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Optics Express

Active stabilization of a fiber-optic two-photon interferometer using continuous optical length control

Seok-Beom Cho, Heonoh Kim

Published May 11, 2016• 12 citations
Publisher

Abstract

The practical realization of long-distance entanglement-based quantum communication systems strongly rely on the observation of highly stable quantum interference between correlated single photons. This task must accompany active stabilization of the optical path lengths within the single-photon coherence length. Here, we provide two-step interferometer stabilization methods employing continuous optical length control and experimentally demonstrate two-photon quantum interference using an actively stabilized 6-km-long fiber-optic Hong-Ou-Mandel interferometer. The two-step active control techniques are applied for measuring highly stable two-photon interference fringes by scanning the optical path-length difference. The obtained two-photon interference visibilities with and without accidental subtraction are found to be approximately 90.7% and 65.4%, respectively.

DOI
10.1364/oe.24.010980

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The practical realization of long-distance entanglement-based quantum communication systems strongly rely on the observation of highly stable quantum interference between correlated single photons. This task must accompany active stabilization of the optical path lengths within the single-photon coherence length. Here, we provide two-step interferometer stabilization methods employing continuous optical length control and experimentally demonstrate two-photon quantum interference using an actively stabilized 6-km-long fiber-optic Hong-Ou-Mandel interferometer. The two-step active control techniques are applied for measuring highly stable two-photon interference fringes by scanning the optical path-length difference. The obtained two-photon interference visibilities with and without accidental subtraction are found to be approximately 90.7% and 65.4%, respectively.
We report on single-photon interference experiments in a Michelson-type interferometer built with two 6-km-long fiber spools, as well as on the active stabilization of the interferometer. A weak coherent light signal was (de-) multiplexed with a strong reference light using wavelength-division multiplexing technique, and real-time feedback control technique was applied for the reference light to actively stabilize the phase fluctuation in the long-armed fiber interferometer. The stabilized interferometer showed phase stability of 0.06 rad, which corresponds to an optical path length fluctuation of 15 nm between the 6-km-long interfering arms. The raw visibility obtained without subtracting noise counts in the single-photon interference experiment was more than 98% for stabilized conditions.
We experimentally demonstrate stable and user-adjustable single-photon interference in a 1 km long fiber-optic Mach-Zehnder interferometer, using an active phase control system with the feedback provided by a classical laser. We are able to continuously tune the single-photon phase difference between the interferometer arms using a phase modulator, which is synchronized with the gate window of the single-photon detectors. The phase control system employs a piezoelectric fiber stretcher to stabilize the phase drift in the interferometer. A single-photon net visibility of 0.97 is obtained, yielding future possibilities for experimental realizations of quantum repeaters in optical fibers and violation of Bell's inequalities using genuine energy-time entanglement.
The quantum clock synchronization based on frequency-correlated photon pairs and HOM interference has shown femtosecond-level precision and great application prospect in numerous fields depending on high-precision time-frequency signals. Due to the difficulty of achieving stable HOM interference fringe after long-distance fiber transmission, this scheme of synchronization is hampered from long-haul field application. Utilizing segmented fibers instead of a single long-length fiber, we achieved the stable observation of the two-photon interference of the lab-developed broadband frequency-correlated photon pairs after 20 km-long fiber transmission, without employing the auxiliary phase stabilization method. Referenced to this interference fringe, the balance of the two fiber arms is achieved with a long-term stability of 20 fs. The HOM-interference-based synchronization over a 20-km fiber link is thus demonstrated, and a minimum stability of 74 fs has been reached at 48 000 s. This result not only provides a simple way to stabilize the fiber-optic two-photon interferometer for long-distance quantum communication systems but also makes a great stride forward in extending the quantum-interference-based synchronization scheme to the long-haul field applications.
A novel scheme to realize stable single-photon interference and router-control is demonstrated in this paper. Phase difference between clockwise and counter-clockwise single-photons is controlled by time-division phase modulation in an optic fiber Sagnac loop. Due to the annular geometric configuration of the Sagnac loop, clockwise and counter-clockwise single-photons travel the same optical path and have the same slowly-varying phase drifts, which are automatically compensated. Furthermore, clockwise and counter-clockwise single-photons encounter the same polarization mode dispersion, which is also automatically compensated at the exit port of the Sagnac interferometer. Long-distance single-mode optic fiber at 1550 nm is used in our experimental realization. Fringe visibility of single-photon interference higher than 98% and fidelity of single-photon router-control higher than 90% have been obtained in a Sagnac fiber loop of 5 km. We also realized a stable single-photon interference in Sagnac fiber loops as long as 27 and 52 km. The corresponding fringe visibilities were higher than 94% and 84%, respectively.
Optical interferometers are crucial for long-distance quantum communications. We propose a novel phase stabilization method employing triple wavelength bands for single-photon interference in optical fiber-based interferometers, incorporating a simplified laser frequency stabilization technique that utilizes a single nonlinear crystal. After stabilization, the residual phase noise of the fiber interferometer was 20 mrad for differential arm lengths (Δ L ) of 0 and 10 m. This represents a 9-fold and 7.5-fold improvement for Δ L = 0 and 10 m, respectively, compared with those observed in previous double-wavelength methods. These findings enhance the visibility and simplicity of quantum key and entanglement distributions in long-distance quantum communication.