SCIENTIFIC PROGRAMS AND ACTIVITIES

June 28,
11:10 a.m.
Room 210

Savannah Garmon, University of Tokyo
Bound state influence on long-time non-exponential decay in open quantum systems

We study the long time non-exponential decay in an open quantum system in which a bound state approaches the energy continuum as the system parameters are varied. We find that at least two regions exist yielding different power law decay behaviors; we term these the long-time near zone and the long-time far zone. In the near zone the survival probability falls off according to a t^-1 power law, while in the far zone it falls off as t^-3. The timescale T_Q separating these two regions is inversely related to the gap between the bound state energy and the continuum threshold, hence when the bound state is absorbed into the continuum for special parameter values, this time scale diverges and the survival probability follows the enhanced t^-1 power law even on asymptotic scales.

[1] S. Garmon, T. Petrosky, L. Simine, and D. Segal, Fortschr. Phys. 61, 261 (2013)

Friday 21-June-2013
11:10 AM
Room MP 606, 60 St. George Street, Toronto

Howard Wiseman, Griffith University
100 years of quantum jump theory: Is there anything new to say for a 2-level atom?

The famous "quantum jump" of an atom from one level to another was first postulated by Bohr in 1913, nearly 100 years ago. Can there still be anything interesting to say about quantum jumps in a two-level atom today, 100 years later? Of course my answer is "yes", as I will be talking about two recent papers in this area [1,2]. The most recent [2] proposes experiments to definitively prove that quantum jumps are not due to emission of a photon, but rather to detection. Such a test is possible because different types of detection lead to different types of jumps. The other [1] shows that this flexibility in causing jumps of different types can always be used creatively to make an atom jump between a two fixed states, although in general these are NOT orthogonal.

[1] R. Karasik and H. M. Wiseman, How Many Bits Does It Take to Track
an Open Quantum System?
Phys. Rev. Lett. 106, 020406 (4 pages) (2011).
[2] Howard M. Wiseman and Jay M. Gambetta, Are Dynamical Quantum Jumps
Detector-Dependent?
Phys. Rev. Lett. 108, 220402 (5 pages) (2012).

June 07,
11:10 a.m.
Room MP 606, 60 St. George Street, Toronto

May 27,
11:10 a.m.
Room MP 606, 60 St. George Street, Toronto

Stefanie Barz (University of Vienna)
Experimental Blind Quantum Computing

Quantum computers are among the most promising applications of quantum-enhanced technologies. Quantum effects such as superposition and entanglement enable computational speed-ups that are unattainable using classical computers. Blind quantum computing shows an additional fundamental advantage of quantum over classical computation: a computation can be made private. Using the framework of measurement-based quantum computing, blind quantum computing enables a nearly-classical client to access the resources of a more computationally-powerful quantum server without divulging the content of the requested computation. Here, we demonstrate the first experimental version of this protocol using polarization-entangled photonic qubits. We show various blind one- and two-qubit gate operations as well as blind versions of the Deutsch's and Grover's algorithms. Further, we address the question if quantum computations can be verified by classical entities which are not able to compute the results themselves. When the technology to build quantum computers becomes available, blind quantum computing and the verification of quantum computations will become an important feature of quantum information processing.

Roman Krems (The University of British Columbia)
Collective excitations of molecules trapped on an optical lattice

Molecules trapped on an optical lattice represent a unique, controllable many-body system which can be used to study dynamics of collective excitations in new regimes. I will discuss the rotational excitations of molecules on an optical lattice leading to rotational Frenkel excitons. Apart from solid hydrogen, there is no other natural system that exhibits rotational excitons. The rotational excitons have unique properties that can be exploited for tuning non-linear exciton interactions and exciton-impurity scattering by applying an external electric field. I will show that this can be used to explore the competing role of the dynamical and kinematic exciton-exciton interactions in excitonic energy transfer and to study quantum localization in a dynamically tunable disordered potential.

