Superconducting qubits as photon counters in axion dark matter searches

Short answer: Axions are hypothetical particles proposed as a dark matter candidate, and haloscope experiments look for them by watching for microwave photons in a magnetic field. A 2025 Physical Review X paper, available as a 2024 arXiv preprint, reports a haloscope with a superconducting-qubit photon counter that speeds up the search by a factor of 20 compared with quantum-limited linear amplifiers. A separate 2024 preprint proposes qubits as direct axion sensors. No axion has been detected in these abstracts.

What is a haloscope search?

The popular summary of the 2025 paper explains the idea. Astrophysical evidence suggests the existence of dark matter, and the axion is one leading candidate. Axions are predicted to interact weakly with electromagnetic fields, so in a strong magnetic field they could convert into microwave photons. The authors say conventional haloscopes scan tunable microwave resonators across the 1 to 50 GHz range using linear amplifiers that are limited by quantum noise, making searches extremely slow. They add that a recent effort with squeezed states achieved a twofold increase in scan speed.

What did the photon counter do?

The abstract describes a haloscope built from a tunable, high quality factor three-dimensional microwave cavity in a magnetic field. A photon counter, operated cyclically, maps an incoming microwave photon onto the state of a superconducting transmon qubit. The protocol continuously monitors both the power emitted by the cavity and the dark count background, and allows the cavity frequency to be tuned to probe different axion masses. The authors report enhancing the search speed by a factor of 20 compared with quantum-limited linear amplifiers, around 7 GHz according to the summary, and say they set a new standard for resonant detectors above 5 GHz. To handle the sensitivity of superconducting circuits to magnetic fields, they separate the counter from the cavity and link them with a coaxial cable. The abstract is about search speed and does not claim a detection.

What does the theory preprint propose?

A 2024 arXiv preprint, which has not necessarily been peer reviewed, proposes a direct axion search using superconducting transmon qubits as quantum sensors. With an external magnetic field, axion dark matter would generate an oscillating electric field that excites the qubit, and that excitation would be the signal. The authors consider the excitation process including the shielding cavity around the qubits and estimate the signal rate. They discuss enhancing the signal with cavity resonance and with entangled quantum sensors built with a quantum circuit, and say that combining the two could reach the parameter region suggested by QCD axion models. This is a theoretical estimate.

How to read this

The 2025 paper and its 2024 arXiv version are the same work, so this post draws on two distinct studies. Search speed is a measure of how fast a mass range can be covered, and does not mean axions were found. This post is based on abstracts and visible summary text, and full texts were not checked. It makes no claim about products.

FAQ

Was an axion detected?

No. The abstracts report search speed and a proposal, not a detection.

How does a qubit count photons?

It maps an incoming microwave photon onto the transmon qubit state, according to the 2025 abstract.

What speed-up was reported?

A factor of 20 over quantum-limited linear amplifiers, per the authors.

Why separate the counter from the cavity?

The 2025 summary says it protects the superconducting circuit from the strong magnetic field needed for axion conversion.