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arXiv:1006.0251 · Physical Review Letters

Strong Coupling of a Spin Ensemble to a Superconducting Resonator

Yuimaru Kubo, Florian R. Ong, Patrice Bertet, D. Vion, V. Jacques, Dingwei Zheng, A. Dréau, Jean-François Roch, Alexia Auffèves, Fedor Jelezko, Jörg Wrachtrup, M.F. Barthe, Philippe Bergonzo, D. Esteve

Published Sep 27, 2010• 642 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1103/physrevlett.105.140502

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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.
We report the experimental realization of a hybrid quantum circuit combining a superconducting qubit and an ensemble of electronic spins. The qubit, of the transmon type, is coherently coupled to the spin ensemble consisting of nitrogen-vacancy centers in a diamond crystal via a frequency-tunable superconducting resonator acting as a quantum bus. Using this circuit, we prepare a superposition of the qubit states that we store into collective excitations of the spin ensemble and retrieve back into the qubit later on. These results constitute a proof of concept of spin-ensemble based quantum memory for superconducting qubits.
Circuit-QED has demonstrated very strong coupling between light and matter, and has the potential to engineer large quantum devices. Hybrid designs have been proposed which couple large ensembles of atomic and molecular systems to the superconducting resonator. We show that one can achieve an effective strong coupling between light and matter for much smaller ensembles (and even a single electronic spin), through the use of an interconnecting quantum system: in our case a persistent current qubit. Using this interconnect we show that one can effectively magnify the coupling strength between the light and matter by over five orders of magnitude $g\ensuremath{\sim}7\text{ }\text{Hz}\ensuremath{\rightarrow}100\text{ }\text{kHz}$ and enter a regime where a single nitrogen-vacancy (NV) electronic spin can shift the cavity resonance line by over $\ensuremath{\sim}20$ linewidths. With such strong coupling between an individual electronic spin in an NV and the light in the resonator, one has the potential build devices where the associated NV nuclear spins can be strongly coupled over centimeters via the superconducting bus.
We study a hybrid quantum system consisting of spin ensembles and superconducting flux qubits, where each spin ensemble is realized using the nitrogen-vacancy centers in a diamond crystal and the nearest-neighbor spin ensembles are effectively coupled via a flux qubit. We show that the coupling strengths between flux qubits and spin ensembles can reach the strong and even ultrastrong coupling regimes by either engineering the hybrid structure in advance or tuning the excitation frequencies of spin ensembles via external magnetic fields. When extending the hybrid structure to an array with equal coupling strengths, we find that in the strong-coupling regime, the hybrid array is reduced to a tight-binding model of a one-dimensional bosonic lattice. In the ultrastrong-coupling regime, it exhibits quasiparticle excitations separated from the ground state by an energy gap. Moreover, these quasiparticle excitations and the ground state are stable under a certain condition that is tunable via the external magnetic field. This may provide an experimentally accessible method to probe the instability of the system.
For quantum sensing based on nitrogen-vacancies (NV) ensembles, microwave antennas can couple the microwave field to the NV center, which leads it to becoming the core of spin manipulation and can directly affect the sensitivity of quantum sensing. The double split-ring resonator is a widely used microwave device for NV ensembles due to the advantages of high radiation efficiency and uniform magnetic field in millimeter-scale areas. But the bandwidth (30 MHz) is quite narrow which limits the application in quantum sensing with NV ensembles. Here, we experimentally achieve continuous tuning of the resonant frequency of the double split-ring resonator by changing the copper sheet position on the edge of the outer ring. The frequency tuning range can reach 80 MHz, up to 2-3 times the bandwidth, which can cover the transition of the electron spin under different magnetic field conditions. The performance of the tunable antenna in the quantum operation of NV centers is verified by optically detected magnetic resonance and Rabi oscillation. This tunable antenna is promising in the fabrication of integrated and arrayed quantum sensors based on NV ensembles.
A new method for detecting the magnetic resonance of electronic spins at low temperature is demonstrated. It consists in measuring the signal emitted by the spins with a superconducting qubit that acts as a single-microwave-photon detector, resulting in an enhanced sensitivity. We implement such an electron-spin resonance spectrometer using a hybrid quantum circuit in which a transmon qubit is coupled to a spin ensemble consisting of nitrogen-vacancy (NV) centers in diamond. With this setup we measure the NV center absorption spectrum at 30 mK at an excitation level of $\ensuremath{\sim}15\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{B}$ out of an ensemble of 1011{10}^{11}1011 spins.