The rotational excitons can also be used as a basis for quantum simulation of condensed matter models that cannot be realized with ultracold atoms. In particular, I will discuss the possibility of engineering the Holstein, breathing-mode and Su-Schrieffer-Heeger polaron models with polar molecules on an optical lattice. I will discuss the phase diagram of a polaron model with mixed breathing-mode and Su-Schrieffer-Heeger couplings and show that it has two sharp transitions, in contrast to pure models which exhibit one (for Su-Schrieffer-Heeger coupling) or no (for breathing-mode coupling) transition. I will show that ultracold molecules trapped in optical lattices can be used to study precisely this mixed Hamiltonian, and that the relative contributions of the two couplings can be tuned with external electric fields, which brings the possibility of observing the polaron transitions within reach of up-coming experiments. Time permitting, I will also discuss dipole blockade of microwave excitations in ensembles of trapped molecules and how it can be used to create quantum phases of interacting spin-1/2 particles without crossing phase transitions.

Daniel Garcia **Seminar Cancelled

Ryo Okamoto, Hokkaido University and Osaka University
Linear optics quantum circuits

Quantum information science addresses how uniquely quantum mechanical phenomena such as superposition and entanglement can enhance communication, information processing, and precision measurement. Photons are appealing for their low-noise, light-speed transmission and ease of manipulation using conventional optical components. However, it has been very difficult to achieve the necessary two-qubit operations since the physical interaction between photons is much too small. In a breakthrough, Knill, Laflamme, and Milburn (KLM) showed that effective nonlinear interactions can be achieved using only linear optical elements, auxiliary photons, and measurement. Inspired by the KLM approach, a number of quantum logic gates using heralded photons and event postselection have been proposed and demonstrated. Furthermore, optical quantum circuits combining these gates have been demonstrated . We experimentally demonstrate a two photon quantum gate (controlled-NOT gate) based on the KLM approach. This result confirms the first step in the original KLM “recipe” for all-optical quantum computation, and should be useful for on-demand entanglement generation and purification. Our other recent progress on linear optics quantum circuit will also be introduced.

Alexander Gaeta (Cornell University)
Exploiting Optical Waveguides for Quantum Information Applications

Optical waveguides provide tight confinement of light over extended lengths which are ideal for nonlinear optical interactions. I will describe the use of a various types of waveguides including photonics crystal fibers and silicon-based nanowaveguides for quantum information applications. For example, we use hollow-core photonics band-gap fibers filled with Rb atoms to enable nonlinear interactions down to the few-photon level. In addition, we can use dispersion engineering in glass and silicon nanowaveguides to produce quantum-correlated photons, frequency translation of quantum states, and photon shaping.

Luca Turin (BSRC Alexander Fleming, Greece)
Is smell a quantum phenomenon?

Our sense of smell is extraordinarily good at molecular recognition: we can identify tens of thousands of odorants unerringly over a wide concentration range. The mechanism by which this happens is still hotly debated. One view is that molecular shape governs smell, but this notion has turned out to have very little predictive power. Some years ago I revived a discredited theory that posits instead that the nose is a vibrational spectroscope, and proposed a possible underlying mechanism, inelastic electron tunneling. In my talk I will review the history and salient facts of this problem and describe some recent experiments both on fruit lies and on humans that go some way towards settling the question.

Mar. 22,
11:10 a.m.
Room 210

Irfan Siddiqi (University of California)
Dissipation Enhanced Coherence in Superconducting Quantum Bits (slides)

A dissipative environment usually transforms a quantum superposition into a classical state. Recent advances in superconducting circuits--the development of robust quantum-noise-limited microwave amplifiers and quantum bits with lifetimes in excess of 100ms--have enabled the use of quantum feedback to actively suppress decoherence. We discuss experiments in which microwave pulses alter the circuit environment to autonomously cool the system to any coherent superposition of ground and excited states. In addition, we also realize weak measurements of the qubit state to implement real-time feedback. Here, the dominant dephasing is measurement induced and the information extracted is used to generate Rabi oscillations which persist indefinitely.

Coupling the surface state of a topological insulator (TI) to an s-wave superconductor is predicted to produce the long-sought Majorana quasiparticle excitations. Such Majorana fermions may be topologically protected from decoherence, and could play a significant role in solid state implementations of a quantum computer. A requisite step in the search for Majorana fermions is to understand the nature and origin of the supercurrent generated between superconducting contacts and a TI.
In this talk, I will discuss transport measurements of DC Josephson effects in TI-superconductor junctions (Bi2Se3-Al) as the chemical potential is moved from the bulk bands into the band gap, or through the true topological regime characterized by the presence of only surface currents. We compare our results to 3D quantum transport simulations to conclude that the supercurrent is largely carried by surface states, due to the inherent topology of the bands, and that it is robust against disorder. We further find that the supercurrent is not symmetric with respect to the conduction and valence bands, and that the Fraunhofer patterns are similar both within and outside of the topological regime.

