Broadband traveling-wave parametric amplifier with more than 20 dB gain in the GHz frequency range

Quantum information technology requires broadband amplifiers with extremely low intrinsic noise. A traveling-wave parametric amplifier based on a series array of one-junction SQUIDs was developed at PTB. This device is characterized by a large gain, wide frequency bandwidth and a potentially low noise at the quantum limit.

The development of many recent innovations in quantum technologies, such as quantum metrology and quantum computers, critically relies on the availability of cryogenic amplifiers with sufficient gain as well as bandwidth and a very low noise that is determined by quantum mechanical principles. At the current stage, even the best cryogenic semiconductor amplifiers exhibit electrical noise at GHz frequencies, which is ten times higher than allowed for quantum sensitive experiments and key applications of quantum circuits. Parametric amplifiers using nonlinear reactive devices, e.g. Josephson junctions, transfer energy from a pump wave to a signal wave and could be applied as alternative as they achieve nearly quantum-limited noise performance.

An innovative parametric amplifier which consists of a microwave transmission line with embedded series arrays of one-junction SQUIDs (rf-SQUIDs) and uses the traveling-wave principle has been developed at PTB [1]. In contrast to the cubic nonlinearity (Kerr-Effect) used in previous circuit designs, our structure is able to produce a quadratic Josephson nonlinearity and thus allows the use of the advantageous three-wave mixing. Furthermore, the phases of the traveling pump and the amplified signal waves could be adjusted efficiently as the Kerr-Effect is almost absent in this setup. This would lead to a large gain in a wide frequency range.

Test circuits have been fabricated using niobium technology with 1100 Nb/AlO_x/Nb Josephson junctions, and were characterized in liquid helium at a temperature of 4,2 K (Figure 1). The circuit parameters were chosen in a way that the microwave transmission line with the rf-SQUID series arrays has an impedance of approximately 50 Ω and can be measured without complex circuit design for impedance matching. The measurements show a power gain of up to 24 dB in a frequency range of approximately 4 GHz to 5 GHz (Figure 2). The gain could be controlled by an external magnetic field and the pump power. Further investigations of the amplifier circuit at a lower temperature (T < 100 mK) are planned to investigate whether the desired minimum amplifier noise at the quantum mechanical limit can be achieved with such devices.

Figure 1: Test chip with superconducting SQUID series arrays of the traveling-wave amplifier in cryogenic sample holder with microwave connectors.

Figure 2: Amplified signal (red dots) and idler signal (green dots) as a function of the pump power. The blue dots indicate the output signal after switching off the pump power.

[1] A. B. Zorin, Phys. Rev. Applied 6 (2016) 034006