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arXiv:2509.02096 · APL Photonics

Highly efficient and broadband optical delay line toward a quantum memory

Guo Yu, Anindya Banerji, Jia Boon Chin, Arya Chowdhury, Alexander Ling

Published Feb 1, 2026• 2 citations
arXivHTMLPDF

Abstract

We demonstrate a high-efficiency, free space optical delay line utilizing a nested multipass cell architecture. This design supports extended optical paths with low loss, aided by custom broadband dielectric coatings that provide high reflectivity across a wide spectral bandwidth. The cell is characterized using polarization-entangled photon pairs, with signal photons routed through the delay line and idler photons used as a timing reference. Quantum state tomography performed on the entangled pairs reveals entanglement preservation with fidelity of 99.6(9)% following a single-transit delay of up to 687 ns, accompanied by a photon retrieval efficiency of 95.390(5)%. The delay is controllable and can be set between 1.8 and 687 ns in ∼12.6 ns increments. These results position this delay line as a strong candidate for all-optical quantum memories and synchronization modules for scalable quantum networks.

DOI
10.1063/5.0305471

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•Nov 23, 2011•0 citations
We demonstrate a high-efficiency, free space optical delay line utilizing a nested multipass cell architecture. This design supports extended optical paths with low loss, aided by custom broadband dielectric coatings that provide high reflectivity across a wide spectral bandwidth. The cell is characterized using polarization-entangled photon pairs, with signal photons routed through the delay line and idler photons used as a timing reference. Quantum state tomography performed on the entangled pairs reveals entanglement preservation with fidelity of 99.6(9)% following a single-transit delay of up to 687 ns, accompanied by a photon retrieval efficiency of 95.390(5)%. The delay is controllable and can be set between 1.8 and 687 ns in ∼12.6 ns increments. These results position this delay line as a strong candidate for all-optical quantum memories and synchronization modules for scalable quantum networks.
Photonic hyperentanglement enables increased information capacity and enhanced functionality for quantum communication and networking. However, synchronization of hyperentangled photon pairs requires maintaining correlations simultaneously across multiple degrees of freedom (DOFs). The preservation of polarization and energy-time entanglement in hyperentangled photon pairs is demonstrated using a free-space optical delay line based on nested Herriott cells. After a delay of 647 ns, a two-photon interference visibility of 93.9(3)% is observed in the energy-time DOF, while a CHSH parameter of 2.758(5) is obtained in the polarization DOF. These results confirm that entanglement correlations in both DOFs are preserved after propagation through the delay line. They demonstrate that free-space optical delay lines are compatible with complex photonic quantum states and provide a promising route toward delay-based memories for synchronization and multiplexing in quantum networks.
Abstract We report a fiber loop quantum buffer based on a low-loss 2 × 2 switch and a unit delay made of a fiber delay line. We characterize the device by using a two-photon polarization entangled state in which one photon of the entangled photon pair is stored and retrieved at a repetition rate up to 78 kHz. The device, which enables integer multiples of a unit delay, can store the qubit state in a unit of fiber delay line up to 5.4 km and the number of loop round-trips up to 3. Furthermore, we configure the device with other active elements to realize integer multiplier and divider of a unit delay of a qubit. The quantum state tomography is performed on the retrieved photon and its entangled photon. We obtain a state fidelity > 94 % with a maximum storage time of 52 μs with an insertion loss of 5.56 dB. To further characterize the storing and retrieving processes of the device, we perform entanglement-assisted quantum process tomography on the buffered qubit state. The process fidelity of the device is > 0.98. Our result implies that the device preserves the superposition and entanglement of a qubit state from a two-photon polarization-entangled state. This is a significant step towards facilitating applications in optical asynchronous transfer mode based quantum networks.
