REVIEW 5 minor 51 references
Grid-state qubits in a superconducting cavity reach combined preparation and measurement error below one in a thousand.
Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →
T0 review · grok-4.5
2026-07-10 22:47 UTC pith:U2ICNT5M
load-bearing objection Solid experimental result: two-order SPAM reduction for single-mode GKP via post-selected sBs + repeated finite-energy measurement, with QEC left intact and magic states from vacuum included.
Quantum error correction of a grid-state qubit with state preparation and measurement errors below 10⁻³
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
When post-selected sBs stabilization is used for state preparation and repeated finite-energy measurements are used for readout, the combined SPAM error of a single-mode grid-state qubit, averaged over the six cardinal states, falls below 7(7) imes10^{-4} (and to 8(5) imes10^{-3} for H-type magic states). This is two orders of magnitude better than previous grid-state experiments and matches typical transmon SPAM levels, without degrading the logical error rate of subsequent autonomous quantum error correction.
What carries the argument
Post-selected small-big-small (sBs) stabilization interleaved with mid-circuit auxiliary measurements, combined with multi-round finite-energy Pauli measurements that retain only all-agree outcomes; together they suppress auxiliary readout errors, finite-energy envelope errors, and photon-loss errors while remaining fully compatible with autonomous QEC.
Load-bearing premise
That discarding every shot in which successive mid-circuit auxiliary outcomes disagree fully removes residual logical and measurement errors without biasing the reconstructed Pauli expectation values used to compute logical fidelity.
What would settle it
Repeat the same post-selected preparation and multi-round measurement protocol on independently prepared cardinal states and check whether the extracted logical fidelity still matches the claimed SPAM figure when an independent tomography method (for example full characteristic-function reconstruction without post-selection) is used as ground truth.
If this is right
- Grid-state logical qubits can now be prepared and read out at error rates comparable to ordinary superconducting qubits, removing a major bottleneck for circuit-level algorithms.
- H-type magic states prepared from vacuum by the same stabilization cycle become practical resource states for non-Clifford gates inside a GKP architecture.
- The same SPAM protocols can be layered under existing autonomous QEC without increasing the logical error per round, enabling longer error-corrected computations.
- Because the protocols are hardware-efficient and use only a single oscillator mode, they lower the resource overhead for multi-mode or multi-qubit GKP processors.
Where Pith is reading between the lines
- The large reduction in SPAM error implies that previous GKP experiments were limited more by auxiliary readout and finite-energy effects than by the intrinsic quality of the oscillator itself.
- If the all-agree policy can be relaxed to a controlled number of allowed flips without reintroducing bias, the exponential survival-probability cost could be mitigated for deeper circuits.
- Extending the same post-selected stabilization to two-mode grid codes would test whether the SPAM improvement scales with the higher protection those codes provide.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript reports experimental SPAM improvement for a single-mode GKP (grid-state) qubit in a superconducting cavity-transmon architecture. Using post-selected sBs stabilization (from vacuum or 9 dB squeezed states) for preparation of the six cardinal states and H-type magic states, combined with repeated finite-energy measurements under an all-agree post-selection policy, the authors achieve a total SPAM error averaged over cardinal states of 7(7)×10^{-4} (survival ~0.24 prep / ~0.39 meas) and 8(5)×10^{-3} for magic states. These protocols are shown to be compatible with autonomous QEC, yielding a logical error per sBs round of 8.1(2)×10^{-3} that is statistically unchanged from the unimproved baseline. Device parameters, reset error, and readout visibility are independently calibrated; logical fidelity is extracted from Pauli expectations via the standard average (Eq. 3).
Significance. If the reported numbers hold under the stated post-selection, this closes a long-standing practical gap for bosonic GKP qubits: SPAM errors that previously limited computational performance are brought two orders of magnitude below prior GKP experiments and into the range of typical transmon SPAM. The work is concrete and hardware-relevant: it re-uses high-performance autonomous sBs QEC for repeat-until-success preparation (including magic states from vacuum), demonstrates that the same protocols do not degrade QEC gain, and supplies transparent calibrations plus a noise model that reproduces the main trends. The result strengthens the case for grid-state qubits as a hardware-efficient route to fault-tolerant computation.
minor comments (5)
- Abstract and Sec. I claim “below 10^{-3}” while the body (Fig. 4a, text) quotes 7(7)×10^{-4} for cardinal states and 8(5)×10^{-3} for magic states; a single clarifying sentence that the headline figure is the cardinal average under all-agree post-selection would avoid any ambiguity.
- Eq. (3) and the surrounding text define FL via Pauli expectations of the six cardinal states; it would help the reader if the precise mapping from the post-selected mid-circuit bit-strings {mk} to ⟨μ0⟩± were written out once (even if standard).
