Photoelectric Spin Readout Technique

• Photoelectric spin readout is a technique that transduces the spin state of defects like NV centers into electrical signals via spin-dependent photoionization.
• Core methods include two-photon ionization, dual-beam protocols, and spin-to-charge conversion, achieving contrasts up to 20–30% and single-shot fidelities over 95%.
• Applications span fast magnetometry, quantum information processing, and scalable sensor design, overcoming limitations of optical detection.

Photoelectric spin readout refers to techniques that transduce the electronic spin state of point defects—most prominently, nitrogen-vacancy (NV) centers in diamond and related solid-state defects—into an electrical signal by exploiting spin-dependent charge/ionization dynamics. These approaches are central to quantum metrology and quantum information, as they offer chip-scale integrability, high bandwidth, and resilience to photon-collection limitations that constrain purely optical readout. Core implementations include spin-to-charge conversion (SCC) schemes, two-photon ionization PDMR (photoelectric detection of magnetic resonance), charge-capture schemes with long-lived traps, and their analogs in wide-bandgap semiconductors and 2D materials.

1. Principles of Photoelectric Spin Readout

The fundamental mechanism shared by most photoelectric spin readout protocols is spin-dependent photoionization, enabled by the distinct internal dynamics of optically active defect centers. In NV– in diamond, the ground state (^3A₂) and optically excited state (^3E) are split into m_s=0, ±1 sublevels by zero-field splitting (D≈2.87 GHz). Under optical excitation (e.g., 532 nm), m_s=0 cycles efficiently between ^3A₂ and ^3E, while m_s=±1 is rapidly transferred—via spin-selective intersystem crossing (ISC)—to a singlet state (^1A₁/ ^1E), which is dark to further excitation and ionization. Consequently, only the unshelved spin sublevel maintains a high probability of two-photon absorption and is efficiently ionized into the diamond conduction band (NV– → NV⁰ + e⁻).

This population difference translates into a spin-dependent photocurrent when charge carriers are swept to electrodes. The signal magnitude and readout contrast are dictated by the branching ratios of radiative decay, ISC rates, and ionization cross sections, and can be modulated via pulsed or continuous-wave optical and microwave protocols (Bourgeois et al., 2015, Hopper et al., 2018).

2. Methods and Protocols

2.1 Two-Photon and Dual-Beam Photoelectric Detection (PDMR)

The canonical PDMR protocol (Bourgeois et al., 2015, Bourgeois et al., 2016, Gulka et al., 2016, Hopper et al., 2018) employs green (λ≈532 nm) or blue (λ≈450 nm) illumination to drive both excitation and ionization. In diamond, two-photon absorption from ^3E populates the conduction band. Simultaneous microwave (MW) excitation at the ground-state splitting frequency interconverts spin populations, modulating the photocurrent via periodic shelving of m_s=±1 into non-ionizable singlet states. Dual-beam PDMR (Bourgeois et al., 2016) uses a pulsed blue ionization beam and a CW green shelving beam, enhancing contrast and selectively rejecting background (e.g., from substitutional nitrogen, N_s⁰) via lock-in amplification referenced to the blue pulse train.

2.2 Spin-to-Charge Conversion (SCC) in Bulk and Single NVs

SCC protocols translate spin populations into charge populations, subsequently read out optically or electrically (Jayakumar et al., 2018, Zhang et al., 2020, Ulibarri et al., 29 Oct 2025). In an ensemble SCC implementation (Jayakumar et al., 2018), a high-intensity orange (594 nm) laser pulse ionizes NV– exclusively in the m_s=0 state; the NV charge state is then read out via differential photoluminescence using weak 594 nm illumination. At the single-NV level, resonant optical excitation (e.g., 637 nm at cryogenic temperatures) selects m_s=0, followed by spin-selective near-infrared-assisted photoionization (e.g., 1064 nm) and high-fidelity charge-state readout; this yields >95% single-shot fidelity under appropriate conditions (Zhang et al., 2020).

2.3 Charge-Capture Detected Magnetic Resonance (CCDMR)

Recent advances exploit photoionization to deposit charge in long-lived traps at a diamond-metal interface, such that subsequent optical stimulation releases a transient current proportional to the stored charge and thus the spin state (Ulibarri et al., 29 Oct 2025). This modality enables temporal decoupling of spin manipulation from readout and persists over hours at room temperature due to deep trap states. The approach supports Rabi, Ramsey, and Hahn-echo protocols with single-shot fidelities ≈95%.

2.4 PDMR in Other Host Systems

Analogous techniques have been demonstrated for defects in silicon carbide (SiC) (Niethammer et al., 2019) and van der Waals materials such as hBN (Ru et al., 12 Nov 2025). In SiC, spin-dependent two-photon ionization of silicon-vacancy (V2) centers generates a measurable photocurrent, with contrasts up to 0.03 % and compatibility with coherent spin manipulation protocols. In hBN, boron-vacancy (VB–) centers exhibit spin-dependent ISC and two-photon ionization, enabling room-temperature, on-chip-compatible electrical spin readout.

3. Signal Generation, Modelling, and Optimization

The photoelectric signal I_ph(m) is proportional to the spin-dependent population in the excited state and the corresponding ionization rate. For NV centers:

• The rate equations may be written as:

$$\frac{dP_{–}}{dt} = -\Gamma_{\text{ion}} P_{–} + \Gamma_{\text{rec}} P_0$$

$$\frac{dP_{0}}{dt} = +\Gamma_{\text{ion}} P_{–} - \Gamma_{\text{rec}} P_0$$

Where $P_{–}$ and $P_0$ are NV– and NV⁰ populations; $\Gamma_{\text{ion}} \propto I^{2}$ is the two-photon ionization rate; $\Gamma_{\text{rec}}$ is the (weaker) recombination rate.

