A Quantum Jump Sensor for Dark Matter Detection

It is remarkable that on a cosmic scale, we do not know what makes up 84% of the matter in the universe. This dark matter has a profound impact on the movement of stars, the formation of galaxies and the patterns in the afterglow of the Big Bang, but we can only hypothesise what its true nature might be. We will build a new type of quantum sensor using a single isolated electron that will be sensitive enough to tell if dark matter is made from certain types of new particles.

It seems likely that most of the missing matter is some new type of substance which barely interacts with ordinary matter electromagnetically. One hint for what this might be comes from the differing symmetries of the strong and weak nuclear forces, which led particle physicists to propose a new particle, the axion. The theory which predicted the axion does not predict its mass, but light axions around 10^9-10^12 times less than the mass of an electron would have been created in the early universe and still be present today as dark matter.

As well as hints from particle physics, there are also indications from cosmology as to the properties of dark matter. Observations of the microwave transition frequencies of hydrogen in the period of the early universe known as the cosmic dawn suggest that it was colder than expected. This was also the period where dark matter could collide with ordinary mater and reduce its temperature. Exotic particles with tiny charges - known as millicharged particles - would account for this observation.

Many experiments have been carried out to detect axions and millicharged particles, but none have been discovered. The most sensitive experiments to detect axions use a strong magnetic field to encourage the axions to decay into microwave photons with a frequency directly related to the axion mass. They then detect those microwaves. Unfortunately, for an important axion mass range, state-of-the art microwave detectors have a fundamental and unavoidable noise source which dwarfs the axion signal. This minimum noise, referred to as the Standard Quantum Limit can be overcome by counting the number of photons which make up the electromagnetic field. No suitable single photon counter exists in the range 30-60 GHz, so we will invent one. The technology we have chosen is a single electron, trapped in a combination of electric and magnetic fields. As the electron absorbs a microwave photon, its quantum orbit changes detectably. A trapped electron is also sensitive to collisions with any millicharged dark matter, also changing its orbit after a collision. This change in orbit can be measured using quantum jump spectroscopy, which was previously used to measure the electron's magnetic moment.

If we could uncover the nature of dark matter, we would finally have understood the most abundant substance in the universe and characterising its precise properties would have implications for many aspects of astrophysics. A discovery of the axion or millicharged particle would only be the start of a new era of particle physics since both particles would be expected to be accompanied by others. In the case of the axion, these could be much heavier Higgs particles, giving an additional strong argument for the construction of a Future Circular Collider at CERN to discover them. Finally, this device is a sensor for the weakest detectable microwave signals, which could be applied to improved microwave astronomy, molecular spectroscopy for the identification of chemical substances and sensing.