Single-Shot Spin Readout

• Single-shot spin readout is a protocol that maps a single spin’s state to a measurable signal using spin-to-charge or spin-to-photon methods, ensuring fidelity per cycle.
• It employs mechanisms such as energy-selective tunneling, Pauli spin blockade, and optical cycling to enable quantum feedback, error correction, and state verification.
• High readout fidelity and rapid discrimination, demonstrated in MOS-SETs, quantum dots, and optical architectures, underpin the scalability of quantum information processing.

Single-shot spin readout is a measurement protocol in which the quantum state of a single spin—electron or nuclear, in a solid-state host or other platform—is projectively measured in a single experimental run, without the need for ensemble averaging or repeated identical measurements. This enables quantum feedback, error correction, state initialization verification, and time-resolved observation of quantum jumps, all crucial for scalable quantum information processing. Single-shot readout is distinguished by the requirement that the probability of success (fidelity) is quantified per measurement cycle, with a discrimination threshold yielding low misassignment error rates for the two (or more) spin basis states.

1. Physical Mechanisms and Device Architectures

Single-shot spin readout is realized by converting the spin state into an observable signal with sufficiently large contrast and short integration time relative to the relevant relaxation or dephasing processes. The primary mechanism is spin-to-charge or spin-to-photon conversion, achieved through energy-selective tunneling, Pauli spin blockade, or state-dependent optical cycling, and read out electrically or optically.

Metal–oxide–semiconductor single-electron transistor (MOS-SET) with donor electron:

• The device consists of an intrinsic silicon substrate, phosphorus donor atoms forming a small ensemble near a SET island, and surface gates for potential control (Morello et al., 2010).
• Electrochemical potentials μ↓, μ↑ of the donor are split by the Zeeman energy $E_Z = gμ_B B$.
• During the read phase, setting $μ_{\downarrow} < μ_{SET} < μ_{\uparrow}$ ensures that a spin-up electron can tunnel out to the SET, while spin-down is blockaded, converting spin information to a transient current in the SET.

Quantum dot gate-defined devices:

• Lateral semiconductor quantum dots are capacitively and tunnel coupled to a charge sensor (quantum point contact, SET, or a quantum-capacitance-based device) (Antoniadis et al., 2022, Hu et al., 2022, Hogg et al., 2022).
• Singlet–triplet, donor–dot, and spin–valley qubits are common, with readout via energy-selective tunneling, Pauli spin blockade, or enhanced latching mechanisms (Harvey-Collard et al., 2017, Baart et al., 2015).

Optical and cavity-based architectures:

• Cavity quantum electrodynamics (cQED) enables ultrafast readout by Purcell-enhanced emission selectively coupled to a cavity mode, discriminating spin states by the presence/absence of emitted photons within nanoseconds (Antoniadis et al., 2022, Wong et al., 30 Oct 2025).
• Nanophotonic resonators coupled to rare-earth or color centers in Si, SiC, or diamond provide high-fidelity optical single-shot readout by counting photons emitted from spin-selective transitions (Gritsch et al., 8 May 2024, Wong et al., 30 Oct 2025, Zhang et al., 2020, Lai et al., 9 Jan 2024, Anderson et al., 2021).

STM-ESR nuclear spin readout:

• Atomic-scale nuclear spins on surfaces are read out by monitoring ESR-induced current changes in a scanning tunnelling microscope probe tip, with the nuclear state resolvable via hyperfine shift of the ESR line (Stolte et al., 11 Oct 2024).

2. Measurement Protocols and Signal Discrimination

Time-resolved pulsed protocols:

• The canonical protocol divides each cycle into load, wait, read, and empty phases (Morello et al., 2010).
  • Load: Initialize with random or definite spin state (depending on initialization fidelity).
  • Wait/Hold: Spin relaxes at characteristic time $T_1$.
  • Read: Gate voltages set so spin-selective tunneling occurs; current or reflectometry signal records a tunneling event (“blip”).
  • Empty: Charge is removed to reset for the next cycle.

Thresholding and histogram analysis:

• Peak current (or demodulated RF voltage, photon count) traces are recorded per cycle.
• States are assigned by whether the measured value exceeds (or does not exceed) a discrimination threshold $I_T$ or $N_{th}$.
• Fidelities are defined as:

$$F_{\downarrow} = 1 - \int_{I_T}^{\infty} N_{\downarrow}(I)\,dI, \quad F_{\uparrow} = 1 - \int_{-\infty}^{I_T} N_{\uparrow}(I)\,dI$$

where $N_{\downarrow}(I)$ and $N_{\uparrow}(I)$ are histograms of measurement outcomes for definite initial states (Morello et al., 2010).

