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Holographic quantum memory in a spin ensemble
Janus Wesenberg
While the building blocks for few-bit quantum computers have already been demonstrated, scaling these systems up to large quantum computers remains a challenge. One problem is to develop physical systems that can reliably store thousands of qubits, while allowing individual addressing of each bit. With this issue in mind researchers, from the University of Oxford, Yale University, and the University of Aarhus in Denmark, have proposed to store hundreds of qubits as spinwaves in an ensemble of billions of electron spins.
While the building blocks for few-bit quantum computers have already been demonstrated, scaling these systems up to large quantum computers remains a challenge. One problem is to develop physical systems that can reliably store thousands of qubits, while allowing individual addressing of each bit.
With this issue in mind researchers, from the University of Oxford, Yale University, and the University of Aarhus in Denmark, have proposed to store hundreds of qubits as spinwaves in an ensemble of billions of electron spins.
During input and output, the qubits are stored as microwave photons in a superconducting coplanar waveguide. From here, the qubit can be transferred to a common mode spinwave in the ensemble. The wave-number of all spinwaves in the ensemble can be shifted by applying a magnetic gradient field to the system, thus allowing multiple independent spinwave eigenmodes to be accessed in a manner akin to the cells of a Turing tape.
Each electron spin interacts very weakly with the waveguide, and transferring a unit of quantum information to a single spin would take a tenth of a second, much too long to be useful. Due to the collective enhancement of the coupling, the interaction between a billion electron spins and the waveguide is nevertheless strong enough to allow the transfer to proceed in a few hundred nanoseconds. Eventually this scheme will be implemented with single photon excitations, but already proof-of-principle experiments have been performed with microwave excitations which give small tip angles in the electron spin states. These experiments have shown that a hundred independent modes can be stored and subsequently read out arXiv:0908.0101.
Citation: "Quantum computing with an electron spin ensemble", J. H. Wesenberg, A. Ardavan, G. A. D. Briggs, J. J. L. Morton, R. J. Schoelkopf, D. I. Schuster and K. Mølmer. Phys. Rev. Lett. 103, 070502 (2009).
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