Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Effect of double pair emission to entanglement based QKD

Stefano Bettelli, Thomas Lorünser, Momtchil Peev, Edwin Querasser, Miloslav Dušek, Lucie Bartůšková, B. Blauensteiner, Hannes Hübel, Andreas Poppe, Anton Zeilinger

Published Jun 1, 2007• 6 citations
Publisher

Abstract

It is well-known that multi-photon signals in quantum key distribution (QKD) with weak laser pulses represent a security threat resulting in a reduced rate for secret-key production. Similarly, multi-pair contributions from spontaneous-parametric-down-conversion (SPDC) sources may cause information leak. In particular, in the case of polarization coding (i.e., when the polarization of photons is used to encode the logical qubits) a security risk occurs when the polarizations show some degree of correlation. In such a case an eavesdropper can perform a photon number splitting (PNS) attack by getting hold of, and analyzing, one photon out of the multi-photon state without causing any disturbance to the polarization of the other ones. It is therefore important to estimate the relative amount of polarization- correlated multi-pairs in SPDC signals. We present and discuss the results of a recent investigation on this subject. In our study, we consider a CW-SPDC-source scheme of Kwiat type, fully based on polarization-entanglement. The down-converted field is produced continuously, and a suitably chosen coincidence window replaces the concept of pulses.

DOI
10.1109/cleoe-iqec.2007.4387006

Related papers

Effect of source statistics on utilizing photon entanglement in quantum key distribution

Radim Hošák, Ivo Straka, Ana Predojević +2 more

A workflow for evaluation of entanglement source quality is proposed. Based on quantum state density matrices obtained from theoretical models and experimental data, we make an estimate of a potential performance of a quantum entanglement source in quantum key distribution protocols. This workflow is showcased for continuously pumped spontaneous parametric down-conversion (SPDC) source, where it highlights the trade-off between entangled pair generation rate and entanglement quality caused by multiphoton nature of the generated quantum states. We employ this characterization technique to show that secure key rate of down-converted photon pairs is limited to 0.029 bits per detection window due to intrinsic multiphoton contributions. We also report that there exists one optimum gain for continuous-wave down-conversion sources. We find a bound for secure key rate extracted from SPDC sources and make a comparison with perfectly single-pair quantum states, such as those produced by quantum dots.

arXiv:2008.07501•Physical Review A•Apr 13, 2021•16 citations

Feasibility of entanglement-based QKD protocols with SPDC and QD sources

Mariia Gumberidze, Vladyslav C. Usenko

We theoretically analyze the feasibility of entanglement-based quantum key distribution (QKD) protocols [1], [2], considering both spontaneous parametric down-conversion (SPDC) and quantum dot (QD) sources. We account for multiphoton emission in SPDC sources and fine-structure splitting (FSS) in QD. In addition, we incorporate imperfect detection, including dark counts and limited efficiency. For SPDC sources, we confirm the presence of vacuum and multiphoton pairs renders them unsuitable for secure device-independent (DI) QKD implementations. Conversely, with QD sources, accounting for the effects of FSS results in reduced secret key rates compared to ideal conditions. Our findings are crucial for the practical implementation of entanglement-based QKD protocols using realistic sources and detectors.

International Conference on Quantum Communications, Networking, and Computing (QCNC)•Mar 31, 2025•0 citations

Quantum Cryptography with Optical Entanglement at 1.5 µm

Alexander V. Sergienko, Martin A. Jaspan, Bahaa E. A. Saleh +1 more

We report on the engineering, preparation, and utilization of polarization entangled-photon states in the telecommunications window of 1.5 µm for secure quantum key distribution. Traditionally, such sources have been based on the use of the process of spontaneous parametric down conversion (SPDC) when nonlinear crystal is pumped by a continuous wave (cw) or a pulsed laser. The pump photon disintegrates spontaneously giving birth to two twin photons that are strongly correlated in frequency, polarization and wave vector (direction of propagation). Photon entanglement results from the fact that the possible values of correlated parameters of twin photons are made indistinguishable from each other either at the generation or at the single-photon detection stage. This leads to non-classical interference effects.