• The visibility, $V = F_{\downarrow} + F_{\uparrow} - 1$, quantifies overall discrimination power.

Threshold-independent and post-processed methods:

• Linear “extrapolation” formulae allow inference of the true state probability without reliance on a single, fixed threshold, improving robustness to drift and variation (Hu et al., 2022).
• Error bars are propagated via uncertainties in dark count rates, visibilities, and device parameters.

Optical protocols:

• Spin-photon interfaces map spin to optical transitions with distinct cycling rates or emission branching ratios (Antoniadis et al., 2022, Gritsch et al., 8 May 2024, Zhang et al., 2020, Wong et al., 30 Oct 2025).
• Fluorescence-based approaches detect photon arrivals in a well-defined readout window; absence (or presence) of photons is assigned to logical spin states.
• Reflection-based protocols utilize cavity reflectivity contrast between spin states, enabling assignment with Poissonian photon statistics and appropriate thresholds (Wong et al., 30 Oct 2025, Farfurnik et al., 2020).

Ancilla-mapping and repetitive readout:

• Nuclear spins are repeatedly mapped onto a coupled electron spin or ancilla, and the measurement is repeated multiple times to boost SNR, yielding binomial-distributed photon or current counts (Lai et al., 9 Jan 2024, Dréau et al., 2012).

3. Spin-to-Charge and Spin-to-Photon Conversion Physics

Spin-dependent tunneling:

• At finite $B$ field, Zeeman spacings render $\Gamma_{\uparrow}$ markedly different from $\Gamma_{\downarrow}$ by energy-selective alignment with the reservoir Fermi level.
• The tunnel-out and tunnel-in rates follow

$$\Gamma_{\uparrow} = \Gamma_{0} f(\mu_{\uparrow} - \mu_{SET}), \quad \Gamma_{\downarrow} = \Gamma_{0} [1 - f(\mu_{\downarrow} - \mu_{SET})]$$

with $f(E)$ the Fermi function (Morello et al., 2010).

• Lifetime broadening, temperature, and barrier transparency set the ultimate distinguishability and speed of the process.

Pauli spin blockade and latching mechanisms:

• Two-electron solid-state systems employ the Pauli principle to effect a blockade for triplets (or certain configurations), so only the singlet relaxes to a new charge state rapidly (Harvey-Collard et al., 2017, Baart et al., 2015).
• Enhanced latching protocols engineer a full charge-difference signal by inducing metastable charge states with long lifetimes, lengthening the measurement window and increasing SNR.

Spin-photon interface and Purcell enhancement:

• In microcavity or nanophotonic systems, cavity QED raises emission rates via the Purcell effect for one spin state, while the other is off-resonant and suppressed (Antoniadis et al., 2022, Wong et al., 30 Oct 2025, Gritsch et al., 8 May 2024).
• Critical parameters include the cyclicity (average number of cycles before spin-flip), branching ratios, collection and detection efficiency, and the cavity-emitter coupling $g$ relative to decay and dephasing rates.

Spin-to-charge conversion in defect centers:

• Spin-dependent use of intersystem crossing or two-photon ionization allows mapping of spin states to long-lived charge configurations, readable by optical or electrical means (Zhang et al., 2020, Anderson et al., 2021).

4. Fidelity, Visibility, and Error Budgets

Error sources:

• Incomplete or incorrect spin-to-charge/photon conversion (state mapping error).
• Detector noise, limited bandwidth, and finite SNR lead to misassignment (overlap of histograms).
• Spin relaxation ($T_1$) during the measurement window can cause state flips prior to detection.
• Dark counts and false positives in photon or current detection.

Fidelity quantification:

• For the Morello et al. device, the optimized discrimination threshold yields $F_{\downarrow} \approx 99\%$, $F_{\uparrow} \approx 93\%$, $V \approx 92\%$, and $T_1$ times approaching 1 s at moderate fields (Morello et al., 2010).
• Cavity-enhanced QD readout demonstrates $F = 95.2 \pm 0.7\%$ in $3~\mathrm{ns}$, with errors primarily due to photon loss and, at longer times, spin-flip events (Antoniadis et al., 2022).
• Threshold-independent approaches yield a 60$\times$ larger operating window in parameter space for 1% accuracy (Hu et al., 2022).

Tuning and optimization:

• The fastest reliable readout balances the integration time required for sufficient SNR against increased exposure to relaxation and environmental noise.
• Optimizing tunnel rates, bandwidth, and detection sensitivity are universal strategies across platforms.
• For photonic systems, maximizing cyclicity and detection efficiency are key to approaching single-shot fidelity limits.

