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Complex electrical structures composed of nanomaterials are widely studied in the research of nanoelectronics. Characterizing the current distribution is important to understand the current conducting mechanism and optimize the device's design. In this work, we employed the nitrogen vacancy centers in diamond as quantum sensors to directly and noninvasively monitor currents in nanowire networks. The sub-micrometer magnetic field imaging was achieved by injecting microwave current into networks and detecting the magnetic resonate spins' population, revealing the internal current paths involved in electrical conduction during electrical annealing. The establishment, breakdown, and reform of current paths were imaged in detail, which are difficult to realize through conventional methods. The mechanism of resistance change and relocating of current pathways was subsequently analyzed. This work demonstrates that a diamond-based quantum microscope is a useful tool to unveil the nanoscale conducting properties of complex conductive networks and guide the design for potential applications.

An insulator–metal transition (IMT) is an emergent characteristic of quantum materials, which have a great amount promise for applications, such as memories, optical switches, and analog brain functions. This is due to their ability to switch between two well-defined states. Thus, the characterization of the state-switching process is essential for the application of these materials. For vanadium dioxide ([Formula: see text]), the phase transition can be determined from temperature, magnetic field, and dielectric constant. In this paper, we propose a diamond quantum sensing approach based on nitrogen-vacancy centers for analyzing phase transitions. By using lock-in-based optically detected magnetic resonance and Rabi measurement protocols, temperature and magnetic field can reflect local IMT information of the circuit, and microwave can determine IMT information of an electrical isolation region. Our multifunctional quantum sensor exhibits local, nondestructive, and integrated measurements, which are useful for reliability testing in IMT technology applications.

With the miniaturization of electrical components at nanoscale, the impact of dimension and shape on the electrical properties of the devices plays an important role in the applications. In this work, we used an ensemble of nitrogen-vacancy (NV) centers in diamond to noninvasively investigate the size effect on electric conductivity at nanoscale. The magnetic noise originated from the random movement of electrons in conductors, which is related to the conductivity, was detected by recording the spin relaxation of NV centers. The results indicate that the conductivity increases with the size of devices at the scale of electron mean free path. By further imaging the magnetic noise of the metallic structure with discontinuous thickness, we demonstrated that the spatial distribution of conductance at nanoscale can be revealed with high density NV center arrays. The results can help to understand the electron transport in nanomaterials. This technique can be used to optimize the design of nanoscale electrical devices.

Optical levitation in vacuum has been proved as a promising platform for precision measurement and research on microscopic thermodynamics and the macroscopic quantum system. The performance of these applications is highly determined by the detection of the trapped particle's displacement, which is linked with the detection signal through a calibration coefficient. However, it is still a challenge to get the calibration coefficient in medium vacuum. Here, we present a nonlinear method to obtain the calibration coefficient with high accuracy to fill the pressure gap between [Formula: see text] and 50 mbar. By averaging the trajectory segments with the same initial displacement, we analyze the relationship between the nonlinearity induced frequency shift and the amplitude, where the calibration coefficient can be obtained without a priori knowledge about mass and temperature of the trapped particle or the assistance with external force. We further apply this method to detect the mass and density of the trapped nanoparticle and observe the mass variation under different pressures. Therefore, it will pave the way to study the physical processes in medium vacuum based on the precision measurement of nanoparticle's mass, density, and other properties.

High fidelity quantum operation of qubits plays an important role in realistic quantum sensing. It becomes more challenging when there are inevitable interactions between qubits in the solid system. We employ a composite pulse sequence to deal with the problems. The electron spin state of the nitrogen-vacancy center in diamond is flipped with high fidelity by the composite pulse at room temperature. In contrast with traditional rectangular pulses, the composite pulse has a wider excitation profile at the same Rabi frequency. Hence, the three sublevels of host nitrogen nuclear spin can be flipped efficiently and simultaneously, which enhances the signal contrast and detection sensitivity of quantum sensing universally. The enhancement effect works over a wide range of bias magnetic fields. Our scheme can be used for sensing temperature, strain, and electric field and can be applied to other spin–spin coupling systems.

