Towards superconductor-spin ensemble hybrid quantum systems

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Example of how spin packets in a condensed matter system can be accessed and studied, adapted from Kubo et al. A diamond slab containing nitrogen-vacancy center spins (arrows) is positioned above a microfabricated superconducting resonator. A magnetic field BNV is applied as shown to tune the NV centers. A microwave signal excites a mode in the resonator with magnetic field Br to manipulate and probe the spin ensemble.

Ensembles of dilute spin impurities in condensed matter systems can be accessed using microelectronic circuits and therefore may offer a way for long-term storage and enhanced processing of quantum information.

The results of three separate papers on supercondutors and electron spin (described by Kubo et al., Schuster et al., and Wu et al.) point the way towards more complex models of solid-state quantum information processing and unique possibilities for studying novel quantum phenomena at the interface between heterogeneous physical systems. The coupling of mesoscopic quantum bits, which work at microwave frequencies, to atomic systems, which have optical transitions, may also lead to coherent coupling of single-photon microwaves to those of visible light, therefore enabling distributed quantum information processing.

Physical Review Letters – Strong Coupling of a Spin Ensemble to a Superconducting Resonator

We report the realization of a quantum circuit in which an ensemble of electronic spins is coupled to a frequency tunable superconducting resonator. The spins are nitrogen-vacancy centers in a diamond crystal. The achievement of strong coupling is manifested by the appearance of a vacuum Rabi splitting in the transmission spectrum of the resonator when its frequency is tuned through the nitrogen-vacancy center electron spin resonance.

Physical Review Letters – High-Cooperativity Coupling of Electron-Spin Ensembles to Superconducting Cavities

Electron spins in solids are promising candidates for quantum memories for superconducting qubits because they can have long coherence times, large collective couplings, and many qubits could be encoded into spin waves of a single ensemble. We demonstrate the coupling of electron-spin ensembles to a superconducting transmission-line cavity at strengths greatly exceeding the cavity decay rates and comparable to the spin linewidths. We also perform broadband spectroscopy of ruby (Al2O3 : Cr3þ) at millikelvin temperatures and low powers, using an on-chip feedline. In addition, we observe hyperfine structure in diamond P1 centers.

Physical Review Letters – Storage of Multiple Coherent Microwave Excitations in an Electron Spin Ensemble

Strong coupling between a microwave photon and electron spins, which could enable a long-lived quantum memory element for superconducting qubits, is possible using a large ensemble of spins. This represents an inefficient use of resources unless multiple photons, or qubits, can be orthogonally stored and retrieved. Here we employ holographic techniques to realize 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.

In summary, we have demonstrated holographic storage of microwave excitations in an electron spin ensemble. By using magnetic field gradients the microwave excitations are encoded in different collective modes of the spin ensemble and are retrieved individually. More robust storage can be achieved by transferring the coherence from electron spin to a coupled nuclear spin. These results show the prospect of using spin ensembles as a memory medium and implementing a quantum-computing scheme with hybrid physical systems.

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