Search

In his 1935 Gedankenexperiment, Erwin Schrödinger imagined a box with a cat and a poisonous substance which has a 50% probability of being released, based on the decay of a radioactive atom. As such, the life of the cat and the state of the poison become entangled, and the fate of the cat is determined upon opening the box. We present an experimental technique that keeps the cat alive on any account. This method relies on the time-resolved Hong–Ou–Mandel effect: two long, identical photons impinging on a beam splitter always bunch in either of the outputs. Interpreting the first photon detection as the state of the poison, the second photon is identified as the state of the cat. Even after the collapse of the first photon’s state, we show their fates are intertwined through quantum interference. We demonstrate this by a sudden phase change between the inputs, administered conditionally on the outcome of the first detection, which steers the second photon to a pre-defined output and ensures that the cat is always observed alive.

We demonstrate quantum logic using narrow linewidth photons that are produced with an a-priory non-probabilistic scheme from a single 87Rb atom strongly coupled to a high- nesse cavity. We use a controlled-NOT gate integrated into a photonic chip to entangle these photons, and we observe non-classical correlations between photon detection events separated by periods exceeding the travel time across the chip by three orders of magnitude. This enables quantum technology that will use the properties of both narrowband single photon sources and integrated quantum photonics.

Summary form only given. One of today's challenges to realize computing based on quantum mechanics is to reliably and scalably encode information in quantum systems. This is particularly crucial when dealing with promising approaches such as linear optics quantum computing (LOQC) in photonic circuits [1,2]. Although this method is in principle scalable, in practice it is limited by the probabilistic nature of spontaneous parametric down-conversion (SPDC) sources used to seed the circuits.Here, we demonstrate a novel tool to deterministically deliver quantum bits of information for LOQC circuits with the help of single photons emitted from a strongly coupled atom-cavity system (Fig. [1a]). Due to the ≈ 500ns coherence time of these photons [3] and our ability to arbitrarily shape their amplitude and phase profile we time-bin encode the information within one photon. Therefore, the preparation of a single photon sub-divided in d time bins allows for the delivery of arbitrary qu-d-its. RCPWe verify the fidelity of the quantum state preparation with time-resolved quantum-homodyne measurements [4], performed by sending single signal and reference photons into an elementary photonic circuit. Photon correlations monitored in a time-resolved manner then allow the state to be partially reconstructed. To obtain more insight into the time-bin encoding of information within one photon, it is helpful to define a `virtual' photonic circuit (Fig. [1b]) where temporal modes are displayed as spatial ones. Due to the photons being much longer than the detector time resolution, the absolute time of each photo-detection is used to associate the event with one of these virtual detectors. In this way, the respective number of cross-correlation signals for qubits and qutrits, respectively, can be found (Fig. [1c]). We obtain photon production efficiencies up to 85% and state-preparation fidelities of F=96% [4]. Due to the timebin encoding taking place upon the photon's generation process, our technique is inherently non-probabilistic, versatile, reconfigurable and not subject to systematic photon losses. Time-bins as an additional degree of freedom in LOQC experiments and an their deterministic photon-generation scheme can be regarded as a big step towards large-scale quantum computing in photonic networks.

We show how to capture a single photon of arbitrary temporal shape with one atom coupled to an optical cavity. Our model applies to Raman transitions in three-level atoms with one branch of the transition controlled by a (classical) laser pulse, and the other coupled to the cavity. Photons impinging on the cavity normally exhibit partial reflection, transmission, and/or absorption by the atom. Only a control pulse of suitable temporal shape ensures impedance matching throughout the pulse, which is necessary for complete state mapping from photon to atom. For most possible photon shapes, we derive an unambiguous analytic expression for the shape of this control pulse, and we discuss how this relates to a quantum memory., 6 pages, 8 figures

Cavity quantum electrodynamics (cavity QED) with single atoms and single photons provides a promising route toward scalable quantum information processing (QIP) and computing. A strongly coupled atom-cavity system should act as a universal quantum interface, allowing the generation and storage of quantum information. This thesis describes the realisation of an atom-cavity system used for the production and manipulation of single photons. These photons are shown to exhibit strong sub-Poissonian statistics and indistinguishability, both prerequisites for their use in realistic quantum systems. Further, the ability to control the temporal shape and internal phase of the photons, as they are generated in the cavity, is demonstrated. This high degree of control presents a novel mechanism enabling the creation of arbitrary photonic quantum bits.

We investigate the feasibility of implementing an elementary building block for quantum information processing. The combination of a deterministic single photon source based on vacuum stimulated adiabatic rapid passage [1,2], and a quantum memory based on electromagnetically induced transparency in atomic vapour is outlined [3]. Both systems are able to produce and process temporally shaped wavepackets which provides a way to maintain the indistinguish ability of retrieved and original photons. We also propose an efficient and robust repeat-until-success quantum computation scheme based on this hybrid architecture. © 2011 IEEE.

We investigate the feasibility of implementing an elementary building block for quantum information processing. The combination of a deterministic single photon source based on vacuum stimulated adiabatic rapid passage, and a quantum memory based on electromagnetically induced transparency in atomic vapour is outlined. Both systems are able to produce and process temporally shaped wavepackets which provides a way to maintain the indistinguishability of retrieved and original photons. We also propose an efficient and robust `repeat-until-success' quantum computation scheme based on this hybrid architecture., Comment: 10 pages, 7 figures

Considerable effort is being made to realize all the elements required for a distributed quantum network [1]. Ideally such a network would be composed of individual nodes acting as quantum gates/memories, connected together by optical links. Recent work in cavity-QED has shown that it is possible to deterministically create single photons. The narrow bandwidth (MHz) and coherence properties of these photons can be controlled to create atom-photon, as well as photon-photon entanglement [2]. The goals of our work are to build on photon sources for implementing the quantum memories and state mapping schemes required in a quantum network.