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Journal of Magnetic Resonance Open

Multiple electron spin resonance echoes observed for paramagnetic defects in diamond at room temperature

Aharon Blank, Boaz Koren, Alexander Sherman

Published Aug 26, 2023• 3 citations
Publisher

Abstract

Magnetic resonance can be used as a testbed for observing, demonstrating, and studying fundamental concepts of quantum cavity interaction with a two-level system at the microwave regime. Such experiments are mostly carried out at low cryogenic temperatures with spin systems that are embedded in a high-quality (Q-factor) superconducting cavity. For example, recent work has shown that under such conditions, and in a regime of high cooperativity and strong collective coupling between an electron spin system and a microwave cavity, multiple spin echoes are observed. These echoes have been analysed as signatures of coherent quantum effects; namely, the photons in the cavity can excite the spin system, which in turn can excite the cavity itself and so on. In this work we show that the use of a unique tailor-made moderate Q cavity in conjunction with diamond crystals having large concentrations of nitrogen vacancy (NV) centers makes it possible to observe such non-linear quantum phenomenon, even at room temperature. A key requirement of our experiments is that the spin system’s thermodynamic (rather than the physical) temperature is cooled to a few K, while high cooperativity and/or strong coupling are not a necessary prerequisite for such observations. The observation of meaningful microwave cavity quantum effects even at room temperature could lead to applications such as quantum memories and quantum sensing.

DOI
10.1016/j.jmro.2023.100133

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Magnetic resonance can be used as a testbed for observing, demonstrating, and studying fundamental concepts of quantum cavity interaction with a two-level system at the microwave regime. Such experiments are mostly carried out at low cryogenic temperatures with spin systems that are embedded in a high-quality (Q-factor) superconducting cavity. For example, recent work has shown that under such conditions, and in a regime of high cooperativity and strong collective coupling between an electron spin system and a microwave cavity, multiple spin echoes are observed. These echoes have been analysed as signatures of coherent quantum effects; namely, the photons in the cavity can excite the spin system, which in turn can excite the cavity itself and so on. In this work we show that the use of a unique tailor-made moderate Q cavity in conjunction with diamond crystals having large concentrations of nitrogen vacancy (NV) centers makes it possible to observe such non-linear quantum phenomenon, even at room temperature. A key requirement of our experiments is that the spin system’s thermodynamic (rather than the physical) temperature is cooled to a few K, while high cooperativity and/or strong coupling are not a necessary prerequisite for such observations. The observation of meaningful microwave cavity quantum effects even at room temperature could lead to applications such as quantum memories and quantum sensing.
Recent experiments demonstrated the cooling of a microwave mode of a high-quality dielectric resonator coupled to optically cooled nitrogen-vacancy (NV) spins in diamond. Our recent theoretical study [arXiv:2110.10950] pointed out the cooled NV spins can be used to realize cavity quantum electrodynamics effects (C-QED) at room temperature. In this article, we propose to modify the setup used in a recent diamond maser experiment [Nature 55, 493-496 (2018)], which features a higher spin transition frequency, a lower spin-dephasing rate and a stronger NV spins-resonator coupling, to realize better microwave mode cooling and the room-temperature CQED effects. To describe more precisely the optical spin cooling and the collective spin-resonator coupling, we extend the standard Jaynes-Cumming model to account for the rich electronic and spin levels of the NV centers. Our calculations show that for the proposed setup it is possible to cool the microwave mode from 293293293 K (room temperature) to 116116116 K, which is about 727272 K lower than the previous records, and to study the intriguing dynamics of the CQED effects under the weak-to-strong coupling transition by varying the laser power. With simple modifications, our model can be applied to, e.g., other solid-state spins or triplet spins of pentacene molecules, and to investigate other effects, such as the operations of pulsed and continuous-wave masing.
In this Letter, we propose that superradiant echoes can be achieved at room temperature by applying a laser illumination and a microwave Hahn echo sequence to a diamond with a high concentration of nitrogen-vacancy (NV) centers placed in a dielectric microwave cavity. We identify that the combined action of two microwave driving pulses and a free evolution imprints a phase grating among NV spin sub-ensembles in frequency space, and multiple re-phasing of the grated spin sub-ensembles leads to multiple superradiant echoes through a collective coupling with the cavity. Furthermore, we show that the superradiant echoes can be actively tailored with the microwave pulses and the laser illumination by modifying the grating parameters, and the multiple re-phasing dynamics is analogous to the one leading to superradiant beats in optical clock system. In the future, the spin sub-ensembles grating and the resulting echoes can be further optimized with dynamical decoupling, which might pave the way for applications in quantum sensing.
A concept combining optics and microwave pulses with the negative charge-state of the nitrogen-vacancy (NV—) center in diamond is demonstrated through experiments that are equivalent to single-qubit gates, and decoherence for this qubit is examined. The spin levels of the ground state provide the two-level system. Optical excitation provides polarization of these states. The polarized state is operated coherently by 35 GHz microwave pulses. The final state is read out through the photoluminescence intensity. Decoherence arises from different sources for different samples. For high-pressure, high-temperature synthetic diamonds, the high concentration of substitutional N limits the phase-memory to a few μs. In a single-crystal CVD diamond, the phase memory time is at least 32 μs at 100 K. 14N is tightly coupled to the electronic spin and produces modulation of the electron–spin echo decay under certain conditions. A two-qubit gate is proposed using this nuclear spin. This demonstration provides a great deal of insight into quantum devices in the solid state with some possibility for real application.
Among the possible defects that can occur in diamonds, one that draws attention is the nitrogen-vacancy (NV) center.Studied since the early 90s, in recent years, they became one of the most promising platforms for field applications of quantum technology at room temperature, especially for quantum sensing.They are sensitive to electric and magnetic fields, temperature, and strain, making these nanosensors a versatile platform for many applications.The NV center's spin state is easy to read and manipulate by optical means, and, most interestingly, it works well at room temperature, which facilitates its use.The great challenge to implement quantum protocols and quantum algorithms is the coherence time of the quantum states.The NV center stands out for having relatively high coherence times at room temperature.This dissertation presents measurements to characterize the electron spin coherence of NV centers in an ultrapure sample of diamond designed for quantum sensing applications.These measurements combine optical e magnetic resonance techniques.For that, a method was implemented using a conventional CCD camera and a clever imaging protocol, making it possible to perform various pulsed microwave experiments on the electron spin, such as Rabi oscillations and Hahn spin-echo, which were used for determining the characteristic relaxation times (T 1 and T 2 ).Results show that this system provides at least two readily accessible qubits that can be used in quantum protocols even at room temperature.
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.
Abstract Multiple-microwave-photon transition is a nonlinear process involving the simultaneous absorption of multiple microwave photons to reach a higher energy state. This phenomenon enables microwave frequency conversion, a crucial technique for spin-wave devices, frequency synthesizers and communication devices. However, it has not been observed in nitrogen-vacancy (NV) centers in diamonds, which serve as a key quantum platform linking flying qubits, microwave photons, and solid-state spins. In this work, we first observed multiple-microwave-photon transitions in NV centers in diamonds, using optically detected magnetic resonance (ODMR) methods with a single laser and microwave beam. We demonstrate that microwave frequencies, effective g-factor, and angular momentum exhibit a 1/n scaling law, but the linewidth exhibits a 1/n^2 scaling law for n-photon transitions between the ground states. Additionally, we detect the hyperfine structure of the ground states and achieve coherent spin manipulation under multi-photon excitation. Our findings offer a novel approach for the precise manipulation of NV center spin states, advancing quantum sensing and quantum information.