Diamond Nitrogen-vacancy Research Articles

Fast Relaxation on Qutrit Transitions of Nitrogen-Vacancy Centers in Nanodiamonds

Thanks to their versatility, nitrogen-vacancy (NV) centers in nanodiamonds have been widely adopted as nanoscale sensors. However, their sensitivities are limited by their short coherence times relative to NVs in bulk diamond. A more complete understanding of the origins of decoherence in nanodiamonds is critical to improving their performance. Here we present measurements of fast spin relaxation on qutrit transitions between the energy eigenstates composed of the $m_s = \pm1$ states of the NV$^-$ electronic ground state in $\sim40$-nm nanodiamonds under ambient conditions. For frequency splittings between these states of $\sim20~$MHz or less the maximum theoretically achievable coherence time of the NV spin is $\sim2~$orders of magnitude shorter than would be expected if the NV spin is treated as a qubit. We attribute this fast relaxation to electric field noise. We observe a strong falloff of the qutrit relaxation rate with the splitting between the states, suggesting that, whenever possible, measurements with NVs in nanodiamonds should be performed at moderate axial magnetic fields ($>60$ G). We also observe that the qutrit relaxation rate changes with time. These findings indicate that surface electric field noise is a major source of decoherence for NVs in nanodiamonds.

Heralded universal quantum computing on electron spins in diamond nitrogen-vacancy centers assisted by low-Q microtoroidal resonators

Heralded universal quantum gates can largely decrease the quantum resource consumed in the integration of a multiqubit quantum circuit. Here, we present a scheme for heralded universal quantum computing, including the construction of the CNOT gate, Fredkin gate, and Toffoli gate on the electron spins in diamond nitrogen-vacancy (NV) centers assisted by low- Q microtoroidal resonators. These quantum gates can achieve unity fidelity, as the infidelity induced by the imperfect input–output process is transferred into photon loss. The clicks on different single-photon detectors can either herald the success of the quantum gates or instruct an immediate recycling procedure, even without reinitializing the NV centers. Besides the heralding, this scheme is also more realistic, observing that the non-monochromatic auxiliary photon and the inhomogeneous NV centers have no adverse impact on fidelity. The high fidelity and efficiency with experimental parameters show that the scheme is feasible with current technology.

Real-Time Charge Initialization of Diamond Nitrogen-Vacancy Centers for Enhanced Spin Readout

A common impediment to qubit performance is imperfect state initialization. In the case of the diamond nitrogen-vacancy (NV) center, the initialization fidelity is limited by fluctuations in the defect's charge state during optical pumping. Here, we use real-time control to deterministically initialize the NV center's charge state at room temperature. We demonstrate a maximum charge initialization fidelity of 99.4$\pm$0.1% and present a quantitative model of the initialization process that allows for systems-level optimization of the spin-readout signal-to-noise ratio. Even accounting for the overhead associated with the initialization sequence, increasing the charge initialization fidelity from the steady-state value of 75% near to unity allows for a factor-of-two speedup in experiments while maintaining the same signal-to-noise-ratio. In combination with high-fidelity readout based on spin-to-charge conversion, real-time initialization enables a factor-of-20 speedup over traditional methods, resulting in an ac magnetic sensitivity of 1.3 nT/Hz$^{1/2}$ for our single NV-center spin. The real-time control method is immediately beneficial for quantum sensing applications with NV centers as well as probing charge-dependent physics, and it will facilitate protocols for quantum feedback control over multi-qubit systems.

