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What Is Quantum Memory And What Is It Good For?

I recently participated in the 2023 OFC Conference and attended a tutorial session on quantum communication. Practical quantum computing may require networking a number of smaller quantum-computing systems, thus requiring transport of quantum entangled particles. Such transport may use “quantum memories” for long distance communication, similar to the use of repeaters for classical communication. It may also use sophisticated machine learning methods and error correction techniques for longer distance communication without quantum repeaters.

Quantum memories are devices that provide quantum computers an analog to the memory used in binary digital devices. Quantum memories store the quantum state of a photon or other entangled particle without destroying the quantum information of that particle. The quantum memory should be able to release an entangled particle with the same quantum state as the stored particle for data retrieval. Whereas digital binary memory stores two states, 0 or 1, a quantum memory can store several states in a quantum superposition. In other words, quantum memories store qubits for later retrieval.

In conventional binary computers various memory technologies are used to store and synchronize various computational processes or to store information that is being transferred from one location to another. Likewise for a quantum computer or for communication using qubits a quantum memory serves a similar purpose. Current quantum memories use photons (light) where the quantum state of a photon is mapped onto a group of atoms and can later be restored as a photon with the same quantum state.

Quantum memories are limited by similar coherence issues that limit the use of quantum computing. They require coherent matter systems so the quantum information stored in the memory isn’t lost due to decoherence. These coherent matter systems could be a very cool gas or a solid-state system.

Quantum memories will enable quantum repeaters that can allow longer distance quantum communication networks. This was an important element in a symposium on “Quantum Information and Optical Communication Networks” at the 2023 OFC as well as looking at alternatives to quantum repeaters for longer distance communication. Researchers from universities from many countries spoke about how to create quantum-based communication networks.

Prem Kumar from Northwestern University led off the symposium talking about engineering challenges for emerging quantum networks. He said that there are two important issues in using quantum states for communication. First, the No-cloning theorem says that it is impossible to duplicate an unknown quantum state and the Heisenberg uncertainty principle says that it is impossible to know a quantum state.

Entangling the quantum states of particles such as photons enables teleportation, which bypasses these quantum mechanical issues, but practical use of entanglement is hard because it is difficult to maintain, so many factors can lead to decoherence and loss of entanglement. There are many operational challenges to creating a quantum internet that uses quantum states. In particular, how does it work with classical communication networks (in order to make this technology affordable) and how can users get access to the entangled particles used in quantum communication (which may require very specialized and expensive equipment) and what sort of software and algorithms would be used to manage a quantum network.

The Northwest McCormick School of Engineering is building the Illinois Express Quantum Network (IEQNET) experimental quantum metro area network. The network will connect Fermilab, Argonne Lab and Northwestern University and will allow quantum and classical light on the same optical fibers. The image below shows three planes of operation for this network, the quantum infrastructure plane at the bottom of the image, the IEQNET control plane in the middle and the quantum network application plane at the top.

There was a demonstration zone at the OFC by a company called NuCrypt of classical and quantum communication using the same fiber. Note that “quantum wrapper” networking protocols are required for qubit transport over a conventional optical network. Note that the IEQNET does not include a quantum repeater technology, which is needed for longer distance quantum communication.

The image below shows quantum networking with this quantum wrapper. Note the QLNA3 with quantum memory clusters that can participate in making a larger quantum computer out of a number of smaller quantum computer systems.

Reza Nejabati from the University of Bristol (in the UK) spoke about secure and scalable quantum computing, including quantum networks. The UK quantum network connects the Bristol QNET to the Cambridge QNET. This network also enabled using the same communication fiber for classical and quantum communication. It uses a dynamic versus a fixed network topology for the entangled photons. They also used machine learning (ML) with a deep neural network (DNN) to estimate and manage the performance of an entanglement-based network.

A useful quantum computer should have more than 1 million Qubits. This is not possible or economical with a single quantum computer. Thus, a scalable quantum computing system will be a network of a large number of smaller quantum processors. A quantum communication network is an important element in creating a useful quantum computer. An FPGA-based interconnection solution provides low-latency and time-deterministic classical communication between remote gate quantum processing units (QPUs) for measurement exchange.

Sophia Economou from Virginia Tech gave a broad overview of quantum error correction coding. This included discussion of long-distance communication using quantum repeaters. These quantum repeaters use a quantum memory to enable retransmission of quantum data over greater distances. She looked at an all-photonic repeater scheme, not requiring a quantum memory or classical signaling between communication nodes using loss-tolerant repeater graphs.

She also looked at deterministic entangled particle generation using matter emitters to create entangled photons. There are two approaches for this, transduction and sequential emission. One uses spatial and the other temporal resources. The sequential emission, such as with quantum dots (QMs) is more practical.

Quantum communication will be an important element in creating practical quantum computing systems. This requires the transport of quantum states in communication networks shared with conventional communication. This may be done with quantum repeaters using quantum memories or other techniques to enhance longer distance quantum entangled particle data transmissions.