Superconducting qubit network (SQN) detectors with different geometries and based on 5 and 10 flux qubits, 8 and 25 transmons have been designed, fabricated and tested.
The SQNs with 8 transmons coupled to the coplanar waveguide were used to experimental study of the waveguide induced long-range interaction between qubits (Fig. 1). The qubits were tuned to a common resonance frequency at 7.898 GHz. Spectroscopic microwave (MW) test measurements (Fig. 2) have given an experimental evidence for a stable collective quantum state in the SQN up to 8 qubits. Time resolved experiments showed reduced group velocities of down to a factor of about 1500 smaller than in vacuum.
We have elaborated the theory of an SQN interacting with a weak MW radiation. The Heisenberg limit of sensitivity is reached in the presence of a strong long-range interaction between qubits. The interaction of single MW photons with an array of transmon in a waveguide cavity resonator has been theoretically investigated.
The coherent collective quantum states have been theoretically studied in disordered SQNs coupled to a low-dissipative resonator and a transmission line. An inductive coupling of SQN to a low-dissipative resonator provides an effective long-range interaction between all qubits. Coupling of an SQN to the transmission line allows one to experimentally access the temporal correlation function of equilibrium/non-equilibrium total polarization (Fig. 3). The collective quantum dynamics occurring in SQNs in presence of a spread of individual qubit frequencies has been studied theoretically. An amplitude of the main resonance drastically increases as the interaction overcomes the disorder, and the collective state is formed. In the presence of a weak non-resonant photon field, the positions of resonances depend on the number of photons, i.e. the collective ac Stark effect is obtained (Fig.4).
The heralded single MW photon source based on Josephson Traveling Wave Parametric Amplifier (JTWPA)-was designed and fabricated (Fig. 5). Measurements of the correlated signals at the output of the JTWPA were performed at T=45 mK.
T-type three terminal SQNs with 10 c-shunted flux qubits have been fabricated (Fig. 6). The first-tone S21 was measure as a function of frequency in the presence of the second-tone (ST) signal applied to port 3. A shift of the position of the resonant transmission dip as a function of the amplitude of the ST signal was observed (Fig.7a). The power at which this shift took place depended on the ST signal frequency (Fig. 7b). Experimental results are in a good agreement with the model based on a non-linear multiphoton interaction between second-tone MW signal and a qubit system of the SQN (Fig.7b). The detector sensitivity of our SQN device is suitable to count a few photons biasing the resonator between port 1 and port 2 at frequency of 7.743 GHz.
Sikivie’s Haloscope at the National Laboratories of INFN in Frascati has been successfully tested and operated at 30 mK.
The Workshop "Searching for Galactic Axions and Superconducting Devices with Quantum Efficiency" have been organised on October 25-29,2021 with the help of SUPERGALAX consortium. Special Session on Quantum Detection in the framework of the WOLTE14 workshop has been organized. The SUPERGALAX activity has been presented on the ASC 22 and on the EUCAS 23 conferences. Several scientific seminars hosted the talks about SUPERGARAX activities have been periodically hold.
