Nitrogen-vacancy Spin Research Articles

Strong driving of a single coherent spin by a proximal chiral ferromagnet

We experimentally investigate the influence of a driven, dynamic vortex magnetization state on an individual nitrogen-vacancy (NV) spin in diamond. The vortex core can be translated within the ferromagnet using an applied magnetic field, allowing us to map out the spatial dependence of the interaction. The vortex displacement is determined using magneto-optical microscopy, while the vortex's influence on the spin is probed using optically detected magnetic resonance to measure the Rabi oscillation frequency between spin levels. We find that the close proximity of the vortex core to the NV (within about 200 nm) leads to more than an order of magnitude enhancement of the Rabi frequency. The NV/vortex interaction differs significantly for transitions to the ${m}_{s}=+1$ and ${m}_{s}=\ensuremath{-}1$ spin states, which we attribute to the chiral nature of the vortex state dynamics. We compare the results with micromagnetic simulations and a simple analytical model to shed light on the mechanisms behind the observed effects.

Dissipatively Stabilized Quantum Sensor Based on Indirect Nuclear-Nuclear Interactions.

We propose to use a dissipatively stabilized nitrogen vacancy (NV) center as a mediator of interaction between two nuclear spins that are protected from decoherence and relaxation of the NV due to the periodical resets of the NV center. Under ambient conditions this scheme achieves highly selective high-fidelity quantum gates between nuclear spins in a quantum register even at large NV-nuclear distances. Importantly, this method allows for the use of nuclear spins as a sensor rather than a memory, while the NV spin acts as an ancillary system for the initialization and readout of the sensor. The immunity to the decoherence and relaxation of the NV center leads to a tunable sharp frequency filter while allowing at the same time the continuous collection of the signal to achieve simultaneously high spectral selectivity and high signal-to-noise ratio.

Diamonds levitating in a Paul trap under vacuum: Measurements of laser-induced heating via NV center thermometry

We present measurements of the electronic spin resonance (ESR) of nitrogen vacancy (NV) centers in diamonds that are levitating in a ring Paul trap under vacuum. We observe ESR spectra of NV centers embedded in micron-sized diamonds at vacuum pressures of 2 × 10−1 mbar and the NV photoluminescence down to 10−2 mbar. Further, we use the ESR to measure the temperature of the levitating diamonds and show that the green laser induces heating of the diamond at these pressures. We finally discuss the steps required to control the NV spin under ultra-high vacuum.

Designing a cavity-mediated quantum cphase gate between NV spin qubits in diamond

While long spin coherence times and efficient single-qubit quantum control have been implemented successfully in nitrogen-vacancy (NV) centers in diamond, the controlled coupling of remote NV spin qubits remains challenging. Here, we propose and analyze a controlled-phase (CPHASE) gate for the spins of two NV centers embedded in a common optical cavity and driven by two off-resonant lasers. In combination with previously demonstrated single-qubit gates, CPHASE allows for arbitrary quantum computations. The coupling of the NV spin to the cavity mode is based upon Raman transitions via the NV excited states and can be controlled with the laser intensities and relative phase. We find characteristic laser frequencies at which the scattering amplitude of a laser photon into the cavity mode is strongly dependent on the NV center spin. A scattered photon can be reabsorbed by another selectively driven NV center and generate a conditional phase (CPHASE) gate. Gate times around 200 ns are within reach, nearly three orders of magnitude shorter than typical NV spin coherence times of around 10 microseconds. The separation between the two interacting NV centers is only limited by the extension of the cavity.

Enhanced Resolution in Nanoscale NMR via Quantum Sensing with Pulses of Finite Duration

The nitrogen vacancy (NV) color center in diamond is an enormously important platform for the development of quantum sensors, including for single spin and single molecule NMR. Detection of weak single-spin signals is greatly enhanced by repeated sequences of microwave pulses; in these dynamical decoupling (DD) techniques, the key control parameters swept in the experiment are the time intervals, $\tau$, between pulses. Here we show that, in fact, the pulse duration offers a powerful additional control parameter. While previously, a non-negligible pulse-width has been considered simply a source of experimental error, here we elucidate the underlying quantum dynamics: we identify a landscape of quantum-state crossings which are usually closed (inactive) but may be controllably activated (opened) by adjusting the pulse-width from zero. We identify these crossings with recently observed but unexpected dips (so called spurious dips) seen in the quantum coherence of the NV spin. With this new understanding, both the position and strength of these sharp features may be accurately controlled; they co-exist with the usual broader coherence dips of short-duration microwave pulses, but their sharpness allows for higher resolution spectroscopy with quantum diamond sensors, or their analogues.

