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Physical Review A

Verifying Energy-time Entanglement via Nonlocal Dispersion Cancellation

Jin-Woo Chae, Heebong Seo, U-Shin Kim, Yoon-Ho Kim

Published Mar 25, 2025• 3 citations
Publisher

Abstract

The authors experimentally demonstrate direct verification of continuous-variable energy-time entanglement via nonlocal dispersion cancellation under realistic telecom conditions, effectively emulating photon propagation through a 379-km optical fiber. They observe a significant violation of the separability criterion by 32 standard deviations, confirming robust entanglement suitable for secure long-distance quantum communication.

DOI
10.1103/physreva.111.l030403

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The authors experimentally demonstrate direct verification of continuous-variable energy-time entanglement via nonlocal dispersion cancellation under realistic telecom conditions, effectively emulating photon propagation through a 379-km optical fiber. They observe a significant violation of the separability criterion by 32 standard deviations, confirming robust entanglement suitable for secure long-distance quantum communication.
The energy-time entangled biphoton source plays a great role in quantum communication, quantum metrology, and quantum cryptography due to its strong temporal correlation and capability of nonlocal dispersion cancellation. As a quantum effect, nonlocal dispersion cancellation is further proposed as an alternative way for nonlocality testing of continuous variable entanglement via the violation of the Bell-like inequality proposed by Wasak et al. [Phys. Rev. A 82, 052120 (2010)]. However, to date there is no experimental report either on the inequality violation or on a nonlocal detection with single-photon detectors at a long-distance transmission channel, which is key for a true nonlocality test. In this paper, we report an experimental realization of a violation of the inequality after 62-km optical fiber transmission at telecom wavelengths with a nonlocal detection based on event timers and a cross-correlation algorithm, which indicates a successful nonlocal test of energy-time entanglement. This work provides feasibility for the strict test of the nonlocality for continuous variables in both long-distance communication fiber channel and free space.
Encoding quantum information via time-bin entangled states has had a profound impact on the development of quantum communications. However, dispersive propagation limits their achievable transmission distances. Here we describe a regime for nonlocal dispersion cancellation where the sum of arrival times of photons undergoing identical dispersion remains highly correlated. We exploit this effect to mitigate dispersive effects in a quantum key distribution fiber link, allowing an increase in the secret key rate by over a factor of 5 after 80 km of optical fiber dispersion.
Quantum key distribution (QKD) enables participants to exchange secret information over long distances with unconditional security. However, the performance of today's QKD systems is subject to hardware limitations, such as those of available nonclassical-light sources and single-photon detectors. By encoding photons in high-dimensional states, the rate of generating secure information under these technical constraints can be maximized. Here, we demonstrate a complete time-energy entanglement-based QKD system with proven security against the broad class of arbitrary collective attacks. The security of the system is based on nonlocal dispersion cancellation between two time-energy entangled photons. This resource-efficient QKD system is implemented at telecommunications wavelength, is suitable for optical fiber and free-space links, and is compatible with wavelength-division multiplexing.
Any practical realization of entanglement-based quantum communication must be intrinsically secure and able to span long distances avoiding the need of a straight line between the communicating parties. The violation of Bell’s inequality offers a method for the certification of quantum links without knowing the inner workings of the devices. Energy-time entanglement quantum communication satisfies all these requirements. However, currently there is a fundamental obstacle with the standard configuration adopted: an intrinsic geometrical loophole that can be exploited to break the security of the communication, in addition to other loopholes. Here we show the first experimental Bell violation with energy-time entanglement distributed over 1 km of optical fibres that is free of this geometrical loophole. This is achieved by adopting a new experimental design, and by using an actively stabilized fibre-based long interferometer. Our results represent an important step towards long-distance secure quantum communication in optical fibres. Practical implementations of quantum communication need to securely deliver information over long distances without line-of-sight. Towards this goal, Cuevas et al.use an actively stabilized interferometer to close the geometry loophole for a Bell inequality violation over 1 km of optical fibre.
Device-independent quantum communication will require a loophole-free violation of Bell inequalities. In typical scenarios where line of sight between the communicating parties is not available, it is convenient to use energy-time entangled photons due to intrinsic robustness while propagating over optical fibers. Here we show an energy-time Clauser-Horne-Shimony-Holt Bell inequality violation with two parties separated by 3.7 km over the deployed optical fiber network belonging to the University of Concepción in Chile. Remarkably, this is the first Bell violation with spatially separated parties that is free of the postselection loophole, which affected all previous in-field long-distance energy-time experiments. Our work takes a further step towards a fiber-based loophole-free Bell test, which is highly desired for secure quantum communication due to the widespread existing telecommunication infrastructure.
The entanglement of quantum systems can produce a variety of nonclassical effects that have practical applications in quantum information science. One example of this is nonlocal dispersion cancellation, in which the effects of dispersion on one photon can be canceled out by the dispersion experienced by a second photon at a distant location. In this paper, we extend the analysis of nonlocal dispersion cancellation to three or more photons. We find that energy-time entanglement of three or more photons can lead to a complete or partial cancellation of dispersion depending on the experimental conditions. These results may be useful in implementing quantum key distribution in networks with three or more nodes.