Quantum Data Held for over One Microsecond on a Tiny Chip

A key component for scalable quantum technologies has been realised, with Priyash Barya and colleagues at University of Illinois Urbana-Champaign and Argonne National Laboratory achieving a breakthrough in quantum memory. The team’s work stores single-photon-level telecom-band optical pulses for over one microsecond within an atomic frequency comb in erbium-doped thin-film lithium niobate, sharply exceeding the limitations of conventional nanophotonic devices due to propagation losses. They verified the quantum characteristics of this storage through demonstrated phase coherence and sub-single-photon noise, alongside the capacity to store up to 20 temporal modes with a bandwidth of 2.2GHz. These findings confirm erbium-doped lithium niobate as a viable platform for on-chip quantum memory at telecom wavelengths, addressing a vital requirement for advancements in photonic quantum computing and quantum networking.

Erbium-doped lithium niobate extends single-photon storage to over a microsecond with high fidelity

Single-photon storage times now exceed 1.2 microseconds, representing a substantial leap from the few nanoseconds typically achieved in nanophotonic devices. This improvement is critical because maintaining quantum information requires minimising decoherence, the process by which quantum states lose their integrity. The longer the storage time, the more opportunities for quantum operations and the greater the potential for building complex quantum circuits. Utilising an atomic frequency comb within an erbium-doped thin-film lithium niobate platform circumvents limitations imposed by signal degradation during photon propagation. An atomic frequency comb, in this context, is a series of precisely spaced frequencies of light, analogous to the teeth of a comb, which allows for the slowing and storage of light pulses through stimulated echo techniques. The erbium-doped lithium niobate provides the necessary optical and acoustic properties for efficient interaction between the light and the material’s atomic structure. A retrieval efficiency of 2.9 percent was demonstrated after a 300 nanosecond delay, and the system can store up to 20 distinct temporal modes of light with an acceptance bandwidth reaching 2.2 gigahertz. This retrieval efficiency, while modest, is a significant step towards practical implementation, and ongoing research focuses on enhancing this parameter through optimisation of the material properties and control pulses.

Phase coherence and sub-single-photon noise detection upon retrieval confirmed the quantum nature of the stored photons, validating the integrity of the quantum state. Phase coherence is a fundamental requirement for quantum information processing, ensuring that the stored photon retains its quantum properties during storage and retrieval. The detection of sub-single-photon noise indicates that the storage process does not introduce significant unwanted photons, which would corrupt the quantum state. Successfully storing 20 distinct temporal modes demonstrated the memory’s capacity to handle complex photonic waveforms, which is important for encoding more information per photon. Each temporal mode represents a different shape or structure of the light pulse, allowing for multiplexing of quantum information. With an acceptance bandwidth of 2.2 gigahertz, the system accommodates a wide range of light frequencies without significant signal distortion, improving data throughput, but current figures do not demonstrate scalability to multiple photons or sustained performance under realistic network conditions. Expanding the bandwidth further would allow for even more complex waveforms and higher data rates, but requires careful consideration of material dispersion and control pulse shaping.

A microsecond of quantum information storage represents a substantial hurdle overcome in the pursuit of practical quantum computers and networks. Quantum computers require the ability to store and manipulate qubits, the quantum equivalent of bits, for extended periods to perform complex calculations. Quantum networks rely on the ability to store and transmit qubits over long distances, necessitating efficient and reliable quantum memories. A precise series of light colours, an atomic frequency comb, now controls and stores light pulses within an erbium-doped lithium niobate crystal, offering a promising alternative to existing methods plagued by signal loss. Traditional optical fibres suffer from significant photon loss, limiting the distance over which quantum information can be transmitted. Erbium-doped lithium niobate is confirmed as a viable material for creating on-chip quantum memory, a key component previously hindered by signal degradation during transmission, although further refinement is still needed to minimise any decay of the stored quantum state and address the persistent challenge of signal loss in all quantum systems. The on-chip integration potential of lithium niobate is particularly attractive, as it allows for miniaturisation and scalability of quantum devices. This achievement now prompts investigation into scaling the system for more complex quantum operations and improving overall efficiency for real-world applications, building on the efficient management and preservation of quantum information provided by the technique. Future research will likely focus on increasing the storage capacity, improving the retrieval efficiency, and demonstrating the ability to store and retrieve multiple photons simultaneously, paving the way for more powerful and versatile quantum technologies. The telecom-band operation is also crucial, as it aligns with the wavelengths used in existing fibre optic networks, facilitating seamless integration with current infrastructure.

The choice of erbium-doped lithium niobate is significant due to its unique combination of properties. Lithium niobate is a well-established material in integrated photonics, offering high optical quality and the ability to guide light efficiently. The incorporation of erbium allows for the creation of an atomic frequency comb through a process called four-wave mixing, where multiple photons interact to generate new frequencies. The erbium ions act as the active medium, providing the necessary energy levels for the storage and retrieval of photons. Furthermore, the thin-film configuration enhances the interaction between light and the material, improving the efficiency of the storage process. This approach represents a departure from previous quantum memory implementations based on rare-earth ion-doped crystals, which often suffer from low storage efficiency and limited bandwidth. The demonstrated storage of 20 temporal modes, each representing a unique spatial or temporal characteristic of the photon, highlights the potential for encoding complex quantum information within the memory. This is a crucial step towards realising more sophisticated quantum communication protocols and quantum computation algorithms.

The researchers successfully stored single photons for over 1 microsecond using erbium-doped lithium niobate, a significant improvement over existing nanophotonic devices. This achievement matters because efficient quantum memory is essential for building scalable quantum computers and networks. The team verified the quantum nature of the stored photons and demonstrated the ability to store up to 20 temporal modes with a bandwidth of 2.2GHz. They intend to focus on increasing storage capacity and retrieval efficiency to further develop this on-chip quantum memory platform.