Optical imaging of localized chemical events using programmable diamond quantum nanosensors

Development of multifunctional nanoscale sensors working under physiological conditions enables monitoring of intracellular processes that are important for various biological and medical applications. By attaching paramagnetic gadolinium complexes to nanodiamonds (NDs) with nitrogen-vacancy (NV) centres through surface engineering, we developed a hybrid nanoscale sensor that can be adjusted to directly monitor physiological species through a proposed sensing scheme based on NV spin relaxometry. We adopt a single-step method to measure spin relaxation rates enabling time-dependent measurements on changes in pH or redox potential at a submicrometre-length scale in a microfluidic channel that mimics cellular environments. Our experimental data are reproduced by numerical simulations of the NV spin interaction with gadolinium complexes covering the NDs. Considering the versatile engineering options provided by polymer chemistry, the underlying mechanism can be expanded to detect a variety of physiologically relevant species and variables.

Measurement of weak static magnetic field with nitrogen-vacancy color center

基于金刚石氮-空位色心对精确测量微弱静磁场进行了探索.金刚石氮-空位色心电子自旋的退相干时间高度依赖于外磁场，而不同的退相干特征时间对磁场的灵敏度不同.对金刚石氮-空位色心电子自旋在不同强度外磁场下的退相干过程进行模拟，得到不同退相干特征时间与磁场大小的高准确度关系，提出了基于响应度最高的退相干特征时间测量静态弱磁场大小和方向的方法，并分析了该方法测量静态弱磁场的灵敏度，证明该方法的测量灵敏度比一般磁场测量仪器更高.

Room-temperature spin-photon interface for quantum networks

Although remarkable progress has been achieved recently, to construct an optical cavity where a nitrogen-vacancy (NV) colour centre in diamond is coupled to an optical field in the strong coupling regime is rather difficult. We propose an architecture for a scalable quantum interface capable of interconverting photonic and NV spin qubits, which can work well without the strong coupling requirement. The dynamics of the interface applies an adiabatic passage to sufficiently reduce the decoherence from an excited state of a NV colour centre in diamond. This quantum interface can accomplish many quantum network operations like state transfer and entanglement distribution between qubits at distant nodes. Exact numerical simulations show that high-fidelity quantum interface operations can be achieved under room-temperature and realistic experimental conditions.

Towards a Room-Temperature Spin Quantum Bus in Diamond via Electron Photoionization, Transport, and Capture

Diamond is a proven solid-state platform for spin-based quantum technology. The nitrogen-vacancy (NV) center in diamond has been used to realize small-scale quantum information processing (QIP) and quantum sensing under ambient conditions. A major barrier in the development of large-scale QIP in diamond is the connection of NV spin registers by a quantum bus at room temperature. Given that diamond is expected to be an ideal spin transport material, the coherent transport of spin directly between the spin registers offers a potential solution. Yet, there has been no demonstration of spin transport in diamond due to difficulties in achieving spin injection and detection via conventional methods. Here, we exploit detailed knowledge of the paramagnetic defects in diamond to identify novel mechanisms to achieve spin injection, transport and detection in diamond at room temperature. Having identified these mechanisms, we explore how they may be combined to realise an on-chip spin quantum bus.

Polarization- and frequency-tunable microwave circuit for selective excitation of nitrogen-vacancy spins in diamond

We report on a planar microwave resonator providing arbitrarily polarized oscillating magnetic fields that enable selective excitation of the electronic spins of nitrogen-vacancy centers in diamond. The polarization plane is parallel to the surface of diamond, which makes the resonator fully compatible with (111)-oriented diamond. The field distribution is spatially uniform in a circular area with a diameter of 4 mm, and a near-perfect circular polarization is achieved. We also demonstrate that the original resonance frequency of 2.8 GHz can be varied in the range of 2–3.2 GHz by introducing varactor diodes that serve as variable capacitors.

