SCIENTIFIC PROGRAMS AND ACTIVITIES

July 11
Stewart Library

Matthew Broome, University of New South Wales
My Quantum Optics Show and Tell: Topology, complexity and biology

Progress in optical quantum computation has started to slow in recent times due to the problems associated with probabilistic quantum gates, lack of good single photon sources and poor non-linear optical materials. However, by looking at other applications besides a fully scalable quantum computer, we see that linear optics alone (beam splitters and phase shifters) is a powerful tool for simulation or emulation of interesting physical systems. In this talk I will discuss some recent results from the University of Queensland's Quantum Technology Lab that employ purely linear optical schemes for this purpose. In particular, I will focus the talk around single- and multi-particle quantum walks for investigating areas from condensed matter science to complexity theory.

Sara Hosseini, Australian National University
Experimental verification of quantum discord in continuous variable states and operational significance of discord consumption

We introduce a simple and efficient technique to verify quantum discord in unknown Gaussian states and certain class of non-Gaussian states. We show that any separation in the peaks of the marginal distributions of one subsystem conditioned on two different outcomes of homodyne measurements performed on the other subsystem indicates correlation between the corresponding quadratures and hence nonzero quantum discord. We also demonstrate that under certain measurement constraints, discord between bipartite systems can be consumed to encode information that can only be accessed by coherent quantum interaction.

Savannah Garmon, Osaka Prefecture University
Bound states, scattering states and resonant states in PT-symmetric open quantum systems

We study the point spectrum and transmission scattering spectrum in extended optical lattice models incorporating balanced elements of energy amplification and attenuation in a central scattering region. These serve as prototype models to illustrate more general concepts relevant to PT-symmetric physics in an open systems context. For a given system geometry, we study two boundary conditions: purely outgoing waves and scattering states. For the boundary condition consisting of purely outgoing waves we obtain the discrete spectrum associated with the scattering region. In this case we reveal that, unlike the Hermitian case, PT-symmetric open quantum systems permit eigenstates with complex-valued eigenvalues to appear in the first Riemann sheet in the complex energy plane. We also demonstrate the presence of and classify two different types of exceptional points appearing in the discrete eigenvalue spectrum. We also demonstrate the presence of what we term a resonance in continuum (RIC) for certain parameter values. Finally, we consider the scattering wave boundary conditions, under which we demonstrate that further imposing PT-symmetry on our scattering state results in a perfect transition through the scattering region.

David McKay (James Franck Institute, University of Chicago)
Cavity QED with superconducting qubits - a multi-pole approach

Superconducting Josephson-junction qubits are an emerging technology for quantum information processing. They offer a scalable, tunable, and coherent platform to study quantum systems. These qubits can be engineered with strong coupling to two or three-dimensional microwave cavities which implements the cavity QED paradigm - coherent coupling of a two-level system to a harmonic oscillator. Cavities enable high-fidelity qubit readout and a common "bus" for qubit-qubit interactions. In this talk, I will discuss our device at the University of Chicago which couples two superconducting transmon-type qubits using a planar multi-cavity (multi-pole) quantum filter. The multi-pole architecture allows for high contrast two-qubit gates; on-resonance the qubit-qubit interactions are strong, but off-resonance the interactions are exponentially suppressed in the number of filter poles. I will outline our adiabatic multi-pole (AMP) entangling gate protocol which we utilize to prepare a Bell state with >90% fidelity in 100ns. The multi-pole architecture is a promising approach towards scalable multi-qubit circuits, lattice based quantum simulation, and photonic registers.

Prof. Mile Gu (Tsinghua University)
Discord as a Consumable Resource

Correlations lie at the heart of our capacity to manipulate information. The fewer the constraints on the correlations we can exploit, the greater our capacity to manipulate information in ways we desire. The rapid development of quantum information science is a testament to this observation. Quantum systems may be so correlated that they are `entangled', such that each of its subsystems possesses no local reality. Exploitation of such uniquely quantum correlations has led to many remarkable protocols that would otherwise be either impossible or in feasible. However, the absence of entanglement does not eliminate all signatures of quantum behaviour. Coherent quantum interactions between separable systems that result in negligible entanglement could still lead to exponential speed-ups in computation. The potential presence of discord within such protocols motivated speculation that discord might prove a better quantifier of the `quantum resource' that coherent interactions exploit to deliver a `quantum advantage'.

