Photon Source May Lower Telecom Detector Costs for QKD

Researchers at Qubitrium B.V. and Qubitrium A.S. have developed a photon-pair source generating approximately 1.5 x 10^6 pairs per second per milliwatt of pump power, a rate designed to significantly reduce hardware costs for quantum key distribution systems. The source emits photons at 548 and 1550 nanometers, achieving 82% detection efficiency on the 548nm arm with a silicon single-photon avalanche diode while transmitting the 1550nm photon through fiber.

This asymmetry allows for a system requiring only one expensive telecom detector, potentially lowering costs compared to conventional two-detector links. The work models a link at 50km, finding that an optimized version could reduce costs to 40% of a link using two superconducting nanowire single-photon detectors.

Telecom Detector Cost Reduction with Nondegenerate Pairs

This performance stems from a novel approach using two periodically poled KTiOPO4 crystals pumped at 405nm, emitting pairs at 548 and 1550nm wavelengths. The initial module, operating in a polarization-correlated configuration, collects 13% of the generated pairs on the 548nm arm and 23% on the 1550nm arm. These collection rates were incorporated into an asymptotic BBM92 model to assess performance. The model compared links using this new source against those employing a conventional dual 1550nm setup.

At a 50km distance, a link utilizing this source and one SNSPD currently costs approximately 25% more per secret bit than a generic link also using one SNSPD, and about six times as much as a generic link requiring two SNSPDs. Researchers found that with optimized target values, specifically, 52% collection and twice the brightness, the cost per bit could be reduced to 40% of that of a link relying on two SNSPDs.

ppKTP Crystal Design for 548 and 1550 nm Emission

The choice of crystal material impacts the robustness of quantum key distribution systems. Researchers opted for periodically poled potassium titanyl phosphate (ppKTP) over lithium niobate (ppLN) due to its higher damage threshold and greater stability against temperature fluctuations when using a 405nm pump laser. This decision addresses a practical concern in maintaining consistent performance over time, a critical factor for real-world deployment of quantum communication technologies.

Type-0 quasi-phase matching was selected for its brightness, despite requiring tighter thermal and spectral control, demonstrating a trade-off between efficiency and stability in the system’s design. Two distinct configurations were designed, beginning with a correlated setup using two ppKTP crystals with orthogonal axes to produce polarization-correlated, but not entangled, photon pairs. A diagonally polarized pump laser passes through both crystals sequentially, initiating the down-conversion process that generates photon pairs at 548nm and 1550nm wavelengths.

Precise temperature stabilization, maintained to within ±0. 1°C for each crystal, is essential for achieving the desired emission wavelengths and maximizing the efficiency of the photon pair generation. Lens systems and filters further refine the output, removing residual pump light and preparing the signal for transmission. A second lens collimates the generated photon pairs, and a long-pass filter eliminates any remaining pump radiation. This careful control over the optical path ensures a clean signal, minimizing noise and maximizing the quality of the entangled photons.

“We prefer ppKTP over ppLN for a UV pump, because it has a higher damage threshold and is more robust to temperature drift,” the researchers state, highlighting the practical advantages of their material selection. The design prioritizes reliable operation and efficient photon pair generation for future quantum communication networks.

BBM92 Key Rate Modeling with Detector Efficiencies

The current design achieves a pair-generation rate of 1.53 MHz per milliwatt of pump power, a figure 2.7 times higher than a comparable source reported elsewhere. This brightness is important because it directly impacts the overall efficiency of quantum key distribution systems, particularly when aiming to reduce reliance on costly detectors. Unequal heralding efficiencies present a challenge, with the 548nm arm registering 10.8% efficiency against 2.5% on the 1550nm arm at 6.3mW.

The registered rates and efficiencies are presented without correction for afterpulse probability, a factor that would likely influence the final performance metrics. Despite these limitations, the product of the two arm efficiencies remains a key determinant of the coincidence rate, impacting the overall security and speed of the quantum communication link. A symmetric 1550nm link requires two telecom-grade detectors, meaning the coincidence rate is affected by the square of the 1550nm detection efficiency. The team’s modeling suggests that with optimized target values, this approach could significantly reduce the cost of quantum communication infrastructure.

