Storage of Multiple Coherent Microwave Excitations in an Electron Spin Ensemble

Abstract

Electron and nuclear spins have good coherence times and an ensemble of spins is a promising candidate for a quantum memory. By employing holographic techniques via field gradients a single ensemble may be used to store many bits of information. Here we present a coherent memory using a pulsed magnetic field gradient, and demonstrate the storage and retrieval of up to 100 weak 10 GHz coherent excitations in collective states of an electron spin ensemble. We further show that such collective excitations in the electron spin can then be stored in nuclear spin states, which offer coherence times in excess of seconds.Instead of storing information in specific locations as in photography and in conventional computer memory, information can be stored in distributed collective modes, as in holography. Advantages include obviating the need for local manipulations and measurements, enhanced coupling to electromagnetic fields, and robustness against decoherence of individual members of the ensemble. This principle has been applied to different light-matter interfaces such as atoms [1][2][3][4], ion-doped crystals [5][6][7][8][9], polar molecules [10-13], or spins [14,15]. Controlled reversible inhomogeneous broadening (CRIB) [8], or gradient echo memory (GEM) [7] schemes which apply external field gradients to address different storage modes have been proposed and observed in gaseous atomic samples [1,2] and in ion-doped solids [6,7].In this Letter, we demonstrate the storage of multiple microwave excitations in an electron spin ensemble. The spin ensembles used as the storage medium are the electron spin of nitrogen atoms in fullerene cages ( 14 N@C 60 ) and phosphorous donors in silicon. The microwave excitations are phase encoded using a static or pulsed field gradient, with the latter allowing for recall in arbitrary order. We have stored up to 100 weak excitations in a spin ensemble and recalled them sequentially. We also demonstrate the coherent transfer of the stored multiple excitations between electron spin and nuclear spin, which will allow much longer storage times [16]. The multimode storage achieved in this way offers prospects of constructing a long-lived quantum memory which could be used for a hybrid quantum computing architecture with superconducting qubits.A quasistatic magnetic field along the z-axis causes the members of the spin ensemble to precess at an angular frequency B(z, t)µg e / , where µ is the Bohr magneton and g e is the electron gyromagnetic ratio. Applying a magnetic gradient of strength G = ∂B(z, t)/∂z for a time τ consequently leads to a difference in precession angle of δθ = (µg e / )Gτ · δz between two spins with separation δz along the z-axis. A gradient pulse thus maps a spin state with a coherent transverse magnetization (such as that generated by a global resonant microwave tipping pulse) to a spin-wave excitation associated with a wave number k = (µg e / )Gτ · z. Each further application of G for duration τ generates a change in the wavevector of the global spin wave mode, by an a...

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