Recent Advances Two Methods to Simultaneously Improve Readout of Silicon Spin Quantum Bits

Quantum computers require a readout method that can discriminate the state of their computational units (i.e., quantum bits), and an ideal method should have three advantages: 1) the ability to read information quickly, much faster than a quantum bit losing its quantum state; 2) high fidelity readout information, which should be above 99% fidelity; and 3) readout sensors should be easily integrated to improve computer performance.

Recently, two research teams from the University of Cambridge, UK, and the University of New South Wales, Australia, published separate papers independently demonstrating their work on improving the readout performance of quantum bits in computers using silicon-based electron spins as quantum bits.

Figure 1 Cambridge (left) New South Wales (right)

UK team: using a "single electron box"

The three key metrics for readout methods in quantum processors are measurement speed, fidelity and footprint. Fast and high fidelity enables mid-range measurements, a necessary feature for many dynamic algorithms and quantum error correction, while a small footprint helps to design scalable, highly linked architectures that can improve computational performance.

Professor G.A. Oakes and his team from the University of Cambridge have developed a compact dispersive charge sensor, the radio-frequency single-electron box (SEB), SEB). The sensor consists of a quantum dot that periodically exchanges individual electrons with a charge bank due to the oscillating electric field of an electrical resonator coupled to it. The magnitude and phase of this exchange can be used as a sensitive probe of the electrostatic environment of the SEB, in our case the state of the electron spin quantum bit in the silicon quantum dot.

The sensor not only requires fewer electrodes than conventional detectors, but also achieves state-of-the-art spin readout fidelity of 99.2% in less than 6 microseconds , on a time scale much shorter than the typical coherence time of electron spin quantum bits. In addition, they have developed a detailed model highlighting the most important technical parameters of the SEB.

Link to the paper：

https://journals.aps.org/prx/abstract/10.1103/PhysRevX.13.011023

Australian team: designing door-based sensors

Semiconductor spin quantum bit devices are among the candidates for large-scale quantum computers, and they use materials and nanofabrication processes that have been very well developed in the computer chip industry over the decades.

The nanoscale size of semiconductor quantum bits (or quantum bits) promises to package the millions of quantum bits that will eventually be needed onto a compact chip. However, such a small quantum bit size poses a double challenge, as the circuitry required for quantum bit control and readout must also be nanoscale and must be densely packed on the chip to become a fully functional quantum processor.

Professor M.R. Hogg and his team from the University of New South Wales have designed an atomic-scale, gate-based sensor that is capable of reading out significantly more than one quantum bit of data. They successfully demonstrated the readout of three quantum bits with 95% fidelity and showed that it is feasible to read 15 quantum bits using the sensor they designed, compared to alternative methods with similar sensor performance that can read only 3-4 quantum bits.

Professor M.R. Hogg said their work shows promise for the development of quantum bit readouts in semiconductor quantum processors, where the on-chip density of compact sensors has the potential to be reduced by another order of magnitude.

Link to original article：

PRX Quantum 4, 010319 (2023) - Single-Shot Readout of Multiple Donor Electron Spins with a Gate-Based Sensor (aps.org)

Reference link：

https://journals.aps.org/prxquantum/abstract/10.1103/PRXQuantum.4.010319