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The Second Quantum Revolution
Published in Jonathan P. Dowling, Schrödinger’s Web, 2020
Quantum memories are ubiquitous requirements for nearly all quantum-information-processing tasks, particularly for quantum computing and communication.56 In all of our discussions so far, we assume that, when an entangled photon pair arrives at Alice’s and Bob’s locations, they have some way of storing the state indefinitely. That storage is a quantum memory. But unlike a classical memory, which stores bits of information, the quantum memory must store qubits of information, maintaining quantum uncertainty, unreality, and entanglement. That is a much taller order. A poor-person’s quantum memory is made by taking the incoming photon and put it into a fiber loop so that it just goes around and around until you need it. But that does not help since the fiber loop is as lossy as the transmission fiber. After a few round trips, the photon is lost. Another approach, one our group championed, is to put a type of quantum error corrector in the loop to refresh the state of the photonic qubit on each round trip. However, the technology for doing that is still years off.57 Another approach is to transduce the qubit state from the photon to another type of hardware platform – such as trapped atoms – either confined in an atom trap or some solid-state material. One promising approach is to use an atomic qubit that sits inside a diamond where storage times on the orders of minutes have been seen.58 (See Figure 5.15.) The biggest problem with these is that it is not a simple task to couple the photons in and out of the diamond, the diamond must be cryogenically cooled, and the wavelengths of photons that work with the diamond are quite a bit different from those that work well with fiber. The memory is long – but a lot of loss goes into getting the photons from the fiber into the diamond and back again. Once more, we point out, like in teleportation, the important thing is not the physical qubit that carries the quantum state, but rather the quantum state itself, regardless of whether it resides on an atom or a photon. For the discussion of quantum repeaters, we will assume that all the nodes in the network, such as A, C, D, and B, shown in Figure 5.14, possess a perfect quantum memory that can store the photons forever without loss or noise.
Entanglement-enabled interferometry using telescopic arrays
Published in Journal of Modern Optics, 2020
Siddhartha Santra, Brian T. Kirby, Vladimir S. Malinovsky, Michael Brodsky
Finite-lifetime quantum memories. Another example of decoherence in the network that leads to an X-state resource is dephasing in quantum memories. To show this, we consider a quantum network which relies on quantum memories and entanglement swapping to distribute the state between the two telescopes as shown in Figure 6. The simplest scheme comprises two sources of entangled Bell-pairs of photons, a set of quantum memories at the telescope sites and an entanglement swapping setup at the middle station. One photon of each entangled pair is stored in a quantum memory at the telescope site before entanglement swapping is performed by a joint measurement on one photon from each pair at a central station (25, 26). The action of an imperfect memory on a qubit σ stored in the memory for a time t can be modelled as dephasing of the off-diagonal elements. Following (26), we describe a single-qubit dephasing using a super operator, , acting on the qubit density matrix, , as where , is the memory coherence time and is the spin Pauli operator along the z-direction.