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Unconditional entanglement interface for quantum networks

Christoph Baune, Jan Gniesmer, Sacha Kocsis, Christina E. Vollmer, Petrissa Zell, Jaromír Fiurášek, and Roman Schnabel
Phys. Rev. A 93, 010302(R) – Published 19 January 2016
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    Abstract

    Entanglement drives nearly all proposed quantum information technologies. By up-converting part of a 1550 nm two-mode squeezed vacuum state to 532 nm, we demonstrate the generation of strong continuous-variable entanglement between widely separated frequencies. Nonclassical correlations were observed in joint quadrature measurements of the 1550 and 532 nm fields, showing a maximum noise suppression 5.5 dB below vacuum. Our versatile technique combines strong nonclassical correlations, large bandwidth, and in principle, the ability to entangle the telecommunication wavelength of 1550 nm with any optical wavelength.

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    • Received 2 October 2015

    DOI:https://doi.org/10.1103/PhysRevA.93.010302

    ©2016 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Entanglement productionQuantum information processing with continuous variables
    Quantum Information, Science & Technology

    Authors & Affiliations

    Christoph Baune1,2, Jan Gniesmer2,3, Sacha Kocsis1,2,4, Christina E. Vollmer2, Petrissa Zell2, Jaromír Fiurášek5, and Roman Schnabel1,2,*

    • 1Institut für Laserphysik und Zentrum für Optische Quantentechnologien, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany
    • 2Institut für Gravitationsphysik, Leibniz Universität Hannover and Max-Planck-Institut für Gravitationsphysik (Albert-Einstein-Institut), Callinstrasse 38, 30167 Hannover, Germany
    • 3Institut für Festkörperphysik, Leibniz Universität Hannover, Appelstrasse 2, 30167 Hannover, Germany
    • 4Centre for Quantum Dynamics and Centre for Quantum Computation and Communication Technology, Griffith University, Brisbane 4111, Australia
    • 5Department of Optics, Palacký University, 17. listopadu 12, 77146 Olomouc, Czech Republic

    • *roman.schnabel@physnet.uni-hamburg.de

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    Vol. 93, Iss. 1 — January 2016

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    • Figure 1
      Figure 1

      Conceptual schematic of an elementary segment of a quantum network where entangled light beams establish quantum correlations between two nodes of the network. Efficient transmission of light is ensured by operating at telecommunication wavelength, and the coupling to local quantum memories (QM) is enabled by frequency up-conversion (SFG) of the transmitted light beams.

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    • Figure 2
      Figure 2

      Illustration of the experimental setup. Squeezed vacuum states of continuous-wave light at 1550 nm are produced in a standing-wave cavity by degenerate type I optical parametric down-conversion, as described in more detail in Ref. [26], and split up at a variable beam splitter. The reflected part is directly sent to a balanced homodyne detection (BHD) while the reflected mode is up-converted to 532 nm and also detected in a BHD. The sum of the BHD signals is recorded by a spectrum analyzer (SA).

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    • Figure 3
      Figure 3

      Characterization of our unconditional quantum interface. Shown is the sum of the two BHD signals at 5 MHz sideband frequency. While the phase ϕ of the BHD at 532 nm was continuously scanned, the phase of the BHD at 1550 nm was set to measure the squeezed quadrature amplitude X̂1550 [(ii), red trace] or antisqueezed P̂1550 quadrature [(i), blue]. The four extremal points represent the following measurement settings. A: Var[X̂1550+X̂532], B: Var[X̂1550−X̂532], C: Var[P̂1550+P̂532], D: Var[P̂1550−P̂532]. The orange trace (iv) was recorded when the 532 nm phase was also fixed, revealing stable nonclassical correlations about 5.5 dB below the vacuum level [(iii), black)]. Note that the traces were recorded successively, and there is no actual meaning in the relative positions of the minima and maxima. None of the traces was corrected for our detection scheme's dark noise [(v), gray].

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    • Figure 4
      Figure 4

      Spectral characterization of the entanglement interface. Trace (i) shows the spectrum when Var[P̂1550−P̂532] is detected, corresponding to point D from Fig. 3. Trace (ii) shows the spectrum for Var[X̂1550+X̂532], corresponding to point A from Fig. 3. Normalized variance values below zero, the vacuum reference, signify nonclassical correlations. The bottom trace (iii) shows the dark noise of our balanced homodyne detector.

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