Abstract
A single-end adaptive-optics (AO) module is experimentally demonstrated to mitigate the emulated atmospheric turbulence effects in a bi-directional quantum communication link, which employs orbital angular momentum (OAM) for data encoding. A classical Gaussian beam is used as a probe to detect the turbulence-induced wavefront distortion in the forward direction of the link. Based on the detected wavefront distortion, an AO system located on one end of the link is used to simultaneously compensate for the forward and backward channels. Specifically, with emulated turbulence and when the probe is turned on, the mode purity of photons carrying OAM ℓ = 1 is improved by ~ 21 % with AO mitigation. We also measured the performance when encoding data using OAM {ℓ = −1, + 2} and {ℓ = −2, + 1} in the forward and backward channels, respectively, at 10 Mbit/s per channel with one photon per pulse on average. For this case, we found that the AO system could reduce the turbulence effects increased quantum-symbol-error-rate (QSER) by ~ 76 % and ~ 74 %, for both channels in the uni-directional and bi-directional cases, respectively. Similar QSER improvement is observed for the opposite direction channels in the bi-directional case.
Highlights
Quantum optical communications holds the promise for secure transfer of data over both free-space and fiber links [1,2,3,4,5,6,7]
There has been interest in quantum systems that utilize a larger number of orthogonal states by using a larger “alphabet” set of characteristics of photons. Such a larger alphabet might enable better performance in quantum communication links, such as: (i) a larger transmission capacity in terms of bits/sec, since an alphabet of 2N possible values can transmit N bits per symbol period; and (ii) a higher photon efficiency in terms of bits/photon, since a single photon can carry N bits of information instead of 1; this is very similar to the difference between binary data encoding using {0, 1} and M-ary data encoding using {0, 1, . . ., M-1} [8, 9]
The phase term of an OAM beam is described as exp(ilθ) in the transverse plane, where θ refers to the azimuthal coordinate and l defines the “charge” carried by the OAM mode. l describes the number of 2π phase shifts occurring in the azimuthal direction and could be a positive, negative, or a zero-value integer, corresponding to clockwise, counterclockwise phase helices, or a conventional non-OAM Gaussian beam with power in the beam center, respectively [15, 16, 26]
Summary
Quantum optical communications holds the promise for secure transfer of data over both free-space and fiber links [1,2,3,4,5,6,7]. There has been interest in quantum systems that utilize a larger number of orthogonal states by using a larger “alphabet” set of characteristics of photons. Such a larger alphabet might enable better performance in quantum communication links, such as: (i) a larger transmission capacity in terms of bits/sec, since an alphabet of 2N possible values can transmit N bits per symbol period; and (ii) a higher photon efficiency in terms of bits/photon, since a single photon can carry N bits of information instead of 1; this is very similar to the difference between binary data encoding using {0, 1} and M-ary data encoding using {0, 1, . To detect an OAM beam at the receiver, an OAM beam can be converted back to a Gaussian-like beam by passing it through a phase distribution that is a conjugate of the transmitter
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