Oleg Astafiev and colleagues at NEC's Nano Electronics Research Laboratories and Japan’s RIKEN national lab have built their maser by placing a nanometre-sized “island” of superconducting aluminium at one end of a microwave resonator that is several millimetres long. The electrons in the superconductor exist in “Cooper pairs”. However, if a voltage is applied to the island, a pair can be broken with one electron leaving the island immediately via a conducting lead and the other being promoted to a higher energy state.

After some time, the second electron also exits, leaving the island in an intermediate energy state. This state eventually decays by emitting a microwave photon into the resonator, where the photon is trapped as a standing wave. The trapped photon can then stimulate the emission of another photon from the island, rapidly filling the resonator with multiple coherent photons.

This amplification process is similar to that which occurs in a conventional maser – but instead of involving many identical atoms or molecules, the process involves only one artificial atom – the superconducting island. Astafiev told physicsworld.com that this gives the device several technological advantages over conventional masers. One is that the wavelength of the microwaves can be tuned very precisely by simply varying the voltages that are applied to the island and changing the resonator frequency. This is unlike conventional masers, which produce microwaves at wavelengths that are fixed by the energy levels of the constituent atoms or molecules.

The device only requires dc voltages to run — unlike a conventional maser, which requires an external radio-frequency power source. As a result the entire maser system can be very small and generate very little heat. This makes it relatively easy to operate at temperatures near 1 K, where aluminium is a superconductor.

According to Astafiev, the maser could be used to create tunable microwave sources and amplifiers that are integrated onto silicon chips. Such devices could be used to study molecular dynamics and other properties of matter. He also believes that single-atom masers could be used as a source of coherent photons in quantum computers that are based on supercomputing quantum bits (qubits). Indeed, the NEC– RIKEN team has been making qubits from tiny islands of superconductor since 1999.