Session B4: Focus Session: New Trends in Quantum Optics

Spinning Photons and Twisting Oscillators

Optomechanics is the study of the interaction between optical radiation and mechanical motion. Typically, an optomechanical system is composed of an optical resonator coupled to a mechanical degree of freedom. Some of the most striking experimental achievements include the quantum ground state preparation for a macroscopic oscillator, the detection of optomechanical quantum back-action, and generation of optomechanically induced transparency and slow light. Most optomechanical systems depend on linear coupling between the optical field and the displacement of the mechanical oscillator. In this talk, I will start instead by discussing the basic quantum mechanics of a generic quadratically coupled optomechanical system, followed by our efforts towards extending optomechanics to torsional and rotational systems. Specifically, I will describe our theoretical proposal to couple a windmill-shaped dielectric to cavity Laguerre Gaussian modes. In addition, I will present an optoacoustic system, composed of a LG mode coupled t surface acoustic waves of a spherical mirror, as a new platform for storage of photons carrying orbital angular momentum. Finally, I will discuss our most recent investigation of the prospects of cooling full rotational motion to the quantum regime.   

Hong-Ou-Mandel Interference with Microwave Photons: Doing Quantum Optics Experiments using Superconducting Circuits

Using modern micro and nano-fabrication techniques combined with superconducting materials we realize quantum electronic circuits in which we create, store, and manipulate individual microwave photons. The strong interaction of photons with superconducting quantum two-level systems allows us to probe fundamental quantum effects of microwave radiation and also to develop components for applications in quantum technology. Previously we have realized on-demand single photon sources which we have characterized using correlation function measurements [1] and full quantum state tomography [2]. For this purpose we have developed efficient methods to separate the quantum signals of interest from the noise added by the linear amplifiers used for quadrature amplitude detection [3]. We now regularly employ superconducting parametric amplifiers [4] to perform nearly quantum limited detection of propagating electromagnetic fields. These enable us to probe the entanglement which we generate on demand between stationary qubits and microwave photons freely propagating down a transmission line [5]. Using two independent microwave single photon sources, we have recently performed Hong-Ou-Mandel experiments at microwave frequencies [6] and have probed the coherence of two-mode multi-photon states at the out-put of a beam-splitter. The non-local nature of such states may prove to be useful for distributing entanglement in future small-scale quantum networks.\\[4pt] [1] D. Bozyigit et al., Nat. Phys. 7, 154 (2011)\newline [2] C. Eichler et al., Phys. Rev. Lett. 106, 220503 (2011)\newline [3] C. Eichler et al., Phys. Rev. A 86, 032106 (2012)\newline [4] C. Eichler et al., Phys. Rev. Lett. 107, 113601 (2011)\newline [5] C. Eichler et al., Phys. Rev. Lett. 109, 240501 (2012)\newline [6] C. Lang et al., Nat. Phys. 9, 345-348 (2013)   

Quantum Teleportation of Dynamics and Effective Interactions Between Remote Systems

Most protocols in Quantum Information Science are discrete in the sense that they consist of a series of unitary operations and measurements which are applied sequentially. However, some implementations are intrinsically continuous. The most prominent example are atomic ensembles interacting with light, where schemes based on the continuous detection of quadrature operators are realized. In this system, protocols can be performed that are intrinsically deterministic and continuous in time. Here we address the question how this property can be exploited by designing primitives that take advantage of this fact. We introduce two protocols which achieve a qualitatively new goal - to control and transmit quantum evolutions between remote locations. We present two related protocols, where we consider two remote systems which evolve according to a given local dynamics. One scheme implements an effective non-local dynamics, where the two remote systems evolve as if they were interacting with each other. The other protocol realizes the quantum teleportation of a time evolution, which uses the dynamics of one system to steer the evolution of the other.   

Experimental entanglement of 60 modes of the quantum optical frequency comb and application to generating hypercubic-lattice cluster states

In the race to build a practical quantum computer in the laboratory, the ability to create very large quantum registers and entangle them is paramount, along with the ability to address the issue of decoherence. With particular regard to scalability, the field-based, continuous-variable (CV) flavor of quantum optics offers notable promise, in particular by enabling ``top down,'' rather than ``bottom up,'' entangling approaches of quantum field modes. It is also important to note the relevance of continuous variables to universal quantum computing, with the recent discovery of a fault tolerance threshold for quantum computing with CV cluster states and nonGaussian error correction. In 2011, some of us generated simultaneously 15 independent 4-mode cluster states over 60 modes of the quantum optical frequency comb (QOFC) of a single optical parametric oscillator (OPO). In this work, we used a single OPO to generate a 60-mode dual-rail cluster state, which is the largest entangled system to date whose subsystems are all simultaneously available. Using the exact same setup, we also generated two copies of a 30-mode dual-rail cluster state. We will then present a new proposal to ``weave'' such massively scalable continuous-variable cluster states into hypercubic-lattice quantum graphs   

Rydberg atoms inside hollow-core photonic crystal fibres

Rydberg atoms have peculiar properties as enhanced sensitivities to AC/DC electric fields or exaggerated strong interactions between them, leading to optical non-linearities on the single photon level. These properties are mostly studied with spectroscopic methods often limited by the free space diffraction limit. This can be avoided by confining Rydberg atoms inside hollow core fibres offering a perfect match of guided light modes with the atomic gas in terms of atom-light coupling. Additionally we choose Kagome type fibres due to their extremely thin structures, promising a reduced atom wall coupling. With coherent three photon spectroscopy we can show that Rydberg atoms can be excited within these fibres up to states of n$=$46 without severe perturbations by the fibre environment.   

Robust quantum receivers for coherent state discrimination

Quantum state discrimination is a central task for quantum information and is a fundamental problem in quantum mechanics. Nonorthogonal states, such as coherent states which have intrinsic quantum noise, cannot be discriminated with total certainty because of their intrinsic overlap. This nonorthogonality is at the heart of quantum key distribution for ensuring absolute secure communications between a transmitter and a receiver, and can enable many quantum information protocols based on coherent states. At the same time, while coherent states are used for communications because of their robustness to loss and simplicity of generation and detection, their nonorthogonality inherently produces errors in the process of decoding the information. The minimum error probability in the discrimination of nonorthogonal coherent states measured by an ideal lossless and noiseless conventional receiver is given by the standard quantum limit (SQL). This limit sets strict bounds on the ultimate performance of coherent communications and many coherent-state-based quantum information protocols. However, measurement strategies based on the quantum properties of these states can allow for better measurements that surpass the SQL and approach the ultimate measurement limits allowed by quantum mechanics. These measurement strategies can allow for optimally extracting information encoded in these states for coherent and quantum communications. We present the demonstration of a receiver based on adaptive measurements and single-photon counting that unconditionally discriminates multiple nonorthogonal coherent states below the SQL. We also discuss the potential of photon-number-resolving detection to provide robustness and high sensitivity under realistic conditions for an adaptive coherent receiver with detectors with finite photon-number resolution.