- Appendix D.2 and Figs. D.2–D.3 already quantify the survival–infidelity trade-off under milder policies; a short pointer in the main text (Sec. II D or II E) would make clear that the all-agree choice is conservative rather than the only viable option.
- Fig. 2b bottom panel and Fig. 3b use single-round vs multi-round finite-energy measurements; labeling the measurement protocol explicitly on each panel would reduce the need to cross-reference the caption.
- A few typographical items: “presqueezing” / “Presqueezing” capitalization is inconsistent; “primarly” (Sec. II E) should be “primarily”; arXiv date stamp appears as July 9, 2026.
Circularity Check
No significant circularity: experimental SPAM measurement under explicit post-selection protocols, not a derivation that reduces to its inputs.
full rationale
The paper's central claim is an empirical measurement of combined SPAM error (ϵ_SPAM = 1 − F_L from the standard average of Pauli expectations over the six cardinal states, Eq. 3) under two explicitly defined protocols: post-selected sBs stabilization (repeat-until-success compatible, all-agree on mid-circuit auxiliary outcomes) and repeated finite-energy measurements (likewise all-agree). These protocols are described in Sec. II C–D and Appendices C–D with concrete pulse sequences, gauge updates, and survival probabilities; the reported numbers (7(7)×10^{-4} for cardinal states, 8(5)×10^{-3} for H-type magic states) are direct experimental outcomes, not predictions obtained by fitting a parameter to a subset of the same data and re-using it. Self-citations (e.g., [24] for the device and sBs implementation, [21, 29–31] for finite-energy measurement and GKP background) supply prior experimental techniques and theory; they are not invoked as uniqueness theorems that force the present result, nor do they smuggle an ansatz that is then re-labeled a first-principles derivation. Simulations in the appendices use an independent noise model (storage decay + transmon T1/Tϕ + classical bit-flip) and recover the same qualitative saturation of ϵ_SPAM, providing consistency checks rather than circular support. The logical-error-per-round comparison (0.0081(2) vs 0.0085(2)) further shows the SPAM protocols do not alter the autonomous QEC performance they are claimed to be compatible with. No step reduces by construction to its own inputs; the work is self-contained experimental reporting.
Axiom & Free-Parameter Ledger
free parameters (3)
- finite-energy envelope Δ =
0.38
- number of sBs rounds N_sBs and RFE rounds N_RFE =
typically 8–11 (prep), 5–8 (meas)
- auxiliary readout visibility V =
0.95(1)
axioms (3)
- domain assumption The system is accurately described by the dispersive Hamiltonian of a cavity mode dispersively coupled to a transmon (including first- and second-order cross-Kerr terms).
- domain assumption sBs stabilization with the listed displacement amplitudes and gauge updates drives the oscillator into the finite-energy GKP code space.
- ad hoc to paper All-agree post-selection on auxiliary mid-circuit outcomes yields an unbiased sample of the logical state whose Pauli expectations equal the true logical fidelity.
read the original abstract
Grid state qubits offer a hardware-efficient approach to large-scale fault-tolerant quantum computing. They access the information redundancy required for quantum error correction by exploiting the large Hilbert space naturally available in harmonic oscillators. Superconducting architectures are particularly suitable to implement grid state qubits due to their fast and high-fidelity operations. Grid states in superconducting circuits enable quantum error correction (QEC) with performance beyond break-even. However, the state preparation and measurements (SPAM) errors of grid states has been a significant limitation to computational performances. In this work, we leverage high-performance QEC to enable repeat-until-success state preparation of both cardinal and magic states of the single-mode grid-state qubit. We combine this with an improved measurement protocol that corrects for both finite-energy envelope and auxiliary qubit readout errors, and increases robustness to photon loss. Our experiments, using both techniques, achieve a combined state-preparation and measurement error below $10^{-3}$. This represents two orders-of-magnitude improvement over the state of the art, bringing this platform on par with standard SPAM error levels measured in transmon qubits.
Figures
Reference graph
Works this paper leans on
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[1]
SPAM error metric To quantify the impact of SPAM errors experimentally, the logical fidelity is measured through the Pauli expec- tation values of six cardinal states of the single-mode grid code. The logical fidelity corresponds to FL = 1 2 + 1 12 X ˆµ0∈{ ˆX0, ˆY0, ˆZ0} (⟨ˆµ0⟩+ − ⟨ˆµ0⟩−),(3) where⟨ˆµ0⟩± =⟨±µ|ˆµ0| ±µ⟩denotes the Pauli expecta- tion value ...
