Radio-frequency reflectometry

Dr Wafa Afzal, Archer Materials Physics Researcher and Alumni of University of Wollongong, explains how the company is performing low-temperature physics using the Radio-frequency (RF) reflectometry technique.

Quantum processors control and read out qubits. Like all advanced quantum computing architectures, the Archer ¹²CQ qubit processor chip will require high fidelity control (data input) and readout (data output) to function. Readout allows for interpretation of quantum calculations while minimising the loss of quantum information to the surrounding environment.

Archer’s qubit technology uses a unique metallic-like carbon nanomaterial. This qubit material is based on an electron spin-1/2 with long coherence times and has the potential for incorporation with existing semiconductor manufacturing processes including complementary metal-oxide-semiconductor (CMOS).

RF reflectometry measurements at Archer involve the nanoscale precision placement of particles embedded within quantum electronic devices. These devices are then incorporated within a specialised circuit and RF reflectometry is used as a measurement tool. The measurements require state of the art cryogenic systems. The low temperatures provide the desired higher frequencies to the devices to measure the magnitude and phase of a reflected radio signal using programmable interfacing.

Low-temperature measurements are required for low-noise, high sensitivity characterisation of the qubit material. RF reflectometry is a measurement method used by many experimental physicists working in the field of quantum computing. It is used for electron charge sensing and for electron spin readout. The technique involves sending a radio frequency signal along a circuit transmission line and then analysing the reflected signal to gather information about impedance changes that can then be used as part of the quantum state readout process.

The radio frequency signal is sent through a tank circuit which interacts with the electron’s charge or spin state in the qubit material. The changes in the reflected signal correspond to the quantum state of the qubit.

The high resistance, typical of quantum devices, used in RF reflectometry is converted to the characteristic impedance of the transmission line by a matching network combining the impedances of an inductor and a capacitor. Impedance matching of the resonator to the input line is important to ensure maximal power transfer from the RF signal to the device. This allows for the detection of subtle changes in the electronic or spin states.

RF reflectometry is usually implemented using high-frequency lock-in techniques and is rarely prone to inverse-frequency and other types of low-frequency noise. This provides a useful measurement bandwidth and measurement speed, that could lead to an ability to address qubits faster than the decoherence time.

RF reflectometry is known to impose minimal disturbance on a spin qubit’s quantum state during measurement, which may assist in preserving the long electron spin coherence times of the unique carbon-based qubit material.

Archer works with tertiary institutes across Australia and internationally to perform the low temperature RF reflectometry measurements. These institutes provide the Archer team access to infrastructure, facilities, and equipment, including dilution refrigerators that can cool quantum electronic devices to a few millikelvin required for RF reflectometry.

To learn more about the technical aspects of RF reflectometry: https://doi.org/10.1063/5.0088229
An example of how RF reflectometry used in carbon-based quantum devices: https://arxiv.org/abs/2007.03588