Feb. 08,
11:10 a.m.
Room 230

Dylan Mahler (UToronto)
Adaptive Quantum State Tomography

Within the past 5 or so years, a number of experiments have revealed the condensation of polariton quasi-particles in quasi 2d inorganic quantum well cavities. A polariton condensate forms when there is a sufficiently high density of exciations in a material sandwiched between two dielectric reflectors that spontaneous symmetry breaking occurs and the exciton gas condenses to form a superfluidic state. This has opened the door to test a number of novel and fundimental theories ranging from the BEC to BCS cross over to Hawking radiation from blackholes. Loosely speaking, the condensation occurs when all the dipole oscillators in the system are driven by a common field mode and spontaneously beging to evolve synchroneously much like the effect of super-radiance. Our work has focused upon the dynamics of condensate formation in organic semiconductor-based systems. I will discuss both the equilibrium and non-equilibrium/steady state regimes using models based upon one and two dimensional arrays of organic chromophores. Time permitting, I will discuss our work on linear arrays of quantum nanorods coupled by a common surface plasmon mode.

Eric Bittner (University of Houston)
Theory and Models of Bose Condensation of Exciton/Polariton in Organic Semiconductor Thin-films

Within the past 5 or so years, a number of experiments have revealed the condensation of polariton quasi-particles in quasi 2d inorganic quantum well cavities. A polariton condensate forms when there is a sufficiently high density of exciations in a material sandwiched between two dielectric reflectors that spontaneous symmetry breaking occurs and the exciton gas condenses to form a superfluidic state. This has opened the door to test a number of novel and fundimental theories ranging from the BEC to BCS cross over to Hawking radiation from blackholes. Loosely speaking, the condensation occurs when all the dipole oscillators in the system are driven by a common field mode and spontaneously beging to evolve synchroneously much like the effect of super-radiance. Our work has focused upon the dynamics of condensate formation in organic semiconductor-based systems. I will discuss both the equilibrium and non-equilibrium/steady state regimes using models based upon one and two dimensional arrays of organic chromophores. Time permitting, I will discuss our work on linear arrays of quantum nanorods coupled by a common surface plasmon mode.

Olga Smirnova (Max-Born Institute, Berlin, Germany)
Time-resolving tunneling dynamics

Time-resolving tunneling is a well-recognized controversial problem. The main difficulty comes from the absence of unambiguous definition of tunneling time. The question becomes even more intriguing in many-body systems. First, many body-interactions during tunneling may delay the electron escape through the barrier. Second, these interactions can be used to record the tunneling dynamics. Many-body interactions during tunneling range from Josefson junction to metal-insulator tunneling, to electron tunneling from atoms and molecules in strong infrared laser fields. In latter case the tunneling barrier is created by the laser field. The corresponding ionization mechanism is called "optical tunneling" to distinguish it from the tunneling in static electric fields.
We show how one can use a combination of multicolor (from infrared to extreme ultraviolet) light fields to time-resolve optical tunneling in one-electron and many-electron systems.

Man-Duen Choi (University of Toronto)
The Taming of the Shrew - Tricks or Treats with Quantum Entanglements

I wish to tame the physical quantum entanglements (in disguise of non-commutative geometry), by means of pure mathematics. Note that the research work along these lines, has been proven to be useful to the foundation of abstract quantum information in the light of (the reality of) quantum computers. This is an expository talk; no background knowledge of quantum information will be assumed in this talk.

We will consider the implementation of a symmetric informationally complete probability-operator measurements (SIC POM) in the Hilbert space of a d-level system by a two-step measurement process: a diagonal-operator measurement with high-rank outcomes, followed by a rank-1 measurement in a basis chosen in accordance with the result of the first measurement. We find that any Heisenberg-Weyl group-covariant SIC POM can be realized by such a sequence where the second measurement is simply a measurement in the Fourier basis, independent of the result of the first measurement. Furthermore, at least for the particular cases studied, of dimension 2, 3, 4, and 8, this scheme reveals an unexpected operational relation between mutually unbiased bases and SIC POMs; the former are used to construct the latter. As a laboratory application of the two-step measurement process, we propose feasible optical experiments that would realize SIC POMs in various dimensions. I am looking forward to meet you.

