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Efficient time-bin qubit analyzer compatible with multimode optical channels

Jeongwan Jin, Sascha Agne, Jean‐Philippe Bourgoin, Yanbao Zhang, Norbert Lütkenhaus, Thomas D. Jennewein

Published Sep 24, 2015• 6 citations
OpenAlex

Abstract

Time-bin encoded photonic qubits are robust against decoherence in birefringent media such as optical fibers, which has led to many successful quantum communication demonstrations, including a Mach-Zehnder interferometer-based system, plug-and-play quantum key distribution (QKD), differential phase shift (DPS) and coherent one-way (COW) protocols, quantum teleportation, and elements of quantum repeaters. Despite these successes, time-bin encoding is considered impractical for turbulent free-space or multimode fiber channels, because spatial and temporal mode distortions hinder the interference required for analysis of time-bin superposition states. Here we present an efficient time-bin qubit analyzer, assisted by passive relay optics, that matches the spatial mode of the interfering paths. Our system demonstrates interference visibility of 89 %, even for strongly distorted optical input modes emerging from multimode fiber under variable angles of incidence. We measure a high level of entanglement between a time-bin qubit and a separate polarization qubit, thereby demonstrating the feasibility of time-bin based quantum communication over optical multimodal links.

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Time-bin encoded photonic qubits are robust against decoherence in birefringent media such as optical fibers, which has led to many successful quantum communication demonstrations, including a Mach-Zehnder interferometer-based system, plug-and-play quantum key distribution (QKD), differential phase shift (DPS) and coherent one-way (COW) protocols, quantum teleportation, and elements of quantum repeaters. Despite these successes, time-bin encoding is considered impractical for turbulent free-space or multimode fiber channels, because spatial and temporal mode distortions hinder the interference required for analysis of time-bin superposition states. Here we present an efficient time-bin qubit analyzer, assisted by passive relay optics, that matches the spatial mode of the interfering paths. Our system demonstrates interference visibility of 89 %, even for strongly distorted optical input modes emerging from multimode fiber under variable angles of incidence. We measure a high level of entanglement between a time-bin qubit and a separate polarization qubit, thereby demonstrating the feasibility of time-bin based quantum communication over optical multimodal links.
We demonstrate two approaches for unbalanced interferometers as time-bin qubit analyzers for quantum communication, robust against mode distortions and polarization effects as expected from free-space quantum communication systems including wavefront deformations, path fluctuations, pointing errors, and optical elements. Despite strong spatial and temporal distortions of the optical mode of a time-bin qubit, entangled with a separate polarization qubit, we verify entanglement using the Negative Partial Transpose, with the measured visibility of up to $0.85\ifmmode\pm\else\textpm\fi{}0.01$. The robustness of the analyzers is further demonstrated for various angles of incidence up to $0.{2}^{\ensuremath{\circ}}$. The output of the interferometers is coupled into multimode fiber yielding a high system throughput of 0.74. Therefore, these analyzers are suitable and efficient for quantum communication over multimode optical channels.
Despite its widespread use in fiber optics, encoding quantum information in photonic time-bin states is usually considered impractical for free-space quantum communication as turbulence-induced spatial distortion impedes the analysis of time-bin states at the receiver. Here, we demonstrate quantum key distribution using time-bin photonic states distorted by turbulence and depolarization during free-space transmission. Utilizing a novel analyzer apparatus, we observe stable quantum bit error ratios of 5.32 %, suitable for generating secure keys, despite significant wavefront distortions and polarization fluctuations across a 1.2 km channel. This shows the viability of time-bin quantum communication over long-distance free-space channels, which will simplify direct fiber/free-space interfaces and enable new approaches for practical free-space quantum communication over multi-mode, turbulent, or depolarizing channels.
Encoding quantum information in photonic time-bin states is typically considered impractical for moving free-space quantum communication due to the difficulties with phase stabilization of distant quantum time-bin interferometers and turbulence of free-space channels. We demonstrate a novel approach using reference frame independent time-bin quantum key distribution that completely avoids the need for active relative phase stabilization while simultaneously overcoming a highly multi-mode channel without any active mode filtering. This scheme enables passive, self-compensating time-bin quantum communication without any mode filtering, mode sorting, adaptive optics, active basis selection, or active phase alignment. We realize a proof-of-concept demonstration using hybrid polarization and time-bin entangled photons that demonstrates a sustained asymptotic secure key rate greater than 0.07 bits/coincidence over a 15 m multi-mode fiber optical channel and showing entanglement correlations over a moving 38.5 dB loss free-space channel, including system losses. The scheme simplifies the use of time-bin encoding and can be readily applied over various spatially multi-mode and fluctuating channels involving rapidly moving platforms, including airborne and satellite systems.
Quantum key distribution is a quantum communication protocol which seeks to address potential vulnerabilities in data transmission and storage. One of the main challenges in the field is achieving high rates of secret key in lossy and turbulent free-space channels. In this scenario, most experimental demonstrations have used the polarization of photons as their qubit carriers, due to the relative robustness of polarization in free-space propagation. Time-bin or phase-based protocols are considered less practical due to the wave-front distortion caused by atmospheric turbulence. However, demonstrations of novel free-space interferometer designs are enabling interferometers to measure multimodal signals with high visibility. That means it is now viable to consider the prospects of implementing time-bin or phase-based protocols, which have demonstrated high key rates and long transmission distances in optical fiber. In this work, we present the possibilities of implementing time-bin protocols in turbulent free-space channels, using the coherent one-way protocol as the example. We present an analysis of the secret key rate and quantum bit error rate of the system, providing the errors due to noise counts, and the extinction ratio of the pulses. Finally, we developed a model to quantify the expected losses for a turbulence free-space channel, specifically for a free-space satellite-to-ground station channel.
The photonic temporal degree of freedom is one of the most promising platforms for quantum communication over fiber networks and free-space channels.In particular, time-bin states of photons are robust to environmental disturbances, support high-rate communication, and can be used in high-dimensional schemes.However, the detection of photonic time-bin states remains a challenging task, particularly for the case of photons that are in a superposition of different time bins.Here, we experimentally demonstrate the feasibility of picosecond time-bin states of light, known as ultrafast time bins, for applications in quantum communications.With the ability to measure time-bin superpositions with excellent phase stability, we enable the use of temporal states in efficient quantum key distribution protocols such as the BB84 protocol.
Two main challenges for quantum networks are state preservation and scaling current infrastructure. Photonic polarization qubits are susceptible to effective decoherence via polarization mode dispersion in optical fibers. This can be circumvented by encoding qubits in the photon’s arrival time, i.e., time-bin encoding. Here, we present measurements on a thin-film lithium niobate integrated-optic device, designed to analyze telecom-wavelength photonic time-bin qubits. By thermo-optically tuning the phase and amplitudes of interfering processes traversing the photonic circuit on the device, we are able to obtain ∼ 83% interference visibility, marking significant progress towards efficient time-bin encoding and analysis with integrated photonics.