2020 Fiscal Year Annual Research Report
Microwave quantum optics with tunable cavity resonators and superconducting qubits
| Project/Area Number |
20F20757
|
|---|
| Research Institution | The University of Tokyo |
|---|
Principal Investigator |
中村 泰信 東京大学, 先端科学技術研究センター, 教授 (90524083)
|
|---|
| Co-Investigator(Kenkyū-buntansha) |
CHANG CHUNG WAI SANDBO 東京大学, 先端科学技術研究センター, 外国人特別研究員
|
|---|
| Project Period (FY) |
2020-07-29 – 2023-03-31
|
| Keywords | multi-qubit readout / quantum state tomography / parametric amplifier / parametric process / quantum computing |
|---|
| Outline of Annual Research Achievements |
We focused on designing new signal measurement tools based on parametric processes. We realized conventional resonator-based approach was a limiting factor for applications in a large-scale superconducting quantum computing system. Thus, we extended our implementation to a more general platform with artificial waveguides, where one well-known example being the Josephson traveling-wave parametric amplifier (JTWPA).
We designed our JTWPA with focus on reduced loss and fabrication complexity, the major obstacles of current implementations. Using EM simulations and S-parameters calculations, we created a new waveguide layout utilizing co-planar lumped-elements. We modelled the 2D layout as a 1D chain of circuit elements to extract the unit cell parameters, followed by creating a model of dispersion-engineered waveguide and its corresponding 2D layout, finally verifying its transmission and bandgap properties. From the waveguide layout, we used spice simulator to construct a numerical model with Josephson junctions included for wide-band parametric process, with the initial focus on amplification. We obtained simulation results consistent with existing JTWPA devices, as well as new designs potentially providing higher gain and bandwidth.
Further, we fabricated prototypes of our JTWPA designs based on standard photolithography and liftoff recipes. We performed preliminary measurement on the prototypes and confirmed the basic functionality as a nonlinear waveguide by observing pump power dependent transmission over wide bandwidth and the bandgap at the designated frequency.
|
| Current Status of Research Progress |
Current Status of Research Progress
3: Progress in research has been slightly delayed.
Reason
We have completed the initial study, design and simulation, now moving onto the fabrication of the device. From the initial measurement, results suggested the device behaved as a nonlinear artificial waveguide which resembled that of a JTWPA. However, our prototype exhibited only a limited gain of 5 dB, significantly below that of the predicted 20 dB or higher gain from simulations. This non-ideal behavior of the current prototypes can be attributed to inhomogeneity of Josephson junctions in terms of their critical current in the prototype device, which is known to cause deviation from ideal gain.
To allow a simplified device fabrication procedure, we employed two liftoff photolithography processes to create the circuit layer and junction layer with aluminum. However, the typical liftoff recipe involves a chemical (TMAH) which unavoidably attack aluminum thin film, introducing uncertainty in junction size when patterning for the junction layer where the circuit layer aluminum already existed and may be attacked. This was later realized from junction resistance measurement and the initial device cooldown.
To mitigate the problem, we were developing alternative liftoff photolithography recipes without the use of the aluminum-attacking chemicals, however the alternative developer/resist combinations tested so far are not fully compatible, the development has thus taken longer time than expected. These have contributed to the delay in realizing a fully functional JTWPA.
|
| Strategy for Future Research Activity |
In FY2021, we will work on optimizing the fabrication which focuses on the homogeneity in Josephson junctions. This will be conducted by two possible approaches: 1. Continue to develop alternative liftoff method with photolithography. This is currently in progress and new recipes utilizing only compatible resist/developers are being tested. 2. If we conclude that liftoff photolithography is impractical for our process, we will move on to use ebeam-lithography to create the junction layer pattern. It is the more conventional approach in Josephson junction fabrications with higher success rate, however it could take more time for making a JTWPA device which typically requires thousands of junctios. The focus will shift to first realizing a fully function JTWPA device, followed by optimization of fabrication time.
After the JTWPA device has reached the expected performance, we will move on to the device characterization based on two types of measurements: 1. Multi-qubit readout experiment. We will evaluate the JTWPA’s performance on both bandwidth and noise by using it as the first stage amplifier in a simultaneous multi-qubit readout. 2. State tomography experiment with the JTWPA. By operating the JTWPA in a three-wave mixing operation, we will evaluate the capability of the device for characterizing other quantum states such as squeeze states and qubit signals.
On finishing the above two experiments and evaluating the device performance, other parametric process such as coherent signal coupling will be studied based on a slightly modified JTWPA device.
|
Research Products
(4 results)