What Happens if you Fire Lasers at Diamond Then Microwave It? A Novel Method Towards Ion Channel Study

Ion channels represent important drug targets, implicated in the pathology of many disease states. Studying ion channel function is challenging, with current technology lacking spatial, i.e. electrophysiology, or temporal, i.e. scanning fluorescent imaging, information, vital for investigation of channel behaviour and populations. Previous research within the Castell lab developed a single molecule fluorescent imagining technique able to visualise ions moving through channels using reactive fluorophores, gaining spatial and short-term temporal data. However, due to photobleaching and the lack of fluorophores for some commonly trafficked ions, this method is still limited. This work attempts to overcome these limitations by utilising nitrogen vacancy (NV) diamond as a fluorophore. NV diamond do not undergo photobleaching and, furthermore, have properties that allow modulation of their fluorescence in response to environmental factors, like magnetic field. This allows for optical sensing of environmental perturbations, for example local ionic concentration. Therefore, integrating NV into the established system would produce a method of ion channel evaluation able to sense any type of ion movement while retaining spatial and temporal data. To use NVs as a fluorophore, sensitivity to relatively small signals (∼5%) had to be achieved. This was done through a serious of physical and analytical techniques, most notably development of a suite of post-hoc image analysis techniques. This has allowed for visual of magnetic fields, down to a single atomic level. Further development of method of magnetic field application have allowed for even finer sensitivity, down to paramagnetic levels. Utilising this sensitivity, an imaging system has been designed which overcomes the limitations of previous methods of ion channel research, allowing for localization of wide range of ion channel function over time.

Optimization of optical control of nitrogen vacancy centers in solid diamond

The nitrogen-vacancy (NV) centers in diamond have the advantages of stable triaxial structure, ultra-long electron spin coherence time and simple optical readout at room temperature. A nitrogen atom in the diamond crystal replaces a carbon atom and a vacancy is generated at the adjacent position, forming a point defect in the <i>C</i><sub>3<i>v</i></sub> space group structure. Its ground state and excited state are both spin triplet states. It is the key to achieving efficient preparation of optical initial state and extracting NV color center’s information in the researches of highly sensitive sensing magnetic detection, temperature detection, biological imaging, quantum computing, etc. However, there was no systematic study on relevant parameters of laser for high-concentration NV color center’s samples in previous experimental studies. Based on a high concentration diamond NV ensemble, we use pulsed optical detection magnetic resonance (ODMR) technology to systematically study the relationship among laser initial polarization time, information reading time and laser power, and the influence of laser incident polarization angle on the accuracy of sensing information. The effects of various laser parameters on the NV1 peak of ODMR on the [111] axis of the NVs of diamond are also investigated. The contrast of ODMR increases firstly with a sigmoid function and then decreases with an e-exponential function as the information reading time increases. The incident polarization angle of the laser is sinusoidal, with a period of 90°. According to the above experimental results, we finally choose the appropriate experimental parameters at 45.8 W/cm<sup>2</sup> (300 μs of polarization, 700 ns, reading time, laser incident angle is 220°) for ODMR test. Compared with previous experimental parameters (polarization time was 50 us, read the time of 3000 ns, laser incident angle was 250°), the experimental results show that the contrast of ODMR increases from 2.1% to 4.6%, and the typical magnetic sensitivity is improved from 21.6 nT/Hz<sup>1/2</sup> to 5.6 nT/Hz<sup>1/2</sup>. The optimization of the optical control of NVs in solid diamond is realized. The above results provide an effective support for the detection of high-sensitivity manipulation sensing based on high-concentration NV ensemble.

Diamond Nitrogen-vacancy Centers and Development to Laser Science

Diamond Nitrogen-vacancy Centers and Development to Laser Science

High-Contrast Two-Frequency Optically Detected Resonances in Diamond Nitrogen-Vacancy Centers for Timekeeping Schemes

Negatively charged nitrogen-vacancy (NV) centers are widely used in a precision metrology due to their unique properties, which allow spin states to be controlled at relatively high temperatures. The NV-centers are most widely used in magnetometry of ultrahigh spatial resolution, because the frequencies of the resonances in their optically detected spectra are magnetically dependent; however, over the past several years, new works have been published on the use of NV centers in a time metrology as well. The effectiveness of NV centers being used in this case is limited due to the strong dependence of the parameters of the centers' energy structure on the magnetic field, temperature, and pressure; nevertheless, their use as microdimensional microwave range frequency references remains very promising. In particular, zero-field level anticrossing can be used to suppress the sensitivity of narrow resonances to magnetic fields for metrological applications. In this letter, we present a method of two-frequency excitation of narrow magnetically independent optically detected resonances in bulk NV diamond in a zero magnetic field, and discuss the possibility of their use in timekeeping schemes.