Quantum optical memories are a key component in a variety of quantum information applications, from extending quantum communication channels to building high-efficiency single-photon sources to enabling protocols requiring multiple synchronized qubits. However, most current photon storage systems utilize light-matter interactions and are therefore not broadband; meanwhile the available broader-bandwidth photon storage systems operate with somewhat shorter storage times or require cryogenic operation. Here we develop a system with multiplexed free-space storage cavities, able to store single photons with high efficiency over variable delays, up to 12.5 µs, and over multiple nanometers bandwidth at room temperature. The system can store multiple qubits encoded in various degrees of freedom (e.g., time-bin, and polarization) simultaneously. The work presented here has demonstrated storage of polarization states for 1.25 µs and retrieval through single-mode fiber with a state fidelity >99% and efficiency 82%.
Abstract Various near-term quantum networking applications will benefit from low-loss, fiber-coupled photonic quantum memory devices with high efficiencies. We demonstrate a fiber-coupled loop-and-switch quantum memory platform with a pass-through efficiency of ~54% and an overall storage efficiency that scales as ~0.5 N +1 where N is the number of storage cycles. We highlight the trade-off between memory lifetime and qubit accessibility in this platform by using two different storage cycle times of ~40 ns and ~0.5 μs, and demonstrate high-fidelity storage and retrieval of ultra-broadband single-photon polarization qubits in both cases.
During the last decade, quantum entanglement has paved the way out to of the lab modern applications such as quantum computation and communication. Today, small scale quantum networks exist already, but they are limited to a few 100 km distance, due to intrinsic fiber transmission losses and non perfect detectors. These networks are typically established using photon pair sources based on spontaneous parametric down conversion (SPDC). Widely used entanglement observables are time-bin and polarization, where the latter one is undoubtedly simpler to analyze due to lack of interferometric devices. It was shown recently, that introducing quantum memories (QM) between the communication parties could increase both communication speed and distance. Such memories already exist, but their narrow absorption bandwidth does not match the broadband emission spectrum of SPDC sources. Our report is twofold. First, we introduce a new polarization entanglement scheme based on a birefringent delay line (BDL). A pair of incoming photons is projected on a polarizing beam splitter (PBS), where vertically polarized photons travel along a polarization maintaining fiber loop accumulating a delay δ compared to horizontally polarized photons, which exit the BDL directly upon reflection at the PBS. Via post selection of only simultaneously arriving photon pair events, we create entangled states of the form α|H>_1|H>_2 + e^(iφ)β|V>_1 |V>_2. The probability amplitudes α and β are simply tuned by controlling the input pair's polarization and the phase φ can be varied by piezo fiber stretcher (PZT) in the delay line. Therefore, any superposition of the polarization entangled Bell states can be created. The scheme may also be used for entanglement distillation of non maximally entangled states, e.g. emitted by quantum dots. We will introduce all requirements needed to create entanglement from such a BDL. Second, we show how to apply this scheme to produce polarization entanglement from one of the most efficient photon pair generators, namely a periodically poled Lithium Niobate waveguide structure (PPLN/W), where the quasi phase matching is optimized for the type-0 SPDC interaction. The pair's wavelength is chosen in the standardized ITU-21 channel of the low loss telecom C-band and a fiber Bragg grating is applied to reduce the initial spectral bandwidth from 5 THz to 540 MHz, which is suitable for some solid-state quantum memories. Using the BDL, we create the maximally entangled states, with near perfect quality, which is proven by a violation of the Bell's inequalities by more than 29 standard deviations. We achieve a source spectral brightness of 211 narrowband polarization entangled pairs created per second, mW of pump power, and MHz of spectral bandwidth, coupled into a single mode fiber. This is on the same level as the best time-bin entanglement sources reported to date. The source is very practical due to the use of standard telecom fiber components. Moreover the choice for polarization instead of time-bin entanglement allows for simple state analysis using only rotating PBSs. This is a non negligible advantage, especially for narrowband photons with long coherence times, where time-bin analysis requires interferometer path length differences on the order of a few meters stabilized to a few tens of nanometers. Non unit visibilities are solely explained by the nonlinear character of our photon pair source, giving rise to double pair contributions. Applying this scheme to deterministic photon pair sources should result in perfect quality entanglement. We believe that our scheme should play an important role for future quantum networking applications.