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Free-space laser communication technologies. Conference

Studies on a time-energy entangled photon pair source and superconducting nanowire single-photon detectors for increased quantum system efficiency

Evan J. Katz, Benjamin Child, Ian R. Nemitz, Brian E. Vyhnalek, Tony Roberts, Andrew Hohne, Bertram M. Floyd, Jonathan R. Dietz, John Lekki

Published Mar 4, 2019• 0 citations
Publisher

Abstract

Time-energy entangled photon pairs are created by a system consisting of a 1064 nm pump diode laser that is fiber coupled to a high generation rate photon pair source. The source is a dual element periodically poled Magnesium Oxide doped Lithium Niobate (MgO:LN) waveguide that upconverts 1064 nm photons to single 532 nm photons in the first stage. In the second stage, the green photons are down converted to time-energy entangled photon pairs at 794 nm and 1614 nm. The output photon pairs are guided by fiber to sorting optics where they are separated and sent into high-efficiency photon detectors. In particular, the 1614 nm photons are detected by a superconducting nanowire with efficiency near 85% and dead time less than 30 ns. Detector output electrical signals are sent to a time tagger with bin resolution as narrow as 25 ps for coincidence counting. The ultimate goal of this setup is to demonstrate a single-source, high efficiency, high data rate, low noise, quantum communication system to enable Earth-space quantum networks. Test results that characterize the time-energy entangled photon pair creation rates of our source will be presented, via measures of accidental and true coincidence rates versus pump current. To reduce noise (accidentals) as much as possible, and for better understanding of our overall quantum system path-efficiency, studies of fluorescence caused by our pump’s 1064 nm and 532 nm photons will be investigated and discussed. Finally, characteristic measurements of our superconducting nanowire detector, such as dead time and detection efficiency versus electrical bias, will be offered. Please verify that (1) all pages are present, (2) all figures are correct, (3) all fonts and special characters are correct, and (4) all text and figures fit within the red margin lines shown on this review document. Complete formatting information is available at http://SPIE.org/manuscripts Return to the Manage Active Submissions page at http://spie.org/submissions/tasks.aspx and approve or disapprove this submission. Your manuscript will not be published without this approval. Please contact author_help@spie.org with any questions or concerns.

