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Spin-to-charge conversion + rf reflectometry ro_s2c

Spin converted into a charge-motion event by Pauli spin blockade or energy-selective tunnelling, then read by an rf-matched charge sensor in microseconds without destroying the qubit.

Spin-to-charge conversion + rf reflectometry

ro_s2c · layer 6 Readout · demonstrated · since 2004
Open on the map

Pauli/energy-selective tunnelling sensed by a gate-based single-electron box (SEB) or an SET; 99.2% in < 6 µs; 99.9% at 100 µs.

Brief →

Key references

[624] G. A. Oakes et al., “Fast High-Fidelity Single-Shot Readout of Spins in Silicon Using a Si… · 2023
[189] P. Steinacker et al., “Industry-compatible silicon spin-qubit unit cells exceeding 99% fid… · 2025

Design space — attributes

(a) carrier affinity
fabricated
(b) time · entangling
—
(c) readout
spin-to-charge + rf reflectometry · 6.3 µs · non-destructive · mid-circuit
(d) mobility
—
(e) control
low-frequency electrical @ room temperature
(f) error structure
coherent / calibration
(g) manufacturing
CMOS foundry 300 mm

Evaluation space — dated attributes

fast readout (Oakes et al., Quantum Motion; rf single-electron box) → 99.2% in < 6 µs
defines: clock (cycle time) · 2023-02 · source
best SPAM → 99.9% (100 µs integration)
defines: error channel · 2025-09 · source

The 99.2% in < 6 µs is Oakes et al., PRX 13, 011023 (2023), Quantum Motion — an rf single-electron box, not an SET.

Standard records

reset / initialisation time → 1.0 µs · typical
Si spins: 'heralded initialization protocol ... repeated the initialization if unsuccessful', with a t_ramp = 1 us ramp at V_J = 1.12 V · 2025-09 · source [D] ⓘ
reset error → not published · typical
Initialisation error is not reported separately from the 99.9% SPAM figure · 2025-09 · source [D] ⓘ
SPAM error → 1.00e-3 · typical
Si spins, Pauli spin blockade + d.c. single-island SET: 'SPAM fidelities reach up to 99.9%' at t_int = 100 us; best device 99.95(8)% · 2025-09 · source [D] ⓘ
QND fidelity / survival → not published · typical
No QND or repeated-readout fidelity is reported for the PSB/SET spin readout · 2025-09 · source [D] ⓘ

Actors & goals — annotations

Silicon / germanium quantum-dot spins — Intel, Diraq, Quantum Motion, HRL→IBM, QuTech/Groove, Quobly, Equal1 · G4 G7
Donor spins in silicon — SQC · G7

Edges

needs: Gate-defined quantum-dot spin (Si/SiGe, SiMOS, Ge)°, Donor spin (P in ²⁸Si)°
° one of several that would do · the usual route, not a strict need

Used by 10 machines (10 primary · 0 alternate)

↗ its register card · paper / product / press: links that name the machine (attribution checked on 20 Sep 2026)

Pictures

Every picture carries the story of what it shows and its credit line.

Fig. 1 — Fig. 1: The foundry-fabricated spin qubit unit cell.a, Scanning electron microscopy of the split-gate device with false colours. The silicon channel…
Figure of a foundry-made silicon spin-qubit cell read without a separate charge sensor. Panel (a) is a false-colour electron micrograph (40 nm scale) of a silicon channel crossed by split gates; two electrons sit under gates L and R, gate L is wired to a resonant circuit whose response reveals the electrons' state, and gate R receives fast pulses. Panel (b) draws the device in cross-section; (c) is the charge map read through the resonator; (d) and (e) show the coupling between the two dots tuned by a gate. From Nature Electronics (2026), Fig. 1, by Hamonic, Urdampilleta and co-authors. It illustrates gate-based radio-frequency readout of a singlet–triplet qubit. “Fig. 1 — Fig. 1: The foundry-fabricated spin qubit unit cell.a, Scanning electron microscopy of the split-gate device with false colours. The silicon channel (khaki) is…” by Hamonic et al., www.nature.com/articles/s41928-026-01654-9, CC BY 4.0 (creativecommons.org/licenses/by/4.0/)
Fig. 1 — Figure 1: (a) Electronic and cryogenic setup. An RF tone PinP is split in two at room temperature (RT). One part serves as local oscillator (LO) for
Three-panel figure. (a) A drawing of the measurement chain from room temperature through 4 K to 50 mK: attenuated input and pump lines, a directional coupler, isolators, a Josephson travelling-wave parametric amplifier (+12 dB), a high-electron-mobility-transistor (HEMT) amplifier (+35 dB) and an IQ demodulator, with an optical micrograph of the spiral inductor forming the tank circuit on gate B3. (b) A false-coloured scanning electron micrograph of a four-dot silicon (SiMOS) device with gates B1–B3 and T1–T3. (c) A charge-stability map of the demodulated signal. It is Fig. 1 of arXiv 2307.14717 by V. Elhomsy and co-authors. It shows where a parametric amplifier sits in gate-based spin-qubit readout. “Fig. 1 — Figure 1: (a) Electronic and cryogenic setup. An RF tone PinP is split in two at room temperature (RT). One part serves as local oscillator (LO)…” by Victor Elhomsy et al., arxiv.org/abs/2307.14717, CC BY 4.0 (creativecommons.org/licenses/by/4.0/)
Fig. 1 — Figure 1: Schematic of the experimental setup. The double quantum dot, located at the 60 mK stage, is coupled to an RF-reflectometry circuit for fas…
A drawn block schematic of a cryogenic measurement set-up across three stages (300 K, 4 K, 60 mK), with two real images: a scanning-electron micrograph of the gate electrodes of a silicon double quantum dot and an optical image of its chip, the imaged area boxed in orange. It is Fig. 1 of arXiv 2608.10325 (August 2026) by Johnathan Bryan, Tim J. Wilson and Hong-Wen Jiang, on real-time FPGA-based (field-programmable gate array) feedback stabilization of a silicon double quantum dot. Radio-frequency reflectometry reads the dot's charge state quickly; a Quantum Machines OPX controller runs a proportional-integral-derivative (PID) loop that corrects gate voltages in real time against drift. “Fig. 1 — Figure 1: Schematic of the experimental setup. The double quantum dot, located at the 60 mK stage, is coupled to an RF-reflectometry circuit for fast…” by Johnathan Bryan, Tim J. Wilson, Hong-Wen Jiang, arxiv.org/abs/2608.10325, CC BY 4.0 (creativecommons.org/licenses/by/4.0/)
ro_s2c · layer 6 Readout · centrality 8.4 · demonstrated · since 2004 · updated 2026-09-30

