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While quantum computers can do interesting things without dedicated memory, memory would provide a lot of flexibility in terms of the sorts of algorithms they could run and how quantum systems can interact with each other and the outside world. Building quantum memory is extremely challenging, as reading to and writing from it both have to be extremely efficient and accurate, and the memory has to do something that's very atypical of quantum systems: hold on to its state for an appreciable length of time.
If we solve the problems, however, quantum memory offers some rather unusual properties. The process of writing to quantum memory is very similar to the process for quantum teleportation, meaning the memory can potentially be transmitted between different computing facilities. And since the storage device is a quantum object, there's the possibility that two qubits of memory in different locations can be entangled, essentially de-localizing the qubit's value and spreading it between two facilities.
In a demonstration of that promise, Chinese researchers have entangled quantum memory at facilities over 20 kilometers apart. Separately, they have also done the entanglement with photons that have traveled through 50 kilometers of optical cable. But the process of transmitting and entangling comes with an unfortunate side effect: it takes so long that the memory typically loses its coherence in the meantime.
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