The detection of microwave photons is crucial for a wide range of fundamental research, as well as the development of new devices. Microwave radiation is associated with the superconducting energy gap in superconductors, the energy gaps in semiconductor nanostructures, and the rotational and vibrational excitations of molecules.

But although single-photon detectors already exist for visible light, it has proved more difficult to make a similar device for microwave photons because they are typically around a thousand times less energetic. Physicists have previously built a detector that is sensitive to single photons in the far-infrared range, but it used strong magnetic fields that – in many cases – would destroy the very effect that the detector was trying to observe.

Now Astafiev and colleagues have designed a non-magnetic device that is based on two electrically connected quantum dots. Quantum dots are nano-sized deposits of one semiconductor embedded in another semiconductor. Since the dot material has an energy bandgap that is smaller than that of the surrounding material, it can trap charge carriers.

The quantum dots in the new device are made from gallium arsenide and aluminium gallium arsenide. When a photon arrives at the first dot, it excites an electron into the conduction band of the dot, and a strong bias voltage transfers this electron to the second quantum dot. This dot acts as a single-electron transistor, which is switched by the electron to register the photon. This one-way transfer of single electrons is crucial because it prevents an excited electron returning to its ground state in the first quantum dot before it can be registered.

Ostafiev and colleagues believe their detector, which is sensitive to photons with wavelengths in the sub-millimetre range, is very versatile. It can detect photons with frequencies as low as 410 GHz – compared with a lower limit of 448 GHz for the earlier device – and the researchers say that it could be tuned to respond to different wavelengths by using quantum dots of different sizes.

The team also says that it should be possible to make the device from silicon only, which would make it compatible with a wide range of existing electronic devices. Since the detector uses no magnetic field, the team is optimistic that it will also be suitable for a wide range of applications.