Tables for select device architectures:

System	Mechanism	Reported Fidelity	Read Time
MOS-SET, Si donor (Morello et al., 2010)	Spin-dep. tunneling	>90% (V 92%)	100 μs
Cavity QD (Antoniadis et al., 2022)	Optical/Purcell	95.2% ±0.7%	3 ns
Donor array, SLQD sensor (Hogg et al., 2022)	Elzerman-style	95%	~μs
Optical Er:Si nanocavity (Gritsch et al., 8 May 2024)	Optical cavity	86.9%	0.71 ms
SiC V2–, nuclear ancilla (Lai et al., 9 Jan 2024)	Ancilla CNOT	99.5% (dual-step)	1.24 ms
STM-ESR 49Ti (Stolte et al., 11 Oct 2024)	Hyperfine/ESR-STM	~98%	>20 ms

5. Experimental Results and Systematic Variations

Key empirical values:

• Tunnel rates: $1/\Gamma_{\uparrow}$ down to $10~\mu$s, $1/\Gamma_{\downarrow}$ $\approx40~\mu$s (Si MOS-SET).
• Spin lifetime $T_1$: up to $1~\mathrm{s}$ in Si; scaling as $K_0 + K_5 B^5$ characteristic of phonon-limited relaxation (Morello et al., 2010).
• Bandwidth/SNR: $\sim 3~\mu$s rise for $\Delta I/I_\mathrm{noise} \gg 1$ at $<200$ kHz electronics (MOS-SET); MHz-range rf/optical systems attain sub-$\mu$s windows and single-photon SNR (Antoniadis et al., 2022, Hogg et al., 2022).
• Parameter-robustness: Threshold-independent protocol retains $>1$\% accuracy over $\sim$60$\times$ larger $(t_\mathrm{read}, V_\mathrm{thresh})$ window (Hu et al., 2022).
• Multiplexing: Donor-defined sensors in Si can reach $\sim$15 qubits per sensor, compared to 3–4 for gate-defined architectures at similar fidelity (Hogg et al., 2022).

Device-specific limitations and remedies:

• Tunnel rate limitations: Set by spatial separation and barrier transparency; can be engineered electrically or via deterministically placed donors.
• Bandwidth/noise: Improved with rf reflectometry and GHz-capable quantum amplifiers.
• Fidelity saturation: Limited by thermal/extrinsic processes or intrinsic cyclicity in photonic approaches.

6. Scalability, Robustness, and Future Prospects

Scalability:

• CMOS-compatible architectures are crucial for integrating large-scale donor arrays or quantum-dot registers with charge or rf sensors, as exemplified by FD-SOI implementations at the 22 nm node (Clarke et al., 15 Oct 2025).
• Spin-photon interfaces with high Purcell factors in nanophotonic structures and spectral multiplexing allow hundreds of spectrally resolved qubits per cavity mode and are compatible with fiber networks for modular quantum computing (Gritsch et al., 8 May 2024, Wong et al., 30 Oct 2025).
• Gate-based and dispersive readout utilizing shared rf or microwave lines supports frequency multiplexing, minimizing footprint and wiring overhead (Zheng et al., 2019, Hogg et al., 2022, Pakkiam et al., 2018).

Parameter robustness and error correction:

• Threshold-independent analysis and error budgeting facilitate operation in the presence of slow drifts and fluctuating device parameters (Hu et al., 2022).
• Readout fidelities $>99\%$ over $\mu$s timescales are within reach with optimized device engineering, enabling fault-tolerant mid-circuit measurements (Oakes et al., 2022, Harvey-Collard et al., 2017, Lai et al., 9 Jan 2024).

Integration and cross-platform considerations:

• SCC techniques mapping quantum state to a long-lived charge or optical state readily interface to scalable electronics or telecom-band photonics (Zhang et al., 2020, Anderson et al., 2021, Gritsch et al., 8 May 2024).
• Hybrid systems combining STM-ESR with atomic-level assembly, and surface-based arrays, create tunable platform for quantum simulation and memory (Stolte et al., 11 Oct 2024).
• Further advances anticipated by improving emitter–cavity coupling, detection efficiency, cryogenic amplifier performance, and implantation precision.

Outlook:

• Single-shot readout underpins high-fidelity initialization, quantum feedback, and repeated error-syndrome cycles for quantum processors.
• The confluence of fast, robust, and scalable protocols across platforms—MOS-SET, donor/quantum-dot, color/rare-earth centers, and engineered photonic devices—positions single-shot spin readout as a mature and indispensable tool for both current and next-generation quantum information systems (Morello et al., 2010, Clarke et al., 15 Oct 2025, Antoniadis et al., 2022, Gritsch et al., 8 May 2024).