© 2026 Aqora Quantum S.A.S.

Terms Privacy Legal Notice

Quantum Communication and Quantum Networking

Entanglement Based Quantum Key Distribution Using a Bright Sagnac Entangled Photon Source

Chris Erven, Deny R. Hamel, Kevin J. Resch, Raymond Laflamme, Gregor Weihs

Published Oct 11, 2012• 1 citation

Abstract

We report on improvements in an entangled free-space quantum key distribution (QKD) system by replacing the original non-collinear type-II spontaneous parametric down-conversion (SPDC) polarization entangled photon source with a new brighter Sagnac interferometric entangled photon source. While the SPDC source was integral to the initial setup of the system, it was limited in photon pair production rate and entanglement quality. Initial experiments with the new Sagnac source have already yielded substantially higher entangled photon rates and improved visibilities. In order to examine the integration of the new source with the QKD system, a local QKD experiment is performed where the source is pumped with 5 mW of power yielding an average raw key rate of 9,423 bits/s and an average final secret key rate of 2,695 bits/s, with an observed average QBER of 2.48%. Initial experiments distributing entangled photons over a single 1,305 m free-space link have seen entangled photon pair coincident detection rates as high as 3,000 cps. Extrapolating based on these initial numbers and previous experiments, we hope to obtain an average secret key rate of 715 bits/s for a two free-space link QKD experiment running the source at full power which will represent an order of magnitude increase over our previous experiments. An additional benefit of the new source is that it has a much narrower bandwidth which will aid in making the system compatible with daylight experiments. However, one drawback of the source is an appreciable double pair emission rate which initial experiments indicate.

Related papers

Entanglement based free-space quantum key distribution

Chris Erven, Christophe Couteau, Raymond Laflamme +1 more

We report on the progress of our real-time entanglement based free-space quantum key distribution (QKD) system which uses polarization entangled photon pairs sent over a variety of free-space optical telescope links to distribute the key. An experiment with one photon from each pair sent over a 1,325 m long free-space link and the other photon detected locally next to the source is described. The system performs the complete QKD protocol including all error correction and privacy amplification algorithms. Over the course of 6.5 hours of communication at night an average raw key rate of 1,398 bits/s with an average quantum bit error rate (QBER) of 4.58% was observed producing an average final key rate of 244 bits/s. We also performed a Bell inequality experiment over two free-space links of 435 m and 1,325 m respectively, producing a total separation of 1,575 m between the two detectors in the experiment. During the Bell inequality experiment we observed an average Bell parameter of 2.51 ± 0.11.

Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Jun 18, 2008•4 citations

Entangled quantum key distribution over two free-space optical links

Chris Erven, Christophe Couteau, Raymond Laflamme +1 more

We report on the first real-time implementation of a quantum key distribution (QKD) system using entangled photon pairs that are sent over two free-space optical telescope links. The entangled photon pairs are produced with a type-II spontaneous parametric down-conversion source placed in a central, potentially untrusted, location. The two free-space links cover a distance of 435 m and 1,325 m respectively, producing a total separation of 1,575 m. The system relies on passive polarization analysis units, GPS timing receivers for synchronization, and custom written software to perform the complete QKD protocol including error correction and privacy amplification. Over 6.5 hours during the night, we observed an average raw key generation rate of 565 bits/s, an average quantum bit error rate (QBER) of 4.92%, and an average secure key generation rate of 85 bits/s.

arXiv:0807.2289•Optics Express•Oct 7, 2008•96 citations

A space-suitable, high brilliant entangled photon source for satellite based quantum key distribution

Erik Beckert, Oliver de Vries, Rupert Ursin +3 more

Quantum Key Distribution (QKD) will be the first applications for quantum technologies, addressing information security needs by providing a tool for quantum encryption. An Entangled Photon Source EPS, the key building block for QKD, is presented that is based on a non-linear, Spontaneous Parametric Down Conversion SPDC process in a temperature controlled ppKTP crystal that is double-side pumped at 405 nm in a sagnac interferometer setup. The polarization entangled photons are emitted at 810 nm and at a rate of up to ca. 300k per second at 8 mW pumping power. Visibilities in the HV and the DA polarization base in the range of 97-99% have been measured. The EPS integration was realized in a ruggedized, minimized setup to prove space-suitability during thermal and mechanical environmental testing.

