Quantum memories: emerging applications and recent advances (1511.04018v2)

Abstract: Quantum light-matter interfaces are at the heart of photonic quantum technologies. Quantum memories for photons, where non-classical states of photons are mapped onto stationary matter states and preserved for subsequent retrieval, are technical realizations enabled by exquisite control over interactions between light and matter. The ability of quantum memories to synchronize probabilistic events makes them a key component in quantum repeaters and quantum computation based on linear optics. This critical feature has motivated many groups to dedicate theoretical and experimental research to develop quantum memory devices. In recent years, exciting new applications, and more advanced developments of quantum memories, have proliferated. In this review, we outline some of the emerging applications of quantum memories in optical signal processing, quantum computation, and nonlinear optics. We review recent experimental and theoretical developments, and their impacts on more advanced photonic quantum technologies based on quantum memories.

• The paper demonstrates that quantum memories map photonic states onto matter states, enabling non-classical state storage and retrieval.
• It details methodologies involving optically-controlled protocols and engineered absorption to balance noise and efficiency.
• Emerging applications include optical signal processing, quantum computing synchronization, and facilitation of non-linear photon interactions.

Quantum Memories: Emerging Applications and Recent Advances

The paper "Quantum Memories: Emerging Applications and Recent Advances" provides an in-depth exploration of the critical role quantum memories play in the future of quantum technologies. Quantum memories, devices that map non-classical states of photons onto stationary matter states and then later retrieve these states, underpin many photonic quantum technologies. They serve as a crucial component in quantum repeaters and can synchronize probabilistic events vital for linear-optics-based quantum computation, thereby paving the way for advanced photonic quantum technologies.

Overview of Quantum Memories

Quantum memories interface light and matter, enabling storage of quantum information encoded in photonic states. Classical optical processors often face substantial limitations concerning bandwidth, wavelength, and pulse duration. Quantum memories can circumvent these by providing coherent, noise-free storage and manipulation of arbitrary optical states, including frequency and temporal modes. This unique capability facilitates their integration into larger quantum systems, offering versatile solutions not possible with classical counterparts.

Implementations and Protocols

The implementations of quantum memories vary significantly across different platforms and protocols. The paper outlines several approaches, typically divided into optically-controlled memories and engineered absorption protocols:

• Optically-Controlled Memories: Leveraging control pulses to create electromagnetically induced transparency (EIT), these systems have on-demand re-emission capabilities but can introduce noise due to intense control fields.
• Engineered Absorption: This strategy uses inhomogeneous broadening, exemplified by gradient echo memory (GEM) and atomic frequency combs (AFC). These approaches tend to ensure noiseless operations at the cost of preset re-emission times unless hybrid methods are used involving both optical controls and engineered absorption.

Physical Platforms

Researchers have pursued various physical implementations, each with distinct advantages and challenges:

• Rare-Earth-Ion Doped Solids: Notable for their narrow linewidths and potential for integration into compact photonic devices, they provide notable coherence times, advantageous for long-term quantum information applications.
• Nitrogen-Vacancy Centers in Diamond: These demonstrate room-temperature operation with long coherence times, facilitating hybrid quantum systems bridging optical and microwave domains.
• Alkali Metal Vapors: With high optical depths, they readily support protocols like EIT and off-resonant Raman memories, although challenges such as atomic motion and transient optical depths remain.
• Raman Scattering in Solids: Frequent exploration uses diamond, which offers broad manipulative capability in the THz regime.

Emerging Applications

The paper elucidates several applications harnessing the ability of quantum memories to manipulate quantum states:

• Optical Signal Processing: Signal processors efficiently manipulate optical modes for integration in quantum networks, encompassing frequency conversion and temporal control among others.
• Optical Quantum Computation: Quantum memories offer deterministic synchronization of non-deterministic quantum operations essential in linear optical quantum computing (LOQC) and assist in creating large photonic cluster states for measurement-based quantum computation.
• Non-Linear Optical Interactions: Quantum memories act as intermediaries for photon-photon interactions, vital for quantum networking and the implementation of logic gates in quantum computation.

Conclusion and Future Outlook

Quantum memories stand as a linchpin in advancing photonic quantum technologies. While overcoming current technical challenges such as noise, efficiency, and storage times remains crucial, the field has made substantial strides. Future research will likely focus on enhancing multimodality, integrability, and robustness in quantum memories to align them closer with practical applications in quantum communication and computation. Furthermore, their potential role in spectrally and temporally multiplexed quantum states may shape next-generation quantum networks.