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arXiv:1006.0242 · Physical Review Letters

High-Cooperativity Coupling of Electron-Spin Ensembles to Superconducting Cavities

David Schuster, Adam Sears, Eran Ginossar, L. DiCarlo, Luigi Frunzio, John J. L. Morton, Hua Wu, G. A. D. Briggs, Bob B. Buckley, D. D. Awschalom, Robert J. Schoelkopf

Published Sep 27, 2010• 475 citations
arXivHTMLPDF

Abstract

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:Cr(3+)) at millikelvin temperatures and low powers, using an on-chip feedline. In addition, we observe hyperfine structure in diamond P1 centers.

DOI
10.1103/physrevlett.105.140501

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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:Cr(3+)) at millikelvin temperatures and low powers, using an on-chip feedline. In addition, we observe hyperfine structure in diamond P1 centers.
We report strong coupling between an ensemble of nitrogen-vacancy center electron spins in diamond and a superconducting microwave coplanar waveguide resonator. The characteristic scaling of the collective coupling strength with the square root of the number of emitters is observed directly. Additionally, we measure hyperfine coupling to (13)C nuclear spins, which is a first step towards a nuclear ensemble quantum memory. Using the dispersive shift of the cavity resonance frequency, we measure the relaxation time of the NV center at millikelvin temperatures in a nondestructive way.
We propose to encode a register of quantum bits in different collective electron spin wave excitations in a solid medium. Coupling to spins is enabled by locating them in the vicinity of a superconducting transmission line cavity, and making use of their strong collective coupling to the quantized radiation field. The transformation between different spin waves is achieved by applying gradient magnetic fields across the sample, while a Cooper pair box, resonant with the cavity field, may be used to carry out one- and two-qubit gate operations.
High-spin systems in molecules and inorganic solids hold significant potential for advancing quantum information storage and processing technologies, owing to their multilevel structure. However, the dynamics of spin-photon interactions in these systems remain underexplored in practical device settings. Here we investigate the quantum properties of an S = 3/2 spin ensemble in a ruby crystal embedded in a high-Tc superconducting microwave cavity. Using electron spin resonance techniques, we measure the coupling strength between the quantized electromagnetic mode of the cavity and the spin ensemble across a broad temperature range and for various crystal axis orientations relative to the applied magnetic field. The coupling strength, governed by thermal polarization of the spins and quantum mechanical transition probabilities, shows quantitative agreement with theoretical predictions based on spin Hamiltonian diagonalization and electromagnetic simulations of the cavity mode. These findings establish impurity spins in solids as controllable multilevel quantum systems and provide critical insights into the factors influencing spin-cavity interactions. This work opens new avenues for optimizing quantum information processing and developing versatile quantum technologies based on high-spin systems. Impurity spins in solids are controllable multilevel quantum systems that can be harnessed for implementing quantum memories. In this work, the authors investigate the coupling strength between spins in a ruby crystal and a quantized electromagnetic mode using electron spin resonance technique.
Group-IV color centers in diamond are promising candidates for quantum networks due to their dominant zero-phonon line and symmetry-protected optical transitions that connect to coherent spin levels. The negatively charged tin-vacancy (SnV) center possesses long electron spin lifetimes due to its large spin-orbit splitting. However, the magnetic dipole transitions required for microwave spin control are suppressed, and strain is necessary to enable these transitions. Recent work has shown spin control of strained emitters using microwave lines that suffer from Ohmic losses, restricting coherence through heating. We utilize a superconducting coplanar waveguide to measure SnV centers subjected to strain, observing substantial improvement. A detailed analysis of the SnV center electron spin Hamiltonian based on the angle-dependent splitting of the ground and excited states is performed. We demonstrate coherent spin manipulation and obtain a Hahn echo coherence time of up to T 2 = 430 μ s . With dynamical decoupling, we can prolong coherence to T 2 = 10 ms , about a sixfold improvement compared to earlier works. We also observe a nearby coupling C 13 spin, which may serve as a quantum memory, thus substantiating the potential of SnV centers in diamond and demonstrates the benefit of superconducting microwave structures. Published by the American Physical Society 2024
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This article reviews efforts to build a new type of quantum device, which combines an ensemble of electronic spins with long coherence times, and a small-scale superconducting quantum processor. The goal is to store over long times arbitrary qubit states in orthogonal collective modes of the spin-ensemble, and to retrieve them on-demand. We first present the protocol devised for such a multi-mode quantum memory. We then describe a series of experimental results using NV (as in nitrogen vacancy) center spins in diamond, which demonstrate its main building blocks: the transfer of arbitrary quantum states from a qubit into the spin ensemble, and the multi-mode retrieval of classical microwave pulses down to the single-photon level with a Hahn-echo like sequence. A reset of the spin memory is implemented in-between two successive sequences using optical repumping of the spins.