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CIRQUSS Report Summary

Project ID: 615767
Funded under: FP7-IDEAS-ERC
Country: France

Mid-Term Report Summary - CIRQUSS (Circuit Quantum Electrodynamics with Single Electronic and Nuclear Spins)

The detection and characterization of paramagnetic species in a sample by Electron Spin Resonance (ESR) spectroscopy has numerous applications, from biochemistry to materials science and even archaeology. ESR spectroscopy detects weak microwave signals emitted by the spins when they are placed in resonance with a microwave cavity. Usual spectrometers are only able to detect large numbers (>10^7) of spins. For many applications however, ESR spectroscopy at the nanoscale would be highly desirable, in particular for quantum information processing applications where spins are used as quantum bits in a quantum computer.
The CIRQUSS project aims at achieving this goal, using the tools developed in the field of superconducting quantum circuits. One central goal of the project is to demonstrate single-spin sensitivity in ESR spectroscopy detection. For that, difficult technological developments are required, in particular the fabrication of nanometric superconducting wires in close proximity to the spin to be detected. This should be combined with high-quality factor resonators, and highly sensitive microwave amplifiers known as Josephson Parametric Amplifiers (JPAs).
While single-spin sensitivity has not yet been achieved, we have made key progress in that direction in the first half of the CIRQUSS project. In particular, we have performed the first detection of ESR spectroscopy of an ensemble of spins using a JPA, at a temperature of 20 millikelvin. The heart of the spectrometer is a micron-scale superconducting resonator, deposited on top of a silicon sample that contains bismuth donors, whose spin we detect. The spectrometer is able to detect as little as 1000 donor spins. This represents a gain of 4 orders of magnitude over the state-of-the-art [1]. Moreover, we have demonstrated a new way to improve its sensitivity by shining so-called squeezed microwave states on the spins. Squeezed states have reduced noise on one of the two field quadratures, and if the phase of the squeezing corresponds to the phase of the emitted echo, its signal-to-noise is improved [2]. Finally, we have demonstrated that spin relaxation occurs by spontaneous emission of microwave photons through the cavity (known as the Purcell effect) [3]. Purcell-limited spin relaxation is mandatory for single-spin detection, and this regime had never been reached so far.
To make further progress on the sensitivity, we need to reduce the transverse dimensions of the resonator (which in these experiments was of 5 microns) down to the few tens of nanometers scale. Using electron-beam lithography, we have developed novel fabrication techniques to do that.
Besides superconducting resonators, other types of quantum circuits are useful for detecting individual spins. Because of their large magnetic moment, superconducting flux-qubits are prime candidates. In order to couple them to spins, long coherence times are needed. We have made an experiment where we improve the flux-qubit coherence times by one order of magnitude over the state-of-the-art, which is encouraging for the next steps. Their properties need however to be better controlled and tuned for our final goal.
[1] Reaching the quantum limit of sensitivity in electron-spin resonance A. Bienfait, J.J. Pla, Y. Kubo, M. Stern, X. Zhou, C.C. Lo, C.D. Weis, T. Schenkel, M.L.W. Thewalt, D. Vion, D. Esteve, B. Julsgaard, K. Moelmer, J.J.L. Morton, and P. Bertet, Nature Nanotechnology 11(3), 253-257 (2015)
[2] Magnetic resonance with squeezed microwaves A. Bienfait, P. Campagne-Ibarcq, A. Holm-Kiilerich, X. Zhou, S. Probst, J.J. Pla, T. Schenkel, D. Vion, D. Esteve, J.J.L. Morton, K. Moelmer, and P. Bertet, arxiv:1610.03329, submitted to Science
[3] Controlling Spin Relaxation with a Cavity A. Bienfait, J.J. Pla, Y. Kubo, X. Zhou, M. Stern, C.C Lo, C.D. Weis, T. Schenkel, D. Vion, D. Esteve, J.J.L. Morton, and P. Bertet, Nature 531, 74 (2016).

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