A quantum memory for orbital angular momentum photonic qubits

Journal name:
Nature Photonics
Volume:
8,
Pages:
234–238
Year published:
DOI:
doi:10.1038/nphoton.2013.355
Received
Accepted
Published online

Abstract

Among the optical degrees of freedom, the orbital angular momentum of light1 provides unique properties2, including mechanical torque action, which has applications for light manipulation3, enhanced sensitivity in imaging techniques4 and potential high-density information coding for optical communication systems5. Recent years have also seen a tremendous interest in exploiting orbital angular momentum at the single-photon level in quantum information technologies6, 7. In pursuing this endeavour, we demonstrate here the implementation of a quantum memory8 for quantum bits encoded in this optical degree of freedom. We generate various qubits with computer-controlled holograms, store and retrieve them on demand using a dynamic electromagnetically induced transparency protocol. We further analyse the retrieved states by quantum tomography and thereby demonstrate fidelities exceeding the classical benchmark, confirming the quantum functioning of our storage process. Our results provide an essential capability for future networks9 exploring the promises of orbital angular momentum of photons for quantum information applications.

At a glance

Figures

  1. Experimental set-up for quantum storage and analysis of OAM qubits.
    Figure 1: Experimental set-up for quantum storage and analysis of OAM qubits.

    An OAM photonic qubit encoded via an SLM is coherently mapped into a large atomic ensemble of cold caesium atoms and retrieved on demand at a later time. The control and OAM qubit fields have linear orthogonal polarizations, co-propagate with an angle of 1.7°, and are separated after the memory interface by a Glan–Taylor polarizing beamsplitter. To fully reconstruct the density matrix of the retrieved qubits, the photons enter into a two-path interferometer, where each path includes a mode projector based on a blazed-fork computer-generated hologram (CGH) and a single-mode fibre. The two paths are arranged in a way to project the photons respectively into the |Lright fence mode (LG left path) and the |Rright fence mode (LG right path). Events are detected at the output of a fibre beamsplitter by single-photon counting modules (APD 1 and APD 2). The relative phase ϕ between the two paths is scanned and experimentally determined by sending a phase-reference beam backward and analysing its spatial structure at the input of the interferometer using a digital camera. Inset: zoom-in false-colour image of the atomic ensemble (length, ~2 mm).

  2. Experimental fringe measurements and phase analysis.
    Figure 2: Experimental fringe measurements and phase analysis.

    Measured counts on APD 1 (dark blue) and APD 2 (light green) for a retrieved |Aright fence state when the relative phase ϕ of the interferometer is scanned. No background correction has been applied and the dotted horizontal line gives the background level. The fitted visibilities are 80 ± 1% for both fringes (96.5 ± 1% with correction). The displayed modes correspond to the images recorded on the camera and used for calibration of the relative phase (see Methods). The dotted white lines in the images show the phase computed by the image analysis routine. The count rate for APD 2 has been multiplied by two to account for the second fibre beamsplitter used to back-propagate the phase-reference beam. The background noise has two main contributions, one from the dark counts of the APD (~100 Hz) and the second due to residual leakage of the control field into the detected mode (~500 Hz), despite spatial filtering via the control-signal angle and additional polarization filtering. Errors were estimated assuming Poissonian statistics.

  3. Quantum tomography of the retrieved OAM qubits.
    Figure 3: Quantum tomography of the retrieved OAM qubits.

    Reconstructed density matrices for the four input states |Rright fence, |Lright fence, |Hright fence = |Rright fence + |Lright fence/ and |Dright fence = |Rright fence + i|Lright fence/ . The mean number of photons per pulse here is , and no background correction has been applied. The first column displays, for each state, its location in the Bloch sphere, the phase pattern imprinted by the SLM, and the associated spatial mode.

  4. Average fidelities of the retrieved qubits and quantum storage.
    Figure 4: Average fidelities of the retrieved qubits and quantum storage.

    The state fidelity, averaged over the six input qubits, is given as a function of the mean photon number per pulse . The purple points correspond to raw data, and the green ones are corrected for background noise. The blue dotted line gives the classical limit for a memory with unity storage and readout efficiency and the red line shows the classical limit for the actual efficiency of our memory device (the pink shaded area represents the error bar on the efficiency). Vertical and horizontal error bars indicate the standard deviations of fidelities and mean photon numbers for the six input states, respectively.

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Affiliations

  1. Laboratoire Kastler Brossel, Université Pierre et Marie Curie, Ecole Normale Supérieure, CNRS, 4 place Jussieu, 75252 Paris Cedex 05, France

    • A. Nicolas,
    • L. Veissier,
    • L. Giner,
    • E. Giacobino,
    • D. Maxein &
    • J. Laurat

Contributions

L.G., L.V., E.G. and J.L. planned the initial experimental set-up for light–matter interfacing, which was constructed by L.G., L.V. and J.L. All authors contributed to the OAM experiment. A.N., L.V. and D.M. designed the generation and characterization system and performed the measurements and data analysis under the supervision of J.L. All authors contributed to discussing the results. J.L., A.N., L.V. and D.M. wrote the manuscript.

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The authors declare no competing financial interests.

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