The performance of superconducting-qubit magnetometers depends not only on magnetic-field encoding during Ramsey interrogation, but also on how efficiently the encoded information is recovered during readout. Squeezed microwaves provide a promising route to improve information extraction by suppressing readout noise, yet how such readout enhancement translates into accessible information for magnetic-field estimation remains unclear. Here we develop an effective detected-mode framework linking projected quadrature noise, state-assignment error, and the classical Fisher information accessible from binary readout outcomes. A finite mismatch between the squeezed quadrature and the discrimination axis produces an optimal squeezing strength through the competition between squeezed and anti-squeezed fluctuations. We further show that squeezed readout improves the readout-limited magnetic-field sensitivity by recovering information otherwise lost during state assignment, with the enhancement remaining robust against quadrature mismatch, transmission loss, and added noise. These results establish a quantitative connection between squeezed-microwave readout and measurement-stage information recovery in superconducting quantum sensing.
The performance of superconducting-qubit magnetometers depends not only on magnetic-field encoding during Ramsey interrogation, but also on how efficiently the encoded information is recovered during readout. Squeezed microwaves provide a promising route to improve information extraction by suppressing readout noise, yet how such readout enhancement translates into accessible information for magnetic-field estimation remains unclear. Here we develop an effective detected-mode framework linking projected quadrature noise, state-assignment error, and the classical Fisher information accessible from binary readout outcomes. A finite mismatch between the squeezed quadrature and the discrimination axis produces an optimal squeezing strength through the competition between squeezed and anti-squeezed fluctuations. We further show that squeezed readout improves the readout-limited magnetic-field sensitivity by recovering information otherwise lost during state assignment, with the enhancement remaining robust against quadrature mismatch, transmission loss, and added noise. These results establish a quantitative connection between squeezed-microwave readout and measurement-stage information recovery in superconducting quantum sensing.
Abstract Quantum Fisher information quantifies the sensitivity encoded in a parameter-dependent quantum state, whereas the performance of a practical sensor is limited by the information that survives the physical channel and can be extracted by the available receiver. We introduce an accessible Fisher-information framework based on a physically admissible family of measurements and use it to separate state-level sensitivity, channel degradation, and receiver limitations. The framework is applied to Ramsey and NOON interferometry, squeezed-state displacement sensing, two-level thermometry, and multiparameter qubit sensing. We show that resource normalization, finite visibility, readout errors, measurement rank, shot allocation, calibration uncertainty, and nuisance parameters can substantially modify the experimentally available advantage. For squeezed-state sensing, homodyne-axis misalignment produces a finite optimal squeezing, while bounded calibration uncertainty leads to a robust operating rule. In the pure-loss model considered here, attenuation reduces the attainable information without shifting this optimum. The results provide practical design rules that connect quantum-enhanced state preparation with realistic measurement constraints and experimentally extractable sensing performance.
The ability to perform high-fidelity quantum nondemolition qubit readout is pivotal for the realization of large and powerful quantum computers. Such readout of superconducting qubits is generally enabled by amplifying the weak dispersive measurement signals using phase-preserving quantum-limited Josephson amplifiers with sufficient gain to dilute the contribution of the added noise by the output chain. Here, we further enhance the qubit readout fidelity by (1) simultaneously measuring the two-mode squeezed states of the amplified readout signals at the signal and idler frequencies of the nondegenerate amplifier and (2) coherently combining them at the classical processing stage following a relative rotation that maximizes the signal to noise ratio of the qubit-encoded readout quadrature. Such readout scheme exhibits enhancement in the readout fidelity for all practical values of amplifier gain and noise added by the output chain and is fully compatible with frequency multiplexed setups used in large quantum processors.
Quantum metrology based on superconducting qubits offers unprecedented precision in measuring fundamental physical quantities, particularly magnetic fields. In this study, we develop a modified quantum phase estimation algorithm utilizing superconducting qubits, enhanced by optimized initial state preparation, significantly improving measurement sensitivity and reducing estimation times compared to conventional techniques. To ensure practical applicability and robustness, we incorporate advanced noise modeling, including intrinsic decoherence effects such as flux-induced noise and Purcell losses. Through detailed analytical modeling and numerical simulations, we systematically evaluate sensitivity and practical implementation constraints across various superconducting qubit architectures—transmon, fluxonium, conventional flux, and C-shunt flux qubits. Our approach quantifies the critical trade-offs between coherence, measurement dynamics, and sensitivity, identifying optimal operational regimes for high-performance quantum sensing. We introduce custom circuit simulation methods enabling the analysis and optimization of arbitrary superconducting quantum devices. Our findings provide a framework for advancing quantum-enhanced magnetometry and promise to facilitate next-generation quantum sensors for fundamental physics research, gravitational field measurements, and quantum-assisted searches for dark matter candidates.
