Towards metrology for and with superconducting qubits

Superconducting circuits are one of the leading technologies for the realization of quantum computers. However, achieving practical, fault-tolerant quantum computers requires solutions to many technological challenges. Metrology for qubits and their peripherical devices could accelerate this development and help gaining more confidence in scientific achievements. The Quantum Electronics Department of PTB is – together with national and international partners – actively pursuing this metrological support and this poster will highlight some of the corresponding activities at PTB:
(i) Concepts of voltage metrology, such as variants of the Josephson arbitrary waveform synthesizer (JAWS), are adopted at PTB for the control of superconducting qubits in order to realize quantum accurate synthesizers with a minimum of amplitude and phase noise. First waveforms at GHz frequencies, that are generated from a JAWS circuit, have been realized. Such devices could help improving qubit operation.
(ii) Parametric amplifiers, that do not contain dissipative elements, enable amplification at the quantum limit of added noise. These devices are ideally suited to amplify read out signals of superconducting qubits. PTB is working on travelling-wave parametric amplifiers (TWPAs) and resonant amplifiers. The latest devices of PTB show 20 dB gain over a bandwidth of 4 GHz and, thus, are extremely promising for qubit readout.
(iii) Using the properties of a two-level system, superconducting qubits can be employed as quantum-accurate and extremely sensitive power sensors at cryogenic temperatures. So far PTB has fabricated Transmon qubits and employed qubit spectroscopy and the AC Stark shift for the demonstration of power sensing. The ultimate goal will be to have dedicated circuit elements, that can be placed everywhere in a superconducting circuit, enabling quantum-accurate power sensing at this position.
(iv) Opto-electronic techniques based on femtosecond lasers allow for quasi non-invasive measurements of high-frequency signals with an unprecedented bandwidth. Based upon these techniques, PTB has constructed a cryogenic sampling oscilloscope with a theoretical bandwidth exceeding 1 THz. This oscilloscope has been used for the characterization of ultrafast photodiodes at 4 K and will be further enhanced for in-situ waveform measurements for the optimization of superconducting circuits.
The poster will summarize these topics, showing the latest results and future prospects.