Frontiers in Optics•Jan 1, 2006•0 citations

Spontaneous Parametric Down Conversion Source Multi-Photon Component Reduction via Photon-Number-Resolving Detector

Ciro Bruscino, Matteo Di Giancamillo, Pasquale Ercolano +11 more

Quantum Key Distribution (QKD) is an optical communication technique that allows two authenticated parties to exchange an encryption key using protocols grounded in the laws of quantum mechanics, making the communication intrinsically secure. A fundamental requirement for key exchange through such protocols is the use of high-performance single-photon detectors and infrared single-photon sources with low multi-photon components. In addition to the rapid progress in single-photon sources (SPSs), achieving the ideal scenario where a source emits only single photons at telecom wavelength with high efficiency remains a theoretical concept. Multi-photon components emitted by non-ideal light sources such as lasers or Spontaneous Parametric Down Conversion (SPDC) sources can open the door to successful attacks from non-legitimate parties. In this work, we demonstrate, by using a Photon-Number-Resolving Detector (PNRD), based on parallel superconducting strips, the possibility of obtaining a single-photon source with extremely multi-photon components starting from a non-single-photon light source, namely an SPDC one. To estimate the reduction of the multi-photon component, we analyzed the second-order autocorrelation function, g(2)(0), of the light source using a PNRD modelled as 8 independent pixels in parallel, in a heralding protocol. We managed to obtain a reduction of the g(2)(0) using a CW SPDC with an emission rate of about 10 MHz using several sampling bin-widths, spanning from less than one nanoseconds to hundreds of nanoseconds. These results align with the goals of the Quantum Italy Deployment project, as sources with suppressed multi-photon components could play a key role in improving the performance of QKD systems.

IEEE Transactions on Applied Superconductivity•Mar 30, 2026•0 citations

The effect of multi-pair signal states in quantum cryptography with entangled photons

Miloslav Dušek, Kamil Brádler

Real sources of entangled photon pairs (like parametric down conversion) are not perfect. They produce quantum states that contain more than only one photon pair with some probability. In this paper it is discussed what happens if such states are used for the purpose of quantum key distribution. It is shown that the presence of "multi-pair" signals (together with low detection efficiencies) causes errors in transmission even if there is no eavesdropper. Moreover, it is shown that even the eavesdropping, that draws information only from these "multi-pair" signals, increases the error rate. Information, that can be obtained by an eavesdropper from these signals, is calculated.

arXiv:quant-ph/0011007•Journal of Optics B Quantum and Semiclassical Optics•Feb 5, 2002•8 citations

Quantum communication with spectrally correlated photons (Conference Presentation)

Piotr Leszek Kolenderski, Mikołaj Lasota, Karolina Sedziak +1 more

Single-photon sources are crucial components for the implementation of quantum communication protocols. However, photons emitted by some of the most popular types of realistic sources are spectrally broadband. Due to this drawback, the signal emitted from such sources is typically affected by the effect of temporal broadening during its propagation through telecommunication fibers which exhibit chromatic dispersion. Such problem can be observed e.g. when using sources based on the process of spontaneous parametric down-conversion (SPDC). In the case of long-distance quantum communication temporal broadening can significantly limit the efficiency of temporal filtering. It is a popular method, which is based on the reduction of the duration time of the detection window, used for decreasing the number of registered errors. In this work we analyzed the impact of the type of spectral correlation within a pair of photons produced by the SPDC source on the temporal width of those photons during their propagation in dispersive media. We found out that in some situations the width can be decreased by changing the typical negative spectral correlation into positive one or by reducing its strength. This idea can be used to increase the efficiency of the temporal filtering method. Therefore it can be applied in various implementations of quantum communication protocols. As an example of the application we subsequently analyzed the security of a quantum key distribution (QKD) scheme based on single photons. It was realized in the configuration with the source of photons located in the middle between the legitimate participants of a QKD protocol (called typically Alice and Bob). We demonstrated that when the information about the emission time of the photons produced by the SPDC source is not distributed to Alice and Bob, the maximal security distance can be considerably extended by using positively correlated photons, while in the opposite case strongly (no matter positively or negatively) correlated photons are optimal. We also found out that the results of our calculation may be very sensitive to the spectral widths of the photons produced by the SPDC source. In addition, we concluded that in realistic situation Alice and Bob would have to optimize their source over both the spectral widths of the generated photons and the type of spectral correlation in order to maximally extend the security distance. The results of our work are, in particular, important for the QKD systems utilizing commercial telecom fibers populated by strong classical signals. In those systems temporal filtering method can be used to reduce not only the dark counts registered by the detection system, but also the channel noise originating from the process of Raman scattering, which is the main factor limiting their performance.

