The approach makes use of a Josephson junction, currently used to make extremely sensitive voltmeters and to detect minute changes in magnetic fields.
Josephson junctions consist of a thin layer of insulator sandwiched between two superconducting layers. Under the right conditions, electrons travel from one superconducting layer to the other with no resistance through the insulator in the middle. When the current reaches a critical level, a finite resistance appears and a voltage develops across the device.
Paolo Solinas, a physicist at SPIN-CNR, was experimenting on Josephson junctions with his colleagues at NEST-CNR, when unusual behaviour was seen. The team found that Josephson junctions placed in an oscillating magnetic field produced voltage pulses.
Turning to theory, the researchers found that an oscillating magnetic field produced a ‘phase’ – a sudden jump in a quantum mechanical property of the superconductor layers. The phase jump in turn produced the voltage pulse. The researchers also found that a regularly time-dependent magnetic field would produce voltage pulses that contained hundreds of harmonics of the original driving frequency, including frequencies thousands of times higher.
“The output of a single device is small,” said Solinas, “but you could build an array of devices to turn the intrinsic low power of a single junction into higher output power.” The team calculated that, by stringing together 1000 Josephson junctions made from niobium and aluminum oxide, a 100MHz input frequency could be converted into a 100pW signal at 50GHz.
The team also found that changing the shape of the Josephson junction changed the amount of power at different output frequencies. In particular, a ring shaped junction produced more power at higher harmonics than a circular or rectangular junction.
According to Solinas, electronic circuits made from Josephson junctions could be millimetres long and be readily integrated into chips.
