Spectroscopic properties of inhomogeneously broadened spin ensembles in a cavity

Kurucz, Z.; Wesenberg, J. H.; Mølmer, Klaus

doi:10.1103/physreva.83.053852

Cited by 92 publications

(111 citation statements)

References 36 publications

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“…Mat. and [20,25,26] that in both curves shown in Fig. 2b P e (τ ) tends towards 0.08 at long times, as is also found with the qubit initially in |g .…”

supporting

confidence: 60%

“…The qubit, bus resonator, and spin ensemble are described by Hamiltonians ). The spin-resonator dynamics can be calculated from this model, as explained in [25,26] and in the Supplementary Material. The sample we use is a polished (110) plate of dimensions 2.2 × 1 × 0.5 mm 3 taken from a type-Ib HPHT crystal which contained 40 ppm of neutral substitutional nitrogen (the P1 centers) as measured by IR absorption.…”

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confidence: 99%

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Hybrid Quantum Circuit with a Superconducting Qubit Coupled to a Spin Ensemble

et al. 2011

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Present-day implementations of quantum information processing rely on two widely different types of quantum bits (qubits). On the one hand, microscopic systems such as atoms or spins are naturally well decoupled from their environment and as such can reach extremely long coherence times [1,2]; on the other hand, more macroscopic objects such as superconducting circuits are strongly coupled to electromagnetic fields, making them easy to entangle [3,4] although with shorter coherence times [5,6]. It thus seems appealing to combine the two types of systems in hybrid structures that could possibly take the best of both worlds. Here we report the first experimental realization of a hybrid quantum circuit in which a superconducting qubit of the transmon type [5,7] is coherently coupled to a spin ensemble consisting of nitrogen-vacancy (NV) centers in a diamond crystal [8] via a frequency-tunable superconducting resonator [9] acting as a quantum bus. Using this circuit, we prepare arbitrary superpositions of the qubit states that we store into collective excitations of the spin ensemble and retrieve back later on into the qubit. We demonstrate that this process preserves quantum coherence by performing quantum state tomography of the qubit. These results constitute a first proof of concept of spin-ensemble based quantum memory for superconducting qubits [10][11][12]. As a landmark of the successful marriage between a superconducting qubit and electronic spins, we detect with the qubit the hyperfine structure of the NV center.Superconducting qubits have been successfully coupled to electromagnetic [13] as well as mechanical [14] resonators; but coupling them to microscopic systems in a controlled way has up to now remained an elusive perspective -even though qubits sometimes turn out to be coupled to unknown and uncontrolled microscopic degrees of freedom with relatively short coherence times [15]. Whereas the coupling constant g of one individual microscopic system to a superconducting circuit is usually too weak for quantum information applications, ensembles of N such systems are coupled with a constant g √ N enhanced by collective effects.This makes possible to reach a regime of strong coupling between one collective variable of the ensemble and the circuit. This collective variable, which behaves in the low excitation limit as a harmonic oscillator, has been proposed [10-12] as a quantum memory for storing the state of superconducting qubits. Experimentally, the strong coupling between an ensemble of electronic spins and a superconducting resonator has been demonstrated 2 spectroscopically [16][17][18], and the storage of a microwave field into collective excitations of a spin ensemble has been observed very recently [19,20]. These experiments were however carried out in a classical regime since the resonator and spin ensemble behaved as two coupled harmonic oscillators driven by large microwave fields. In the perspective of building a quantum memory, it is instead necessary to perform experiments at the level of a...

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“…Mat. and [20,25,26] that in both curves shown in Fig. 2b P e (τ ) tends towards 0.08 at long times, as is also found with the qubit initially in |g .…”

supporting

confidence: 60%

mentioning

confidence: 99%

Hybrid Quantum Circuit with a Superconducting Qubit Coupled to a Spin Ensemble

et al. 2011

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“…In our explicitly time-dependent study we will demonstrate the so-called 'cavityprotection effect' 5,7 , which was predicted in these proposals but has remained so far unobserved.…”

mentioning

confidence: 70%

“…At present, the main source of decoherence in these systems is inhomogeneous spin broadening, which limits their performance for the coherent transfer and storage of quantum information [5][6][7] . Here we study the dynamics of a superconducting cavity strongly coupled to an ensemble of nitrogen-vacancy centres in diamond.…”

mentioning

confidence: 99%

“…Here we study the dynamics of a superconducting cavity strongly coupled to an ensemble of nitrogen-vacancy centres in diamond. We experimentally observe how decoherence induced by inhomogeneous broadening can be suppressed in the strong-coupling regime-a phenomenon known as 'cavity protection' 5,7 . To demonstrate the potential of this e ect for coherent-control schemes, we show how appropriately chosen microwave pulses can increase the amplitude of coherent oscillations between the cavity and spin ensemble by two orders of magnitude.…”

