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arXiv:2610.30250

Continuous Measurement of Quantum Axion Dark Matter

Yunjia Bao, Dhong Yeon Cheong, Nicholas L. Rodd, Joey Takach, Lian-Tao Wang, Kevin Zhou

hep-phgr-qchep-exquant-phSubmitted Oct 21, 2026• Announced Oct 23, 2026• 0 citations
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Abstract

We provide a fully quantum-mechanical description of the continuous measurement of wavelike dark matter. Our focus is cavity haloscopes, although our results inform a broad range of axion searches. We extend the input-output formalism by treating the axion as a quantum field, finding how its quantum state is imprinted onto higher-order moments of observables of the outgoing transmission-line modes. We further develop a novel measurement-record formalism, which computes the joint probability distribution of all measurement outcomes and is well suited to studying the axion in the time domain. With it, we compute the exact axion-induced number distribution for the first time, demonstrating that it is not Poisson. We argue that virialized axion dark matter is expected to be in a Gaussian quantum state, and show that number or quadrature measurements can distinguish this from the predictions of a coherent state or even a Bose-Einstein condensate. Finally, we show that intrinsically quantum effects, such as those from Fock or squeezed axion states, are far too weak to detect.

Comments
66+28 pages, 8 figures

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We provide a fully quantum-mechanical description of the continuous measurement of wavelike dark matter. Our focus is cavity haloscopes, although our results inform a broad range of axion searches. We extend the input-output formalism by treating the axion as a quantum field, finding how its quantum state is imprinted onto higher-order moments of observables of the outgoing transmission-line modes. We further develop a novel measurement-record formalism, which computes the joint probability distribution of all measurement outcomes and is well suited to studying the axion in the time domain. With it, we compute the exact axion-induced number distribution for the first time, demonstrating that it is not Poisson. We argue that virialized axion dark matter is expected to be in a Gaussian quantum state, and show that number or quadrature measurements can distinguish this from the predictions of a coherent state or even a Bose-Einstein condensate. Finally, we show that intrinsically quantum effects, such as those from Fock or squeezed axion states, are far too weak to detect.
We propose a paradigm for quantum enhanced axion dark matter search, which does not rely on power measurements. We propose to measure directly the axion amplitude and phase in an interferometric protocol at the quantum limit, using a non-linear cavity. In addition, we introduce gyromagnetic modes as wide mass range transducers for axion signals compatible with standard haloscope designs. We expect this scheme to offer an improvement of at least 4 orders of magnitude in figure of merit and at least 2 orders of magnitude in mass window with respect to standard haloscopes. Owing to its generality, our proposed protocol has the potential to speed up axion search but also the search for dark photons or other cosmological objects, such as galactic masers.
We report an axion dark matter search with a haloscope equipped with a microwave photon counter. The haloscope is a tunable high quality factor three-dimensional microwave cavity placed in a magnetic field. The photon counter, operated cyclically, maps an incoming microwave photon onto the state of a superconducting transmon qubit. The measurement protocol continuously monitors the power emitted by the haloscope cavity as well as the dark count background and enables tuning of the cavity frequency to probe different axion masses. With this apparatus, we enhance by a factor of 20 the search speed that can be reached with quantum-limited linear amplifiers and set a new standard for probing the existence of axions with resonant detectors above 5 GHz.