Spin-to-charge conversion + rf reflectometry

Spin converted into a charge-motion event by Pauli spin blockade or energy-selective tunnelling, then read by an rf-matched charge sensor in microseconds without destroying the qubit.

Verdict. The readout every silicon architecture uses and the term that sets the silicon clock — 6 µs at 99.2% or 100 µs at 99.9%, with no published device delivering both.

Key references

Λ = error-suppression factor per code-distance step; QBI = DARPA Quantum Benchmarking Initiative (Stage A concept → B R&D plan → C government V&V); G1–G7 = the report's goal classes (see Actors & economics).

Identity & lineage

Spin carries no charge, so a tunnelling event is made spin-conditional — Pauli spin blockade (PSB; two-spin parity, scale = singlet–triplet splitting) or energy-selective Elzerman tunnelling (single spin, needing Zeeman ≫ kT) — and a charge sensor reports it. The sensor is a single-electron transistor or a gate-based single-electron box read via quantum capacitance, in an LC (inductor–capacitor) tank near 0.1–2 GHz. Elzerman single-shot readout dates to 2004. Attributes: readout — spin-to-charge, ~6 µs, non-destructive, mid-circuit-capable; control — low-frequency drive from room temperature, sensor in CMOS, carrier immobile.

Physics & limits

Per-shot signal-to-noise is the phase shift integrated against amplifier noise over a window capped by T1 and floored by tank bandwidth — the whole trade: 99.2% under 6 µs D[624] against 99.9% SPAM at 100 µs D[189]. Elzerman readout dies as kT approaches the Zeeman energy, so hot (≈1 K) operation costs readout first; Pauli spin blockade survives there but answers parity. Relaxation inside the window returns a false ground state, and sensor charge noise walks the threshold, so measurement error is drifting and correlated, not independent as decoders assume. Quantum-limited amplification and in-fridge demodulation move the floor.

Engineering state of the art

table
year figure who tag+key
2023-02-23 99.2% single-shot in <6 µs, rf single-electron box Quantum Motion D[624]
2025-09-24 99.9% SPAM at 100 µs, 300 mm SiMOS Diraq D[189]

Dominant term: SNR at short windows, T1 at long; no foundry device publishes sub-10 µs readout above 99.5%.

The ZQCS channel count is a trade-press restatement of a vendor launch, not a primary release. Not published: a readout-only fidelity, separated by randomised benchmarking, behind the Diraq/imec 99.9% quoted as SPAM.

Manufacturing, materials & supply chain

Built in the qubit's own 300 mm CMOS stack, the sensor adds no materials risk; the gate-based box, needing only an electrode and a tank, is the foundry-favoured form. Instrumentation is bought, not built: Zurich Instruments' SHFQC-LRT gives ≤32 µs weighted integration C[625], and Quantum Machines' OPX1000 lists nine spin customers including Diraq, HRL, imec and Equal1 C[449]. Frequency multiplexing keeps line count sub-linear: the wall at 10³ is tank spacing across the band; at 10⁴–10⁶ it is aggregate readout bandwidth and amplifier count, not the sensor. HRL's 4 K controller (366 DACs, ≤3.5 W) shows the chain can move inside the fridge D[190].