Sept. 7
11:10 a.m.
MP 408

Anna Sanpera (Universitat Autònoma Barcelona)
The entanglement spectrum in many-body ultracold bosonic gases

We investigate quantum phases with spinor bosonic gases using quantum information tools. We show that in finite quantum spin chains when approaching a quantum phase transition, the Schmidt gap, i.e. the difference between the two largest eigenvalues of the reduced density matrix $\lambda_{1},\lambda_{{2}}$, signals the critical point and
scales with universal critical exponents related to the relevant operators of the corresponding conformal theory describing the perturbation from the critical point. Such scaling behavior allows to identify explicitly the Schmidt gap as a local order parameter.

Aug.10

Ryo Okamoto (Hokkaido University and Osaka University) 
Demonstration of Adaptive Quantum Estimation with Photons'

Quantum theory is inherently statistical. This entails repetition of experiments over a number of identically prepared quantum objects, if one wants to know the "true state" or the "true value" of the parameter that specifies the quantum state. In applications, one needs to design the estimation procedure in such a way that the estimated value of the parameter should be close to the true value (consistency), and that the uncertainty of the estimated value should be as small as possible (efficiency). To realize these requirements, an adaptive quantum estimation (AQE) was proposed, and recently was proved to have the strong consistency and asymptotic efficiency.
In the presentation, we will report the first experimental demonstration of AQE. The angle of a half wave plate that initializes the linear polarization of input photons is estimated using AQE. The statistical analysis of these results verifies the strong consistency and asymptotic efficiency of AQE. It is expected that AQE will provide a useful methodology in the broad area of quantum information processing, communication, and metrology.

July 20
Stewart Library

Pavel Ginzburg (King’s College London)
Quantum aspects of light-matter interaction affected by plasmonic nano-structures (abstract )

Manipulation of optical near fields in vicinity of quantum emitters can significantly improve various tasks, relying on efficiency, polarization and directionality of extracted light. So called optical antennas, employing the phenomenon of localized plasmon resonances [1], were shown to provide some of the desired functionalities [2] and serve as very promising components for quantum information devises [3], where operation on few photons level is required.
Plasmonic nanostructures are also perfect candidates for the realization of various concepts for the improvement of nonlinear effects, since, generally, nonlinear optical phenomena are proportional to higher powers of the driving field, motivating the quest for the local electromagnetic field enhancement. For example, novel and very promising phenomena of spontaneous two-photon emission from semiconductors [4] was enhanced by three orders of magnitude, using array of plasmonic nano antennas[5].
In this contribution we will discuss resent progress in light emission devices, enhanced or rely on subwavelength plasmonic resonators. The general concept of such configurations is depicted on Fig. 1. The emphasis will be on rigorous quantum description of various linear [6] and nonlinear [7] processes on the nano-scale, involving the presence of active/absorbing and dispersive material components [8].
References:
1. S. A. Maier, Plasmonics: Fundamentals and Applications, Springer Science + Business Media LLC: New York, 2007.
2. A. G. Curto, G. Volpe, T. H. Taminiau, M. P. Kreuzer, R. Quidant, and N. F. van Hulst, Science 329, 930 (2010).
3. A. V. Akimov, A. Mukherjee, C. L. Yu, D. E. Chang, A. S. Zibrov, P. R. Hemmer, H. Park, and M. D. Lukin, Nature 450, 402 (2007).
4. A. Hayat, P. Ginzburg, and M. Orenstein, Nature Photonics 2, 238 (2008). 5. A. Nevet, N. Berkovitch, A. Hayat, P. Ginzburg, S. Ginzach, O. Sorias, and M. Orenstein, Nano Lett. 10, 1848 (2010).
6. P. Ginzburg and A. V. Zayats, Opt. Express 20, 6720-6727 (2012). 7. A. N. Poddubny, P. Ginzburg, P. A. Belov, A. V. Zayats, and Y. S. Kivshar, submitted to Phys. Rev. Lett. arXiv:1206.1036v1
8. N. A. R. Bhat, and J. E. Sipe, Phys. Rev. A 73, 063808 (2006).