Proposal for a quantum delayed-choice experiment with a spin-mechanical setup

We describe an experimentally feasible protocol for performing a variant of the quantum delayed-choice experiment with massive objects. In this scheme, a single nitrogen-vacancy (NV) center in diamond driven by microwave fields is dispersively coupled to a massive mechanical resonator. A double-pulse Ramsey interferometer can be implemented with the spin-mechanical setup, where the second Ramsey microwave pulse drives the spin conditioned on the number states of the resonator. The probability for finding the NV center in definite spin states exhibits interference fringes when the mechanical resonator is prepared in a specific number state. On the other hand, the interference is destroyed if the mechanical resonator stays in some other number states. The wavelike and particlelike behavior of the NV spin can be superposed by preparing the mechanical resonator in a superposition of two distinct number states. Thus a quantum version of Wheeler's delayed-choice experiment could be implemented, allowing of fundamental tests of quantum mechanics on a macroscopic scale.

Exploiting bistable pinning of a ferromagnetic vortex for nitrogen-vacancy spin control

The strong, localized magnetic field produced by the core of a ferromagnetic vortex provides a platform for addressing and controlling individual nitrogen-vacancy (NV) center spins in diamond. Translation of a vortex state in a thin ferromagnetic disk or wire can be understood as motion through an effective pinning potential, arising from the defects in the material. Coupling an NV spin to a vortex state in a proximal ferromagnet imprints the pinning landscape onto the spin transitions. Quantitative characterization of the pinning potential is necessary to control the spin-vortex system. First, we map the effective pinning potential by raster scanning the vortex core through a permalloy disk and measuring the hysteretic vortex displacement vs. magnetic field using differential magneto-optical microscopy. Second, we demonstrate that the interaction between the vortex and a nearby NV spin can be characterized using the pinning map and the path taken by the vortex core through the landscape. Finally, we identify locations of bistability in the pinning landscape, and use them to manipulate the nitrogen vacancy spin in a controlled bimodal fashion by switching the spin on and off resonance with a driving field on a ∼ 10 ns timescale at room temperature.

Demonstration of a Coherent Electronic Spin Cluster in Diamond.

An obstacle for spin-based quantum sensors is magnetic noise due to proximal spins. However, a cluster of such spins can become an asset, if it can be controlled. Here, we polarize and readout a cluster of three nitrogen electron spins coupled to a single nitrogen-vacancy spin in diamond. We further achieve sub-nm localization of the cluster spins. Finally, we demonstrate coherent spin exchange between the species by simultaneous dressing of the nitrogen-vacancy and the nitrogen states. These results establish the feasibility of environment-assisted sensing and quantum simulations with diamond spins.

Efficient creation of dipolar coupled nitrogen-vacancy spin qubits in diamond

Coherently coupled pairs or multimers of nitrogen-vacancy defect electron spins in diamond have many promising applications especially in quantum information processing (QIP) but also in nanoscale sensing applications. Scalable registers of spin qubits are essential to the progress of QIP. Ion implantation is the only known technique able to produce defect pairs close enough to allow spin coupling via dipolar interaction. Although several competing methods have been proposed to increase the resulting resolution of ion implantation, the reliable creation of working registers is still to be demonstrated. The current limitation are residual radiation-induced defects, resulting in degraded qubit performance as trade-off for positioning accuracy. Here we present an optimized estimation of nanomask implantation parameters that are most likely to produce interacting qubits under standard conditions. We apply our findings to a well-established technique, namely masks written in electron-beam lithography, to create coupled defect pairs with a reasonable probability. Furthermore, we investigate the scaling behavior and necessary improvements to efficiently engineer interacting spin architectures.

Electron spin control of optically levitated nanodiamonds in vacuum.

Electron spins of diamond nitrogen-vacancy (NV) centres are important quantum resources for nanoscale sensing and quantum information. Combining NV spins with levitated optomechanical resonators will provide a hybrid quantum system for novel applications. Here we optically levitate a nanodiamond and demonstrate electron spin control of its built-in NV centres in low vacuum. We observe that the strength of electron spin resonance (ESR) is enhanced when the air pressure is reduced. To better understand this system, we investigate the effects of trap power and measure the absolute internal temperature of levitated nanodiamonds with ESR after calibration of the strain effect. We also observe that oxygen and helium gases have different effects on both the photoluminescence and the ESR contrast of nanodiamond NV centres, indicating potential applications of NV centres in oxygen gas sensing. Our results pave the way towards a levitated spin–optomechanical system for studying macroscopic quantum mechanics.

