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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Demonstrating hybrid quantum effects with two entangled laser beams

Boris Hage, Jiří Janoušek, Seiji Armstrong, Thomas Symul, J. Bernu, Helen M. Chrzanowski, Ping Koy Lam, Hans‐A. Bachor

Published May 1, 2011• 0 citations
Publisher

Abstract

We report on the preparation of entangled two mode squeezed states of yet unseen quality. Based on a measurement of the covariance matrix we found for the famous EPR-variance a value of 0.36. Furthermore, two mode quantum state tomography was used to extract single photon Fock states solely based on homodyne detection, demonstrating the strong quantum features of this entangled pair of laser beams. The probability for a single photon in this ensemble measurement exceeded 2/3.

DOI
10.1109/cleoe.2011.5943418

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We report on the preparation of entangled two mode squeezed states of yet unseen quality. Based on a measurement of the covariance matrix we found for the famous EPR-variance a value of 0.36. Furthermore, two mode quantum state tomography was used to extract single photon Fock states solely based on homodyne detection, demonstrating the strong quantum features of this entangled pair of laser beams. The probability for a single photon in this ensemble measurement exceeded 2/3.
We have reconstructed the quantum state of optical pulses containing single photons using the method of phase-randomized pulsed optical homodyne tomography. The single-photon Fock state 1> was prepared using conditional measurements on photon pairs born in the process of parametric down-conversion. A probability distribution of the phase-averaged electric field amplitudes with a strongly non-Gaussian shape is obtained with the total detection efficiency of (55+/-1)%. The angle-averaged Wigner function reconstructed from this distribution shows a strong dip reaching classically impossible negative values around the origin of the phase space.
A feature of quantum mechanical states is delocalised coherence. We directly observe this coherence up to the three photon level. Explicitly, we carry out two-mode weak-field homodyne of both a single photon split at a beam splitter and a two-mode squeezed state. While our measurements are insufficient to quantify or witness entanglement directly, with improved collection efficiency and measurement stability we aim to show that this can be achieved with such a scheme. Thus we have shown how such a detector can directly probe coherence in both the discrete and continuous variable pictures.
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We present a continuous-variable experimental analysis of a two-photon Fock state of free-propagating light. This state is obtained from a pulsed nondegenerate parametric amplifier, which produces two intensity-correlated twin beams. Counting two photons in one beam projects the other beam in the desired two-photon Fock state, which is analyzed by using a pulsed homodyne detection. The Wigner function of the measured state is clearly negative. We developed a detailed analytic model which allows a fast and efficient analysis of the experimental results.
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.