A design for a new broad-bandwidth amplifier for detecting single microwave photons has been unveiled by physicists at the University of California, Berkeley and the Lawrence Berkeley National Laboratory in the US. The "Josephson travelling-wave parametric amplifier" uses a technique called "resonant phase matching", which is expected to boost the gain in the amplifier by more than 10 dB compared with existing Josephson parametric amplifiers (JPAs). The new device is also predicted to have a bandwidth of 3 GHz, and together these properties should allow it be used in quantum circuits that operate at multiple frequencies, as well as finding use in extremely sensitive astronomical detectors.

The readout and control of superconducting quantum bits (qubits) involves the detection of extremely weak microwave signals containing as few as one photon. This can be done using a JPA, which incorporates one or more Josephson junctions, each consisting of a tiny slice of insulator sandwiched between two superconductor contacts. A parametric amplifier works by modulating a circuit parameter such as capacitance or inductance. In a JPA, the Josephson junction behaves like a nonlinear inductor and plays the role of a modulated parameter when it is "pumped" by a microwave signal.

"Our new result is important because JPAs are the most advanced amplifiers available today for making low-noise measurements on systems such as superconducting qubits," explains team member Kevin O'Brien of the Nanoscale Science and Engineering Center at the University of California, Berkeley.

Cavity-free operation

Although JPAs can reach the so-called quantum limit for minimum added noise, they do suffer from having a relatively narrow bandwidth. This is because the gain of the JPA is boosted by coupling the Josephson junction to a resonant cavity, which puts severe limits on the frequencies at which the JPA can operate. Travelling-wave parametric amplifiers (TWPAs) avoid this problem by dispensing with the cavity altogether. Instead, the amplification occurs in a much longer microwave transmission line. However, this creates another problem: high gain can only be achieved when the nonlinear processes taking place in the system are phase matched.

"We show that by adding many resonant elements into the transmission line, we can achieve both phase matching and exponential gain over a broad bandwidth," O'Brien explains.

The researchers, led by Berkeley's Xiang Zhang, integrated these elements by making the transmission line from 2000 unit cells, with each cell comprising a Josephson junction and a resonator. The resonators modify the phase velocity of the pump signal, which allows the pump to efficiently transfer energy throughout the entire device.

Swings and JPAs

To put things more simply, an example of parametric amplification in real life is a playground swing, says O'Brien. "While the JPA is a circuit in which the Josephson junction is the nonlinear element and the pump a microwave field, a swing is a nonlinear oscillator driven by moving your centre of mass. Both the swing's amplitude and the amplitude of a weak microwave signal will increase over time, as energy transfers from your movements or the microwave pump," he explains.

"This is all very well if the phase of the pump is correct, but it is another matter if the pump does not have the right phase relationship with the oscillations. In this case, energy transfer from the pump can be less efficient, and in some cases the pump can actually reduce the oscillation's amplitude. Children learn how to optimize this pump phase through trial and error, and we can now optimize Josephson junction TWPA gain thanks to resonant phase matching," he continues.

The team is now busy fabricating devices based on its design and is also using these to perform measurements on superconducting qubits.

The work is descibed in Physical Review Letters.