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Quantum 2.0 Conference and Exhibition

Feasibility study of frequency-encoded photonic qubits over a free-space channel

Stéphane Vinet, Wilson Wu, Yujie Zhang, Thomas D. Jennewein

Published Jan 1, 2025• 0 citations
Publisher

Abstract

We demonstrate a novel interferometric approach to decode frequency-bins, transmitted over free-space channels, without any adaptive optics or modal filtering. Moreover, we investigate the phase stability requirements for frequency-bin encoded satellite communication.
DOI
10.1364/quantum.2025.qm3a.2

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Frequency-bin quantum encoding shows great promise for quantum communication given its high-dimensional scaling, compatibility with photonic integrated circuits and synergy with classical optical communication technology. However, to date all demonstrations have been performed over single-mode and static channels, while the transmission over fluctuating and turbulent channels has not been addressed. We propose and demonstrate a novel approach that leverages field-widened interferometers to decode frequency-bins transmitted over free-space channels without any adaptive optics or modal filtering. Moreover, we investigate the phase stability requirements so that frequency-bin encoding could be feasible for satellite to ground quantum links. Our passive approach expands the versatility of frequency-bin encoding, paving the way towards long-range and fluctuating channels.
Frequency-bin quantum encoding shows great promise for quantum communication, given its high-dimensional scaling, compatibility with photonic integrated circuits, and synergy with classical optical communication technology. However, to date, all demonstrations have been performed over single-spatial mode and static channels, while the transmission over fluctuating and turbulent channels has not been addressed. We propose and demonstrate a novel approach that leverages field-widened interferometers to decode frequency-bins transmitted over free-space channels without any adaptive optics or modal filtering. Moreover, we investigate the phase stability requirements so that frequency-bin encoding could be feasible for satellite-to-ground quantum links. Our passive approach expands the versatility of frequency-bin encoding, paving the way towards long-range and fluctuating 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.
Abstract Frequency-bin entangled photons can be efficiently produced on-chip which offers a scalable, robust and low-footprint platform for quantum communication, particularly well-suited for resource-constrained settings such as mobile or satellite-based systems. However, analyzing such entangled states typically requires active and lossy components, limiting scalability and multi-mode compatibility. We demonstrate a novel technique for processing frequency-encoded photons using linear interferometry and time-resolved detection. Our approach is fully passive and compatible with spatially multi-mode light, making it suitable for free-space and satellite-to-ground applications. As a proof-of-concept, we utilize frequency-bin entangled photons generated from a high-brightness multi-resonator source integrated on-chip to show the ability to perform arbitrary projective measurements over both single- and multi-mode channels. We report the first measurement of the joint temporal intensity between frequency-bin entangled photons, which allows us to certify entanglement by violating the Clauser-Horne-Shimony-Holt (CHSH) inequality, with a measured value of ∣ S ∣ = 2.32 ± 0.05 over multi-mode fiber. By combining time-resolved detection with energy-correlation measurements, we perform full quantum state tomography, yielding a state fidelity of up to 91%. We further assess our ability to produce non-classical states via a violation of time-energy entropic uncertainty relations and investigate the feasibility of a quantum key distribution protocol. Our work establishes a resource-efficient and scalable approach toward the deployment of robust frequency-bin entanglement over free-space and satellite-based links.
We demonstrate a novel, passive method for processing frequency-bin encoded quantum information using linear interferometry and time-resolved detection, eliminating the need for electro-optic modulation or nonlinear optics. This approach is compatible with multimode propagation, a crucial step for free-space quantum communication, and favorable for size, weight, and power (SWaP) requirements. Using a high-brightness multi-resonator source, we generate frequency-bin entangled photons and show the ability to perform arbitrary projective measurements in both single- and multi-mode propagation scenarios. For the first time, we measure the joint temporal amplitude of the generated state, directly linking its entangled nature to the temporal oscillation of non-local correlations. We leverage this to certify entanglement with a Bell curve visibility exceeding 95%. Our technique enables full quantum state tomography, yielding a state fidelity of up to 91% for a maximally entangled state. This work paves the way for robust entanglement deployment over free-space and satellite-based links.
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.