Introducing the quantum capacitance parametric amplifier

—by Abdallah El Kass

A new sub-species of amplifier used to consolidate and robustly deliver signals inside a dilution refrigerator, the quantum capacitance parametric amplifier (QCPA), forms a building block of the quantum machine and reliably boosts the amplitude of a signal while limiting the added noise in the process.

With the QCPA, we demonstrate a new kind of low-noise amplifier that may find use in the readout chain of a quantum machine or other millikelvin experimental applications (https://arxiv.org/abs/2304.13227).  Compared to other members of the parametric amplifier family, the QCPA can handle higher power levels and is indifferent to the existence of a magnetic field—both characteristics suitable for semiconductor qubits.  Furthermore, it can be monolithically integrated on-chip with semiconductor qubits.

The achievement of the QCPA was unique as it required the intersection of several disciplines and expertise starting with the growth of the GaAs heterostructure (performed overseas by collaborators) characterised by record electron mobilities, the engineering and design of the amplifier circuit, the subsequent fabrication of the device at the USYD cleanroom, and the testing capabilities at millikelvin temperatures in a dilution fridge with signal and noise characterisation.  With invaluable advice from other members of the Quantum Nanoscience Laboratory and colleagues at USYD, I handled the design, local fabrication at the USYD cleanroom, and the testing and characterisation of the device.

The prototype QCPA may be improved on multiple frontiers, such as via the implementation of a wideband travelling-wave topology of the QCPA that targets the readout of many qubits in semiconductor platforms.

Dr Abdallah El Kass is a Research Fellow at The University of Sydney, working with Chief Investigator Prof. David Reilly.  He completed his PhD in electrical engineering on millikelvin electronics at the quantum–classical interface in 2021.  He is interested in cryogenic electronic readout interfaces to the quantum hardware and has worked on the cryogenic characterisation of SiGe bipolar transistors and its qubit readout potentials.  This work is part of EQUS’ Quantum Engines and Instruments research program.