Quantum Science and the Search for Axion Dark Matter

The dark-matter puzzle is one of the most important open problems in modern physics. The ultralight axion is a well-motivated dark-matter candidate, conceived to resolve the strong-$CP$ problem of quantum chromodynamics. Numerous precision experiments are searching for the three nongravitational interactions of axionlike dark matter. Some of the searches are approaching fundamental quantum limits on their sensitivity. This Perspective describes several approaches that use quantum engineering to circumvent these limits. Squeezing and single-photon counting can enhance searches for the axion-photon interaction. Optimization of quantum spin-ensemble properties is needed to realize the full potential of spin-based searches for the electric dipole moment and the gradient interactions of axion dark matter. Several metrological and sensing techniques, developed in the field of quantum information science, are finding natural applications in this area of experimental fundamental physics.

Popular Summary

What is the Universe made of? Decades of precise measurements indicate that, at present, only about 5% of the of the Universe's energy density is in the form of nucleons, electrons, photons, or neutrinos—the standard particles whose properties and interactions we understand. Most of the matter in the Universe is in the form of dark matter, which has feeble interactions (or none at all) with the standard particles, aside from gravity. For many decades scientists have tried to detect nongravitational interactions of dark matter in a laboratory, but so far there has been no unambiguous detection.

Recent technological advances in the field of quantum technology can guide and accelerate the progress toward discovery. This is especially true for direct searches for ultralight axionlike dark matter. Many of the ongoing and proposed experiments in this field are approaching, or have already reached, sensitivity levels where quantum resources are needed to achieve their scientific goals. This is also where several of the quantum approaches and devices, having matured in quantum information science, can find a natural application. Experiments that search for interactions of axionlike dark matter with electromagnetic fields can incorporate squeezing or single-quantum detection of microwave photons, making use of superconducting devices or Rydberg atom sensors. Quantum engineering of spin ensembles will play the key role in searches for the electric dipole moment and the gradient interactions of axionlike dark matter.

Whether or not one of these techniques helps discover the axion depends, of course, on its unknown mass and interactions. Whatever the outcome, quantum technology is already letting us look in unexplored places, and perhaps unexpected discoveries will be made.

Figure 1

The potential QCD axion dark-matter mass range (bottom scale) and the corresponding Compton frequencies (top scale). The ADMX experiment aims to cover the mass range between $2.5$ and $8.3\text{μ}\mathrm{eV}$ and future modifications will seek to double the upper end to $16.5\text{μ}\mathrm{eV}$ [89, 90, 92]. This is only a small fraction of the possible mass range. Approaches based on quantum engineering are an important part of the efforts to broaden the search. Squeezing of quantum fluctuations of the electromagnetic field and photon counting will extend the search at higher masses and radio-frequency quantum sensors will enable lumped-element-based experiments at lower masses. Still lower masses will be explored by experiments that make use of quantum spin ensembles.

Figure 2

A conceptual schematic of the ADMX haloscope. A resonant cavity is placed in a strong magnetic field created by a superconducting magnet inside a dilution refrigerator. Dark-matter axions are converted to photons via the electromagnetic interaction in Eq. (3). The resulting electromagnetic signal occupies a narrow band near the axion Compton frequency ${\nu }_a$. It is coupled out of the cavity and into a sensitive amplifier and detection chain. The search for the unknown axion mass is performed by tuning the cavity resonance.

Figure 3

(a) A conceptual schematic of the HAYSTAC haloscope. The detection scheme is similar to that of the ADMX, consisting of a resonant cavity in a strong magnetic field. HAYSTAC operates at microwave frequencies, where zero-point quantum fluctuations of the electromagnetic field are the dominant noise source. (b) The frequency dependence of the Johnson-Nyquist noise originating in the cavity ($N_1$) and reflected from the cavity ($N_2$), plotted as a function of the detuning from cavity resonance. The noise spectral density is in units of single-quadrature vacuum noise $h{\nu }_c/4$, where ${\nu }_c$ is the cavity frequency. The gray band indicates the range of frequencies where internal cavity noise dominates. (c) By squeezing one of the quadratures of the vacuum state incident on the cavity, HAYSTAC is able to reduce the reflected noise and overcouple the cavity, which broadens the frequency band over which internal cavity noise dominates [the gray region; note that the $x$ axis spans the same scale as in (b)]. This increases the sensitive bandwidth at each cavity tuning step and therefore speeds up the axion search.

Figure 4

A conceptual schematic of the CASPEr search for the EDM and gradient interactions of axionlike dark matter. The axionlike-dark-matter field interacts with the spin ensemble via the oscillating effective magnetic field $B^{\ast }$. Experiments search for the resulting tilt and precession of the spin-ensemble magnetization $M$, when the spin Larmor frequency is on resonance with the axion Compton frequency ${\nu }_a$. The search for the unknown axion mass is performed by varying the leading field $B_0$, which tunes the spin Larmor frequency.