Hybrid Quantum Device with Nitrogen-Vacancy Centers in Diamond Coupled to Carbon Nanotubes.

We show that nitrogen-vacancy (NV) centers in diamond interfaced with a suspended carbon nanotube carrying a dc current can facilitate a spin-nanomechanical hybrid device. We demonstrate that strong magnetomechanical interactions between a single NV spin and the vibrational mode of the suspended nanotube can be engineered and dynamically tuned by external control over the system parameters. This spin-nanomechanical setup with strong, intrinsic, and tunable magnetomechanical couplings allows for the construction of hybrid quantum devices with NV centers and carbon-based nanostructures, as well as phonon-mediated quantum information processing with spin qubits.

Fast nanoscale addressability of nitrogen-vacancy spins via coupling to a dynamic ferromagnetic vortex.

The core of a ferromagnetic vortex domain creates a strong, localized magnetic field, which can be manipulated on nanosecond timescales, providing a platform for addressing and controlling individual nitrogen-vacancy centre spins in diamond at room temperature, with nanometre-scale resolution. Here, we show that the ferromagnetic vortex can be driven into proximity with a nitrogen-vacancy defect using small applied magnetic fields, inducing significant nitrogen-vacancy spin splitting. We also find that the magnetic field gradient produced by the vortex is sufficient to address spins separated by nanometre-length scales. By applying a microwave-frequency magnetic field, we drive both the vortex and the nitrogen-vacancy spins, resulting in enhanced coherent rotation of the spin state. Finally, we demonstrate that by driving the vortex on fast timescales, sequential addressing and coherent manipulation of spins is possible on ∼100 ns timescales.

Spatially resolved detection of complex ferromagnetic dynamics using optically detected nitrogen-vacancy spins

We demonstrate optical detection of a broad spectrum of ferromagnetic excitations using nitrogen-vacancy (NV) centers in an ensemble of nanodiamonds. Our recently developed approach exploits a straightforward CW detection scheme using readily available diamond detectors, making it easily implementable. The NV center is a local detector, giving the technique spatial resolution, which here is defined by our laser spot, but in principle can be extended far into the nanoscale. Among the excitations, we observe the propagating dipolar and dipolar-exchange spinwaves, as well as dynamics associated with the multi-domain state of the ferromagnet at low fields. These results offer an approach, distinct from commonly used optically detected magnetic resonance techniques, for spatially resolved spectroscopic study of magnetization dynamics at the nanoscale.

Entangling a single NV centre with a superconducting qubit via parametric couplings between photons and phonons in a hybrid system

We propose an efficient scheme for generating entangled states between a single nitrogen-vacancy (NV) centre in diamond and a superconducting qubit in a hybrid set-up. In this device, the NV centre and the superconducting qubit couple to a nanomechanical resonator and a superconducting coplanar waveguide cavity, respectively, while the microwave cavity and the mechanical resonator are parametrically coupled with a tunable coupling strength. We show that, highly entangled states between the NV centre and the superconducting qubit can be achieved, by means of the Jaynes–Cummings interactions in the NV-resonator and qubit-cavity subsystems which transfer the entanglement between the vibration phonons and the cavity photons to the NV centre and the superconducting qubit. This work may provide interesting applications in quantum computation and communication with single NV spins and superconducting qubits.

Electron Paramagnetic Resonance of a Single NV Nanodiamond Attached to an Individual Biomolecule

Electron paramagnetic resonance (EPR), an established and powerful methodology for studying atomic-scale biomolecular structure and dynamics, typically requires in excess of 1012 labeled biomolecules. Single-molecule measurements provide improved insights into heterogeneous behaviors that can be masked in ensemble measurements and are often essential for illuminating the molecular mechanisms behind the function of a biomolecule. Here, we report EPR measurements of a single labeled biomolecule. We selectively label an individual double-stranded DNA molecule with a single nanodiamond containing nitrogen-vacancy centers, and optically detect the paramagnetic resonance of nitrogen-vacancy spins in the nanodiamond probe. Analysis of the spectrum reveals that the nanodiamond probe has complete rotational freedom and that the characteristic timescale for reorientation of the nanodiamond probe is slow compared with the transverse spin relaxation time. This demonstration of EPR spectroscopy of a single nanodiamond-labeled DNA provides the foundation for the development of single-molecule magnetic resonance studies of complex biomolecular systems.