Stabilizing fiber-based interferometers is a challenge. Here we show a visibility of 0.96 ±0.03 (phase resolution σλ = 0.012λ) by only single-photon processing with fiber phase variations of 230°/s. Applications range from coherent optical communication to advanced quantum communication protocols.
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.
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.
Coherent optical fiber networks are extremely sensitive to thermal, mechanical, and acoustic noise, which requires elaborate schemes of phase stabilization with dedicated auxiliary lasers, multiplexers, and photodetectors. This is particularly demanding in quantum networks operating at the single-photon level. Here, we propose a simple method of phase stabilization based on single-photon counting and apply it to quantum fiber networks implementing single-photon interference on a lossless beamsplitter and coherent perfect absorption on a metamaterial absorber. As a proof of principle, we show dissipative single-photon switching with visibility close to 80%. This method can be employed in quantum networks of greater complexity without classical stabilization rigs, potentially increasing efficiency of the quantum channels.
Optical interferometers are extensively used in fundamental physics studies, gravitational wave detection, quantum metrology, topological photonics, and quantum information processing. Fiber-based interferometers are compact, mechanically robust, and cheap, and thus are ubiquitously deployed. However, the optical phase in fiber interferometers is sensitive to ambient perturbation, resulting in compromised phase-sensing precision. Therefore, phase control, shifting, and stabilization of fiber interferometers are essential. Methods to create stable interference patterns and to lock a fiber interferometer at arbitrary phase have been shown, which however are sophisticated, bulky, and delicate, preventing wider application in harsh environments outside laboratories or in space. Here we demonstrate an innovative method for arbitrary-phase locking of fiber unbalanced Mach-Zehnder interferometers. Compared with existing methods, our method is much simpler. We showcase the preparation and characterization of a narrow-band, energy-time-entangled photon state generated in integrated nonlinear microresonators, where two-photon interference visibility reaches 0.993(6). Our method constitutes a critical building block for photonic quantum networks, and is useful to emerging single-photon interference in curved space-time that facilitates exploration of the interface of quantum mechanics and general relativity.
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.
Time is a practical and robust degree of freedom for the encoding of quantum information. Qubits encoded in so-called `time-bins', allowing a discrete superposition of two potential arrival times, have their entanglement preserved even over long propagation distances in standard fiber networks [1]. Time has also been used for the preparation of more complex quantum systems, such as hyper-entangled and cluster states [2]. These qualities put time-bin encoding at the center of applications ranging from quantum state preparation through to quantum communications and information processing. One of the hallmarks of the scheme is that a nonlinear element has to be pumped with phase-coherent double-pulses, which can be achieved by using unbalanced interferometers [1], making their phase stability the key determinant of both state generation and analysis accuracy. However, the phase ambiguity arising from the periodicity of feedback signals [3], as well as the difficulty of stabilizing on interference maxima/minima, leave progress to be made in fiberbased interferometry for time-bin encodings.