Quantum Communications and Quantum Imaging•Sep 19, 2017•0 citations
It is well-known that multi-photon signals in quantum key distribution (QKD) with weak laser pulses represent a security threat resulting in a reduced rate for secret-key production. Similarly, multi-pair contributions from spontaneous-parametric-down-conversion (SPDC) sources may cause information leak. In particular, in the case of polarization coding (i.e., when the polarization of photons is used to encode the logical qubits) a security risk occurs when the polarizations show some degree of correlation. In such a case an eavesdropper can perform a photon number splitting (PNS) attack by getting hold of, and analyzing, one photon out of the multi-photon state without causing any disturbance to the polarization of the other ones. It is therefore important to estimate the relative amount of polarization- correlated multi-pairs in SPDC signals. We present and discuss the results of a recent investigation on this subject. In our study, we consider a CW-SPDC-source scheme of Kwiat type, fully based on polarization-entanglement. The down-converted field is produced continuously, and a suitably chosen coincidence window replaces the concept of pulses.
A workflow for evaluation of entanglement source quality is proposed. Based on quantum state density matrices obtained from theoretical models and experimental data, we make an estimate of a potential performance of a quantum entanglement source in quantum key distribution protocols. This workflow is showcased for continuously pumped spontaneous parametric down-conversion (SPDC) source, where it highlights the trade-off between entangled pair generation rate and entanglement quality caused by multiphoton nature of the generated quantum states. We employ this characterization technique to show that secure key rate of down-converted photon pairs is limited to 0.029 bits per detection window due to intrinsic multiphoton contributions. We also report that there exists one optimum gain for continuous-wave down-conversion sources. We find a bound for secure key rate extracted from SPDC sources and make a comparison with perfectly single-pair quantum states, such as those produced by quantum dots.
We theoretically analyze the feasibility of entanglement-based quantum key distribution (QKD) protocols [1], [2], considering both spontaneous parametric down-conversion (SPDC) and quantum dot (QD) sources. We account for multiphoton emission in SPDC sources and fine-structure splitting (FSS) in QD. In addition, we incorporate imperfect detection, including dark counts and limited efficiency. For SPDC sources, we confirm the presence of vacuum and multiphoton pairs renders them unsuitable for secure device-independent (DI) QKD implementations. Conversely, with QD sources, accounting for the effects of FSS results in reduced secret key rates compared to ideal conditions. Our findings are crucial for the practical implementation of entanglement-based QKD protocols using realistic sources and detectors.
We report on the engineering, preparation, and utilization of polarization entangled-photon states in the telecommunications window of 1.5 µm for secure quantum key distribution. Traditionally, such sources have been based on the use of the process of spontaneous parametric down conversion (SPDC) when nonlinear crystal is pumped by a continuous wave (cw) or a pulsed laser. The pump photon disintegrates spontaneously giving birth to two twin photons that are strongly correlated in frequency, polarization and wave vector (direction of propagation). Photon entanglement results from the fact that the possible values of correlated parameters of twin photons are made indistinguishable from each other either at the generation or at the single-photon detection stage. This leads to non-classical interference effects.
Quantum Key Distribution (QKD) is an optical communication technique that allows two authenticated parties to exchange an encryption key using protocols grounded in the laws of quantum mechanics, making the communication intrinsically secure. A fundamental requirement for key exchange through such protocols is the use of high-performance single-photon detectors and infrared single-photon sources with low multi-photon components. In addition to the rapid progress in single-photon sources (SPSs), achieving the ideal scenario where a source emits only single photons at telecom wavelength with high efficiency remains a theoretical concept. Multi-photon components emitted by non-ideal light sources