In this presentation, I will give a brief tutorial of quantum discord. I then introduce and demonstrate an operational method to use discord as a physical resource. I show that under certain measurement constraints, discord between bipartite systems can be consumed to encode information that can only be accessed by coherent quantum interactions. The inability to access this information by any other means allows us to use discord to directly quantify this `quantum advantage'. I will outline recent experiments done at the Australian National University and the University of Queensland, where we experimentally encoded information within the discordant correlations separable states. The amount of extra information recovered by coherent interaction is quantified and directly linked with the discord consumed during encoding. I survey the potential applications of this phenomena, in both certification of entangling operations, and protecting the benefits of entanglement in entanglement breaking noise.

May 9, 2014
Room 210

Prof. Norbert Lütkenhaus (University of Waterloo)
Beating Classical Communication Resources by Quantum Communication

The area of quantum communication complexity searches for communication tasks that can be solved more efficiently using quantum states as carriers of information rather than their classical counterparts. Some of these protocols show an exponential savings in communication resources when operating in the quantum domain. Will it be possible to realize this abstract advantage in practical quantum optical implementations? The long-term goal would be to find protocols that convince a classical optical communication engineer that using the quantum domain is preferable. But there is no bonus point for saying 'quantum' ... we need to measure success in terms of resources as counted by the classical optical communication engineer.

In this talk we will present a specific quantum communication protocol that can be implemented using laser pulses and linear optics and which beats classical communication. The measure is usage of Hilbert space dimensions, which translates to a significant reduction in required optical power levels and leakage of information. While this alone might not convince a classical optical communication engineer (yet?), we will show how our findings change the current view of communication complexity as a purely theoretical field without practical impact.

Prof. Sven Höfling (University of St Andrews)
Integrated Quantum Photonics

Quantum information processing is an emerging field which promises secure communication or computational speed-ups for certain important computational problems if they are tackled with quantum computers. This has stimulated intense research on a variety of quantum bit (qubit) carriers and quantum technological platforms. Single photons are a prime qubit for the propagation and processing of quantum information, as they can be transmitted over long distances with low loss and manipulated by linear optical elements. However, the production, processing and detection of single photons is still mostly realized using bulky free-space or fiber-optic devices, posing severe challenges if more complex quantum circuits with high functionality going beyond a few photonic qubits are considered. Waveguide integrated quantum photonic circuits provide a route to overcome such limitations [1], where we target in this work the full integration of active and passive quantum devices on a single GaAs chip.

For the development of a quantum integrated photonics platform on GaAs, we develop a waveguide platform for the integration of single photon sources based on InGaAs quantum dots (QDs), superconducting single photon detectors, electro-optic tuners, directional couplers and splitters. Single-photon sources coupled to waveguides are realized by embedding QDs in photonic crystal cavities [2]. In order to tune individual QDs spectrally for indistinguishable photon emission, on-chip electrical control is established. Waveguide single-photon detectors are demonstrated by patterning superconducting NbN nanowires on top of ridge waveguides, resulting in high efficiency and low jitter [3]. They can also be arranged to implement on-chip photon auto-correlation measurements [4] or photon number resolved detection [5]. The presented building blocks are a key for integrated quantum photonics based quantum information processing and results towards this goal and key features of this platform will be presented.

In conclusion, we have demonstrated the key building blocks of a scalable quantum photonic integrated circuit. They are based on the same GaAs/AlGaAs material basis and therefore can be integrated on the same chip. This should open the way to solid-state quantum processing with several tens of qubits.