DOI
10.1117/12.2508736

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quant-pharXiv:2610.41709•Nov 2, 2026
Time-energy entangled photon pairs are created by a system consisting of a 1064 nm pump diode laser that is fiber coupled to a high generation rate photon pair source. The source is a dual element periodically poled Magnesium Oxide doped Lithium Niobate (MgO:LN) waveguide that upconverts 1064 nm photons to single 532 nm photons in the first stage. In the second stage, the green photons are down converted to time-energy entangled photon pairs at 794 nm and 1614 nm. The output photon pairs are guided by fiber to sorting optics where they are separated and sent into high-efficiency photon detectors. In particular, the 1614 nm photons are detected by a superconducting nanowire with efficiency near 85% and dead time less than 30 ns. Detector output electrical signals are sent to a time tagger with bin resolution as narrow as 25 ps for coincidence counting. The ultimate goal of this setup is to demonstrate a single-source, high efficiency, high data rate, low noise, quantum communication system to enable Earth-space quantum networks. Test results that characterize the time-energy entangled photon pair creation rates of our source will be presented, via measures of accidental and true coincidence rates versus pump current. To reduce noise (accidentals) as much as possible, and for better understanding of our overall quantum system path-efficiency, studies of fluorescence caused by our pump’s 1064 nm and 532 nm photons will be investigated and discussed. Finally, characteristic measurements of our superconducting nanowire detector, such as dead time and detection efficiency versus electrical bias, will be offered. Please verify that (1) all pages are present, (2) all figures are correct, (3) all fonts and special characters are correct, and (4) all text and figures fit within the red margin lines shown on this review document. Complete formatting information is available at http://SPIE.org/manuscripts Return to the Manage Active Submissions page at http://spie.org/submissions/tasks.aspx and approve or disapprove this submission. Your manuscript will not be published without this approval. Please contact author_help@spie.org with any questions or concerns.
Entangled photon pairs are created by a system consisting of a 1064 nm pump diode laser that is fiber coupled to a high generation rate photon pair source. The source is a dual element periodically poled potassium titanyl phosphate (KTP) waveguide that up-converts 1064 nm photons to single 532 nm photons in the first stage. In the second stage, the green photons are down converted to energy entangled photon pairs at 800 nm and 1600 nm. The output photon pairs are guided by fiber to sorting optics where they are separated and sent into high-efficiency photon detectors. In particular, the 1600 nm photons are detected by a superconducting nanowire with efficiency over 60% and dead time less than 50 ns. Detector output electrical signals are sent to a time tagger with bin resolution as narrow as 25 ps for coincidence counting. The ultimate goal of this setup is to demonstrate a singlesource, high efficiency, high data rate, quantum communication system to enable Earth-space quantum networks. Of particular interest is a source of entangled photons that is amenable to utilization in aircraft and spacecraft under rigorous flight and environmental conditions. Test results that characterize the entangled photon pair creation and detection capabilities of our system will be presented.
A periodically poled MgO – doped LiNbO3 (MgO:LN) non-degenerate photon pair source is utilized for spontaneous parametric down-conversion of 532 nm photons into time-energy entangled pairs of 794 and 1614 nm photons. The entangled photons are separated using previously detailed sorting optics, such that each wavelength is independently directed through one of two modified Mach-Zehnder interferometers – also known as a Franson interferometer – after which they are fiber-optically guided to high-efficiency photon detectors. Output from the detectors is sent to a high resolution time tagger, where coincidences between the entangled photons are recorded. By varying the length of the long path in one Mach-Zehnder interferometer, it is possible to observe high visibility sinusoidal fringes in the measured coincidence rates (while no variation is seen in single photon detection rates). These fringes – due to interference between the photon probability amplitudes – are indicative of a violation of the Bell inequality, and confirm inconsistencies with local hidden variable theory for the correlations of the time-energy entangled photon pairs.
The time-energy entangled photon source is a core resource in quantum key distribution, quantum teleportation, and quantum metrology. The output of orthogonally polarized photon pairs can enhance the effective coincidence counts of the entangled source in these applications. Additionally, an all-fiber entangled source offers small size and high mobility advantages. However, most current orthogonally polarized all-fiber time-energy entangled sources are limited to a bandwidth of around 1 nm, which is detrimental to the effective utilization of channel resources in fiber networks and restricts precision in quantum metrology. To address this issue, we employ a fiber-coupled 775 nm narrow linewidth pump laser in a periodically poled lithium niobate (PPLN) waveguide to achieve type-II spontaneous parametric down-conversion (SPDC). This results in an all-fiber orthogonally polarized time-energy entangled photon source with a bandwidth greater than 7 nm. We measured the single counts, coincidence counts, coincidence to accidental coincidence ratio (CAR), single photon spectra, and joint spectral intensity of the entangled source, calculating the Schmidt number S = 7.8. Furthermore, we measured Hong-Ou-Mandel (HOM) interference by inserting two 2 nm bandpass filters, and the interference visibility remained around 95.8% after continuous operation for 3 hours. The experimental results indicate that while maintaining an all-fiber structure, this source possesses a broadband range, low noise, and high stability, providing efficient and reliable biphoton resources for applications in quantum communication, quantum networks, and precision metrology.
Fast and efficient detection of single photons with high timing accuracy is a crucial requirement in most quantum optics experiments and enables novel sensing and imaging solutions. Superconducting nanowire single-photon detectors (SNSPD) achieve technology-leading performance in terms of detection efficiency, dark count rate, timing jitter, and detector dead times. However, conventional SNSPDs with high system detection efficiency typically rely on resonant enhancement of the absorption efficiency, thus only achieving attractive detector benchmarks over narrow spectral windows. Waveguide-integrated SNSPDs allow for leveraging the wideband material absorption in superconducting nanowires by absorbing light in a traveling-wave geometry but have been limited to low system detection efficiencies due to interface losses when coupling to optical fibers. Here, we show how high system detection efficiencies of 22%–73% are realized over a broad wavelength range from 532 nm to 1640 nm in a single waveguide-integrated SNSPD device. We accomplish efficient coupling between optical fibers and waveguide-integrated nanowire detectors by employing a 3D interface, produced in direct laser writing, that relies on total internal reflection for achieving a broad transmission bandwidth. We further find low timing jitter of 25.7 ps and detector decay times of 9.8 ns, allowing for single-photon counting with high repetition rates up to 100 MHz. Our work paves the way for an efficient single-photon detector solution that combines the spectral requirements of an extremely wide range of quantum optics experiments in a single device. The coupling approach and SNSPD-integration with nanophotonic circuits are further well-suited for realizing large-scale detector arrays.
Superconducting nanowire single photon detectors (SNSPDs) have separately demonstrated high efficiency, low noise, and extremely high speed when detecting single photons. However, achieving all of these simultaneously has been limited by detector subtleties and tradeoffs. Here, we report an SNSPD system with <80 ps timing resolution, kHz noise count rates, and 76% fiber-coupled system detection efficiency in the low-flux limit at 1550 nm. We present a model for determining the detection efficiency penalty due to the detection recovery time, and we validate our method using experimental data obtained at high count rates. We demonstrate improved performance tradeoffs, such as 68% system detection efficiency, including losses due to detector recovery time, when coupled to a Poisson source emitting 100 million photons per second. Our system can provide limited photon number resolution, continuous cryogen-free operation, and scalability to future imaging and GHz-count-rate applications.
In a commercial entanglement-distribution system over telecom fibre, the single-photon detectors are the largest part of the hardware cost. In entanglement-based quantum key distribution (QKD), both photons of each pair must be detected. With two telecom photons, each arm needs either an InGaAs single-photon avalanche diode (SPAD) of low efficiency or a superconducting nanowire single-photon detector (SNSPD) with its own cryostat. We study a highly nondegenerate photon-pair source that needs a telecom detector on one arm only. Two periodically poled KTiOPO4_44​ (ppKTP) crystals pumped at 405~nm emit pairs at 548 and 1550~nm. A Si-SPAD with 82\% efficiency detects the 548~nm photon at the source node, and only the 1550~nm photon travels through fibre. A first, unoptimised module in a polarisation-correlated configuration generates about 1.5×1061.5\times10^61.5×106 pairs/s per mW of pump power. It collects 13\% of the pairs on the 548~nm arm and 23\% on the 1550~nm arm. In an asymptotic BBM92 model, we compare its links with generic 2×15502\times15502×1550~nm links that use a bright published source at the same pump power. At 50~km, our link with one SNSPD costs about 25\% more per secret bit than the generic link with one SNSPD. With optimistic target values of 52\% collection and twice the brightness, it costs about 40\% as much as the generic link with two SNSPDs. Our link without a cryostat then costs about 70\% less than the generic link with one SNSPD. If every link runs at its own best pump power, the two-SNSPD link is again the cheapest. The source can also serve clock synchronisation and detector calibration, which need only time-correlated pairs. All key rates assume the entangled configuration of the source, which is under development.