such as lasers or Spontaneous Parametric Down Conversion (SPDC) sources can open the door to successful attacks from non-legitimate parties. In this work, we demonstrate, by using a Photon-Number-Resolving Detector (PNRD), based on parallel superconducting strips, the possibility of obtaining a single-photon source with extremely multi-photon components starting from a non-single-photon light source, namely an SPDC one. To estimate the reduction of the multi-photon component, we analyzed the second-order autocorrelation function, g(2)(0), of the light source using a PNRD modelled as 8 independent pixels in parallel, in a heralding protocol. We managed to obtain a reduction of the g(2)(0) using a CW SPDC with an emission rate of about 10 MHz using several sampling bin-widths, spanning from less than one nanoseconds to hundreds of nanoseconds. These results align with the goals of the Quantum Italy Deployment project, as sources with suppressed multi-photon components could play a key role in improving the performance of QKD systems.
Real sources of entangled photon pairs (like parametric down conversion) are not perfect. They produce quantum states that contain more than only one photon pair with some probability. In this paper it is discussed what happens if such states are used for the purpose of quantum key distribution. It is shown that the presence of "multi-pair" signals (together with low detection efficiencies) causes errors in transmission even if there is no eavesdropper. Moreover, it is shown that even the eavesdropping, that draws information only from these "multi-pair" signals, increases the error rate. Information, that can be obtained by an eavesdropper from these signals, is calculated.
Single-photon sources are crucial components for the implementation of quantum communication protocols. However, photons emitted by some of the most popular types of realistic sources are spectrally broadband. Due to this drawback, the signal emitted from such sources is typically affected by the effect of temporal broadening during its propagation through telecommunication fibers which exhibit chromatic dispersion. Such problem can be observed e.g. when using sources based on the process of spontaneous parametric down-conversion (SPDC). In the case of long-distance quantum communication temporal broadening can significantly limit the efficiency of temporal filtering. It is a popular method, which is based on the reduction of the duration time of the detection window, used for decreasing the number of registered errors. In this work we analyzed the impact of the type of spectral correlation within a pair of photons produced by the SPDC source on the temporal width of those photons during their propagation in dispersive media. We found out that in some situations the width can be decreased by changing the typical negative spectral correlation into positive one or by reducing its strength. This idea can be used to increase the efficiency of the temporal filtering method. Therefore it can be applied in various implementations of quantum communication protocols. As an example of the application we subsequently analyzed the security of a quantum key distribution (QKD) scheme based on single photons. It was realized in the configuration with the source of photons located in the middle between the legitimate participants of a QKD protocol (called typically Alice and Bob). We demonstrated that when the information about the emission time of the photons produced by the SPDC source is not distributed to Alice and Bob, the maximal security distance can be considerably extended by using positively correlated photons, while in the opposite case strongly (no matter positively or negatively) correlated photons are optimal. We also found out that the results of our calculation may be very sensitive to the spectral widths of the photons produced by the SPDC source. In addition, we concluded that in realistic situation Alice and Bob would have to optimize their source over both the spectral widths of the generated photons and the type of spectral correlation in order to maximally extend the security distance. The results of our work are, in particular, important for the QKD systems utilizing commercial telecom fibers populated by strong classical signals. In those systems temporal filtering method can be used to reduce not only the dark counts registered by the detection system, but also the channel noise originating from the process of Raman scattering, which is the main factor limiting their performance.