Electron-Induced State Conversion in Diamond NV Centers Measured with Pump-Probe Cathodoluminescence Spectroscopy.

Nitrogen-vacancy (NV) centers in diamond are reliable single-photon emitters, with applications in quantum technologies and metrology. Two charge states are known for NV centers, NV0 and NV–, with the latter being mostly studied due to its long electron spin coherence time. Therefore, control over the charge state of the NV centers is essential. However, an understanding of the dynamics between the different states still remains challenging. Here, conversion from NV– to NV0 due to electron-induced carrier generation is shown. Ultrafast pump–probe cathodoluminescence spectroscopy is presented for the first time, with electron pulses as pump and laser pulses as probe, to prepare and read out the NV states. The experimental data are explained with a model considering carrier dynamics (0.8 ns), NV0 spontaneous emission (20 ns), and NV0 → NV– back transfer (500 ms). The results provide new insights into the NV– → NV0 conversion dynamics and into the use of pump–probe cathodoluminescence as a nanoscale NV characterization tool.

Quantum information processing with closely-spaced diamond color centers in strain and magnetic fields [Invited

Electron and nuclear spins of diamond nitrogen-vacancy (NV) centers are good candidates for quantum information processing as they have long coherence time and can be initialized and read out optically. However, creating a large number of coherently coupled and individually addressable NV centers for quantum computing has been a big challenge. Here we propose methods to use high-density diamond NV centers coupled by spin-spin interaction with an average separation on the order of 10 nm for quantum computing. We propose to use a strain gradient to encode the position information of each NV center in the energy level of its excited electron orbital state, which causes a shift of its optical transition frequency. With such strain encoding, more than 100 closely-packed NV centers below optical diffraction limit can be read out individually by resonant optical excitation. A magnetic gradient will be used to shift the electron spin resonant (ESR) frequencies of NV centers. Therefore, the spin state of each NV center can be individually manipulated and different NV centers can be selectively coupled. A universal set of quantum operations for two-qubit and three-qubit system is introduced by careful design of external drives. Moreover, entangled states with multiple qubits can be created by this protocol, which is a major step towards quantum information processing with solid-state spins.

Extending the Quantum Coherence of a Near-Surface Qubit by Coherently Driving the Paramagnetic Surface Environment.

Surfaces enable useful functionalities for quantum systems, e.g., as interfaces to sensing targets, but often result in surface-induced decoherence where unpaired electron spins are common culprits. Here we show that the coherence time of a near-surface qubit is increased by coherent radio-frequency driving of surface electron spins, where we use a diamond nitrogen-vacancy (NV) center as a model qubit. This technique is complementary to other methods of suppressing decoherence and, importantly, requires no additional materials processing or control of the qubit. Further, by combining driving with the increased magnetic susceptibility of the double-quantum basis, we realize an overall fivefold sensitivity enhancement in NV magnetometry. Informed by our results, we discuss a path toward relaxation-limited coherence times for near-surface NV centers. The surface-spin driving technique presented here is broadly applicable to a wide variety of qubit platforms afflicted by surface-induced decoherence.

Entanglement purification for memory nodes in a quantum network

The entanglement between quantum memory nodes is a prerequisite in a quantum network, and the diamond nitrogen-vacancy (NV) center is a promising candidate serving as a quantum memory node. Here, we investigate the possibility of achieving an entanglement purification protocol (EPP) for entangled NV centers in distant diamonds. To construct the EPP, we design a nondestructive parity-check detector (PCD) utilizing an auxiliary polarization-entangled photon pair, which makes our EPP less time consuming and insensitive to the phase fluctuation of the optical path length. The satisfied fidelity of an NV center pair after purification and efficiency of obtaining a purified NV center pair with our EPP can be obtained with current experimental techniques in the realistic condition. This EPP is useful for a quantum network in which NV centers are used as quantum memory nodes.