Quantum sensing estimates a physical parameter encoded in the state of a probe; with independent spin probes the precision follows the standard quantum limit. Studies of sensing precision often assume that the parameters entering the model, such as the noise, are known. In practice these parameters are not always known, and a mismatch between the assumed and actual values induces a systematic error. Here we study single-qubit Ramsey magnetometry of a DC magnetic field under an unknown detuning between the actual and nominal spin frequencies: A first pulse puts the qubit into a superposition of its two states, the field to be sensed then adds a relative phase during an exposure stage, and a second pulse enables the readout. In our setting, the field acts only during the exposure stage, whereas the detuning acts throughout the whole protocol. We analyze how the detuning biases the estimate, preventing the total estimation error from following the standard quantum limit. We then construct a composite-pulse preparation and readout that exploits the difference in the intervals over which the field and the detuning act to cancel the detuning to first order. We evaluate the performance of this composite-pulse protocol and show that it suppresses the detuning-induced bias.
We study quantum enhancement of sensitivity using squeezed light in a multiparameter quantum sensor, the hybrid dc and rf optically pumped magnetometer (hOPM) [M. Lipka , Multiparameter quantum sensing and magnetic communication with a hybrid dc and rf optically pumped magnetometer, , 034054 (2024)]. Using a single spin ensemble, the hOPM acquires both the dc field strength (scalar magnetometry) and resonantly detects one quadrature of the ac magnetic field at a chosen frequency (rf magnetometry). In contrast to the Bell-Bloom scalar magnetometer [C. Troullinou , Squeezed-light enhancement and backaction evasion in a high sensitivity optically pumped magnetometer, , 193601 (2021)], the backaction evasion in the hOPM is incomplete, leading to a nontrivial interplay of the three quantum noise sources in this system: photon shot noise, spin projection noise, and measurement backaction noise. We observe these interactions using squeezed light as a tool to control the distribution of optical quantum noise between S 2 and S 3 polarization Stokes components, and the resulting effect on readout quantum noise and measurement backaction. These results demonstrate quantum-enhanced sensitivity in a continuously operating multiparameter sensor and reveal fundamental trade-offs between sensitivity, backaction, and bandwidth.
This document presents Version 6.1 of a proposed quantum magnetic sensor based on acollectively prepared, spin-squeezed ensemble of superconducting qubits.The central objective is to determine, experimentally and reproducibly, whether quantumstate preparation can improve magnetic-field estimation relative to a matched unsqueezedsuperconducting-qubit reference operated with the same device, readout chain, resource budget,and total cycle time.A critical physical refinement of the present version is that a superconducting qubit is notassumed to couple to magnetic field through the bare electron gyromagnetic ratio.Instead, the magnetic transduction coefficient is defined operationally asκB,i =⃓⃓⃓⃓∂ωi∂B⃓⃓⃓⃓,or, for flux-mediated devices,κB,i =⃓⃓⃓⃓∂ωi∂Φi⃓⃓⃓⃓Mi,where Miis the effective field-to-flux coupling coefficient.This formulation permits transmon, SQUID-loop, fluxonium, or related architectures withoutconflating artificial-qubit dynamics with physical electron-spin Zeeman coupling.The quantum-state resource is characterized using the optimized Wineland spin-squeezingparameterξ2R =N minn⊥ Var(Sn⊥ )|⟨S⟩|2.Values below unity constitute a metrological spin-squeezing criterion under the statedcollective-spin model.The primary metrological endpoint is not ξ−1R alone.Instead, the proposed experiment directly compares the measured magnetic-field sensitivityper square root bandwidth of the squeezed protocol against a matched coherent/unsqueezedprotocol:Gpair =ηB,cohηB,sq. The principal confirmatory criterion is that the preregistered 95% confidence interval forthe paired gain exclude unity after accounting for preparation overhead, contrast, decoherence,readout noise, calibration uncertainty, device-to-device variation, and cooldown-to-cooldownvariation.The framework includes Hamiltonian modeling, collective squeezing, Ramsey interferometry,classical and quantum Fisher information, open-system dynamics, magnetic transductioncalibration, null tests, blind analysis, preregistration, hierarchical statistics, uncertainty budgets,replication criteria, falsification rules, data schemas, and a reproducible Python referenceimplementation.No experimental result is claimed.