Role in the stack

Required by quantum-dot and donor spins, with no replace or conflict edges — the load-bearing readout node in both the silicon/germanium quantum-dot spin architecture and the donor-spin architecture. The register lists 10 machines using it, among them QM-One, RacQ (Bell-1), the 11-qubit donor processor and the HRL exchange-only silicon spin processor (18 EO qubits from 54 quantum dots). With exchange gates in tens of nanoseconds, readout at 6–100 µs sets derived clock = sum of the syndrome round: gate layers + transport + readout + reset: 8.5×10⁻⁶ s at the fast end, 6.3 µs of it readout. Verification: the two headline figures are different devices with different sensors — a speed–fidelity curve, not one system — and the 99.9% bundles initialisation. Neither is replicated independently.

Actors & economics

Who.
| Organisation | Role | Country | What they do | Evidence |
|---|---|---|---|---|
| Quantum Motion | developer | UK | Single-electron-box readout, 99.2% in <6 µs | D[624] |
| Diraq | developer | Australia | 99.9% SPAM on 300 mm imec SiMOS | D[189] |
| Silicon Quantum Computing | developer | Australia | Readout of donor nuclear registers | Dfact |
| HRL Laboratories | developer | USA | Readout sequenced by 4 K cryo-CMOS | Dfact |
| Zurich Instruments | supplier | Switzerland | Sells the reflectometry instruments | C[625] |
| Quantum Machines | supplier | Israel | Readout stack for nine spin teams | C[449] |

Money. 2025-02-25 · Quantum Machines · Series C · $170 M · PSG Equity, Intel Capital · closed Cfact. 2025-11-06 · Diraq, Quantum Motion, SQC · QBI Stage B · ≤$15 M each · DARPA · selected fact. 2026-05-07 · Quantum Motion · Series C · $160 M · DCVC, Kembara · closed Cfact. 2026-05-21 · Diraq · CHIPS LOI · ≤$38 M · US Commerce · non-binding fact.

Market & supply chain. Zurich Instruments and Quantum Machines supply the rf chain nearly every spin team buys rather than builds C[449], [625]; cryogenic amplification is the second pinch point. Pays into G2, G3, G7.

IP & standards. No dated patent family for spin-to-charge conversion or rf reflectometry in a named database as of 4 Sep 2026.

Roadmaps & track record. Diraq (2026-08-27 · 150 k physical, 1 k logical by 2029 · unmet; its 2026-07-09 release said "thousands by 2029") Rfact. Zurich Instruments (2026-03-09 · ZQCS for "several thousand qubits" · shipping) P[526]. No vendor promises a dated readout rate; instruments ship on schedule, qubit counts do not.

Strategic reading. Readout is the one silicon subsystem bought rather than built, so control-electronics vendors keep leverage whichever qubit wins; if the gate-based box displaces the sensor transistor, the foundry captures it and per-qubit-sensor designs lose twice.

Outlook & open questions

Confirm or demote in 12–24 months: ≥99.9% below 20 µs on a foundry device; ≥16 sensors multiplexed on one line with a crosstalk number. Best case 2029: >100 sensors read under 10 µs each, demodulated in-fridge. Worst case: readout stays the clock-setting term and the largest syndrome-cycle error. Open: (1) tanks per quantum-limited amplifier; (2) whether parity readout suffices without a per-qubit ancilla; (3) how correlated threshold drift looks to a decoder.

References
  1. [189] P. Steinacker et al., “Industry-compatible silicon spin-qubit unit cells exceeding 99% fidelity,” Nature, vol. 646, no. 8083, pp. 81–87, Sep. 2025, doi: 10.1038/s41586-025-09531-9. D
  2. [190] Members of the HRL Quantum Team and Collaborators, “A digitally controlled silicon quantum processing unit,” arXiv:2604.16216, Apr. 2026. D
  3. [449] Quantum Machines, “Semiconductor Spin Qubits,” Aug. 11, 2026. [Online]. Available: https://www.quantum-machines.co/qubit-types/semiconductor-spin-qubits/ C
  4. [526] Zurich Instruments, “Zurich Instruments Launches the ZQCS Quantum Control System to Master the Long-Lived Logical Qubit Challenge,” Mar. 9, 2026. [Online]. Available: https://www.zhinst.com/americas/en/news/zurich-instruments-launches-zqcs-quantum-control-system-master-long-lived-logical-qubit/ P
  5. [624] G. A. Oakes et al., “Fast High-Fidelity Single-Shot Readout of Spins in Silicon Using a Single-Electron Box,” Phys. Rev. X, vol. 13, no. 1, Art. no. 011023, Feb. 2023, doi: 10.1103/PhysRevX.13.011023. D
  6. [625] Zurich Instruments, “SHFQC-LRT Long Readout Time,” Jan. 30, 2025. [Online]. Available: https://www.zhinst.com/en/products/shfqc-lrt-long-readout-time C