[1] A. Politi, J. C. F. Matthews, and J. L. O'Brien, Science 325, 1221 (2009).
[2] T. B. Hoang, J. Beetz, L. Midolo, M. Skacel, M. Lermer, M. Kamp, S. Höfling, L. Balet, N. Chauvin, and A. Fiore, Appl. Phys. Lett. 100, 061122 (2012).
[3] J. P. Sprengers, A. Gaggero, D. Sahin, S. Jahanmirinejad, G. Frucci, F. Mattioli, R. Leoni, J. Beetz, M. Lermer, M. Kamp, S. Höfling, R. Sanjines, and A. Fiore, Appl. Phys. Lett. 99, 181110 (2011). [4] D. Sahin, A. Gaggero, T. B. Hoang,1 G. Frucci, F. Mattioli, R. Leoni, J. Beetz, M. Lermer, M. Kamp, S. Höfling, and A. Fiore, Opt. Express 21, 11162 (2013).
[5] D. Sahin, A. Gaggero, Z. Zhou, S. Jahanmirinejad, F. Mattioli, R. Leoni, J. Beetz, M. Lermer, M. Kamp, S. Höfling, and A. Fiore, Appl. Phys. Lett. 103, 111116 (2013).

Optical channels, such as fibers or free-space links, are ubiquitous in today's telecommunication networks. A complete physical model of these channels must necessarily take quantum effects into account in order to determine their ultimate performances. Specifically, Gaussian bosonic quantum channels have been extensively studied over the past decades given their importance for practical purposes. In spite of this, a longstanding conjecture on the optimality of Gaussian encodings has yet prevented finding their communication capacity. In this talk we will present a recent result that solves this conjecture and establishes the ultimate achievable bit rate under an energy constraint. We will conclude discussing further implications of our result.

Prof. Marko Loncar, Harvard University
Quantum Nanophotonics and Nanomechanics with Diamond

Diamond possesses remarkable physical and chemical properties, and in many ways is the ultimate engineering material - “the engineer’s best friend!” For example, it has high mechanical hardness and large Young’s modulus, and is one of the best thermal conductors. Optically, diamond is transparent from the ultra-violet to infra-red, has a high refractive index (n = 2.4), strong optical nonlinearity and a wide variety of light-emitting defects. Finally, it is biocompatible and chemically inert, suitable for operation in harsh environment. These properties make diamond a highly desirable material for many applications, including high-frequency micro- and nano-electromechanical systems, nonlinear optics, magnetic and electric field sensing, biomedicine, and oil discovery. One particularly exciting application of diamond is in the field of
quantum information science and technology, which promises realization of powerful quantum computers capable of tackling problems that cannot be solved using classical approaches, as well as realization of secure communication channels. At the heart of these applications are diamond’s luminescent defects—color centers—and the nitrogen-vacancy (NV) color center in particular. This atomic system in the solid-state possesses all the essential elements for quantum technology, including storage, logic, and communication of quantum information.
I will review recent advances in nanotechnology that have enabled fabrication of nanoscale optical devices and chip-scale systems in diamond that can generate, manipulate, and store optical signals at the single-photon level. Examples include a room temperature source of single photons based on diamond nanowires 1 and plasmonic appertures 2, as well as single-photon generation and routing inside ring 3 and photonic crystal resonators fabricated directly in diamond 4. In addition to these quantum applications I will present our recent work on diamond based on-chip frequency combs, as well as diamond
nanomechanical resonators.

1. T.M. Babinec, B.M. Hausmann, M. Khan, Y. Zhang, J. Maze, P.R.
Hemmer, M. Lon?ar, "A bright single photon source based on a diamond
nanowire," Nature Nanotechnology, 5, 195 (2010)

2. J.T. Choy, B.M. Hausmann, T.M. Babinec, I. Bulu, and M. Lon?ar,
"Enhanced Single Photon Emission by Diamond-Plasmon Nanostructures.,"
Nature Photonics, 5, 738 (2011)

3. B.J.M. Hausmann, et al, "Integrated Diamond Networks for Quantum
Nanophotonics", Nano Letters, 12, 1578 (2012)

4. M.J. Burek, et al, “Free-standing mechanical and photonic
nanostructures in single-crystal diamond”, Nano Letters, 12, 6084 (2012)

