Electron spin resonance detected by a superconducting qubit
Yuimaru Kubo, Igor Diniz, Cécile Grèzes, Takahide Umeda, Junichi Isoya, Hitoshi Sumiya, T. Yamamoto, Hiroyuki Abé, S. Onoda, Takeshi Ohshima, V. Jacques, A. Dréau, Jean-François Roch, Alexia Auffèves, D. Vion, D. Esteve, Patrice Bertet
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 ${10}^{11}$ spins.
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∼15\ensuremathμB out of an ensemble of 1011 spins.
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.
Superconducting circuits are promising components for future quantum information technologies such as quantum computing, quantum simulation, and quantum sensing. Superconducting quantum interference devices (SQUIDs) and superconducting flux qubits can be used as a sensitive magnetic field sensor to detect local electron spin resonance (ESR). These types of sensor are characterized by good sensitivity and high spatial resolution for electron spins. In fact, we have achieved a sensitivity of 20 spins/Hz with a sensing volume of 6 fl combining a flux qubit and qubit readout using a Josephson bifurcation amplifier. As a demonstration, we measured the ESR spectrum of erbium electron spins in a Y2SiO5 crystal and nitrogen vacancy centers in diamond. In both cases, we obtained reasonable electron spin parameters by fitting a simulation to the spectrum. In this way, we proved superconducting circuits to be promising magnetic field sensors for evaluating the properties of solid materials.
Quantum entanglement is a kernel of quantum computation and quantum communication. We introduce a theoretical scheme to achieve the entanglement between two separated quantum nodes in a hybrid system. The proposed hybrid system based on diamond nitrogen-vacancy (NV) center spin ensemble is coherently coupled to a superconducting quantum circuit consisting of two quantum nodes and a quantum channel. Each node in our setup is composed of an NV center spin ensemble magnetically coupled to a superconducting coplanar resonator. The NV center spin ensemble composed of N identical and non-interacting NV spins, is placed in the magnetic field antinode of the superconducting coplanar resonator where the coupling is maximized. An array of superconducting quantum interference devices (SQUIDs) is inserted in the central conductor of resonator to make its frequency tunable with the magnetic flux threading through the SQUID loops. This flux is generated by passing current through an on-chip wire, so that the resonator can be brought in resonance with the NV center spins without changing their Zeeman splitting. Quantum qubits encoded into two separate nodes are connected by a vacuum superconducting coplanar resonator that is used as a quantum channel. This setup can potentially take the best elements of each individual system:NV center spin ensemble with longer coherence time capable of preparing, storing and releasing photonic quantum information, and the superconducting quantum circuits are easy to manipulate externally and can perform quantum logic gates to control quantum information rapidly. In order to realize the entanglement between two separated quantum nodes, firstly, we make a canonical transformation and obtain the Hamiltonian of the system that is reduced to two NV center spin ensembles resonantly coupled to a single mode of the superconducting coplanar resonator. Then we put forward the hybrid NV center spin-photon qubit encoding. In this hybrid encoding, the NV center spin and photon degrees of freedom enter on an equal footing into the definition of the qubit, in which case, quantum channel will switch on when three superconducting coplanar resonators are in resonance with each other, and all the manipulations can perform simply by tuning the frequencies of the superconducting coplanar resonators. Under the precise control of the evolution time, high fidelity entanglement between two separated quantum nodes is achieved. We show that this proposal can provide high fidelity quantum entanglement under realistic conditions, both in the resonant and the dispersive interaction cases. This hybrid quantum system will exhibit long coherence time and possess features like easy fabrication, integratability, and potential scalability. Furthermore, the quantum node composed of an NV center spin ensemble magnetically coupled to a superconducting coplanar resonator can be respectively integrated, which has practical applications in the realization of quantum information transmission and quantum entanglement among multiple quantum nodes.
Nitrogen-vacancy (NV) centers in diamond and superconducting qubits are two promising solid-state quantum systems for quantum science and technology, but the realization of controlled interfaces between individual solid-state spins and superconducting qubits remains fundamentally challenging. Here, we propose and analyze a hybrid quantum system consisting of a magnetic skyrmion, an NV center, and a superconducting qubit, where the solid-state qubits are both positioned in proximity to the skyrmion structure in a thin magnetic disk. We show that it is experimentally feasible to achieve strong (coherent or dissipative) coupling between the NV center and the superconducting qubit by using the as an intermediary. This allows coherent information transfer and nonreciprocal responses between the NV center and the superconducting qubit at the single quantum level with high controllability. The proposed platform provides a scalable pathway for implementing quantum protocols that synergistically exploit the complementary advantages of spin-based quantum memories, microwave-frequency superconducting circuits, and topologically protected magnetic excitations.
Electron spin resonance (ESR) is a powerful spectroscopy method which allows to identify and characterize paramagnetic species. It usually relies on detecting microwave radiation absorbed or emitted by the spins into a microwave resonator tuned to their Larmor precession frequency. Because of the weak spin-microwave coupling, conventional ESR spectroscopy has a low sensitivity. Recent experiments have demonstrated that superconducting quantum circuits have the potential to enhance the spin detection sensitivity to single spin detection; by namely using high quality factor small mode volume resonator. However, these demonstrations have been realized in very restrictive conditions, using well-known spin systems. To probe samples coming from chemistry or biology, quantum-circuit enabled spectrometers have to be adapted to be robust to losses. They have typically two origins: the dielectric losses introduced by the samples, mostly aqueous, and losses coming from the strong magnetic field required to tune the electronic spins to the resonator frequency. This PhD work centered on implementing a resonator that would achieve the same ESR sensitivity while being robust to dielectric losses and strong magnetic fields. Using NbTiN films deposited on sapphire, microwave resonators were shown to have quality factors above Q=105 in magnetic field, even when samples with dielectric losses above tan d > 0.05 were introduced. A study of the resonator intrinsic losses showed marked drops at multiple specific magnetic fields: a signature of coupling to spurious systems. Over the course of the PhD, mitigation nanofabrication techniques for these contaminants were developed, such as hard mask etching and annealing, to remove all but one of this contaminants. Finally, test of the spectrometer on a benchmark species (BDPA, α,γ-Bisdiphenylene-β-phenylallyl, a known ESR marker) were realized.
High-fidelity quantum operation of qubits plays an important role in magnetometry based on nitrogen-vacancy (NV) centers in diamonds. However, the nontrivial spin-spin coupling of the NV center decreases signal contrast and sensitivity. Here, we overcome this limitation by exploiting the amplitude modulation of microwaves, which allows us to perfectly detect magnetic signals at low fields. Compared with the traditional double-quantum sensing protocol, the full contrast of the detection signal was recovered, and the sensitivity was enhanced three times in the experiment. Our method is applicable to a wide range of sensing tasks, such as temperature, strain, and electric field.