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[2]
Results As the number of repeated finite-energy measurement rounds increases, the total SPAM error decreases and rapidly saturate after only a few repetitions, as shown in Fig. 3(b). For a single round, SPAM fidelity is primarily limited by readout errors of the auxiliary qubit, which are signif- icantly suppressed through repetition and postselection. As...
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[3]
Experimental device The experimental device is composed of a supercon- ducting double-post microwave cavity machined in high- purity Al. A superconducting chip made on a sapphire substrate with a strip line resonator, a Purcell filter and a transmon is coupled to the storage mode through the waveguide leading to the storage cavity. The resonator, filter a...
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[4]
Hamiltonian parameters In the operating regime, where each modes are far de- tuned from each-other, the static part of the system can be described by the following dispersive Hamiltonian, ˆHdisp./ℏ=ω sˆa†ˆa+ωqˆb†ˆb+ω rˆc†ˆc| {z } dressed modes + Ks 2 ˆa†2ˆa2 + Kq 2 ˆb†2ˆb2 + Kr 2 ˆc†2ˆc2 | {z } dressed self-Kerr + 2χsqˆa†ˆaˆb†ˆb+ 2χ qrˆb†ˆbˆc†ˆc| {z } cro...
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[5]
The complete reset sequence is composed of two identical blocks of pulses
Auxiliary qubit reset As mentioned in the main text, the auxiliary qubit re- set is performed through a drive between the|f0⟩ ↔ |g1⟩ of the qubit-resonator sub-system. The complete reset sequence is composed of two identical blocks of pulses. Each block is composed of aπ ef pulse on the auxiliary qubit, followed by a microwavef0g1drive between the |f0⟩ ↔ ...
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[6]
Mid-circuit measurements calibration The readout of the auxiliary is benchmarked by eval- uating the readout visibility, defined as V=p g|g −p g|e,(A1) wherep g|k is the probability of assigning the readout re- sult to|g⟩when preparing in a state|k⟩withk∈g, e. The mid-circuit measurements of the auxiliary qubit are performed using the protocol illustrated...
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[7]
ECD gate In the experiments, the echoed conditional displace- ment (ECD) gates are implemented over a total duration of 450 ns, comprising displacement pulses of duration 50 ns, interaction time 100 ns, andπpulses of duration 10 ns. The amplitude of the final displacement pulse in the ECD sequence is scaled by a factorζ ECD, calibrated to cancel any resid...
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A circuit with depthN= 7, corre- sponding to the number of ECD and rotation pairs, is used here
Storage presqueezing state preparation Presqueezing of 9 dB is implemented using a numer- ically optimized sequence of ECD gates and auxiliary qubit rotations. A circuit with depthN= 7, corre- sponding to the number of ECD and rotation pairs, is used here. The phase space orientation of the presqueez- ing is adjusted for the different target states. For| ...
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[9]
Characterization of the storage coherence times To measure the relaxation time of the storage mode, the system is first initialized in a single-photon Fock state. After a free evolution time, the population of the single-photon state is measured by mapping the on photon state|1⟩onto the excited state of an auxiliary qubit,|e⟩. This mapping, as well as the...
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[10]
Postselected stabilization cycles The cycles of stabilization rounds are defined in this section. Based on the ordering of the different types of sBs rounds, the total number of rounds is chosen such that after a certain numbers of cycles, the state is in the trivial gauge (see appendix C 3), i.e. the gauge where the grid state has +1 value for stabilizer...
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[11]
Simulations In Fig. C.1, simulation results of|±Z⟩state prepa- ration starting from the vacuum and a squeezed state are presented. The survival probability (a) and the ex- pectation value of ˆZ, ˆSp and ˆSx measurements (b)-(c) are shown as a function of the number of rounds. The squeezing allows to start the sequence with the logical in- formation alread...
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[12]
Gauge Our state preparation protocols contains sBs rounds where the big ECD corresponds to a stabilizer or a Pauli operator. Pauli rounds leave the state in a modified codespace, where the peaks are not on the original phase space lattice points, but are shifted by half a lattice spac- ing. We say that such a modified codespace has non- trivial gauge. We ...
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[13]
Improved ˆYmeasurements Here, we investigate how the choice of Pauli representa- tive affects logical Pauli ˆYmeasurement in the repeated- measurement protocol. For Pauli ˆZand Pauli ˆX, the available representatives of each operator are collinear and therefore commute with a trivial zero symplectic phase. In contrast, Pauli ˆYadmits two orthogonal repre-...
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[14]
Survival probability vs infidelity tradeoff The postselection procedure becomes increasingly resource-intensive as system size grows due to the ex- ponential decrease of the survival probability. For state preparation, this is not a problem since repeat-until- success approaches, which are fully compatible with the introduced protocols, can be implemented...
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