Free-space laser communication technologies. Conference•Mar 4, 2019•6 citations

Spectrally engineered collinear type-0 SPDC source with enhanced spectral brightness for entanglement distribution

Dong-Gil Im, Jungmo Lee, Kyungdeuk Park +3 more

Entangled photon sources with high spectral brightness are important resources for photonic quantum information processing, particularly in quantum communication and quantum networking where usable photon flux of entangled photons is often constrained by channel loss and source inefficiency. Here, we demonstrate a spectrally engineered type-0 spontaneous parametric down-conversion (SPDC) source with enhanced spectral brightness for entanglement distribution. By pumping a 30-mm ppKTP crystal with an ultra-narrowband laser slightly detuned from degeneracy, photon-pair generation is concentrated into a narrow spectral bandwidth while retaining the strong nonlinear interaction of type-0 phase matching. The source produces a coincidence rate of 44.6 kHz corresponding to a detected spectral brightness of 0.507 MHz/mW/nm. We further integrate the source into a Sagnac interferometer to generate polarization-entangled photon pairs and demonstrate entanglement distribution through a 2.56 km free-space round-trip channel. Our results show that spectral engineering provides a practical route to compact, spectrally bright entangled-photon sources for quantum communication applications.

quant-pharXiv:2606.24036•Jun 24, 2026•0 citations

Strategies to achieve high brightness in collinear Sagnac polarization entangled photon source

Jay B. Bhatt, Parthkumar V. Sakhiya, Adarsh Jain +2 more

Entangled photon source is a basic building block of various quantum technologies such as quantum communication, QKD, quantum metrology, etc. Spontaneous parametric down conversion (SPDC) process is widely reported method for generation of entanglement due to its ease of implementation and stable performance. In this paper, we report design and development of type-II Sagnac loop based entangled photon source with a high brightness of $8.42\times 10^{4}$ pairs/s/mW with 98.18% visibility, a CHSH inequality violation of S=2.777±0.006 and state fidelity of 97.8%. An automated test setup has also been developed for carrying out the measurements. The essential formulated design procedure to achieve optimal performance has been discussed and presented. The measured performance of developed source is also compared to other reported sources having the same configuration. In this work, higher brightness is achieved with comparable fidelity to previously reported works. This methodology can be extended to any collinear Sagnac loop based entangled photon sources.

IEEE Microwaves, Antennas, and Propagation Conference (MAPCON)•Dec 11, 2023•0 citations

Pulsed Sagnac source of narrow-band polarization-entangled photons

Onur Kuzucu, Franco N. C. Wong

We demonstrate pulsed operation of a bidirectionally pumped polarization Sagnac interferometric down-conversion source and its generation of narrow-band, high-visibility polarization-entangled photons. Driven by a narrow-band, mode-locked pump at 390.35 nm, the phase-stable Sagnac source with a type-II phase-matched periodically poled ${\text{KTiOPO}}_{4}$ crystal is capable of producing 0.01 entangled pair per pulse in a 0.15-nm bandwidth centered at 780.7 nm with 1 mW of average pump power at a repetition rate of 31.1 MHz. We have achieved a mean photon-pair generation rate of as high as 0.7 pair per pulse, at which multipair events dominate and significantly reduce the two-photon quantum-interference visibility. For low generation probability $\ensuremath{\alpha}$, the reduced visibility $V=1\ensuremath{-}\ensuremath{\alpha}$ is independent of the throughput efficiency and of the polarization analysis basis, which can be utilized to yield an accurate estimate of the generation rate $\ensuremath{\alpha}$. At low $\ensuremath{\alpha}$ we have characterized the source entanglement quality in three different ways: average quantum-interference visibility of 99%, the Clauser-Horne-Shimony-Holt $S$ parameter of $2.739\ifmmode\pm\else\textpm\fi{}0.119$, and quantum state tomography with 98.85% singlet-state fidelity. The narrow-band pulsed Sagnac source of entangled photons is suitable for use in quantum information processing applications such as free-space quantum key distribution.

arXiv:0710.5390•Physical Review A•Mar 7, 2008•87 citations

We report on improvements in an entangled free-space quantum key distribution (QKD) system by replacing the original non-collinear type-II spontaneous parametric down-conversion (SPDC) polarization entangled photon source with a new brighter Sagnac interferometric entangled photon source. While the SPDC source was integral to the initial setup of the system, it was limited in photon pair production rate and entanglement quality. Initial experiments with the new Sagnac source have already yielded substantially higher entangled photon rates and improved visibilities. In order to examine the integration of the new source with the QKD system, a local QKD experiment is performed where the source is pumped with 5 mW of power yielding an average raw key rate of 9,423 bits/s and an average final secret key rate of 2,695 bits/s, with an observed average QBER of 2.48%. Initial experiments distributing entangled photons over a single 1,305 m free-space link have seen entangled photon pair coincident detection rates as high as 3,000 cps. Extrapolating based on these initial numbers and previous experiments, we hope to obtain an average secret key rate of 715 bits/s for a two free-space link QKD experiment running the source at full power which will represent an order of magnitude increase over our previous experiments. An additional benefit of the new source is that it has a much narrower bandwidth which will aid in making the system compatible with daylight experiments. However, one drawback of the source is an appreciable double pair emission rate which initial experiments indicate.

We report on the progress of our real-time entanglement based free-space quantum key distribution (QKD) system which uses polarization entangled photon pairs sent over a variety of free-space optical telescope links to distribute the key. An experiment with one photon from each pair sent over a 1,325 m long free-space link and the other photon detected locally next to the source is described. The system performs the complete QKD protocol including all error correction and privacy amplification algorithms. Over the course of 6.5 hours of communication at night an average raw key rate of 1,398 bits/s with an average quantum bit error rate (QBER) of 4.58% was observed producing an average final key rate of 244 bits/s. We also performed a Bell inequality experiment over two free-space links of 435 m and 1,325 m respectively, producing a total separation of 1,575 m between the two detectors in the experiment. During the Bell inequality experiment we observed an average Bell parameter of 2.51 ± 0.11.

