Optical quantum memory (1002.4659v3)

Abstract: Quantum memory is important to quantum information processing in many ways: a synchronization device to match various processes within a quantum computer, an identity quantum gate that leaves any state unchanged, and a tool to convert heralded photons to photons-on-demand. In addition to quantum computing, quantum memory would be instrumental for the implementation of long-distance quantum communication using quantum repeaters. The importance of this basic quantum gate is exemplified by the multitude of optical quantum memory mechanisms being studied: optical delay lines, cavities, electromagnetically-induced transparency, photon-echo, and off-resonant Faraday interaction. Here we report on the state-of-the-art in the field of optical quantum memory, including criteria for successful quantum memory and current performance levels.

• The paper details how optical quantum memory synchronizes quantum processes and enables long-distance quantum communication via quantum repeaters.
• It evaluates various storage approaches—including EIT, DLCZ, and photon-echo techniques—highlighting their efficiencies and operational challenges.
• Performance metrics like fidelity, efficiency, multimode capacity, and delay-bandwidth product are rigorously analyzed to guide future research.

Optical Quantum Memory

The paper "Optical Quantum Memory" explores a pivotal area in quantum information science, probing the role of quantum memory in quantum computing and quantum communication. It highlights optical quantum memory as an essential component for synchronizing processes within quantum computers and enabling long-distance quantum communication through quantum repeaters. This paper provides a comprehensive state-of-the-art overview of various optical quantum memory mechanisms, assessing the criteria for successful quantum memory and current performance levels.

Introduction and Performance Criteria

Optical quantum memory involves the storage of quantum states of light, essentially qubits, in a medium that can preserve the information and release it on demand. The paper elaborates on the performance criteria which include worst-case fidelity, efficiency, transfer coefficients, and multimode capacity. These metrics guide the evaluation of how effectively a quantum memory can store and retrieve quantum information with high fidelity.

1. Worst-case fidelity: This involves maintaining the fidelity of the stored quantum state above a certain threshold, enabling the utilization of fault-tolerant quantum error correction techniques.
2. Efficiency: Defined as the ratio of the stored pulse energy to the retrieved pulse, this metric provides insight into the storage process's effectiveness but doesn’t account for noise contamination.
3. Multimode capacity: This defines how many optical modes can be simultaneously stored within the memory, varying significantly depending on the storage mechanism.
4. Storage time and delay-bandwidth product: The capability to store information for a sufficient duration to perform necessary quantum tasks.

Realizations of Optical Quantum Memory

The paper explores various methodologies for realizing optical quantum memory, covering a spectrum from optical delay lines and cavities to advanced interaction phenomena.

• Optical Delay Lines and Cavities: These represent the most straightforward approach but are generally limited by fixed storage times and trade-off constraints between storage efficiency and delay-bandwidth product.
• Electromagnetically-Induced Transparency (EIT): Through EIT, photons can be slowed, stopped, and stored in atomic media. However, challenges remain in maintaining high optical density without incurring detrimental processes, such as four-wave mixing.
• Duan-Lukin-Cirac-Zoller (DLCZ) Protocol: This protocol facilitates entanglement between distant atomic ensembles allowing transfer of quantum states between light and matter, notably servicing quantum repeater applications.
• Photon-echo Quantum Memory: The controlled reversible inhomogeneous broadening (CRIB) and atomic frequency combs (AFC) methods are advanced photon-echo techniques promising high-efficiency storage with potential for multi-mode capabilities.
• Off-resonant Faraday Interaction: This utilizes the phase shifts induced in a medium by off-resonant light to achieve a form of memory without direct excitation of the medium’s atoms, thus minimizing noise impact.

Implications and Future Directions

The research addressed in this paper points towards significant theoretical and practical implications for quantum technologies. As quantum memory develops, its integration into larger quantum systems appears promising, especially in overcoming current limitations associated with storage efficiency, fidelity, and external noise factors.

Future research and technological exploration promise to enhance quantum memory capabilities further, such as improving multi-mode storage and combining different mechanisms' strengths into single-system solutions. The advancements in this field are likely to underpin developments in robust, scalable quantum networks and support evolving quantum computing architectures.

In conclusion, the paper provides a detailed exploration of optical quantum memory, emphasizing its critical role in advancing quantum information science. It presents an insightful examination of the current technologies and lays the groundwork for future research directions that could significantly enhance the functionality and integration of quantum memory systems within quantum technologies.