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confidence: 99%

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Protecting a spin ensemble against decoherence in the strong-coupling regime of cavity QED

et al. 2014

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Hybrid quantum systems based on spin ensembles coupled to superconducting microwave cavities are promising candidates for robust experiments in cavity quantum electrodynamics (QED) and for future technologies employing quantum mechanical e ects [1][2][3][4] . At present, the main source of decoherence in these systems is inhomogeneous spin broadening, which limits their performance for the coherent transfer and storage of quantum information [5][6][7] . Here we study the dynamics of a superconducting cavity strongly coupled to an ensemble of nitrogen-vacancy centres in diamond. We experimentally observe how decoherence induced by inhomogeneous broadening can be suppressed in the strong-coupling regime-a phenomenon known as 'cavity protection' 5,7 . To demonstrate the potential of this e ect for coherent-control schemes, we show how appropriately chosen microwave pulses can increase the amplitude of coherent oscillations between the cavity and spin ensemble by two orders of magnitude.The processing of quantum information requires special devices that can store and manipulate quantum bits. Hybrid quantum systems 2 combine the advantages of different systems to overcome their individual physical limitations. In this context superconducting microwave cavities have emerged as ideal tools for realizing strong coupling to qubits 3,4,[8][9][10][11][12] for the transfer of excitations on the singlephoton level 13,14 . For the storage of quantum information the negatively charged nitrogen-vacancy (NV) centres in diamond show great potential, especially owing to their long coherence times (up to one second 15 ) and to the combination of microwave and optical transitions which makes them an easily accessible and controllable qubit 16 . Coherently passing quantum information between such a spin and a cavity requires that they are strongly coupled to each other. As has recently been shown 4,10-12 , this limit can be reached by collective coupling to a large spin ensemble, in which case the coupling strength is increased by the square root of the ensemble size. However, this collective coupling comes with a considerable downside: in a solid-state environment a spin is always prone to inhomogeneous broadening. In particular, for an ensemble of NV centres, magnetic dipolar interaction with excess nuclear and electron spins in the diamond crystal leads to an inhomogeneous broadening of the spin transition 17 , which acts as the dominant source of decoherence. Several approaches, including echo-type refocusing techniques 18,19 , have been suggested to overcome this limitation. Here we will concentrate on recent theoretical proposals which rely on the specific shape of the inhomogeneous spectral In our homodyne detection measurements, the input microwave signal is split into two paths, both serving as a reference signal as well as for testing and controlling our experiment. Outside the cryostat both signal paths are combined by a frequency mixer and the quadratures I and Q are recorded with a fast analog-to-digital converter (ADC) ...

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Room‐Temperature Quantum Memories Based on Molecular Electron Spin Ensembles

et al. 2021

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Superconducting quantum bits are addressed by means of microwave radiation and quantum memories in this context thus need to be able to store microwave photon states. To this end, resonant structures for electromagnetic radiation can be used to strongly couple quantum bits and quantum memories to quantized cavity modes of the electromagnetic field. The strong coupling generates a hybrid quantum system that allows mapping the qubit state onto the cavity field state. [6] This strategy has given rise to the field of cavity quantum electrodynamics and has already been used to couple two superconducting qubits together via a cavity bus. [7] A second application of quantum memories is in quantum repeaters for quantum communication. Because telecom wavelengths are in the near-infrared, for this application, optical quantum memories are required that store optical photon states. [8] Considering the choice of material platform for designing microwave quantum memories, electron spins in solids can be easily addressed by microwave pulses and have been shown to possess excellent coherence times up to seconds, for some systems even up to room temperature. [9][10] Unfortunately, the coupling of a single electron spin to a photon is very weak. Although resonant structures can be tailored to improve single-spin coupling, [11][12][13] the weakness of the coupling renders addressing individual spins by means of coupling to cavity modes very challenging. This can be overcome by using an electron spin ensemble rather than single spins, because the coupling strength of a spin ensemble to an electromagnetic resonator mode is proportional to the square root of the number of electron spins in the ensemble. [14] This then leads to the general idea of the implementation of a microwave quantum memory using spin ensembles and microwave resonators (Figure 1). The state to be stored is encoded in a weak microwave pulse and sent to the hybrid quantum system constituted of an electron spin ensemble that is strongly coupled to a microwave resonator, where it is stored. When the quantum state is needed again, a strong microwave pulse is sent to the resonator, which leads to deterministic retrieval of the quantum state that is emitted as a microwave photon to be used as desired.Strong coupling between cavity and electron spin ensembles has been observed in a variety of spin systems, including organic radicals, [15][16][17][18][19][20] phosphorous dopants in silicon, [21,22] P1 and nitrogen vacancy (NV) defects in diamond, [23][24][25][26][27][28][29][30][31][32] N@ C 60 , [33] rare earth dopants in oxides, [34,35] transition metal Whilst quantum computing has recently taken great leaps ahead, the development of quantum memories has decidedly lagged behind. Quantum memories are essential devices in the quantum technology palette and are needed for intermediate storage of quantum bit states and as quantum repeaters in long-distance quantum communication. Current quantum memories operate at cryogenic, mostly sub-Kelvin temperatures and require extens...

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Spectroscopic properties of inhomogeneously broadened spin ensembles in a cavity

Cited by 92 publications

References 36 publications

Hybrid Quantum Circuit with a Superconducting Qubit Coupled to a Spin Ensemble

Hybrid Quantum Circuit with a Superconducting Qubit Coupled to a Spin Ensemble

Protecting a spin ensemble against decoherence in the strong-coupling regime of cavity QED

Room‐Temperature Quantum Memories Based on Molecular Electron Spin Ensembles

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