We report on the first real-time implementation of a quantum key distribution (QKD) system using entangled photon pairs that are sent over two free-space optical telescope links. The entangled photon pairs are produced with a type-II spontaneous parametric down-conversion source placed in a central, potentially untrusted, location. The two free-space links cover a distance of 435 m and 1,325 m respectively, producing a total separation of 1,575 m. The system relies on passive polarization analysis units, GPS timing receivers for synchronization, and custom written software to perform the complete QKD protocol including error correction and privacy amplification. Over 6.5 hours during the night, we observed an average raw key generation rate of 565 bits/s, an average quantum bit error rate (QBER) of 4.92%, and an average secure key generation rate of 85 bits/s.

Quantum Key Distribution (QKD) will be the first applications for quantum technologies, addressing information security needs by providing a tool for quantum encryption. An Entangled Photon Source EPS, the key building block for QKD, is presented that is based on a non-linear, Spontaneous Parametric Down Conversion SPDC process in a temperature controlled ppKTP crystal that is double-side pumped at 405 nm in a sagnac interferometer setup. The polarization entangled photons are emitted at 810 nm and at a rate of up to ca. 300k per second at 8 mW pumping power. Visibilities in the HV and the DA polarization base in the range of 97-99% have been measured. The EPS integration was realized in a ruggedized, minimized setup to prove space-suitability during thermal and mechanical environmental testing.

Entangled photon sources with high spectral brightness are important resources for photonic quantum information processing, particularly in quantum communication and quantum networking where usable photon flux of entangled photons is often constrained by channel loss and source inefficiency. Here, we demonstrate a spectrally engineered type-0 spontaneous parametric down-conversion (SPDC) source with enhanced spectral brightness for entanglement distribution. By pumping a 30-mm ppKTP crystal with an ultra-narrowband laser slightly detuned from degeneracy, photon-pair generation is concentrated into a narrow spectral bandwidth while retaining the strong nonlinear interaction of type-0 phase matching. The source produces a coincidence rate of 44.6 kHz corresponding to a detected spectral brightness of 0.507 MHz/mW/nm. We further integrate the source into a Sagnac interferometer to generate polarization-entangled photon pairs and demonstrate entanglement distribution through a 2.56 km free-space round-trip channel. Our results show that spectral engineering provides a practical route to compact, spectrally bright entangled-photon sources for quantum communication applications.

Entangled photon source is a basic building block of various quantum technologies such as quantum communication, QKD, quantum metrology, etc. Spontaneous parametric down conversion (SPDC) process is widely reported method for generation of entanglement due to its ease of implementation and stable performance. In this paper, we report design and development of type-II Sagnac loop based entangled photon source with a high brightness of

8.42\times 10^{4}

pairs/s/mW with 98.18% visibility, a CHSH inequality violation of S=2.777±0.006 and state fidelity of 97.8%. An automated test setup has also been developed for carrying out the measurements. The essential formulated design procedure to achieve optimal performance has been discussed and presented. The measured performance of developed source is also compared to other reported sources having the same configuration. In this work, higher brightness is achieved with comparable fidelity to previously reported works. This methodology can be extended to any collinear Sagnac loop based entangled photon sources.

We demonstrate pulsed operation of a bidirectionally pumped polarization Sagnac interferometric down-conversion source and its generation of narrow-band, high-visibility polarization-entangled photons. Driven by a narrow-band, mode-locked pump at 390.35 nm, the phase-stable Sagnac source with a type-II phase-matched periodically poled

{\text{KTiOPO}}_{4}

crystal is capable of producing 0.01 entangled pair per pulse in a 0.15-nm bandwidth centered at 780.7 nm with 1 mW of average pump power at a repetition rate of 31.1 MHz. We have achieved a mean photon-pair generation rate of as high as 0.7 pair per pulse, at which multipair events dominate and significantly reduce the two-photon quantum-interference visibility. For low generation probability $\ensuremath{\alpha}$, the reduced visibility $V=1\ensuremath{-}\ensuremath{\alpha}$ is independent of the throughput efficiency and of the polarization analysis basis, which can be utilized to yield an accurate estimate of the generation rate $\ensuremath{\alpha}$. At low $\ensuremath{\alpha}$ we have characterized the source entanglement quality in three different ways: average quantum-interference visibility of 99%, the Clauser-Horne-Shimony-Holt

S

parameter of $2.739\ifmmode\pm\else\textpm\fi{}0.119$, and quantum state tomography with 98.85% singlet-state fidelity. The narrow-band pulsed Sagnac source of entangled photons is suitable for use in quantum information processing applications such as free-space quantum key distribution.