QuTe Workshop – March 31st 2015

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Invited Speakers:

A list of the invited speakers, their institutional affiliations and their talk abstracts may be seen below. Where possible, a title and abstract of the speakers talk have been provided.

  • Christopher Gerry (Lehman College, New York): Professor Gerry’s research is in theoretical physics and is within the broad areas of quantum optics, quantum information processing, and foundations of quantum theory. More specifically, I am working on properties and methods of generating nonclassical states of light, high-resolution interferometry with nonclassical states of light and Bose-Einstein condensates, cavity quantum electrodynamics, quantum information processing with coherent states and their superpositions, tests of quantum mechanics against local realistic theories on a macroscopic scale.


Title: Photon-Number Parity-Based Quantum Optical Interferometry

Abstract: I will review our work, and that of others, on the prospect of using the photon-number parity operator as the proper observable for quantum optical interfereometry for attaining the Heisenberg limit of sensitivity of phase-shift measurements as required in experiments to detect gravitational waves such as LIGO and VIRGO projects. Photon-number parity is a strictly quantum observable with no classical analog. Used in interferometry, where the photon-number parity of one of the output beams is monitored, and with certain types of nonclassical states of light passing through the interferometer, the noise level of the measurement is reduced below the standard quantum limit and approaches the Heisenberg limit. Special nonclassical states of light are required to attain Heisenberg-limited sensitivities even with this measure. This include the so-called N00N states and their superpositions, twin-Fock states and their superpositions, and states obtained by mixing coherent and single-mode squeezed vacuum states of light, the light proposed sources proposed by Caves in 1981. The connection between photon-number parity measurements and the Cramer-Rao bound obtained from quantum the quantum Fisher information will be discussed.  Furthermore, the use of parity measurements increases the resolution of the measurements even if only classical light passes through the interferometer. A recent experiment on photon-number parity based interferometry will be discussed. Other possible uses for photon-number parity measurements in the context of quantum technology will be discussed as well.


  • Pieter Kok (University of Sheffield): Dr Kok’s research focusses mainly on quantum information processing with optical systems. This encompasses not only quantum computing and quantum communication, but also information extraction via metrology, and quantum lithography. Pieter also continues to work on relativistic quantum information theory.


Title: Quantum Imaging and Metrology

Abstract: Quantum technologies can be used to improve the performance of a variety of applications, from communication and computing to imaging and metrology. In this talk, I will review the concept of the Fisher information, which underlies classical and quantum metrology, and show how it is extended to new regimes in both metrology and imaging. I will illustrate these results with new applications in quantum imaging.


  • Roger Colbeck (The University of York): Dr Colbeck’s research spans many aspects of quantum information theory with a particular focus on quantum cryptography, as well as the foundations of quantum mechanics.


Title: Device-independence: what it can provide and open challenges.

Abstract: I will motivate device-independence, explaining how it could eliminate certain vulnerabilities of future cryptographic technologies. I will then summarize what we can do device-independently before looking at open challenges, both from a practical and theoretical viewpoint.


  • Vincent Boyer (The University of Birmingham): Dr Boyer is a lecturer in the group of Cold Atoms and is part of the Midlands Ultracold Atom Research Centre. His research bridges between the fields of Ultracold Atom physics and Quantum Optics in order to develop new quantum technologies impacting the way we process spatial (visual) information. He also participates in collaborative projects aiming to produce the next generation of quantum sensors based on atom interferometry, for instance to detect the gravitational field created by objects under the ground.


Title: Smoothing out the quantum roughness of light for improved imaging.

Abstract: When using classical light, optical measurements and light-based sensing methods are limited by the unavoidable quantum fluctuations in phase and amplitude, also known as shot noise. We investigate methods of manipulating the quantum noise locally across the transverse profile of a beam of light, hereby reducing the “quantum roughness” which is present in the output of even the quietest lasers. I will review our latest experimental results as well as the prospects of using this quantum light illumination to improve the optical resolution of certain super-resolution schemes beyond the quantum noise limit.


  • Zlatko Papic (Perimeter Institute, Canada): Dr Papic is a theorist in condensed matter physics and works on topological phases, graphene, fractional quantum Hall effect, quantum information (covering entanglement and matrix-product states/tensor networks in condensed matter systems and nonequilibrium quantum dynamics of interacting disordered systems).