• The maximum contrast $C$ is dictated by the spin-dependent difference in both ISC rates and ionization pathways, theoretically reaching 20–30% for ideal NVs but limited by competing backgrounds (N_s, charge traps) and incomplete population transfer (Bourgeois et al., 2015, Hrubesch et al., 2016).
• Monte Carlo simulations and rate-equation models are widely used to optimize ionization pulse length, power, and recovery steps for maximum contrast and SNR. Optimal detection is typically achieved at pulse durations 100–200 ns and moderate–high optical powers, balanced to prevent ionization saturation or recombination bottlenecks (Hrubesch et al., 2016).

4. Experimental Implementation and Performance Metrics

Experimental realization requires lithographically defined electrodes on single-crystal diamond or other host materials, with micron/sub-micron gaps to optimize electric field and collection efficiency. Confocal or widefield excitation is used, typically at intensities orders of magnitude above optical saturation. Key parameters and performances are summarized in the table below (see (Jayakumar et al., 2018, Bourgeois et al., 2016, Gulka et al., 2016, Hrubesch et al., 2016, Ulibarri et al., 29 Oct 2025, Zhang et al., 2020, Ru et al., 12 Nov 2025)):

Protocol	Max Contrast	SNR (per cycle)	Single-shot Fidelity	Bandwidth	Notable Features
Bulk SCC (594 nm)	12%	1.0	–	~100 Hz	10× speedup vs. PL readout
Dual-beam dPDMR	9%	–	–	–	Lock-in filtering, defect rejection
Pulsed Elec. Readout	17% (optimal)	–	–	~5 MHz	Monte Carlo-optimized
CCDMR (trap-based)	7%	~10 (1.5 s)	~95%	~0.1–10 Hz	Temporal decoupling, long-term retention
Single-shot SCC (cryogenic)	–	–	>95%	~kHz–MHz	Fault-tolerant regime
SiC V2 (4H)	0.03%	1–10 (3600 s)	–	~1 kHz	Ambient operation
hBN VB– (2D)	0.5%	–	0.5 (single-shot est.)	~kHz (1 s int.)	2D materials, on-chip compatible

SCC-based protocols have demonstrated an order of magnitude higher SNR per measurement and a tenfold reduction in required integration time for millisecond-long sensing pulse sequences compared to standard fluorescence (Jayakumar et al., 2018). Charge-capture schemes currently have bandwidth limited by trap-release times but offer excellent retention and integrability (Ulibarri et al., 29 Oct 2025).

5. Applications and Comparative Advantages

Photoelectric spin readout enables high-sensitivity, chip-integrated quantum sensors and new modalities of quantum-state detection. Notable applications include:

• Fast, high-contrast magnetometry and T₁ relaxometry in bulk and near-surface NV ensembles with video-rate field imaging (Jayakumar et al., 2018).
• All-electrical DEER (double electron–electron resonance) protocols addressing both NV and bath spins with electrical detection, enabling noise characterization and bath engineering without optics (Rubinas et al., 30 Sep 2025).
• Room-temperature spin-state readout in wide-bandgap semiconductors and 2D materials without reliance on high-NA optics, supporting on-chip and wafer-scale fabrication (Ru et al., 12 Nov 2025).
• Quantum computation readout schemes approaching/surpassing the error threshold for fault-tolerant operation via SCC with NIR photoionization (Zhang et al., 2020).
• Long-term storage and temporal decoupling of manipulation/readout steps in trap-based architectures, facilitating complex pulse protocols and imaging (Ulibarri et al., 29 Oct 2025).

Photoelectric readout is not bounded by the photon-collection losses of confocal PL detection; carrier collection efficiencies can approach unity. Electrical readout is amenable to CMOS integration, multiplexed addressability, and wafer-level scaling for practical devices.

6. Limitations, Challenges, and Future Optimization

While photoelectric readout protocols are maturing rapidly, key limitations include:

• Background currents from unintended photoionization of substitutional nitrogen (N_s^0) or other defects, mitigatable by dual-beam or lock-in methodologies (Bourgeois et al., 2016).
• Incomplete shelving or repumping efficiency, setting a ceiling on achievable spin contrast, particularly in materials with non-ideal defect environments (Bourgeois et al., 2015).
• Recombination and trap-state dynamics restricting duty cycle and effective measurement rates in charge-capture schemes (Ulibarri et al., 29 Oct 2025).
• Lower contrast in some alternative hosts (e.g., 4H-SiC, hBN) due to host-specific ISC/ionization cross-sections, although device engineering is closing this gap (Niethammer et al., 2019, Ru et al., 12 Nov 2025).
• For ultra-high-fidelity SCC, the requirement of low spin-flip rates or sufficiently strong photoionization to outcompete spin-relaxation (Zhang et al., 2020).

Continued optimization strategies include increasing laser power and spot overlap, engineering host materials for improved NV:N ratios and trap densities, integration of ultrafast readout electronics, and the development of multi-color or resonant excitation schemes. Device miniaturization—nanofabricated electrodes, integrated MW delivery, and hybrid material stacks—will further promote scalability and performance.

7. Outlook and Significance

Photoelectric spin readout now offers a suite of techniques with competitive or superior speed, SNR, and integrability relative to conventional ODMR-based optical methods. Its extension from bulk diamond NVs to single spins, spin environments, and low-dimensional hosts substantiates its central role in next-generation quantum sensors and information processors across a diverse range of solid-state platforms (Jayakumar et al., 2018, Bourgeois et al., 2016, Gulka et al., 2016, Ulibarri et al., 29 Oct 2025, Ru et al., 12 Nov 2025). These protocols pave the way for scalable, all-electrical, room-temperature quantum architectures deployable in chip-scale technologies and hybrid quantum-electronic systems.