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PRX Quantum

Cavity Entanglement and State Swapping to Accelerate the Search for Axion Dark Matter

Kelly Wurtz, Benjamin Brubaker, Yue Jiang, E. P. Ruddy, Daniel Palken, Konrad W. Lehnert

Published Dec 10, 2021• 30 citations
Publisher

Abstract

In cavity-based axion dark matter detectors, quantum noise remains a primary barrier to achieving the scan rate necessary for a comprehensive search of axion parameter space.Here, we introduce a method of scan rate enhancement in which an axion-sensitive cavity is coupled to an auxiliary resonant circuit through simultaneous two-mode squeezing (entangling) and state-swapping interactions.We show analytically that when combined, these interactions can amplify an axion signal before it becomes polluted by vacuum noise introduced by measurement.This internal amplification yields a wider bandwidth of axion sensitivity, increasing the rate at which the detector can search through frequency space.With interaction rates predicted by circuit simulations of this system, we show that this technique can increase the scan rate up to 15-fold relative to the scan rate of a detector limited by vacuum noise.

DOI
10.1103/prxquantum.2.040350

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hep-phCross-listedarXiv:2610.31717•Oct 23, 2026
In cavity-based axion dark matter detectors, quantum noise remains a primary barrier to achieving the scan rate necessary for a comprehensive search of axion parameter space.Here, we introduce a method of scan rate enhancement in which an axion-sensitive cavity is coupled to an auxiliary resonant circuit through simultaneous two-mode squeezing (entangling) and state-swapping interactions.We show analytically that when combined, these interactions can amplify an axion signal before it becomes polluted by vacuum noise introduced by measurement.This internal amplification yields a wider bandwidth of axion sensitivity, increasing the rate at which the detector can search through frequency space.With interaction rates predicted by circuit simulations of this system, we show that this technique can increase the scan rate up to 15-fold relative to the scan rate of a detector limited by vacuum noise.
In cavity-based axion dark matter detectors, quantum noise remains a primary barrier to achieving the scan rate necessary for a comprehensive search of axion parameter space. Here we introduce a method of scan rate enhancement in which an axion-sensitive cavity is coupled to an auxiliary resonant circuit through simultaneous two-mode squeezing (entangling) and state swapping interactions. We show analytically that when combined, these interactions can amplify an axion signal before it becomes polluted by vacuum noise introduced by measurement. This internal amplification yields a wider bandwidth of axion sensitivity, increasing the rate at which the detector can search through frequency space. With interaction rates predicted by circuit simulations of this system, we show that this technique can increase the scan rate up to 15-fold relative to the scan rate of a detector limited by vacuum noise.
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.
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.
The nature of dark matter is a fundamental puzzle in modern physics. A major approach of searching for dark matter relies on detecting feeble noise in microwave cavities. However, the quantum advantages of common quantum resources such as squeezing are intrinsically limited by the Rayleigh curse—a constant loss places a sensitivity upper bound on these quantum resources. In this paper, we propose an in situ transient control to mitigate such Rayleigh limit. The protocol consists of three steps: in-cavity quantum state preparation, axion accumulation with tunable time duration, and measurement. For the quantum source, we focus on the single-mode squeezed state (SMSS), and the entanglement-assisted case using signal-ancilla pairs in two-mode squeezed state (TMSS), where the ancilla does not interact with the axion. From quantum Fisher information rate evaluation, we derive the requirement of cavity quality factor, thermal noise level and squeezing gain for quantum advantage. When the squeezing gain becomes larger, the optimal axion accumulation time decreases, which reduces loss and mitigates the Rayleigh curse—i.e., the quantum advantage increases with the squeezing gain. Overall, we find that TMSS is more sensitive in the low-temperature limit. In the case of SMSS, as large gain is required for an advantage over vacuum, homodyne detection is sufficient to achieve optimality. Whereas, for TMSS, anti-squeezing and photon counting is optimal. Thanks to recent advances in magnetic field-resilient in-cavity squeezing and rapidly coupling out for photon counting, the proposed protocol is compatible with axion detection scenario.
Microwave cavities in strong magnetic fields are among the most sensitive detectors for axion dark matter detection. Its sensitivity, however, is fundamentally limited by the second order nature of axion photon energy conversion, which yields an extremely weak signal. Here we propose a feasible approach to break this limitation, by pre-exciting the target cavity mode to generate a first order power response signal in situ. The resulting signal amplitude scales linearly with the axion-photon coupling parameter, which is in sharp contrast to the quadratic scaling of conventional schemes, and is detectable via standard quadrature balanced homodyne detection system, even though its phase remains stochastic. Numerical simulations demonstrate that this pre-excited seed field can effectively suppress thermal noise, and thus the detection sensitivity of the system can approach the lower quantum noise limit. Consequently, the sensitivities predicted by the KSVZ and DFSZ models could be surpassed. This proposal might offer a pathway to a major leap in axion dark matter searches, as it is within reach of current technology.