April 14
Stewart Library

Dr. Ben Fortescue, (Southern Illinois University)
Tolerating Qubit Loss in Quantum Error Correction

Standard fault-tolerant quantum error correction (QEC) schemes can protect quantum information against arbitrary errors on individual qubits, but with the implicit assumption that the underlying physical systems remain within the qubit subspace. Fault-tolerance may therefore be lost under "leakage" errors (such as a photonic qubit being absorbed) which violate this assumption, and techniques for modifying QEC to cope with generic leakage can be very costly in terms of the extra operations required. I will discuss a more specific "loss" model of leakage, applicable to many implementations (photonic ones especially), and show that it allows for fault-tolerance to be analysed and achieved at much less cost in common Calderbank-Shor-Steane (CSS) codes.

April 4
Room 210

Angela Sestito, Università della Calabria, Italy
Understanding Quantum Mechanics: a formal analysis of non-locality theorems

Per se, quantum theory entails no violation of the locality principle; conflicts between them arise if further conditions, which do not
belong to the genuine set of quantum postulates, are required to hold. In this talk we explore consequences of adopting the criterion of reality (R) stated by Einstein, Podolsky and Rosen in 1935. In
particular, we will analyze two possible interpretations of (R) and implications that follow if we adopt them in connection with the nonlocality theorem of Hardy: the contradiction - arising if a "wide" interpretation is adopted- does not arise in connection with a "strict" interpretation of (R). We conclude that if the strict interpretation of (R) is adopted the theorem fails in proving the inconsistency between quantum mechanics and locality. Finally, we propose an ideal experiment enabling the simultaneous assignment of the objective values of two incompatible properties of a system made up of two separated non-interacting spin particles when the strict interpretation of (R) is adopted.

Color centers are atomic defects in diamond that possess electronic and nuclear spins. The rapid progress of experiments with color centers in diamond indicates that they are promising systems for quantum information processing, and more important for quantum sensing (imaging) under ambient conditions.
We have devised strategies to achieve highly sensitive measurement of weak signals, such as magnetic field, electric field, pressure, and temperature in the presence of ambient noise, while achieving nanometer spatial resolution. We are exploring various applications of these strategies ranging from quantum information processing, fundamental physics, to material science as well as biology including the structure of proteins and the possible role of quantum effects in biological functions.

Prof. Gerd Leuchs, Max Planck Institute for the Science of Light, Erlangen, Germany
Distributing entanglement via separable states

Whether or not entanglement is necessary pre-­requisite for quantum information protocols had been debated ever since the first experiments on NMR quantum computing, which were performed successfully with separable systems. There were several hints that entanglement is sufficient in such applications but that it is not absolutely necessary. The first was that the entanglement in a Werner state [1] vanishes discontinuously as the state is gradually tuned towards more mixedness. Yu and Eberly [2] found another example of a discontinuous disappearance of entanglement. In a seminal paper Ollivier and Zurek [3] introduced quantum discord as a measure for quantum correlations, which does not show such discontinuities. Quantum discord can be seen as a measure for the entanglement, which can be extracted from a separable system in a mixed state. Along this line, three experiments recently demonstrated distributing entanglement with separable states [4,5,6].

References:
1. R.F. Werner, Phys. Rev. A 40, 4277 (1989)
2. H. Ollivier, W.H. Zurek, Phys. Rev. Lett. 88, 017901 (2002) 3. T. Yu and J.H. Eberly, Phys. Rev. Lett. 93, 140404 (2004) 4. A. Fedrizzi et al., Phys. Rev. Lett. 111, 230504 (2013) 5. C.E. Vollmer et al., Phys. Rev. Lett. 111, 230505 (2013) 6. Ch. Peuntinger et al., Phys. Rev. Lett. 111, 230506 (2013)

Prof. Guifre Vidal, Perimeter Institute
Tensor Networks for Quantum Many-body Systems

Quantum many-body systems are hard to study because the associated Hilbert space, containing all possible many-body states, is huge: its dimension grows exponentially in the system size. In recent years, however, progress in our understanding of quantum entanglement has revealed that a large class of many-body states of interest are highly atypical and such that we can actually efficiently represent them with a mathematical structure called tensor network. As a result, it is now possible to accurately simulate, say, a quantum spin chain made of thousands of interacting spins. In this Colloquium I will (i) review the computational challenge posed by quantum many-body systems; (ii) summarize our current understanding of many-body entanglement; and (iii) give a gentle introduction to tensor networks as an efficient description of many-body states.

