Hiroki Morishita

We investigated the spatial resolution of scanning photocurrent microscopy (SPhM) for imaging single nitrogen-vacancy (NV) centers in diamond in detail and compared it with that of confocal laser scanning microscopy (CLSM). While CLSM suffers from degraded spatial resolution due to fluorescence saturation, SPhM maintains a high spatial resolution despite the high-power laser excitation required. Numerical simulations of SPhM and CLSM images showed good agreement with experimental data and further indicated that SPhM achieves superior spatial resolution. Additionally, the signal-to-noise ratio of the photoelectrically detected magnetic resonance spectrum is comparable to that of the optically detected magnetic resonance spectrum in our home-made spectrometers. These findings suggest that SPhM can enhance the performance of NV-based quantum sensors.

We developed a compact, portable quantum sensor module for pulsed measurements using diamond nitrogen-vacancy (NV) centers, an excitation laser diode of 3 mW and a microwave source of 15 dBm. This module allowed the evaluation of ensemble diamond NV centers for measuring magnetic fields by continuous wave— optically detected magnetic resonance (ODMR), Rabi oscillation, pulsed ODMR, Ramsey measurement and spin-echo measurement. The sensitivity through pulsed measurements reached 40 nT/ Hz at 1 Hz, which achieved higher sensitivity by one order, especially at a frequency around 1 Hz compared with previously reported compact modules using diamond NV centers.

The realization of electroluminescence (EL) of negatively charged nitrogen vacancy (NV⁻) centers is important toward all-electrical control of diamond quantum devices. In this study, we demonstrated electrical excitation and detection of EL of single NV⁻ centers by using lateral diamond p⁺–i(n⁻)–n⁺ diodes. It had been grown by homoepitaxy using the plasma enhanced chemical vapor deposition technique. We introduced a lightly phosphorus doped i(n⁻) layer to stabilize the negative state of NV centers. It was estimated that the efficiency of the electrical excitation rate of the NV center was more than 30 times enhanced by introducing lateral diamond p⁺–i(n⁻)–n⁺ diodes structure compared with the previous vertical diode. Furthermore, the EL of a single NV⁻ center embedded in the i(n⁻) layer region was characterized. The results show that the charge state of the single NV centers can be manipulated by the voltage applied to the p⁺–i(n⁻)–n⁺ diode, where the emission of EL is increasingly dominated by NV⁻ in the range of 30 to 50 V.

Color centers in wide-bandgap semiconductors, including diamond and silicon carbide (SiC), are attractive systems for quantum information and quantum sensor devices with excellent spin properties at room temperature. In addition, nuclear spins in crystals are expected to serve as the quantum memory and to enhance the sensitivity of quantum sensors with the combination with color centers as a result of an extremely long spin coherence time. Although the spin state of both color centers and nuclear spins coupled through hyperfine interactions is usually optically read out, an electrical readout technique is important for miniaturizing and integrating devices. In the present study, we report the electrical detection of silicon vacancy (V2) centers in 4H-SiC by photocurrent-detected magnetic resonance (PDMR) using a frequency-sweep technique. We electrically observe the spin coherence of the V2 centers and clearly resolve the hyperfine splitting of the electron spin signal for the V2 centers coupled with next-nearest-neighbor ²⁹Si atoms. In addition, we apply PDMR to electron–nuclear double resonance (PD-ENDOR) to detect nuclear magnetic resonance of ²⁹Si at room temperature and find that this method can resolve nuclear spins coupled with neighboring electron spins in the V2 centers. The realization of PD-ENDOR is expected to be a critical step toward the development of electrically driven integrated quantum devices.

In TIG arc welding, technicians use a rolling technique to ensure high quality welds. In this rolling technique, the cathode is rotated while welding. However, the rolling method is difficult and complicated. As a countermeasure, it has been proposed that a magnetic field is applied to rotate the arc. When a magnetic field is applied, it is difficult to measure the arc behavior and the convection of the molten pool. Hence, numerical methods for the arc have been developed [1] , [2] . Moreover, the arc behavior is analyzed with applying magnetic field [3] . In this paper, molten pool with applying axial magnetic field in TIG arc welding transient was analyzed. Specifically, analytical model was constructed for solid, liquid, and gas, and four forces were calculated for the molten pool. The four forces are drag force of plasma flow, shear stress of Marangoni effect, buoyancy, and electromagnetic force. As a result, a spiral flow velocity was generated in the molten pool with the application of axial magnetic field. In addition, the flow velocity of the molten pool was more affected by the axial magnetic field than that of the arc. Therefore, it was possible that transient analysis of molten pool with applying axial magnetic field in TIG arc welding.

The extension of spin coherence times is a crucial issue for quantum information and quantum sensing. In solid-state systems, suppressing noise through various techniques has been demonstrated. On the other hand, an electrical control for suppression is important toward individual controls of on-chip quantum-information devices. Here, we show electrical control for extension of the spin coherence times of 40-nm-deep ion-implanted single-nitrogen-vacancy center spins in diamond by suppressing magnetic noise. We apply 120 V dc across two contacts spaced by 10 μm. The spin coherence times, estimated from a free-induction decay and a Hahn-echo decay, are increased up to about 10 times (reaching 10 μs) and 1.4 times (reaching 150 μs), respectively. From the quantitative analysis, the dominant decoherence source, depending on the applied static electric field, is elucidated. Electrical control for extension can deliver a sensitivity enhancement to the dc sensing of temperature, pressure, and electric (but not magnetic) fields, opening up an alternative technique in solid-state quantum-information devices.

We demonstrate electrical detection of the¹⁴N nuclear spin coherence of NV centres at room temperature. Nuclear spins are candidates for quantum memories in quantum-information devices and quantum sensors, and hence the electrical detection of nuclear spin coherence is essential to develop and integrate such quantum devices. In the present study, we used a pulsed electrically detected electron-nuclear double resonance technique to measure the Rabi oscillations and coherence time (T₂) of¹⁴N nuclear spins in NV centres at room temperature. We observedT₂≈ 0.9 ms at room temperature, however, this result should be taken as a lower limit due to limitations in the longitudinal relaxation time of the NV electron spins. Our results will pave the way for the development of novel electron- and nuclear-spin-based diamond quantum devices.