Precision experiments increasingly target weakly coupled waves, including axion dark matter and gravitational radiation. Such waves are commonly described as classical fields, yet they could exist in quantum states with no classical counterpart. We exhibit two severe obstructions to detecting nonclassical effects, both independent of the mode occupancy. First, realistic detectors cannot resolve the fundamental modes of a field; instead they couple to coarse-grained "effective" modes, which often washes out nonclassical effects. Second, all nonclassical effects are suppressed by extra powers of the weak coupling, making them much harder to detect than the waves themselves. We prove this in general, and explicitly show how the suppression arises for quadrature and number statistics, entanglement, and decoherence. The suppression can in principle be overcome given suitable quantum resources, such as highly squeezed detector states, but the required parameters are far beyond current experimental capabilities. We use the axion cavity haloscope as an explicit example, although our conclusions apply to many ultralight dark matter searches, and rule out proposals to establish the quantization of gravity from observations of gravitational waves.
Many experiments that interrogate fundamental theories require detectors whose sensitivities are limited by the laws of quantum mechanics.In cavity-based searches for axionic dark matter, vacuum fluctuations in the two quadratures of the cavity electromagnetic field limit the sensitivity to an axion-induced field.In an apparatus designed to partially mimic existing axion detectors, we demonstrate experimentally that such quantum limits can be overcome through the use of squeezed states.By preparing a microwave cavity in a squeezed state and measuring just the squeezed quadrature, we enhance the spectral scan rate by a factor of 2.12 Æ 0.08.This enhancement is in excellent quantitative agreement with a theoretical model accounting for both imperfect squeezing and measurement.
Quantum phase measurements offer a complementary route to axion searches. We show that axion-photon interactions can imprint both Aharonov-Bohm (AB) and Berry phases in experimentally motivated quantum setups. For a coherently oscillating axion dark matter background, the induced effective current generates a time dependent magnetic flux in an rf-SQUID, leading to a measurable voltage signal through the Josephson phase. For representative benchmarks, this AB phase search reaches the minimum axion-photon coupling gaγγmin∼7.8×10−14 GeV−1g_{aγγ}^{\mathrm{min}}\sim 7.8\times10^{-14}~\mathrm{GeV}^{-1}gaγγmin​∼7.8×10−14 GeV−1 at axion mass ma∼10−10 eVm_a\sim 10^{-10}~\mathrm{eV}ma​∼10−10 eV, with projected sensitivity that can improve on existing limits in that parameter space by roughly one to two orders of magnitude. We also identify a geometric phase observable in a Mach-Zehnder interferometer with an adiabatically rotating magnetic field, providing a proof-of-principle phase-based probe of meV-scale axions even when they do not constitute the dark matter, although sensitivity on the coupling remains weaker than current bounds with conservative tabletop benchmarks. Extending the analysis to a three level photon-axion quasiparticle (AQP)-axion system, with the AQP realized in a topological magnetic insulator, we find a potentially measurable THz Berry phase dominated by the AQP sector, furnishing a nontrivial validation of the formalism in a richer coupled system. These setups establish quantum phase observables as a useful new framework for axion searches, with immediate phenomenological promise in superconducting circuits and longer term potential in quantum enhanced interferometry.
The hypothetical axion particle (of unknown mass) is a leading candidate for dark matter (DM).Many experiments search for axions with microwave cavities, where an axion may convert into a cavity photon, leading to a feeble excess in the output power of the cavity.Recent work [Backes et al., Nature 590, 238 (2021)] has demonstrated that injecting squeezed vacuum into the cavity can substantially accelerate the axion search.Here, we go beyond and provide a theoretical framework to leverage the benefits of quantum squeezing in a network setting consisting of many sensor cavities.By forming a local sensor network, the signals among the cavities can be combined coherently to boost the axion search.Furthermore, injecting multipartite entanglement across the cavities-generated by splitting a squeezed vacuum-enables a global noise reduction.We explore the performance advantage of such a local, entangled sensor network, which enjoys both coherence between the axion signals and entanglement between the sensors.Our analyses are pertinent to next-generation DM-axion searches aiming to leverage a network of sensors and quantum resources in an optimal way.Finally, we assess the possibility of using a more exotic quantum state, the Gottesman-Kitaev-Preskill (GKP) state.Despite a constant-factor improvement in the scan time relative to a single-mode squeezed state in the ideal case, the advantage of employing a GKP state disappears when a practical measurement scheme is considered.