Study And Optimization Of Working Environment For Transmon Qubits

I have been studying the optimization method of working environment for transmon qubit since I went in the Solid-State Quantum Computation Group.At first,we know little about transmon since its new in our lab.However,thanks to the rich experience we gained in semiconductor quantum dot measurements during these years,I learned a lot while facing more challenges.After five years' learning and experiments,I have built up a whole fabrication system and measurement system for the transmon qubit.I spent most of my time in designing and optimizing the working environment,especially the low temperature part.We have successfully improved the average qubit decay time from 0.6 ?s to 5 ?s,and we are still improving it.Transmon qubit system is built by aluminum superconducting circuits.We have to cool it down to extremely low temperature,typically 30 mK,which is far low than aluminum's critical temperature.Even in such environment,noises are large and fatal to the qubit performance.We find that the noises come from the working environment and the measurement method.It is hard to eliminate all the noises,but it's worthing doing so.As a result,we improved the coherence time of transmon qubit by 10 times,and we learned quite a lot about qubit measurement methods.This thesis mainly includes the following parts:1.We briefly introduced the development of quantum computing and three qubit systems.We noticed that the quality of quantum chip not only relied on the qubit system and fabrication quality,but also was restricted by external environment fluctuations.It is effective to improve qubit coherence times by optimizing its working environment.2.We introduced the cavity QED system and the circuit QED system.In dispersive-strong coupling region,the circuit QED system has important applications in quantum information processing.We analyzed the interaction between transmon qubit and the CPW,and we presented the main noise sources affecting transmon qubit's lifetime.We also presented the quantum chip design and design thoughts.3.We studied the controlling method of transmon.and we obtained its operation speed.My first work was building a measurement system for transmon,and we acquired qubits with 0.6 ?s coherence times.4.Since the ratio of coherence time to operation time determines the upper bound of operation fidelity,it is critical to improve qubit's coherence time.My second work was optimizing transmon's low temperature working environment.Multi-layer shields were introduced,which are effective to protect the qubit from magnetic and infrared radiation noises.Besides,we introduced the Purcell filter and the J?Amp.By doing so,we successfully improved averaged qubit T1 to 5 ?s.5.J-Amp can amplify single microwave photon signal efficiently,while adding noise as low as quantum fluctuation.My third work was studying J-Amp design and measurements.Based on J-IMPA type,we acquired 20 dB gain with average noise temperature below 400 mK in different operation modes.6.Further,we considered the affection of qubit package on extending quantum chip.Wire-bonding would never useful,since it would cause severe contact impedance mutation and signal crosstalk,which resulted in coherence time decreasing.My fourth work was design a new three-dimentional qubit package method based on pogo-pin bonding technique.It linked the signal through the third dimention,and it solved the drawbacks of wire-bonding.Besides,this design could suppress signal leackage through the chip surface,which is affective in improving qubit coherence times.7.Quantum chip measurement relies on electronics and instrument set up.I designed a measurement system to replace commercial instruments.With basic extensible ability,we improved the process efficiency by constructing a hardware controlling unit using FPGA.Latancy in less than 200 ns can be achieved,which not only solves the realtime signal generation and processing,but also supports us to realize realtime quantum error correcting in the future.8.Finally,we presented methods for optimization controlling signals.We investigated the impact of LO phase noise on qubit dephasing,and we presented a solution to reduce noise-induced operation errors.Finally,I concIuded a measurement procedure,which can help us optimizing the controlling parameters.The main innocations of this thesis are:1.We designed two Purcell filters to shield the qubit from decaying through the readout cavity.They gave external suppression of 20 dB in qubit frequency range,while had no influence on qubit readout signal.2.For the first time,we investigated J-IMPA under flux-pump mode,we found out that the impedance converter can still work,and we also observed traditional J-Amp modes.We acquire J-IMPA with 20 dB gain and noise temperature below 400 mK.Under extreme operation parameters,we obtained gain up to 29 dB and gain-bandwidth up to 1.2 GHz.We found that the J-Amp could offer single shot readout abitlity with at least 87%readout fidelity.3.We improved a three-dimensional qubit packaging with pogo-pin bonding technogy.It enalbed us with higher quality bonding contact and method to expand the qubit number without increasing chip size.Besides,this packaging improved the noise isolation from the qubit environment,and also decreased signal leakage on chip.4.For the first time,we regarded the qubit measurement system as an independent research project.We gave detailed design for a 40-channel system,which can be further extended.Particually,we offered a solution which can achieve real-time feedback control with hardware latancy in less than 200ns.5.We investigated the relationship between the LO phase noise of qubit control&measurement system and the qubit dephasing time.We gave a set of design parameters which can reduce the system error rate under 0.034%.