Design of diplexer for implementation in SPA

One of the major requirements for superconducting qubit hardware to work efficiently is high-fidelity, quantum non-demolition qubit readout. The qubits and the cavity where the qubits are placed are present in a cryogenic setup at a temperature around 10mK. To read the state of qubit at room temperature, it is necessary to amplify the weak microwave signal without adding much noise. The amplification can be done only by using quantum-limited amplifiers. One example of a superconducting quantum limited amplifier is the SNAIL parametric amplifier (SPA). SNAIL Parametric Amplifier (SPA) is a phase preserving superconducting amplifier. The amplifier has a non-linear element where the signal to be amplified interacts with a strong microwave pump at double the signal frequency. It is necessary to separate the amplified signal tone from the pump tone on the output of the device in order to protect the qubit from the strong microwave pump. In this report, we have studied how to add a diplexer to a SPA circuit to achieve this required separation between signal and pump tones.

A diplexer is a device that divides the signal from a common port into different frequencies. It consists of two bandpass filters attached to the common port at one end and to the respective signal port at the other. The diplexer here directs the signal from qubit and pump tone to the amplifier which is connected to the common port. Thus, the diplexer along with a circulator is used to direct the signal from qubit to the amplifier and the amplified signal to the observer at room temperature. In this project, different diplexers with filters of different order were designed. The response (linewidth) of these diplexers was observed both in the absence and presence of the amplifier and they were compared. The report concludes by noting the difficulties in designing the perfect diplexer and presents future design methodologies to obtain a diplexer and amplifier, fulfilling the requirements- wide passband, low insertion loss at passband and high isolation between signal and pump ports.