Superconducting amplifiers offer high performance at low power

Researchers at the National Astronomical Observatory of Japan (NAOJ) have devised a new concept of superconducting microwave low-noise amplifiers for use in radio wave detectors for radio astronomy observations, and successfully demonstrated a high-performance cooled amplifier with power consumption three orders of magnitude lower than that of conventional cooled semiconductor amplifiers.

Described in the journal Applied Physics Letters, the team’s work is expected to contribute to the realisation of large-scale multi-element radio cameras and error-tolerant quantum computers, both of which require a large number of low-noise microwave amplifiers.

The researchers utilised an SIS mixer, a device which is named after its structure — a very thin film of insulator material sandwiched between two layers of superconductors (S-I-S). In a radio telescope, cosmic radio waves collected by an antenna are fed into an SIS mixer and the output signal is amplified by low-noise semiconductor amplifiers. An SIS mixer operates in a very low temperature environment, as low as 4 Kelvin (-269°C), and the amplifiers are also operated at that temperature.

To improve the performance of radio telescopes, researchers are developing a large-format radio camera equipped with 2D arrays of SIS mixers and amplifiers. However, the power consumption is a limiting factor. The typical power consumption of a semiconductor amplifier is about 10 mW, and by assembling 100 sets of detectors, the total power consumption reaches the maximum cooling capability of a 4 Kelvin refrigerator. The research team came up with a simple but innovative idea to realise a superconductor amplifier by connecting two SIS mixers, whereby they would exploit the basic functions of the SIS mixer: frequency conversion and signal amplification.

“The most important point is that the power consumption of an SIS mixer is, in principle, as low as microwatts,” said research team leader Associate Professor Takafumi Kojima. “This is three orders of magnitude less than that of a cooled semiconductor amplifier.”

After obtaining successful preliminary results in 2018, the team advanced both the theoretical studies of the system and the physical implementation of its various components. In the end, the research team optimised the system and realised an ‘SIS amplifier’ with 5–8 dB (three to six times) gain below the frequency of 5 GHz and a typical noise temperature of 10 K, which is comparable to the current cooled semiconductor amplifiers such as HEMT and HBT, but with much lower power consumption.

“By changing the configuration of the components, we can further improve the gain and low-noise performance of an SIS amplifier,” Kojima said. “The idea of connecting two SIS mixers has broader applications for making various electronics that have functions other than amplification.”

This low-noise, low-power-consumption amplifier is also expected to be suitable for large-scale quantum computers. Currently available quantum computers are small scale, with less than 100 qubits, but larger-scale, error-tolerant, general-purpose quantum computers will require more than 1 million qubits. To handle a large number of qubits, a large number of amplifiers must be installed, and a dramatic reduction in amplifier power consumption is required.

NAOJ has experience in the development of superconducting receivers for a number of radio telescopes, including the Nobeyama 45 m radio telescope which began operation in 1982. NAOJ is also currently working to upgrade the superconducting receivers to improve the performance of the Atacama Large Millimeter/submillimeter Array (ALMA), which is operated in the Republic of Chile in cooperation with East Asia, Europe and North America. Of the 10 types of receivers (corresponding to 10 different frequency bands) currently installed on ALMA, three were developed by NAOJ, as were the SIS chips at the heart of these receivers.

Image caption: An ‘SIS amplifier’ using two SIS mixers, one at each end. Image credit: NAOJ.