The pursuit of a practical quantum computing platform is a major scientific and technological goal. While superconducting circuits and trapped ions have shown progress, semiconductor spin qubits offer potential for greater scalability and integration. Nuclear spins, with their long coherence times due to environmental isolation, are attractive candidates for quantum computation and memory. Nitrogen-vacancy (NV) centers in diamond are particularly appealing due to their high fidelity single- and two-qubit gates, and demonstrated quantum error correction protocols. Previous work has shown control of multiple nuclear spins via a single NV electron spin and probabilistic entanglement distribution between NV electron spins using a spin-photon interface at cryogenic temperatures. However, the probabilistic nature and limited entanglement rates pose challenges for scalable quantum computation. This paper introduces an approach using an electrically-read electron-nuclear spin gate on a diamond electronic chip at room temperature, potentially overcoming many technological hurdles associated with existing methods. The researchers leverage their previously developed technique of photoelectric detection of magnetic resonance (PDMR), which offers superior spatial resolution compared to optical readout because it’s limited only by the size of the electrodes, rather than the diffraction limit. Recent advancements in NV production yield and precise implantation techniques now make nanoscale NV placement feasible, enabling individual readout of NV spins at distances suitable for dipolar interactions, crucial for creating small entangled qubit arrays. PDMR's advantages include insensitivity to excited-state saturation at high photon fluxes, a benefit stemming from the longer charge carrier recombination lifetime in diamond. This work focuses on demonstrating the electrical readout of a fundamental two-qubit system (NV electron and ¹⁴N nuclear spin), a significant step towards nanoscale NV qubit systems, as electrically detected readout of a single NV nuclear spin has been previously elusive.

The paper extensively reviews the existing literature on quantum computing platforms, highlighting the advantages and limitations of various approaches including superconducting circuits, trapped ions, and semiconductor spin qubits. It emphasizes the potential of nuclear spins in solids, particularly in diamond, for their long coherence times and use in quantum algorithms, memories, and sensors. The authors cite numerous studies demonstrating the high fidelity of quantum gates using NV electron and nuclear spins, along with the achievements in entanglement distribution between NV centers using optical detection. They also discuss previous work on electrical detection of electron spins using PDMR, noting its potential for improved spatial resolution and insensitivity to saturation effects. Finally, the review touches on advancements in NV production techniques, highlighting how their improved yield and implantation precision are enabling the creation of nanoscale NV arrays. Existing research on the electrical readout of nuclear spins, particularly the lack of single-spin electrical readout, is also presented as motivation for the current work. The review emphasizes the challenges of scaling up the quantum computation in diamond because of the probabilistic nature of entanglement and the limited entanglement rates previously achieved (on the order of 40 Hz).

The researchers used a commercial high-pressure high-temperature (HPHT) diamond with intrinsic single NV defects. Coplanar interdigitated contacts with a 3.5 µm gap were fabricated on the diamond surface using optical lithography. A yellow-green (561 nm) laser was used to minimize interference from other diamond defects that have a photoionization onset at around 550nm. The photocurrent was measured using a lock-in detection technique. Individual NV centers were selected for study. DC current-voltage (I-V) characteristics were measured to identify three current components: NV photocurrent, non-NV photocurrent, and dark current. Signal-to-background contrast (SBC) was calculated to optimize the bias voltage. The NV center was imaged using both optical and electrical methods for comparison. Electrical imaging showed significantly improved spatial resolution in all three dimensions. To read out the nuclear spin, the ¹⁴N nuclear spin was polarized to the m_I = |+1⟩ state using a magnetic field of ~510 G, approaching the excited-state level anti-crossing (ESLAC). A pulsed lock-in envelope readout technique was used. A theoretical model based on the Lindblad master equation was developed to describe the photoelectric readout process. The model included charge state transitions, time-dependent spin polarization, and charge carrier readout at the ESLAC. Time traces of the optical signal from different initial spin states were recorded to determine power-dependent ionization rates. The model successfully reproduced experimental photoluminescence (PL) curves and predicted electron and hole currents. Coherent electron and nuclear spin rotations were performed using RF pulses and microwave (MW)-assisted readout. The nuclear spin resonance frequency was measured by varying the RF pulse frequency, and Rabi oscillations were observed by varying the RF pulse duration. MW-free readout was also demonstrated, showing comparable contrast to MW-assisted readout. The theoretical model accurately described both cases. The methodology also includes details on the experimental setup, including the laser, objective, diamond sample preparation (interdigitated contacts, MW line), and data acquisition.

