Quantum Memory

Managing the Interaction Dance between Photons and Qubits

Quantum memories harness and preserve the properties of quantum bits, or qubits, that are the core of quantum communication and computation. Quantum memories store the quantum state of a photon or other entangled particle without destroying the quantum information of that particle. In short, quantum memory is used to temporally store quantum states and to retrieve them later with the original quantum state maintained, undisturbed, or modified. Memory systems are crucial enablers for quantum tasks including various processing tasks, synchronization, error correction and purification and they play a critical role in the implementation of quantum network repeaters.

By temporarily storing quantum states, quantum memory serves a critical role in teleportation-based quantum repeaters which enable long distance quantum communications beyond the loss limitation in optical fibers. More on Quantum Repeaters in a future post! Historically, quantum memories have been developed along two distinct paths: optically controlled memories and engineered absorption.

Solid State and Optical Systems

Solid state systems. Solid state quantum memories store quantum information in solid-state systems, which are typically easier to integrate into electronic devices compared to other forms of quantum memories, such as those based on atomic systems. This type of quantum memory includes nitrogen-vacancy (NV) centers in diamonds. NV centers leverage relatively easy optical preparation systems and readout (retrieval) of electronic and nuclear spin states along with relatively long coherence times – an important consideration for quantum memory.

Optical systems. Optical systems for quantum memories involve using light to store and manipulate quantum information. An example of this type of memory is rare-earth ion doped crystals that use quantum properties of light to store and retrieve quantum information. Rare-earth ion-doped crystals exhibit excellent coherence properties at cryogenic temperatures. This is particularly important for application of quantum memories in long-distance quantum communications that require storage times greater than milliseconds.

Rare-earth ion-doped crystals have initially been used for implementing storage of time-bin qubits using the AFC protocol. AFC (Atomic Frequency Comb – more below) is a quantum memory technique that stores photonic qubits in a non-homogenous solid-state media with comb-like structure. The stored quantum data is re-emitted after a fixed, programmable time set by the comb spacing.

Absorption Systems

Atomic ensembles use a large group of atoms as a medium to store quantum information. This approach leverages the collective quantum states of atoms to create a robust and efficient storage mechanism for quantum information. These systems are distinct from solid-state quantum memories in that they typically use gases or vapors of atoms as the medium for quantum storage. Examples of atomic ensemble quantum memories include rubidium vapor cells, cavity QED (quantum electrodynamics), and cold atoms using electromagnetically induced transparency (EIT).

• Quantum Memory using Rubidium vapor cells

These systems store and retrieve quantum states of light, typically using rubidium gas atoms in warm or room-temperature that exist in superpositions of multiple energy states simultaneously, which can be influenced by electromagnetic fields. Compared with cold-atom memories, warm rubidium cells are simpler to assemble and operate at or near room temperature, which is advantageous for deployment in real-world quantum networks and field tests. They are also compatibility with existing fiber networks, making them a leading approach for quantum repeater nodes to enable long-distance and quantum communication links.

• Cavity Quantum Electrodynamics – Cavity QED

Cavity QED approaches are actively being developed for quantum memory systems because they enable strong and stable (coherent) interactions between light and matter. Cavity QED uses resonant optical or microwave cavities to confine electromagnetic modes and enhance interactions with quantum emitters such as atoms, ions, or artificial qubits.

An optical cavity, also called a resonating cavity or an optical resonator is an arrangement of mirrors or other optical elements that traps light at specific resonant frequencies so that photons bounce back and forth many times and strongly interact with a quantum emitter (such as an atom, ion, quantum dot, or superconducting qubit). By matching the cavity resonance to an internal transition of the emitter, the electromagnetic field is confined to a small mode volume which boosts the strength and coherence of light–matter interactions beyond what is possible in free space.

• Circuit Quantum Electrodynamics - Circuit QED

Another approach known as Circuit QED is also exploring interactions between light and matter where a single photon within a single mode cavity coherently couples them to a quantum atom. In Circuit QED the photon is stored in a one-dimensional on-chip microwave resonator and the quantum object is not a natural atom but an artificial one.

• Electromagnetically Induced Transparency (EIT)

EIT is an optical phenomenon in atoms that uses quantum interference to induce transparency into an otherwise resonant and opaque medium. EIT systems create and retrieve the stored excitation without destroying the quantum state, and can be used for slow light, light storage and quantum memory applications.

EIT is a quantum interference phenomenon in three-level systems that generates a narrow transparency window, enabling impactful control over light propagation. It utilizes a strong control field alongside a weak probe field to create destructive interference that minimizes absorption and enhances nonlinear optical effects.

EIT-based systems are preferred for long storage times, ease of implementation, and situations where high-fidelity operation is essential without demanding bandwidth requirements. They are especially useful when low control field power and simple setup are priorities. EIT underpins diverse applications including quantum memories, switchable photonic devices, high-resolution spectroscopy, and slow-light propagation in various platforms.

• Raman-based systems

These systems excel in high-bandwidth, high-speed quantum memories, accommodating ultrashort pulses and higher repetition rates. They have recently demonstrated near-unity efficiency and fidelity, rivaling the best EIT systems, although they require careful control of technical noise and strong drive fields.

Overall, the choice between EIT and Raman depends on the needs: for broadband or high-speed storage, Raman-based cavity schemes are emerging as the performance leaders; for established high-fidelity but narrower bandwidth quantum memories, EIT remains a robust standard.

Atomic Frequency Comb (AFC) Storage Protocol

AFC memory is a quantum memory technique that stores photonic qubits in inhomogeneously broadened ensembles by preparing an absorption profile with a comb-like structure.

Inhomogeneously broadened ensembles refer to large groups of atoms or ions whose optical transitions have a range of frequencies, rather than a single well-defined frequency. This enables AFC systems to store quantum states of light by collectively absorbing photons as coherent excitations spread across many atoms with different resonance frequencies.

When a photon is absorbed, it creates a collective excitation across the atomic ensemble, and re-emits after a fixed, programmable time set by the comb spacing. It is particularly well-suited for multimode and telecom-band storage in solid-state media.

Wrapping up

This article simply scratches the surface of topics related to quantum memory systems and technologies. Quantum memory is a foundational, and EXTREMELY COMPLEX set of technologies required for implementing advanced quantum networks, quantum computing synchronization, and practical long-distance quantum communication.

Researchers around the globe are working to maintain quantum coherence in quantum memory over time by addressing environmental noise and decoherence, balancing storage time (coherence) with bandwidth and retrieval speed, and integrating memory systems with other quantum systems, as well as improving the efficiency of photon-memory coupling, and improving scalability.

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