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arXiv:1112.4855 · Physical Review Letters

Tomography of a High-Purity Narrowband Photon from a Transient Atomic Collective Excitation

Andrew MacRae, Travis Brannan, R. Achal, Alexander I. Lvovsky

Published Jul 17, 2012• 105 citations
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Abstract

We demonstrate efficient heralded generation of high purity narrow-bandwidth single photons from a transient collective spin excitation in a hot atomic vapor cell. Employing optical homodyne tomography, we fully reconstruct the density matrix of the generated photon and observe a Wigner function reaching the zero value without correcting for any inefficiencies. The narrow bandwidth of the photon produced is accompanied by a high generation rate yielding a high spectral brightness. The source is, therefore, compatible with atomic-based quantum memories as well as other applications in light-atom interfacing. This Letter paves the way to preparing and measuring arbitrary superposition states of collective atomic excitations.

DOI
10.1103/physrevlett.109.033601

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We demonstrate efficient heralded generation of high purity narrow-bandwidth single photons from a transient collective spin excitation in a hot atomic vapor cell. Employing optical homodyne tomography, we fully reconstruct the density matrix of the generated photon and observe a Wigner function reaching the zero value without correcting for any inefficiencies. The narrow bandwidth of the photon produced is accompanied by a high generation rate yielding a high spectral brightness. The source is, therefore, compatible with atomic-based quantum memories as well as other applications in light-atom interfacing. This Letter paves the way to preparing and measuring arbitrary superposition states of collective atomic excitations.
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We experimentally demonstrate that a nonclassical state prepared in an atomic memory can be efficiently transferred to a single mode of free-propagating light. By retrieving on demand a single excitation from a cold atomic gas, we realize an efficient source of single photons prepared in a pure, fully controlled quantum state. We characterize this source using two detection methods, one based on photon-counting analysis and the second using homodyne tomography to reconstruct the density matrix and Wigner function of the state. The latter technique allows us to completely determine the mode of the retrieved photon in its fine phase and amplitude details and demonstrate its nonclassical field statistics by observing a negative Wigner function. We measure a photon retrieval efficiency up to 82% and an atomic memory coherence time of 900 ns. This setup is very well suited to study interactions between atomic excitations and use them in order to create and manipulate more sophisticated quantum states of light with a high degree of experimental control.
This thesis presents the experimental demonstration of an atomic source of narrowband nonclassical states of light. Employing four-wave mixing in hot atomic Rubidium vapour, the optical states produced are naturally compatible with atomic transitions and may be thus employed in atom-based quantum communication protocols. We first demonstrate the production of two-mode intensity-squeezed light and analyze the correlations between the two produced modes. Using homodyne detection in each mode, we verify the production of two-mode quadrature-squeezed light, achieving a reduction in quadrature variance of 3 dB below the standard quantum limit. Employing conditional detection on one of the channels, we then demonstrate the generation of single-photon Fock states as well as controllable superpositions of vacuum and 1-photon states. We fully characterize the produced light by means of optical homodyne tomography and maximum likelihood estimation. The narrowband nature of the produced light yields a resolvable temporal wave-function, and we develop a method to infer this wave function from the continuous photocurrent provided by the homodyne detector. The nature of the atomic process opens the door to a new direction of research: generation of arbitrary superpositions of collective atomic states. We perform the first proof-of-principle experiment towards this new field and discuss a proposal for extending this work to obtain full control over the collective-atomic Hilbert space.
A single-photon Fock state has been generated by means of conditional preparation from a two-photon state emitted in the process of spontaneous parametric down-conversion. A recently developed high-frequency homodyne tomography technique has been used to completely characterize the Fock state by means of a pulse-to-pulse analysis of the detectors' difference photocurrent. The density matrix elements of the generated state have been retrieved with a final detection efficiency of about 57%. A comparison has been performed between the phase-averaged tomographic reconstructions of the Wigner function as obtained from the measured density-matrix elements and from a direct Abel transform of the homodyne data. The ability of our system to work at the full repetition rate of the pulsed laser (82MHz)(82\phantom{\rule{0.3em}{0ex}}\mathrm{MHz})(82MHz) substantially simplifies the detection scheme, allowing for more ``exotic'' quantum states to be generated and analyzed.