The first series of JTWPAs were designed and fabricated in Niobium (Nb) technology. Pretesting at a temperature of 4.2 K indicated a relative gain (difference in the output signal with the pump signal switched on and off) of more than 12 dB in a bandwidth of 1.4 GHz. Although these first tests have clearly shown proof of principle for the operation of the project’s JTWPA, the achieved gain was smaller than expected and a large number of strong, higher frequency mixing products were observed. Simulations have highlighted that for high-gain and wide-bandwidth operation the design of the JTWPA must mitigate higher-order terms.

To impede the amplification of signals with frequencies above the pump frequency a significant phase mismatch between the pump and the higher frequency signals should be established. The project determined that this can be realized by an appropriate adjustment of the JTWPA parameters. Such circuit parameters also cause a small phase mismatch between the pump and the low-frequency signals to be amplified, limiting the achievable gain.

To avoid this phase difference, the project worked out several alternative strategies:

It is expected that with the improved concepts and designs the gain and bandwidth can be increased to reach the targeted parameters for the JTWPA. New JTWPA circuits based on the novel concepts are now being designed and fabricated in Nb and in Al technology. Both types of JTWPA samples are to be examined at mK temperatures after pretesting of the Nb samples at 4.2 K.

A low-temperature (cryostat) setup at RHUL is now operational for the characterization of both resonator and traveling wave parametric amplifier devices at the many photon levels at sub-100 mK temperatures. Microwave equipment to support noise squeezing measurements is available for deployment on the low-temperature measurement system when suitable parametric amplifier devices become available.

In addition, a new 300 mK continuous-cycle low-temperature (cryostat) system at NPL is now ready for use. The NPL low-temperature cryostat system will be used for wide bandwidth characterization at 4-12 GHz. Improvements have been made to this measurement system to include low-temperature switching of rf lines between test samples and calibration circuits. The use of a thermal noise source as a reference standard is planned for absolute measurements. A new suite of scientific measurement and data analysis software has been written to enable fully automated measurements of devices in the 300 mK apparatus.

A further low-temperature measurement setup at PTB, based on a dilution refrigerator, will shortly be completed with the aim of characterizing the JTWPA circuits from objective 1 at mK temperatures with a focus on their linearity, dynamic range, and noise, using standard microwave generators and thermal noise sources. Moreover, NPL and PTB are operating simple 4He dip probes for quick characterization of Nb devices.

Several parametric amplifier samples fabricated in niobium technology have been characterized so far and tests have been carried out on single niobium Josephson junctions and aluminum junctions.

The two low-temperature cryostat systems at RHUL and NPL will together form the metrology platform for microwave measurements for the characterization of the JTWPA device and other quantum devices. The NPL 300 mK system is configured for wide-band (4-12 GHz) characterization of JTWPA devices.

Measurements of the project’s first series of JTWPAs designed and fabricated in Nb technology have been carried out in liquid helium on NPLs 4 K probe. The results showed clear proof of three-wave mixing, a strong second harmonic generation of the pump frequency, and many higher-order mixing products, in line with the circuit simulations. The measurements also included “single tone spectroscopy” with the aim of observing the harmonic generation in the device to compare with the results from numerical modeling. Work to reconcile the observed harmonic generation with the computer models is ongoing.

Both a numerical and an analytical model for JTWPA behavior up to the 5-th harmonic of the pump frequency have been developed by NPL and RHUL. An objective comparison of the models has been published as a peer-reviewed scientific paper [1]. Initial comparisons with practical measurements have been made and work is continuing to validate the experimental data for absolute values of the JTWPA rf reflection and transmission parameters in order to obtain a more robust comparison with the models.

The results from this analysis will inform future JTWPA designs, in particular the requirements for dispersion engineering. The new designs were modified by reducing the cut-off frequency of the microwave transmission line, the plasma frequency of the SQUIDs, and by applying appropriate phase matching techniques. The next steps will be the characterization of JTWPAs, comprising the new concepts for the phase matching, from several consortium members in Nb and Al technology at 300 mK.

[1] T. Dixon, J.W. Dunstan, G.B. Long, J.M. Williams, P.J. Meeson, and C.D. Shelly, “Capturing Complex Behavior in Josephson Traveling-Wave Parametric Amplifiers”, Phys. Rev. Applied 14, 034058 (2020) https://doi.org/10.1103/PhysRevApplied.14.034058

The first series of rf-SETs in Al technology has been designed and fabricated, the design includes the rf-SET and the tank circuit for impedance matching to the 50 Ohm transmission line or JTWPA on the same chip. The designed tank circuit resonance frequency is about 4 GHz. The rf measurement setup in the cryogen-free dilution refrigerator at LanU for high-frequency characterization of rf-SETs has been accomplished and is ready for device characterization.

The measurement setup is based on the rf-reflectometry technique and includes attenuators and a directional coupler in the drive line and a directional coupler, isolators, and cold semiconductor amplifiers in the receiver line. The JTWPA’s from objective 1 will be inserted in between the isolators and the cold semiconductor amplifiers after the characterization of the JTWPAs has been completed in objective 3 and the performance of the rf-SETs has been verified.

Designs for Al nano-SQUIDs have been optimized and the first batch of DC Nb nano-SQUIDs has been fabricated. First characterizations of these preliminary DC nanoSQUIDs in Nb technology were accomplished at 4.2 K. In addition to this, Nb rf-nanoSQUIDs (both single Josephson Junction and arrays), incorporated in resonators, have been designed and fabricated.

The coupling scheme for impedance matching of the SQUID sensor to the JTWPA has been further elaborated. An on-chip coupling scheme for matching the low-impedance nano-SQUID sensor and the JTWPA, based on tuneable direct coupling with a quarter-wavelength resonator, has been developed. Also, a second alternative approach was studied by embedding the nano-SQUID into an LC resonator with a resonance frequency of about 4-6 GHz and coupling it to the JTWPA with a passive directional coupler.