
[1] F. Rose, MOG, F. Piéchon, Phys. Rev. B 88, 125438 (2013).
[2] MOG, G. Montambaux, F. Piéchon, EPL 105, 57005 (2014).
- Riadh Rebhi (Centre for Quantum Technologies, NUS): Trapped ions, precision measurements and beyond
Laser cooled and trapped ions are known for precision measurements, quantum emulation and quantum information processing. Single or a few ions forming a Coulomb crystal provides most pristine environment, however there are many possibilities of precision experiments even with cold non-neutral gas as is formed in an ion trap before crystallization. This presentation will involve our experimental contribution towards the understanding of many-body physics geared for fundamental tests of the Standard Model in the low energy sector. Given the potential of trapped ion systems, our next generation of traps based on surface electrodes will be introduced along with our experiments in progress on the generation Berry phases in Abelian and non-Abelian geometries.
- Thibault Vogt: Excitation of a quantum gas to Rydberg states
Rydberg atoms constitute a paradigmatic system for the study of quantum many-body physics. Very large dipole-dipole interaction between Rydberg atoms leads to the dipole blockade, at the heart of promising proposals for quantum simulation or studies of strongly correlated systems [1-2]. Dipole blockade has already been used to excite spatially organized structures of Rydberg atoms [3], realize single photon sources [4] or implement quantum gates [5]. In this talk, I will present the recent developments of our experimental setup for achieving highly coherent Rydberg excitation. I will present preliminary spectroscopic measurements obtained for the excitation of 87Rb Rydberg atoms in a nearly degenerate gas. I will also discuss our progress towards the detection of Rydberg atoms based on the interaction enhanced absorption imaging technique [6].
[1] Weimer, H., et al. Nature Physics 6:382-388 (2010).
[2] Pupillo, G., et al. PRL 104 223002 (2010).
[3] Schausz, P., et al. Nature 491 87-91 (2012).
[4] Peyronel et al. Nature 488 58-60 (2012).
[5] Isenhower et al. PRL 104 010503 (2010).
[6] Günter et al. Science 342, 954-956 (2013).
- Pinaki Sengupta Strange correlations in symmetry protected topological phases
Recently, a “strange correlator” has been proposed as a direct probe for the topological character of interacting Symmetry Protected Topological (SPT) phases. Using projective quantum Monte Carlo, we are able to directly access the strange correlator in a variety of phases, as well as to examine its critical behavior at the quantum phase transition between trivial and non-trivial symmetry protected topological phases. After finding the expected long-range behavior in these two symmetry conserving phases, we go on to verify the topological nature of two-leg and three-leg spin-1 Heisenberg antiferromagnetic ladders. This demonstrates the power of the strange correlator in distinguishing between trivial and non-trivial symmetry protected topological phases.
[1] K. Wierschem and P. Sengupta, "Quenching the Haldane Gap in Spin-1 Heisenberg Antiferromagnets", Phys. Rev. Lett. 112, 247203 (2014).
[2] K. Wierschem and P. Sengupta, "Strange correlations in spin-1 Heisenberg antiferromagnets", Phys. Rev. B 90, 115157 (2014).
- Gong Jiangbin (Physics Department, NUS) Coherence-induced correction to quantum adiabatic theorem: Influence of Interband-coherence on adiabatic pumping in periodically driven systems
Adiabatic pumping can serve as a means to detect topological phase transitions in periodically driven systems. For initial states prepared as a Wannier state of a Floquet quasi-energy band, the adiabatic pumping can be simply connected with the Chern number of the occupied band. By contrast and much more remarkable, for general and easy-to-prepare initial states possessing coherence between different Floquet quasienergy bands, adiabatic pumping is found to be comprised by two components independent of the pumping time scale: a weighted integral of the Berry curvature summed over all Floquet quasienergy bands, plus an inter-band-coherence induced correction. It is stressed that the found correction is always there no matter how slowly a pumping cycle is executed. In addition to probing topological phase transitions, adiabatic pumping is now also anticipated to be useful in manifesting coherence and decoherence effects in periodically driven systems. Our results are also generally useful towards a better understanding of the quantum adiabatic theorem.
- Vincent Josse: Weak (and Anderson) localization of ultracold atoms
- Quantum Information
- Tomasz Paterek (SPMS-NTU): Random correlations and quantum entanglement
We show that expectation value of squared correlations measured along random local directions is a quantifier of quantum entanglement in pure states which can be directly experimentally assessed by a measurement on two copies of the state. Entanglement can therefore be measured by parties who do not share a common reference frame and do not even possess a well-defined local reference frame. We show that already a single-qubit-per-party reference and rotationally invariant measurements are sufficient to quantify entanglement and violate a Bell inequality. We also provide entanglement witness solely in terms of random correlations, capable of detecting entanglement in mixed states, and emphasize how data gathered for a single measurement setting reliably detects entanglement.
