# Quantum Optics INRIM

## Quantum communication

Quantum key distribution (QKD) is a strategic technology that allows two distant parties to share encryption keys with an unprecedented level of security based on the laws of quantum mechanics. Although QKD protocols can be proven unconditionally secure in theory, in practice any deviations of the real system from the idealised model could introduce vulnerabilities. For QKD technology to become a viable real-world solution, end-users need confidence in it, and this requires the characterisation of physical parameters of the practical QKD system devices.

Our group takes advantage of PiQuET - Piemonte Quantum Enabling Technology - infrastructure and instruments to develop characterization methods and protocols for the measurement of the key parameters of commercial QKD devices, testing both active QKD components (as SPADs and SNSPDs) and assembled QKD modules of complete QKD systems (as IdQuantique Clavis3 and Cerberis3 system). We test and develop counter-measures on attacks on QKD systems, as the back-flash attack, analyzing, through a portable optical time domain reflectometer (OTDR) operating at single photon level, the light reflected back from detectors and commercial QKD modules. We also test innovative QKD protocols (as Twin-Field QKD) and Trusted Nodes transmission on real fiber, in collaboration with QN5, on the Italian Quantum Backbone, a 1800 km-long fiber infrastructure now used for high-level time and frequency dissemination services and experiments. In collaboration with QN3 group, we take advantage of single photon radiometry facilities to provide traceable measurements. QKD technology is currently the subject of standardisation efforts at the European and at international level and our group makes its expertise available to all those organisations addressing standardisation issues in QKD and quantum technology in general.

## Highlights

### Coherent phase transfer for real-world twin-field quantum key distribution

Quantum mechanics allows distribution of intrinsically secure encryption keys by optical means. Twin-field quantum key distribution is one of the most promising techniques for its implementation on long-distance fiber networks, but requires stabilizing the optical length of the communication channels between parties. In proof-of-principle experiments based on spooled fibers, this was achieved by interleaving the quantum communication with periodical stabilization frames. In this approach, longer duty cycles for the key streaming come at the cost of a looser control of channel length, and a successful key-transfer using this technique in real world remains a significant challenge. Using interferometry techniques derived from frequency metrology, we develop a solution for the simultaneous key streaming and channel length control, and demonstrate it on a 206 km field-deployed fiber with 65 dB loss. Our technique reduces the quantum-bit-error-rate contributed by channel length variations to <1%, representing an effective solution for real-world quantum communications.

### A study to develop a robust method for measuring the detection efficiency of free-running InGaAs/InP single-photon detectors

The challenges faced in a comparison of measuring the detection efficiency of free-running InGaAs/InP single-photon avalanche detectors (InGaAs/InP SPAD) were studied by four European National Metrology Institutes (NMIs) meeting at a single laboratory. The main purpose of this study is to develop a trustable measurement technique and to provide a snapshot of the methods used by the four NMIs for measuring such photon-counting detectors at telecom wavelengths in order to establish proper procedures for characterising such devices. The detection efficiency measurements were performed using different experimental setups and reference standards with independent traceability chains at the wavelength of 1550 nm. A dedicated model to correct the dead time and dark count effects on the SPAD’s free-running counting process was developed, allowing the correct value of the photon rate impinging on the detector to be recovered from simple ratemeter measurements. The detection efficiency was measured for mean photon number per pulse between 0.01 and 2.4, corresponding to photon rates between approximately 1100 photon/s and 193,000 photon/s, respectively. We found that the measured values reported by the participants are all consistent within the stated uncertainties, proving the consistency of the measurement approach developed.

### Quantifying backflash radiation to prevent zero-error attacks in quantum key distribution

Single-photon avalanche diodes (SPADs) are the most widespread commercial solution for single-photon counting in quantum key distribution applications. However, the secondary photon emission that arises from the avalanche of charge carriers that occurs during the detection of a photon may be exploited by an eavesdropper to gain information without inducing errors in the transmission key. In this paper, we characterize such backflash light in gated InGaAs/InP SPADs and discuss its spectral and temporal characterization for different detector models and different operating parameters. We qualitatively bound the maximum information leakage due to backflash light and propose solutions for preventing such leakage.

## Most relevant pubblications

C. Clivati, A. Meda, S. Donadello, S. Virzì, M. Genovese, F. Levi, A. Mura, M. Pittaluga, Z. Yuan, A.J. Shields, M. Lucamarini, I. Degiovanni, D. Calonico "Coherent phase transfer for real-world twin-field quantum key distribution" Nature Communications 13, 157 (2022)

H. Georgieva, A. Meda, S. Raupach, H. Hofer, M. Gramegna, I. Degiovanni, M. Genovese, M. Lopez, S. Kueck, “Detection of ultra-weak laser pulses by free-running single-photon detectors: modeling dead time and dark counts effects” Submittet to Quantum Science and Technology

M López, A Meda, G Porrovecchio, RA Starkwood, M Genovese, G Brida, M Šmid, CJ Chunnilall, IP Degiovanni, S Kück, “A study to develop a robust method for measuring the detection efficiency of free-running InGaAs/InP single-photon detectors”,EPJ Quantum Technology 7 (1), 14

M. Mondin, F. Daneshgaran, F. Di Stasio, S. Arnon, J. Kupferman, M. Genovese, I. Degiovanni, F. Piacentini, P. Traina, A. Meda, M. Gramegna, I. Bari, O. Khan, M. Khan, “Analysis, Design and Implementation of an End-to-End QKD Link”,Advanced Technologies for Security Applications, pp.55-64, Springer, Dordrecht

A. Meda, I. P. Degiovanni, A. Tosi, Z. Yuan, G. Brida and M. Genovese, “Quantifying the backflash radiation to prevent zero-error attacks in quantum key distribution” Light: Science & Applications, 2017, 6, e16261,

M. L. Rastello, I. P. Degiovanni, A. G. Sinclair, S. Kück, C. J. Chunnilall, G. Porrovecchio, M. Smid, F. Manoocheri, E. Ikonen, T. Kubarsepp, D. Stucki, K. S. Hong, S. K. Kim, A. Tosi, G. Brida, A. Meda, F. Piacentini, P. Traina, A. Al Natsheh, J. Y. Cheung, I. Müller, R. Klein and A. Vaigu, “Metrology for industrial quantum communications: the MIQC project” Metrologia, 2014, 51(6), S267-S275,

F. Piacentini, A. Meda, P. Traina, H. Kee Suk, I. Degiovanni, G. Brida, M. Gramegna, I. Ruo Berchera, M. Genovese, M. L. Rastello, “Measurement facility for the evaluation of the backscattering in fiber: realization of an OTDR operating at single photon level” International Journal of Quantum Information, 2014, 12 (02), 1461014