Superconducting amplifiers provide high performance, low power consumption

Researchers have devised a new concept of superconducting microwave low-noise amplifiers for use in radio wave detectors for radio astronomy observations, and demonstrated a high-performance cooled amplifier with power consumption three orders of magnitude lower than that of conventional cooled semiconductor amplifiers. This result is expected to contribute to the development 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 used an SIS mixer. The mixer 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 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 capacity of a 4 Kelvin refrigerator. The research team led by Takafumi Kojima, an associate professor at the National Astronomical Observatory of Japan (NAOJ), has developed an innovative idea to realise a superconductor amplifier by connecting two SIS mixers. The team exploits the basic functions of a 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. This is three orders of magnitude less than that of a cooled semiconductor amplifier,” Kojima said.

After obtaining successful preliminary results in 2018, the researchers advanced the theoretical studies of the system and the physical implementation of its various components. 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 lower power consumption.

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

This low-noise, low-power-consumption amplifier is also suitable for large-scale error-tolerant 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 one million qubits. To handle a large number of qubits, a large number of amplifiers must also be installed, with reductions needed in amplifier power consumption.

NAOJ has experience in the development of superconducting receivers for a number of radio telescopes. NAOJ is also currently working to upgrade the superconducting receivers to improve the performance of the Atacama Large Millimetre/submillimetre Array (ALMA), which is operated in the Republic of Chile in cooperation with East Asia, Europe and North America. Of the 10 types of receiver (corresponding to 10 different frequency bands) currently installed on ALMA, three were developed by NAOJ and the SIS chips used in these receivers were also developed and produced in the cleanroom of the NAOJ Advanced Technology Center (ATC).

The research results were published in Applied Physics Letters.

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