Metrology and instrumentation for superconducting quantum circuits
In superconducting quantum information processing (SQIP) components such as qubits, Josephson junctions, micro-resonators and quantum phase slip junctions are used to create electronic circuits that can manipulate quantum information.
The strength of superconducting technology is its scalability and how it an essential platform for quantum computingbuilds on our experience in fabricating microelectronic circuits. The field has seen huge progress in the past 15 years and superconducting circuits now form and quantum sensing.
At NPL we are working on a range of projects to develop the tools and measurement techniques needed to support the development of superconducting quantum computers and circuits. We are also developing metrology and instrumentation specifically tailored to calibrate and understand these devices in detail.
Metrology of materials for superconducting quantum devices and circuits
Left: chip with superconducting resonators used to probe the properties of defects Right: Mapping the changes in device decoherence over time and frequency
Defects and impurities at interfaces and surfaces are one of the problems preventing further scaling up of superconducting quantum computers, and in general they significantly limit the performance of quantum sensors and devices. These defects, generally known as two-level systems (TLS), are not only a source of decoherence but also cause the parameters of qubits to change over time making calibration and tune-up difficult.
At NPL we have developed new measurement techniques for studying TLS which can be applied to a wide variety of circuits and will become increasingly important as complexity increases and multi-layer technology become more widely used. They include:
- direct measurements of TLS-induced noise
- microwave spectroscopy
- micro-Electron Spin Resonance (ESR) which allow us to detect the chemical fingerprint of impurities and defects.
Metrology of superconducting qubits
As the performance of superconducting qubits has increased so has the demands on the measurement methods used. As we continue to scale up quantum computers, we will need quantitative and reproducible methods for characterising qubits and circuits.
At NPL we are developing standardised methods for measuring qubits and analysing their parameters. Standardised metrics can then be used, for example, to optimise fabrication processes.
Five superconducting qubits on a chip made from aluminium
Novel instrumentation for metrology at millikelvin temperatures
Superconducting quantum circuits are operated at temperatures just above absolute zero. By using modern cryogenics we can operate at temperatures as low as 10 mK. Unfortunately, this often means that we can no longer use conventional measurement instruments to characterise circuits. NPL is therefore developing new instrumentation that can operate under the same challenging conditions as quantum circuits.
Near field scanning microwave microscope
Scanning probe techniques are very important for the development of microelectronics and are routinely used to image and study conventional microelectronic circuits. Since superconducting quantum circuits are operated at millikelvin temperatures only limited information can be collected by studying them at room temperature. In addition, the circuits are operated at high (gigahertz) frequencies so we need an instrument which can spatially map the response at these microwave frequencies.
The tip of the NSMM is scanned over the surface of a sample superconducting quantum chip
At NPL we have developed a near-field scanning microwave microscope (NSMM) that is operated at 10 mK and can simultaneously image and map the frequency response of a sample with nanometre resolution. Importantly, this allows us study ’live‘ superconducting circuits and qubits. Developing an instrument which can be used, not only to image circuits but also, to locate any defects/impurities which are detrimental will make it easier to optimise the performance of large-scale quantum circuits.
Read the article in Nature's Scientific Reports: Near-Field Scanning Microwave Microscopy in the Single Photon Regime
Absolute power quantum sensor
The power meter is very important when working with microwave electronics and it determines the level of power at a point in a circuit. Conventional power meters cannot be used with circuits cooled to millikelvin temperatures.
The APQS can be plugged into the line at a point of interest
Some measurements can be done at room temperature before the circuit is cooled down, but the microwave losses of components and cables change dramatically as they are cooled resulting in large measurement errors.
At NPL we are developing an Absolute Power Quantum Sensor (APQS). The sensor, which is based upon superconducting qubit technology, is designed to be installed next to or incorporated into the circuit of interest. The APQS is absolute, meaning it does not need to be calibrated against an external reference. It operates in a broad frequency band at low temperatures (mK) and does not disturb the microwave transmission line when not in use. This makes it ideal for system characterisation of superconducting quantum computers or microwave components at low temperatures.
Superconducting circuits for fundamental metrology
The rapid progress in fabrication and characterisation of superconducting materials and circuits means that we are now able to design, build and test novel superconducting devices in a way that would have been impossible just a few years ago.At NPL we are using our expertise and state-of-the art facilities to explore a range of new superconducting devices that one day could find use in fundamental metrology.
Coherent quantum phase slip junctions
Coherent quantum phase slip (CQPS) junctions made from disordered superconductors are being developed for use in electrical current metrology. Current standards based on CQPS devices have the potential to revolutionise the way we realise the SI ampere and would allow us to measure currents with unprecedented accuracy and precision.
This work is done within the framework of the H2020 project Quantum e-leaps
Dilution refrigerotor used to cool samples down to mK temperatures
Topological insulator nanowires
Topological insulators (TI) are a new class of materials with unique electrical transport properties. At NPL we are exploring the high frequency properties of TI nanowires and ribbons to see how these materials can be used in electrical metrology.
This work is done within the framework of the H2020 project HiTIMe
Circuit element made out of highly disordered