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arXiv:2305.02965 · IET conference proceedings.

Squeezed light coexistence with classical communication over 10 km optical fiber

Adnan A. E. Hajomer, Huy Quang Nguyen, Melis Pahalı, Ulrik L. Andersen, Tobias Gehring

Published Nov 13, 2023• 3 citations
arXivHTMLPDF

Abstract

We report the first coexistence experiment of 1550 nm single-mode squeezed light with a 1310 nm classical light over a 10 km fiber channel, using a locally generated local oscillator. Using real-time phase locking, we measure up to 0.5 dB of squeezing.
DOI
10.1049/icp.2023.2235

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Abstract We demonstrate the generation, 40 km fiber transmission, and homodyne detection of single-mode squeezed states of light at 1550 nm using real-time phase control of a locally generated local oscillator (LO), often called a ‘real LO’ or ‘local LO’. The system was able to stably measure up to around 3.7 dB of noise suppression with a phase noise uncertainty of around 2.5°, using only standard telecom-compatible components and a field-programmable gate array. The compactness, low degree of complexity and efficacy of the implemented scheme makes it a relevant candidate for long distance quantum communication in future photonic quantum networks.
Squeezed light is a crucial resource for continuous-variable (CV) quantum information science. Distributed multi-mode squeezing is critical for enabling CV quantum networks and distributed quantum sensing. To date, multi-mode squeezing measured by homodyne detection has been limited to single-room experiments without coexisting classical signals, i.e., on "dark" fiber. Here, after distribution through separate fiber spools (5 km), -0.9 ± 0.1-dB coexistent two-mode squeezing is measured. Moreover, after distribution through separate deployed campus fibers (about 250 m and 1.2 km), -0.5 ± 0.1-dB coexistent two-mode squeezing is measured. Prior to distribution, the squeezed modes are each frequency multiplexed with several classical signals-including the local oscillator and conventional network signals-demonstrating that the squeezed modes do not need dedicated dark fiber. After distribution, joint two-mode squeezing is measured and recorded for post-processing using triggered homodyne detection in separate locations. This demonstration enables future applications in quantum networks and quantum sensing that rely on distributed multi-mode squeezing.
We investigate the coexistence of clock synchronization protocols with quantum signals in a common single-mode optical fiber. By measuring optical noise between 1500 nm to 1620 nm we demonstrate a potential for up to 100 quantum, 100 GHz wide channels coexisting with the classical synchronization signals. Both "White Rabbit" and pulsed laser-based synchronization protocols were characterized and compared. We establish a theoretical limit of the fiber link length for coexisting quantum and classical channels. The maximal fiber length is below approximately 100 km for off-the-shelf optical transceivers and can be significantly improved by taking advantage of quantum receivers.
One-sided device-independent Quantum Key Distribution (QKD) targets a mathematically proven secure communication between two parties without the need to trust the devices on one side. An appealing kind of high-speed implementation uses GHz-bandwidth two-mode squeezed states, distributed via an optical fiber link. The remote measurements, however, require the precise control of the phase with respect to an optical homodyne local oscillator. Here, we report what is believed to be the first transmission of single-mode squeezed vacuum states with GHz bandwidth over a 1 km optical fiber into a neighboring building combined with phase-stabilized measurements. We directly measure a squeeze factor of up to 4 dB. Our result is an important step towards high-speed QKD with enhanced security level for km-scale areas, e.g., government districts.
The integration of quantum key distribution (QKD) and classical optical communication has attracted widespread attention. In this Letter, we experimentally demonstrate a real-time co-propagation of 1 Tbps for 10 classical channels with one discrete-variable QKD channel in the weakly coupled few-mode fiber (FMF). Based on the selection of optimal device parameters and wavelength assignment of classical channels, as well as the optimization of equipment performance, a secure key rate of as high as 2.7 kbps of coexistence transmission of QKD and classical optical communication can be achieved using a 100.96 km weakly coupled FMF. Therefore, this study is a step toward realizing long-distance quantum-classical coexistence transmission.