These calculations consider detector dead time and afterpulsing effects within a BBM92 model. A first, unoptimized module in a polarization-correlated configuration generates about 1.5× ${10}^6$ pairs/s per mW of pump power. Further simulations explored the impact of improved collection efficiency on both secret-key rate and cost.

548 nm Si-SPAD Performance in the Photon Source

This performance relies on a highly nondegenerate photon-pair source emitting at 548 and 1550 nanometers, designed to require a telecom detector on only one transmission arm. Current measurements reveal heralding efficiencies of 0.111 for the 548nm channel and 0.046 for the 1550nm channel, values corrected for detector dead time and raw efficiencies of 9.7% and 1.6% respectively. These efficiencies, combined with a Si-SPAD efficiency of 82% during measurement, contribute to the overall system performance.

Non-Entangled Configuration and Polarization Correlation

The source generates approximately 1. 53 MHz/mW exceeds that of a comparable source reported by Ref., which achieved 0. 56 MHz/mW. While the source is designed for eventual entanglement distribution, current analysis focuses on its performance as a polarization-correlated system, treating the emitted photon pairs as a separable mixture rather than a coherent, entangled state. The source can also serve pair-based clock synchronisation and the calibration of telecom detectors. These uses need only the time correlation of the pairs, not their entanglement.

The team’s approach allows for applications beyond QKD, including pair-based clock synchronization and calibration of telecom detectors, which do not require entangled photons. Raw registered rates on each arm, along with coincidence counts, are used to calculate heralding efficiencies and the overall pair-generation rate.

The analysis rests on the understanding that the source’s physical asymmetry, requiring a telecom detector on only one arm, offers a potential cost advantage. But this advantage is contingent on demonstrating entanglement in future iterations. The researchers state, emphasizing this work as a foundational step toward a commercially viable system.

Beyond QKD: Applications of Nondegenerate Photon Pairs

While designed with quantum key distribution in mind, the source’s capabilities extend to applications requiring correlated photon pairs without the need for entanglement, such as precise clock synchronization and calibration of sensitive telecom detectors. These alternative uses rely solely on the timing correlation of the generated photons, bypassing the complexities of establishing and verifying entanglement. This approach contrasts with conventional entanglement-based QKD systems, which typically demand high-performance detectors on both ends of the communication link.

The potential for applications beyond QKD is significant, as the source can be deployed for tasks needing only correlated photon arrival times. “Clock synchronisation with photon pairs and detector calibration with correlated photons need only the time correlation of the pairs, so the present module could serve them,” the researchers state. a QKD link built on this source could potentially derive its timing from its own coincidence counts, eliminating the need for additional hardware. This flexibility positions the source as a versatile tool for a range of quantum technologies, even before the entangled configuration is fully developed and tested.

Source Module Characterization and Thermal Stabilization

The module’s performance was assessed using an asymptotic BBM92 model with four detectors per receiver, allowing for a direct cost comparison with existing quantum communication links. Initial measurements of the unoptimized module reveal a cost of approximately $376 per secret Gbit over a two-year period at a 50km distance, when paired with a single superconducting nanowire single-photon detector.

For context, a generic link using a bright published source with a single detector costs $301 over the same period, while a link employing two such detectors reaches a cost of $66. These figures highlight the current economic challenge of implementing this source, despite its potential for cost reduction through optimization.

The study outlines a target set of improvements, including doubling the source’s brightness and achieving a collection efficiency matching a published source, which would bring the cost of a link with one detector down to approximately 40% of a link relying on two detectors. These target values are based on measurements taken on the optical bench, indicating a pathway toward practical implementation. The work demonstrates that while the initial module presents a higher cost per bit than some existing solutions, targeted improvements in collection and brightness can unlock substantial savings in future quantum communication networks.

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