Quantum Memories in Free Space

 

 

 In quantum technologies, the important point is to be able to transmit the information-carrying quantum bits (qubits) from the transmitter to the receiver without loss or damage. This is one of the main purposes. For this, physicists are developing quantum memories that allow us to store information-loaded photons for certain periods of time to be transmitted at any time. Quantum memories are essential components for applications such as quantum information processing, quantum networks, and quantum repeaters.

There are quantum devices developed by various methods in order not to lose photons or disrupt their function during the storage process of qubits. In today's article, I will talk about an alternative approach besides the material quantum memories produced.

Scientists from the University of Illinois Urbana-Champaign, Truman State University, and RightHand Robotics, Inc. have succeeded in producing a quantum memory in free space. Before describing quantum memories operating in free space, let's talk about their counterparts in the literature.

One of them is item memories. Matter memories involve the conversion of photons into electronic or spin states of a cloud or atomic lattice, offering the storage and release of photons within a specified storage time range and potentially high accuracy. However, these memories have the disadvantages of operating at extremely high or low temperatures and storing a narrow wavelength, which reduces efficiency and increases cost. Another example of our type of memory is 'delay line memories'. Delay line memories, unlike matter memories, do not convert photons to a different state, but instead add a fiber optic cable to the system, delaying the photon's arrival by a time determined by the distance of travel (and the refractive index of the delay line material). This approach is inexpensive and simple in terms of cost but includes a few features. For example, the latency of the fiber cannot be changed by a significant amount. Fiber delays eliminate the costly overhead of item memories and the strict wavelength bandwidth, but they still have a low loss in a given wavelength band only.

Upon minimizing these barriers, our researchers developed a memory type based on the idea of delay line memories. This new quantum memory stores photons in a series of multiplexed empty spaces instead of storing them in a fiber. The manufactured memory is constructed with three multiplexed optical delay lines that can be sequentially switched with varying storage times (12.5 ns, 125 ns, and 1.25 µs) to achieve the optimum balance of storage time and efficiency. In addition, the memory allows us to avoid fiber losses and DOF limitations by operating in free space at room temperature, not requiring the high costs of matter memories.

Quantum memories that we can store in free space seem to allow for more robust, high-performance use in some quantum information processing protocols. If you want to have more detailed and technical information, you can reach the article in the references section.

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Reference

Nathan T. Arnold, Michelle Victora, Michael E. Goggin, Paul G. Kwiat, "Free-
space photonic quantum memory," Proc. SPIE 12446, Quantum Computing,
Communication, and Simulation III, 1244606 (8 March 2023); 

doi: 10.1117/12.2649350