Research On Superconducting RSFQ Circuit For Qubit Control

Quantum computing based on superconducting Josephson junction is one of the most promising candidates for the realization of quantum computer.To achieve a scalable quantum computer,we must solve the problem of large-scale integration of qubits and their control and readout circuits.Because of its high speed,lower power consumption and compatible process with superconducting qubits,the superconducting Rapid Single Flux Quantum(RSFQ)circuitry has been shown to be a very promising choice for the control and readout system of superconducting qubits.Since RSFQ is a magnetic flux-based logic,a sound choice of the qubit would be RF-SQUID,who can be easily manipulated by magnetic field.In this paper,we study the design,simulation,fabrication and characterization of RF-SQUID qubit and RSFQ circuitry.First,a self-aligned lift-off process was used to fabricate the RF-SQUID chip,which was designed as a parallel gradiometer,and a millikelvin measure system was constructed under the dilution refrigerator to characterize the RF-SQUID qubit.With the using of DC-SQUID,we characterized its basic parameters and observed resonant tunneling by preparing initial states.Then,we improved the layout of RF-SQUID by adding a shunted resistance to the DC-SQUID and changing RF-SQUID to a Compound Josephson junction(CJJ)RF-SQUID.Then,we fabricated this chip by adding a resistance layer.After fabrication,its basic parameters were measured in the dilution refrigerator around 4.2 K.By decreasing the barrier height,we can monitor the states of qubit continually and obtain the energy difference between the values of two potential minima.For the superconducting RSFQ circuit,its structure is quite complex.We used WRspice to simulate the circuit parameters and designed the layout according to the simulation parameters.At the same time,we also use InductEX to extract the inductance in the layout and optimize the layout.After that,we developed a seven-photolithography Nb based superconducting circuit fabrication process,successfully fabricated the chip and measured its basic parameters under the He3 refrigerator.Finally,we optimized the layout and added a layout with the T flip-flop(TFF)control RF-SQUID qubit circuit.In addition,using the same method as superconducting RSFQ circuits,we designed and fabricated a superconducting microwave source based on RL relaxtion oscillator.