Prof. Arjendu Pattanayak, Carleton College
Surprises in the quantum-classical transition for computed Lyapunov exponents

Completely classical behavior is very different from completely quantum-mechanical behavior, particularly for nonlinear or chaotic systems, even though the transition between the two happens as a function of controllable parameters, such as the size of the system or environmental effects. I report on recent work exploring this multi-parameter transition. The first set of results is on the behavior of calculated quantum Lyapunov exponents Lambda for a Duffing oscillator system as a function of effective action beta as well as the system damping parameter Gamma. In general Lambdas decrease as beta increases (chaos decreases as the system becomes more quantal, as expected). However, we identify anomalous regions where Lambdas
increase with beta, including going from negative to positive with increasing beta; and also regions where the quantum results do not tend smoothly to the classical results. All anomalous results correspond to windows of regularity embedded in a larger chaotic parameter regime, which inverts the usual paradigm: The classically regular behavior is the most challenging for quantum-classical correspondence. I also report on progress on various other projects including (a) studying how classical control algorithms work in controlling quantum systems across this parameter regime and (b) on the control of the quantum chaos.

Nov. 22
Room 210

Dr. Pascale Senellart, CNRS
Cavity quantum electrodynamics with semiconductor quantum dots

A semiconductor quantum dot is a promising system to develop a solid-state quantum network. Like real atoms, quantum dots can emit single photons, polarization entangled photon pairs, indistinguishable photons... Moreover, the spin of a carrier trapped in a quantum dot can present long coherence times and be used as a stationary quantum bit that one can optically manipulate and measure. However, the scalability of a quantum dot based quantum network requires implementing a highly efficient single photon-single quantum dot interface so as to collect every photon emitted by a quantum dot and symmetrically, to ensure that every photon sent onto the device interacts with the quantum dot. Controlling the spontaneous emission of a quantum dot in a cavity is an efficient way to build such an interface. In this talk, we will present our recent results along this research line.
We have developed an in-situ lithography technique to deterministically insert a single quantum dot into a pillar optical microcavity. In the light-matter weak coupling regime, we obtain ultrabright sources of quantum light. We demonstrate sources of indistinguishable single photons with brightness as large as 79 % collected photon per pulse. With coupled pillar cavities, we also fabricate bright sources of polarization entangled photon pairs. The potential of these sources for quantum information processing is demonstrated by implementing an entangling controlled-NOT gate. In the light matter strong coupling regime, we demonstrate optical non-linearities for only 8 incident photons per pulse. Finally, we present a novel photonic structure and a technology allowing the electrical control of the devices, a critical step for the scalability of a quantum network based on semiconductor quantum dots.

Oct. 4
Room 210

Dr. Nicolas Menicucci (The University of Sydney)
A gigantic, 10,000-mode, continuous-variable cluster state

Cluster states are an entangled resource state that enable quantum computing using adaptive measurements alone. This is surprising when one considers what this means: one can quantum compute simply by *looking* at a quantum systems in a particular way! The continuous-variable incarnations of these states are simple to make using lasers and can be scaled up with ease. In this talk, I will describe the theoretical underpinnings of measurement-based quantum computation using continuous-variable systems, and I will report on their experimental realization, including the recent demonstration of a 10,000-mode (!) cluster state. This is the largest entangled state ever created to date in which every constituent quantum system is individually addressable. Issues related to error correction and fault tolerance -- many of which remain open problems -- will also be discussed.