Logic NOT gates of the intensity–noise correlation and squeezing in a diamond nitrogen-vacancy center

Color defects in a diamond nitrogen-vacancy (NV) center represent an important physical system for emergent quantum technologies such as logic gates and registers at room temperature. We realized an optical logic NOT gate by inducing a phase shift in a diamond NV center under nonlinear Kerr effect of spontaneous parametric four-wave mixing (SP-FWM). Also, we discussed the intensity–noise correlation and intensity-difference squeezing of SP-FWM and composite signals (SP-FWM+fluorescence) under self-phase modulation (SPM) and cross-phase modulation (XPM) induced by the nonlinear Kerr effect in V-type and double V-type atomic systems. Our experimental results provide an advanced technique of switching contrast by approximately 90%. Different parameters, such as the power of input laser beam and boxcar gate position, were also configured to control the SPM and XPM, which determine the switching phenomenon for correlation and squeezing.

Stimulated Emission Depletion Microscopy with Diamond Silicon Vacancy Centers

The spatial resolution and fluorescence signal amplitude in stimulated emission depletion (STED) microscopy is limited by the photostability of available fluorophores. Here, we show that negatively-charged silicon vacancy (SiV) centers in diamond are promising fluorophores for STED microscopy, owing to their photostable, near-infrared emission and favorable photophysical properties. A home-built pulsed STED microscope was used to image shallow implanted SiV centers in bulk diamond at room temperature. The SiV stimulated emission cross section for 765-800 nm light is found to be (4.0 +/- 0.3) x 10^(-17) cm^2, which is approximately 2-4 times larger than that of the negatively-charged diamond nitrogen vacancy center and approaches that of commonly-used organic dye molecules. We performed STED microscopy on isolated SiV centers and observed a lateral full-width-at-half-maximum spot size of 89 +/- 2 nm, limited by the low available STED laser pulse energy (0.4 nJ). For a pulse energy of 5 nJ, the resolution is expected to be ~20 nm. We show that the present microscope can resolve SiV centers separated by <150 nm that cannot be resolved by confocal microscopy.

Quantum diamond spectrometer for nanoscale NMR and ESR spectroscopy.

Nitrogen-vacancy (NV) quantum defects in diamond are sensitive detectors of magnetic fields. Owing to their atomic size and optical readout capability, they have been used for magnetic resonance spectroscopy of nanoscale samples on diamond surfaces. Here, we present a protocol for fabricating NV diamond chips and for constructing and operating a simple, low-cost 'quantum diamond spectrometer' for performing NMR and electron spin resonance (ESR) spectroscopy in nanoscale volumes. The instrument is based on a commercially available diamond chip, into which an NV ensemble is ion-implanted at a depth of ~10 nm below the diamond surface. The spectrometer operates at low magnetic fields (~300 G) and requires standard optical and microwave (MW) components for NV spin preparation, manipulation, and readout. We demonstrate the utility of this device for nanoscale proton and fluorine NMR spectroscopy, as well as for the detection of transition metals via relaxometry. We estimate that the full protocol requires 2-3 months to implement, depending on the availability of equipment, diamond substrates, and user experience.

Absorption-free superluminal light propagation in a diamond nitrogen vacancy center

We investigate the dispersion and absorption properties of a weak probe field in a four-level tripod system in a diamond nitrogen vacancy (NV) center. We apply a static magnetic field to the system and it is shown that the slope of dispersion changes from positive to negative only by varying either the intensity or relative phase of applied fields. Then, the transparent subluminal light propagation switches to the transparent superluminal light propagation. It is demonstrated that the group velocity of light pulse can be also controlled by a static magnetic field. Additionally, it is shown that the static magnetic field induces the amplification without inversion in the NV center. Unlike the atomic gas system, the obtained results in the NV center can be experimentally observed at room temperature.

Adiabatic population transfer of dressed spin states with quantum optimal control

We report theoretical studies of adiabatic population transfer using dressed spin states. Quantum optimal control using the algorithm of Chopped Random Basis (CRAB) has been implemented in a negatively charged diamond nitrogen vacancy center that is coupled to a strong and resonant microwave field. We show that the dressed spin states are highly effective in suppressing effects of spin dephasing on adiabatic population transfer. The numerical simulation also demonstrates that CRAB-based quantum optimal control can enable an efficient and robust adiabatic population transfer.

