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Quantum Electronics and Laser Science Conference

Quantum area gate: experimental demonstration

Christoph M. Greiner, B. Boggs, T. Wang, THOMAS W. MOSSBERG

Published Nov 13, 2002• 0 citations
Publisher

Abstract

Summary form only given. In quantum computing, the precise control of input variables used to initialize computational systems is vital for reliable system performance. A parameter that lends itself naturally to the preparation of coherent atomic dynamics used in computation is optical pulse area, i.e. time-integrated resonant Rabi frequency. It is therefore desirable to identify phenomena and demonstrate devices that provide the functionality that allows the controlled manipulation of this parameter. Here, we report the first observation of a novel phenomenon governing the pulse area transmission through an optical cavity containing atoms exhibiting ultraslow atomic decoherence.

DOI
10.1109/qels.2001.961904

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Summary form only given. In quantum computing, the precise control of input variables used to initialize computational systems is vital for reliable system performance. A parameter that lends itself naturally to the preparation of coherent atomic dynamics used in computation is optical pulse area, i.e. time-integrated resonant Rabi frequency. It is therefore desirable to identify phenomena and demonstrate devices that provide the functionality that allows the controlled manipulation of this parameter. Here, we report the first observation of a novel phenomenon governing the pulse area transmission through an optical cavity containing atoms exhibiting ultraslow atomic decoherence.
Summary form only given. An underlying premise of quantum computing is the preservation of system coherence over long time scales. Here, we report studies of atom-cavity dynamics wherein behavior observed is distinctively related to the ultraslow atomic decoherence found in one such system. In our experiment, the atomic decoherence time is four orders of magnitude longer than the empty-cavity lifetime, making the atomic coherence the longest-lived system parameter. The phenomenon we observe here may be loosely thought of as cavity-mediated self-driven optical nutation, i.e., optical nutation driven by the active atoms own coherent emission into the cavity mode.
When a coherently prepared atomic medium is confined in an optical cavity, the atomic coherence and interference manifested by electromagnetically induced transparency (EIT) can be manipulated and enhanced by the collective atom–cavity coupling and the cavity feedback. We present an experimental study of the light transmission through an optical cavity coupled with coherently prepared cold Rb atoms. In the frequency domain, the cavity transmission spectrum exhibits spectral peaks representing the cavity normal modes associated with the multi-atom vacuum Rabi splitting and the intra-cavity dark state manifested by the combined EIT effects of the absorption suppression and the steep normal dispersion. In the time domain, a light pulse coupled into the cavity mode propagates with a slow group velocity and the propagation time delay is significantly increased by the cavity feedback.
Summary form only given.Decoherence, i.e., the rapid destruction of the phase relation between two quantum states of a system caused by the entanglement of these two states with two different states of the environment, is the main limiting factor for quantum information processing. For this reason, decoherence control is now becoming a rapidly expanding field of investigation. Decoherence control would be particularly useful in high-Q electromagnetic cavities. In fact, one of the few experimental realizations of a quantum gate is the Caltech's quantum phase gate, demonstrating conditional quantum dynamics between polarized modes in a high-finesse optical cavity. Moreover, the progressive decoherence of a mesoscopic Schrodinger cat state has been observed for the first time in the experiment of Brune et al. (1996) where the linear superposition of two coherent states of a field in a microwave cavity, with classically distinct phases, has been generated and detected. Here we propose a photodetection-mediated feedback scheme based on a very simple idea: whenever a photon leaks out from the cavity and is detected by a photodetector, a feedback loop supplies the cavity mode with another photon, using a single ion trapped in the cavity and appropriately shaped laser pulses.
Decoherence-free subspaces allow for the preparation of coherent and entangled qubits for quantum computing. Decoherence can be dramatically reduced, yet dissipation is an integral part of the scheme in generating stable qubits and manipulating them via one- and two-bit gate operations. How this works can be understood by comparing the system with a three-level atom exhibiting a macroscopic dark period. In addition, a dynamical explanation is given for a scheme based on atoms inside an optical cavity in the strong-coupling regime and we show how spontaneous emission by the atoms can be highly suppressed.
Semiconductor nanostructures such as quantum dots (QDs) have offered unique opportunities to investigate quantum optical effects in solid-state systems. These include quantum interference, Rabi oscillations, as well as photon antibunching, and were previously observable only in isolated atoms or ions. In addition, QDs can be integrated into optical microcavities, making them attractive for applications in quantum information processing and high efficiency quantum light sources. Despite much progress towards these goals, one area that was little explored is coherent control of such solid-state quantum emitters in cavities. The main technical hurdle lies in overcoming the laser background scattering. By using a sample structure in which QDs are embedded in a planar Fabry-Perot cavity and by using an orthogonal excitation geometry, we have achieved a nearly complete elimination of laser background scattering. This in turn allows us to show resonantly controlled light emission of quantum dots in the cavity including (a) Rabi flopping using pulse control, (b) direct observation of Mollow triplets in the frequency domain, and (c) simultaneously measured first-order and second order photon-photon correlations.
We demonstrate the coherent control of spontaneous emission for a three-level atom located within a photonic band-gap structure with one resonant frequency near the edge of the photonic band gap. Spontaneous emission from the three-level atom can be totally suppressed or strongly enhanced depending on the relative phase between the steady-state control laser coupling the two upper levels and the pump laser pulse used to create an excited state of the atom in the form of a coherent superposition of the two upper levels. Unlike the free-space case, the steady-state inversion of the atomic system is strongly dependent on the externally prescribed initial conditions. This nonzero steady-state population is achieved by virtue of the localization of light in the vicinity of the emitting atom. It is robust to decoherence effects provided that the Rabi frequency of the control laser field-atom interaction exceeds the rate of dephasing interactions. As a result, such a system may be relevant for a single-atom, phase-sensitive, optical memory device on the atomic scale. The protected electric dipole within the photonic band gap provides a basis for a qubit to encode information for quantum computations.