The study successfully demonstrated room-temperature photoelectric readout of a single ¹⁴N nuclear spin coupled to an NV electron spin. The researchers achieved this by leveraging the excited-state level anti-crossing (ESLAC) condition, which enhances the sensitivity of the readout. High-contrast nuclear magnetic resonance spectra and coherent spin rotations were observed, even with long intervals between laser excitation pulses. The use of a pulsed lock-in envelope readout technique allowed for effective measurement of low average photocurrents. The researchers developed a theoretical model based on the Lindblad master equation, successfully reproducing experimental data and providing insights into the spin-dependent photocurrent dynamics. They also successfully modeled the observed differences in ODMR and PDMR contrast. Notably, the electrical imaging showed a threefold reduction in axial size compared to optical imaging, highlighting the superior spatial resolution of the PDMR technique. The model demonstrated that the photo-ionization dynamics of electrons and holes and the associated currents closely follow the photoluminescence (PL) time traces. However, their amplitudes show different power dependencies. The study shows how to achieve a high contrast nuclear spin readout even with long pulse sequences which would otherwise hinder effective signal capture. The successful demonstration of MW-free readout offers further advantages for applications where MW driving is undesirable. These findings collectively represent a crucial advance toward the development of scalable room-temperature quantum computing devices based on NV centers in diamond.

This research successfully addresses the significant challenge of electrically detecting a single nuclear spin at room temperature, a key step in creating scalable quantum computing systems based on NV centers. The use of PDMR offers a substantial improvement in spatial resolution compared to optical methods, moving beyond the diffraction limit to a resolution determined solely by the electrode size. This enables the creation of dense arrays of qubits, crucial for scaling. The ability to control and read out nuclear spins at room temperature significantly simplifies device fabrication and operation. The theoretical model developed offers a comprehensive framework for understanding the complex interplay between electronic and nuclear spins, providing a basis for further optimization of quantum control and readout techniques. The demonstration of high-contrast Rabi oscillations even with long RF pulses highlights the potential for performing complex quantum operations using this method. The achievement of MW-free readout is particularly significant as it reduces potential noise and heating issues in practical devices. The results have broad implications for both quantum computing and sensing applications, opening up new avenues for the development of highly sensitive and compact quantum devices.

This paper presents the first demonstration of room-temperature photoelectric detection of a single nuclear spin in a diamond NV center. The authors achieved this by utilizing the ESLAC condition to enhance readout sensitivity, and their theoretical model accurately predicted experimental observations. The high contrast observed, even with long pulse sequences, underscores the potential for complex quantum operations. The demonstration of MW-free readout offers significant practical advantages. This work represents a substantial step towards the development of scalable electronic quantum processors based on NV centers and paves the way for future research in site-selective individual electron spin polarization at nanoscale distances.

While the study demonstrates a significant advance, some limitations exist. The current photocurrent detection electronics do not allow for sufficiently fast gating to achieve optimal spin contrast, limiting the maximum achievable contrast. Further development of high-speed, low-noise current preamplifiers would improve the results. The study focuses on a single nuclear spin; future work should investigate the scalability to larger arrays of coupled nuclear spins and the challenges of site-selective polarization at very short inter-qubit distances. The theoretical model makes certain assumptions (neglecting some degrees of freedom) that could be refined for even higher accuracy. The influence of the diamond lattice environment, beyond what is captured in the current theoretical model, should also be further investigated.