The negatively charged nitrogen vacancy (NV) center in diamond is increasingly attractive as a solid-state quantum sensor with high sensitivity and spatial resolution. However, the host non-zero nitrogen nuclear spin of the NV center decreases the signal contrast and sensitivity because of hyperfine coupling. Here, we apply multi-frequency synchronous manipulation to the NV center to recover the signal contrast and enhance the sensitivity. The enhancement effect works over a wide range of bias magnetic fields. Compared to the single-frequency-driven sensing process, a sensitivity enhancement factor of 2.7 is achieved at a low bias magnetic field. Such multi-frequency-controlled sensitivity enhancement works also for sensing temperature, strain, and electric field and can be applied to other spin–spin coupling systems.

We demonstrated a fiber-integrated diamond-based magnetometer in this paper. In the system, the fluorescence of nitrogen vacancy (NV) centers in nanodiamonds deposited on a tapered fiber was coupled to the tapered fiber effectively and detected at the output end of the fiber. By using this scheme, optically detected electron spin resonance spectra were recorded for single NV centers. The results confirmed that such a tapered fiber-nanodiamond system can act as a magnetometer. Featured with excellent portability, convenient fabrication, and potential for further integration, the constructed system has been demonstrated to be a practical magnetometer prototype.

The conversion between two nitrogen vacancy (NV) center charge states (NV0 and NV−) was experimentally studied with laser in the range from 378 to 492 nm. The wavelength and power dependent charge state conversion was proved to be single-photon process at short wavelength and two-photon process at long wavelength. Also, the presence of metastable state in NV− significantly affected the conversion process, which can be used to increase NV− population from 72.5% to 80.5% by applying a magnetic field under 532 nm excitation. The method can be used for improving the fidelity of charge state polarization and subsequently optimizing the quantum control with NV center and resolution in the magnetometric and biological sensoring.

An in-line high efficient polarizer, composed of tapered fiber on the Au thin film, is theoretically proposed and experimentally demonstrated. The protocol is based on the high efficient adiabatic conversion of transverse magnetic mode from tapered fiber into surface plasmon (SP) and attenuates quickly in metal film. On the contrary, the transverse electric polarized light is influenced hardly in the whole process. The polarization extinction ratio higher than 500:1 (≈27 dB) is obtained in our experiment. Our demonstration offers a potential way to manipulate the polarization of light in integrated circuit and may inspirit more attention to surface plasmon based devices for polarization controlling.

We experimentally studied the transmission spectrum of a coupled resonator structure in which a low-Q microdisk and a high-Q microtoroid indirectly interact with each other mediated by a fiber taper. Asymmetric Fano resonances were observed and could be controlled to change periodically by adjusting the distance between the two microresonators. It is revealed that the Fano resonance originates from the coupling of the two modes belonging to the two microresonators. The observed period of distance change is around 8 μm, which shows good agreement with the theoretical prediction by the beat of multiple propagating modes in the fiber taper.

Magnetic resonance and fluorescence spectra of nitrogen-vacancy (NV) color centers ensemble in high purity diamond sample were measured, with temperature ranging from 5.6 K to 295 K. Both microwave and optical transition energies have similar nonlinear temperature dependent changes, which might mainly originate from the local thermal expansion. As the frequency shifts will reduce the fidelity of resonant quantum control, the present results demonstrate the necessity of taking temperature fluctuation into consideration. For temperature below 100 K, the transition energies show tendencies to be constant, which indicate higher stability and performance in applications with NV centers.

Polarization dependence of the exciton-plasmon-photon conversion in silver nanowire-quantum dots structure was investigated using a scanning confocal microscope system. We found that the fluorescence enhancement of the CdSe nanocrystals was correlated with the angle between the excitation light polarization and the silver nanowire direction. The polarization of the emission was also related with the nanowire direction. It was in majority in the direction parallel with nanowire., 4 pages, 5 figures

We experimentally characterize the Q-factor in a silica microsphere with the poly(methyl methacrylate) coating. Experimental results demonstrated that the Q-factor of the coated microcavity is higher than 108. Moreover, this coating is able to eliminate scattering from the unsmoothed surface and tune the optical field in the cavity. In the cavity interaction with quantum dots, the coating can not only increase the Q-factor of the microsphere with deposited quantum dots but also draw the maximal field to the interface where the quantum dots exist. Therefore, this coating method can highly enhance the interaction strength, and it will contribute to the cavity quantum electrodynamics study.

We propose a scheme to realize controlled phase gate between two single photons through a single quantum dot in a slow-light photonic crystal waveguide. Enhanced Purcell factor and large β-factor lead to high gate fidelity over broadband frequencies compared to cavity-assisted system. The excellent physical integration of this photonic crystal waveguide system provides tremendous potential for large-scale quantum information processing.