- Leonid A. Krivitsky (DSI-ASTAR: Interaction of Biological Photodetectors with Quantum Light
The ability to control light at a quantum level can be extremely useful in addressing biological problems. Interfacing biological objects with non-classical (quantum) light allows to enhance the precision of biological measurements, fosters the development of more precise models of biological processes, and allows us to reveal the possible role of quantum effects in neurobiology and perception. Rod cells of the retina are natural photodetectors, and they are perfect candidates for studies of biological interfaces with quantum light. Rod cells convert incident light into electrical currents, which are then sent to the brain via the optics nerve. They are responsive at the discrete photon level, and highly-sensitive techniques for the readout of their electrical response are readily available. We interface rod cells with light sources of different photon statistics [1, 2]. We show that similar to commercially available photodetectors, used for quantum optics and quantum communication, rod cells are able to discriminate between thermal and coherent light [1]. We also interface rod cells with a heralded single photon source, realized using the parametric down conversion [2]. This approach allows us (1) to demonstrate single photon sensitivity of rod cells without relying on statistical modeling (2) to measure their quantum efficiency without pre-calibrated devices, and (3) to assess the intrinsic noise of bio-chemical mechanisms of the phototransduction without the interference from multiphoton events. The results are relevant to ongoing studies of manifestation of quantum effects in phototransduction, vision, and photosynthesis.
[1] N. Sim et al., Measurement of Photon Statistics with Live Photoreceptor Cells, Phys. Rev. Lett. 109, 113601 (2012) (Editor’s suggestion).
[2] N. M. Phan et al., Interaction of Fixed Number of Photons with Retinal Rod Cells, Phys. Rev. Lett. 112, 213601 (2014) (Editor’s suggestion)
- Qinghai Wang (NUS): Stabilizing Non-Hermitian Systems by Periodic Driving
The time evolution of a system with a time-dependent non-Hermitian Hamiltonian is in general unstable with exponential growth or decay. A periodic driving field may stabilize the dynamics because the eigenphases of the associated Floquet operator may become all real. This possibility can emerge for a continuous range of system parameters with subtle domain boundaries. It is further shown that the issue of stability of a driven non-Hermitian Rabi model can be mapped onto the band structure problem of a class of lattice Hamiltonians. As an application, we show how to use the stability of driven non-Hermitian two-level systems to simulate a spectrum analogous to Hofstadter’s butterfly that has played a paradigmatic role in quantum Hall physics.
- Dario Poletti (SUTD) Quantum Heat Engines: Otto cycles in Power-Law Trapping Potentials
To be announced
- Teck Seng Koh (MOE) A semiconductor quantum dot spin-charge hybrid qubit
The similarities between gated quantum dots and the transistors in modern microelectronics have led to great interest in the development of qubits in semiconductor quantum dots. Although quantum dot spin qubits have demonstrated long coherence times, their manipulation is often slower than desired for important future applications, such as factoring. Furthermore, scalability and manufacturability are enhanced when qubits are as simple as possible. Previous work has increased the speed of spin qubit rotations by making use of integrated micromagnets, dynamic pumping of nuclear spins or the addition of a third quantum dot. Here, I will talk about a three-electron double quantum dot qubit that is a hybrid of spin and charge. It is simple, requiring neither nuclear-state preparation nor micromagnets. Unlike previous double-dot qubits, the hybrid qubit enables fast rotations about two axes of the Bloch sphere. I will report recent experimental and theoretical results in quantum control and process tomography.
- Joe Fitzsimmon (SUTD) Towards quantum homomorphic encryption
To be announced
- Zilong Chen (DSI-ASTAR) Generation of atomic spin squeezed states by cavity-aided measurements
To be announced
- Hui-Khoon Ng (Yale-NUS & CQT) Fighting quantum noise
In recent years, there have been more and more indications of the potential power of quantum information processing, with many theoretical proposals attempting to harness quantum phenomena to perform a larger and larger range of useful tasks. But these miracles all come to naught unless we can get the noise afflicting existing experimental quantum information systems under control. In the spirit of spurring collaboration in this new UMI, I review the current theoretical and experimental status of quantum information protection schemes, and highlight some of the critical missing pieces.
- Tomasz Paterek (SPMS-NTU): Random correlations and quantum entanglement