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Laser & Photonics Review

Phase Recovery and Stability: Arbitrary Phase Access for Stable Fiber Interferometers (Laser Photonics Rev. 15(7)/2021)

Piotr Roztocki, Benjamin MacLellan, Mehedi Islam, Christian Reimer, Bennet Fischer, Stefania Sciara, Robin Helsten, Y. Jestin, Alfonso Carmelo Cino, Sai T. Chu, Brent E. Little, David Moss, Michael Kues, Roberto Morandotti

Published Jul 1, 2021• 2 citations
Publisher

Abstract

Practical systems for optical interference are critical to coherent signal processing in devices and networks. In article number 2000524, Roberto Morandotti, Michael Kues, and co-workers demonstrate an easily-deployable scheme for the retrieval of relative phase in fiber interferometric systems. The use of bright and dim/photon-level reference signals is contrasted, with pulsed references shown to enable temporal multiplexing with quantum signals.

DOI
10.1002/lpor.202170041

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Practical systems for optical interference are critical to coherent signal processing in devices and networks. In article number 2000524, Roberto Morandotti, Michael Kues, and co-workers demonstrate an easily-deployable scheme for the retrieval of relative phase in fiber interferometric systems. The use of bright and dim/photon-level reference signals is contrasted, with pulsed references shown to enable temporal multiplexing with quantum signals.
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.
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.
Abstract Well‐controlled yet practical systems that give access to interference effects are critical for established and new functionalities in ultrafast signal processing, quantum photonics, optical coherence characterization, etc. Optical fiber systems constitute a central platform for such technologies. However, harnessing optical interference in a versatile and stable manner remains technologically costly and challenging. Here, degrees of freedom native to optical fibers, i.e., polarization and frequency, are used to demonstrate an easily deployable technique for the retrieval and stabilization of the relative phase in fiber interferometric systems. The scheme gives access (without intricate device isolation) to <1.3 × 10−3 π rad error signal Allan deviation across 1 ms to 1.2 h integration times for all tested phases, ranging from 0 to 2π. More importantly, the phase‐independence of this stability is shown across the full 2π range, granting access to arbitrary phase settings, central for, e.g., performing quantum projection measurements and coherent pulse recombination. Furthermore, the scheme is characterized with attenuated optical reference signals and single‐photon detectors, and extended functionality is demonstrated through the use of pulsed reference signals (allowing time‐multiplexing of both main and reference signals). Finally, the scheme is used to demonstrate radiofrequency‐controlled interference of high‐dimensional time‐bin entangled states.
Narrow-linewidth lasers with absolute frequency anchoring are essential for precision metrology, coherent sensing, and emerging quantum technologies beyond laboratory environments. Optical cavities and interferometers provide exceptional short-term spectral purity but lack intrinsic absolute frequency references. Atomic transitions, in contrast, provide stable frequency anchors but offer limited discrimination sensitivity. Recent hybrid approaches have demonstrated the combination of compact optical resonators with atomic references, yet achieving the Hz-level regime remains challenging. Here, we present a hybrid architecture that enables simultaneous realization of Hz-level linewidth and atomic-referenced frequency stability. An external-cavity diode laser is first stabilized to a fiber interferometer to achieve Hz-level spectral purity, while the interferometer is subsequently anchored to an 87Rb D2 transition via modulation transfer spectroscopy to suppress long-term drift and define the laser frequency relative to the atomic transition. This dual-stabilization scheme realizes a compact atomic-referenced laser with a 3.4-Hz linewidth (1-rad integrated-phase method), a minimum fractional frequency stability of 3.4x10-14 at 0.56 s, and 9x10-13 at 100 s. This architecture establishes a practical and scalable route toward compact and field-deployable atomic-referenced narrow-linewidth lasers for precision metrology and quantum technologies.