Title: Topological phases in graphene.

Abstract: Over the past years, a remarkable variety of novel correlated states was discovered in graphene and its bilayer in a magnetic field. These states exhibit previously unseen richness due to an interplay of electron spin, valley and orbital degrees of freedom. They are furthermore distinguished by their high degree of tunability, e.g. via the in-plane magnetic or the perpendicular electric field, which allows one to probe their properties in a more flexible and direct way than in GaAs semiconductor systems. In this talk I will present a theoretical overview of the strongly correlated states that exhibit topological order and arise in graphene and its bilayer, in particular focusing on those that give rise to quasiparticles with non-Abelian statistics (like Majorana fermions). I will also discuss a recent experiment demonstrating that electric field, applied perpendicular to the basal plane of bilayer graphene, is a useful knob to tune the quantum phase transitions between different topological states. These results illustrate the potential of bilayer graphene as a model platform to study complex emergent phases of matter and transitions between them via symmetry breaking.


  • Naomi Nickerson (Oxford-Imperial): Building a working quantum computer is a huge experimental challenge, and one of the major problems faced is that of scalability. Small numbers of qubits can be well controlled but increasing this to bigger numbers becomes very difficult. One way of reducing these problems is by using a distributed system. Here the quantum computer is composed of a number of nodes each containing only a few qubits. Each node is a like a small quantum computer which can do good operations. But between nodes only very noisy operations are possible. Naomi’s research looks at how a system like this can be used to carry out quantum computation, and how it’s possible to protect this computation from errors.


Title: Scalable network quantum technologies with lossy and noisy photonic links.

Abstract: Exquisite quantum control has now been achieved in small ion traps, in nitrogen-vacancy centers and in superconducting qubit clusters. We can regard such a system as a universal cell with diverse technological uses from communication to large-scale computing, provided that the cell is able to network with others and overcome any noise in the interlinks. Here we consider a network architecture for fault tolerant quantum computation, where small cells are connected through probabilistic entanglement generation using optical links, and show that with loss-tolerant entanglement purification this is feasible with the noisy and lossy links that are realistic today. With a modestly complex cell design, and using a surface code protocol with a network noise threshold of 13.3%, we find that interlinks that attempt entanglement at a rate of 2 MHz but suffer 98% photon loss can result in kilohertz computer clock speeds (i.e., rate of high-fidelity stabilizer measurements). Improved links would dramatically increase the clock speed.


  • Henning Schomerus (Lancaster University): Professor Schomerus researches quantum phenomena occurring for electrons and photons in small quantum systems. This has lead him to the study of graphene, topological insulators and superconductors, quantum and atom optical systems, microlasers and photonic structure.


Title: Amplification and absorption in topological photonic systems.

Abstract: Topological photonic systems generate robust modes whose properties are well controlled. A difference to the original electronic context, from which these concepts are borrowed, are photon creation and annihilation processes, which induce a new class of exploitable symmetries but also serve as an extra source of noise. I illustrate these concepts for two examples: PT-symmetric Lasers and the selective amplification of a topologically induced defect mode.


  • Ahsan Nazir (The University of Manchester): Dr Nazir is a theoretician working on open quantum systems and currently leads the Manchester Noisy Quantum Systems Group (MaNQS).


Title: Probing light-matter entangled states through environmental transitions.

Abstract: In this talk I shall show that emitter-cavity dressed states may be probed through interactions with a thermal environment in solid-state cavity QED systems. This is true even in regimes that can be described semiclassically in the absence of such an environment, for example when the emitter-cavity coupling strength is dominated by cavity losses. For this experimentally relevant case, I shall outline how bath-induced dressed state transitions lead to asymmetries in the cavity emission properties, which are absent otherwise. This behaviour is attributed to the quantum nature of the environment and its resulting sensitivity to the joint eigenstructure of the cavity and emitter. This heralds a failure of the semiclassical approach, and challenges the notion that coupling to a thermal bath supports a more classical description of the system. Bath-induced asymmetries also persist over wider regions of parameter space, including the Fano and quantum strong coupling regimes.


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