Nanotesla sensitivity magnetic field sensing using a compact diamond nitrogen-vacancy magnetometer

Solid state sensors utilizing diamond nitrogen-vacancy (NV) centers are a promising sensing platform that can provide high sensitivity and spatial resolution at high precision. Such sensors have been realized in bulky laboratory-based forms; however, practical applications demand a miniaturized, portable sensor that can function in a wide range of environmental conditions. Here, we demonstrate such a diamond NV magnetic field sensor. The sensor head fits inside a 11×7×7 cm3 3D-printed box and exhibits sub-10 nT/Hz sensitivity over a 125 Hz bandwidth. We achieve efficient fluorescence collection using an optical filter and diode in contact with the diamond, which is cut at the Brewster angle to maximize the coupling of 532 nm pump light. We discuss the potential of this flexible approach to achieve sub-nT/Hz shot noise limited sensitivity suitable for detection of a wide range of low-level magnetic fields, particularly those from electrical power systems and from biological sources.

A monolithic immersion metalens for imaging solid-state quantum emitters

Quantum emitters such as the diamond nitrogen-vacancy (NV) center are the basis for a wide range of quantum technologies. However, refraction and reflections at material interfaces impede photon collection, and the emitters’ atomic scale necessitates the use of free space optical measurement setups that prevent packaging of quantum devices. To overcome these limitations, we design and fabricate a metasurface composed of nanoscale diamond pillars that acts as an immersion lens to collect and collimate the emission of an individual NV center. The metalens exhibits a numerical aperture greater than 1.0, enabling efficient fiber-coupling of quantum emitters. This flexible design will lead to the miniaturization of quantum devices in a wide range of host materials and the development of metasurfaces that shape single-photon emission for coupling to optical cavities or route photons based on their quantum state.

Microscopic Imaging of the Stress Tensor in Diamond Using in Situ Quantum Sensors

The precise measurement of mechanical stress at the nanoscale is of fundamental and technological importance. In principle, all six independent variables of the stress tensor, which describe the direction and magnitude of compression/tension and shear stress in a solid, can be exploited to tune or enhance the properties of materials and devices. However, existing techniques to probe the local stress are generally incapable of measuring the entire stress tensor. Here, we make use of an ensemble of atomic-sized in situ strain sensors in diamond (nitrogen-vacancy defects) to achieve spatial mapping of the full stress tensor, with a submicrometer spatial resolution and a sensitivity of the order of 1 MPa (10 MPa) for the shear (axial) stress components. To illustrate the effectiveness and versatility of the technique, we apply it to a broad range of experimental situations, including mapping the stress induced by localized implantation damage, nanoindents, and scratches. In addition, we observe surprisingly large stress contributions from functional electronic devices fabricated on the diamond and also demonstrate sensitivity to deformations of materials in contact with the diamond. Our technique could enable in situ measurements of the mechanical response of diamond nanostructures under various stimuli, with potential applications in strain engineering for diamond-based quantum technologies and in nanomechanical sensing for on-chip mass spectroscopy.

Generation of Entangled and Hyperentangled Bell States on Photon Systems Assisted by Diamond Nitrogen-Vacancy Centers Coupled with Whispering-Gallery-Mode Microresonators

Entanglement plays an important role in quantum information science and technology. Here we propose two schemes for the generation of entangled Bell states in the polarization degree of freedom (DOF) and hyperentangled Bell states in both the polarization and spatial-mode DOFs on two-photon systems, assisted by diamond nitrogen-vacancy (NV) centers coupled with whispering-gallery-mode microresonators as a result of cavity quantum electrodynamics (QED). Compared with previously published schemes, our new proposed schemes are deterministic and flexible for entangling independent photons. By using numerical calculations, we show that the present schemes can work in both the weak- and strong-coupling regimes of cavity QED. Therefore, the schemes exhibit high experimental feasibility with current or near-future technologies, which can be integrated with protocols for the realization of quantum communications and even future complex quantum networks.