Researchers Prepare Persistent Comb and Achieve 23.3% Storage Efficiency

A programmable quantum memory operating at telecom wavelengths has been demonstrated by Chengdong Yang and colleagues at College of Engineering and Applied Sciences, in collaboration with University of Science and Technology of, Nanjing University, and Hefei National Laboratory. The cavity-enhanced quantum memory, fabricated in an isotopically purified lithium niobate microring resonator, achieves an on-chip storage efficiency of 23.3 ±0.5% for 100-ns storage. The ability to store and retrieve time-energy-entangled photons, verified through an entanglement-witness violation exceeding 11 standard deviations, presents erbium-doped thin-film lithium niobate as a promising platform for spectrally multiplexed quantum communication and computation.

Efficient storage and retrieval of time-energy-entangled photons in integrated lithium niobate

On-chip storage efficiency reached 23.3 ±0.5% for 100-nanosecond storage, exceeding previous erbium-doped lithium-niobate devices limited to efficiencies below 3% by over seven-fold. This breakthrough surpasses a vital threshold for practical quantum networks, which were previously hindered by insufficient light storage capacity. The device, fabricated from isotopically purified lithium niobate, utilises long-lived hyperfine shelving states to persistently prepare a high-contrast atomic frequency comb, enhancing light-matter interaction.

Furthermore, the integrated platform enables frequency-selective routing of retrieved photons at rates up to 20MHz with minimal inter-channel crosstalk, below 10−4, and successfully stores time-energy-entangled telecom photons, confirming the quantum nature of the storage process. The device boasts a single-component atomic frequency comb lifetime of 277.6 ±52.6 seconds, underpinning its efficient light storage capabilities. A microring resonator enhances light-matter interaction, bypassing limitations of earlier devices reliant on less effective waveguide technologies. While these results establish a promising programmable interface for quantum networks, achieving scalable, room-temperature operation and extending storage times beyond 100 nanoseconds remain significant hurdles. The long-lived hyperfine shelving states within the erbium ions support persistent atomic frequency comb preparation.

High-efficiency quantum memory utilising isotopically purified lithium niobate microring resonators

An on-chip storage efficiency of 23.3 ±0.5% was achieved for 100-nanosecond storage of telecom photons, a substantial improvement over previous erbium-doped lithium-niobate devices limited to efficiencies below 3%. This cavity-enhanced quantum memory utilises an isotopically purified erbium-doped thin-film lithium niobate microring resonator, demonstrating a key advance in integrated quantum photonics. The device also enables frequency-selective routing of retrieved photons with inter-channel crosstalk below 10−4, enabling precise control over quantum information pathways. Stored time-energy-entangled telecom photons verified the quantum nature of the storage process, violating an entanglement-witness bound by over 11 standard deviations.

A current storage time of 100 nanoseconds presents a limitation for long-distance quantum communication applications. This builds upon previous work utilising naturally abundant erbium ensembles for storage of weak coherent states and non-classical light, and recent studies with isotopically purified erbium exploiting long-lived spin states, by integrating high efficiency, spectral tunability, and electrical control within a single device. The intrinsic electro-optic response of lithium niobate allows for frequency-selective storage and routing of photons at rates up to 20MHz, with minimal signal loss between channels.

Earlier erbium-doped lithium-niobate devices relied on Ti-diffused or laser-written waveguides, resulting in weak optical confinement and material-induced decoherence, which limited storage efficiency. This new approach, utilising thin-film lithium niobate, overcomes these limitations by combining high-Q nanophotonic resonators with impedance matching to the erbium ensemble. Future work should focus on scalability, specifically fabricating and interconnecting multiple microring resonators to create more complex quantum systems. This technology is envisioned as a programmable light-matter interface, essential for building spectrally multiplexed quantum networks capable of transmitting information using multiple wavelengths of light simultaneously.

High-efficiency quantum memory utilising a lithium niobate microring resonator

A novel quantum memory device has achieved 23.3 ±0.5% on-chip storage efficiency for 100-nanosecond intervals. This represents a substantial improvement over previous erbium-doped lithium-niobate systems, which were constrained to efficiencies below 3%. The device utilises an isotopically purified erbium-doped thin-film lithium niobate microring resonator, a tiny circular structure designed to trap and manipulate light. Long-lived hyperfine shelving states, a property of the erbium ions, enable persistent preparation of an atomic frequency comb, in effect a precise arrangement of colours within the telecom band.

The comb sustains a single-component lifetime of 277.6 ±52.6 seconds, providing a stable foundation for quantum information storage. Impedance matching within the cavity further enhances storage performance by optimising light interaction within the resonator. Time-energy-entangled telecom photons were also successfully stored and retrieved, confirming the quantum nature of the storage process by exceeding an entanglement-witness bound by over 11 standard deviations.

A current storage duration of only 100 nanoseconds limits its application in long-distance quantum communication. Further development is needed to extend storage times for practical use. Previous work with erbium-doped lithium-niobate relied on Ti-diffused or laser-written waveguides, which suffered from weak optical confinement or material-induced decoherence, hindering performance. This demonstration of on-chip quantum memory, utilising erbium-doped lithium niobate, establishes a platform capable of both efficient light storage and precise frequency control, a crucial combination for scalable quantum networks. The achieved storage efficiency of 23.3% alongside frequency-selective routing at 20MHz represents a substantial advance beyond previous devices, enabling manipulation of individual photon wavelengths. Verification of quantum storage through time-energy entangled photon retrieval confirms the potential of this material system; however, extending storage duration beyond the current 100-nanosecond limit remains crucial for enabling practical long-distance quantum communication.

The researchers demonstrated a quantum memory using an erbium-doped lithium niobate microring resonator, achieving a storage efficiency of 23.3% for 100-nanosecond storage of telecom photons. This is significant because efficient and programmable quantum memories are essential for building future quantum networks capable of transmitting information securely over long distances. The device also enabled frequency-selective control of retrieved photons at rates up to 20MHz, with minimal signal interference between channels. The authors aim to extend the storage duration beyond 100 nanoseconds to improve its suitability for long-distance quantum communication.