The number of quantum bits embedded in superconducting quantum computers is rapidly increasing. However, there are still many challenges for achieving a continuous increase in the number of qubits. One of the crucial requirements is to reduce the size of electrical instruments for controlling qubits [1]. In general, quantum computers require separate electronic circuits for a single-qubit gate and a two-qubit gate. This has increased the scale of electronic circuits. If two kinds of control signals for a single-qubit gate and two-qubit gate can be output from one electronic apparatus, the scale of the electronic apparatus is reduced. The control signals for the two-qubit gate are frequency-multiplexed microwave signals. By applying the wideband DA converter to electrical instruments, the band of the control signal can be output from one electrical instrument. However, since the amplitude difference between the single-qubit gate and the two-qubit gate is large, it is difficult to adjust the amplitude difference only by electrical instruments.
In this study, we design an on-chip filter that can suppress the amplitude of the microwave for a single-qubit gate and transmit that for a two-qubit gate in order to compensate for the difference in amplitudes required for the single-qubit gate and two-qubit gate. The difference in amplitude guaranteed by the filter must be at least 20 dB. To create a high-pass filter with 20 dB attenuation, we used a technique to make both bandpass and notch with a single resonator [3]. In oursample, an input microwave signal for controlling the qubit is transferred from the back of the chip. To realize a compact on-chip filter, the filter was designed on the backside of the chip with a single resonator. Simulations using the circuit model show a difference in amplitude of more than 20 dB between the single-qubit and two-qubit gate, and a bandwidth of 100 MHz on the single-qubit gate side and 500 MHz on the two-qubit side with relatively flat characteristics. We will report on the performance of the on-chip filter using the electromagnetic field simulator COMSOL.

[1] Almudever, Carmen G., et al. "The engineering challenges in quantum computing." Design, Automation & Test in Europe Conference & Exhibition (DATE), 2017. IEEE, 2017.
[2] J.M. Chow, et al. "A simple all-microwave entangling gate for fixed-frequency superconducting qubits“ Physical Review. Lett. 107, 080502 (2011).
[3] Bronn, Nicholas T., et al. "Reducing spontaneous emission in circuit quantum electrodynamics by a combined readout/filter technique." IEEE Transactions on Applied Superconductivity 25.5 (2015): 1-10.

Keywords: Quantum Computer, Cross Resonance, Filter