Toward a universal silicon quantum computer SQC's new room-temperature, interference-resistant spin quantum bit readout technology

Scientists at SQC have developed a new method to make the critical readout phase of spin quantum bits faster, easier, and less susceptible to interference. ramp measurement technique for robust and high-fidelity spin quantum bit readout" [1], was published in Science Advances.

01Quantum bit readout, need to reduce environmental noise interference

The readout phase is the third key step in the problem solving process of quantum computers. It is divided into three phases.

Setup - the "initialization phase" sets the device to the exact state where it is ready to encode the problem into the computer.

Computation - the "control phase" where the quantum bits interact and computation takes place.

The result - the "readout phase" requires careful measurement of the final state of the quantum bits to determine the result or calculate the answer. The final stage needs to be fast, accurate and robust so that the results encoded in the quantum bits do not drift and make mistakes.

However, quantum bits are extremely sensitive to their environment and environmental noise can interfere with their measurements.

Traditionally, when measuring the spin state of a quantum bit, the measurement is based on whether the constant electrical signal changes. In SQC's new protocol, the electrical signal is a "ramp" and the measurement depends on when the signal changes. By using a variable voltage ramp, the readout is now robust to ambient noise without the need for time-consuming calibration.

This result is further evidence of SQC's focus and progress on error-correcting quantum computers in its technology roadmap.

SQC Roadmap

02Slope spin measurement: high fidelity readout in low field/high temperature environments

To date, three main single readout processes have been developed: energy selective measurement (ESM), time selective measurement (TSM), or blocking by a second exchange-coupled auxiliary quantum bit using bubbly spin. In all cases, the fidelity of the readout process is limited by the temperature of the system and must remain below the quantum bit spin state energy difference.ESM relies on the energetic separation of the quantum bit states, providing a state-dependent charge transition; in contrast, TSM relies on quantum bit states with different tunneling rates for a given charge transition.

In this experiment, the team demonstrated another measurement technique, ramp spin measurement (RSM), which is resilient to high temperature/low magnetic field operation; it scales better to a large number of quantum bits compared to ESM. the fundamental difference between RSM and ESM is that the tunneling rate of the electron spin state varies continuously with time during the protocol , rather than being fixed. The team experimentally demonstrated that by using RSM, we can maintain >99% spin readout fidelity within a measurement time of EZ ≈ 7kB, which is comparable in time to ESM, but the RSM technique can be used for approximately twice the temperature of ESM.

Comparison of ESM and RSM techniques. a) Schematic diagram of the electrochemical potential of ESM and the corresponding voltage pulse. b) Corresponding signal of a nearby charge sensor during ESM. During the readout phase, the spin-up state is detected as a characteristic point in the charge sensor signal. c) Schematic diagram of the electrochemical potential of RSM. The loading (yellow) and empty (blue) phases are the same as those of the ESM. In the readout phase, the electrochemical potential is a continuous ramp. d) Corresponding charge sensor signal of the RSM. A sudden change in the charge sensor signal before a certain threshold time indicates the presence of spintronics.

Comparing the minimum ratio of magnetic field to electron temperature and readout time possible for different readout methods to achieve the specified readout fidelity: RSM and optimally tuned ESM can achieve the same fidelity as practical ESM (ϵ = 0) at twice the temperature.

In the experimental setup, the team finally obtained 99.89 ± 0.02% (99.95 ± 0.01%) ground-state to charge transition visibility at B = 1.5 T ( B = 0.8 T) with an electron temperature of ~110 mK through optimized ramp rates; electron spin state initialization experiments with ramp times less than 100 μs saturated at random initialization, while ramp times greater than 10 ms saturates at initialization ∣↓>, and intermediate time scales for initialization ramps can be used to achieve arbitrary spin fractions.

Verification of slope spin initialization protocol

03Readout performance advantage, promising extension to large quantum bit systems

In this experiment, the SQC team proposed and demonstrated a readout technique for semiconductor spin quantum bits that achieves high readout fidelity in low field/high temperature environments and is robust to electrical noise. The readout protocol is a combination of energy-selective spin readout and time-dependent spin readout, and offers many practical advantages over ESM and TSM.

Finally, the article states [2], "The advantages of RSM over ESM will allow simple single, low magnetic field measurements that can be easily extended to large quantum bit systems."

Reference links：

[1]https://www.science.org/doi/10.1126/sciadv.abq0455

[2]http://sqc.com.au/2022/09/08/sqc-scientists-develop-a-better-way-to-measure-qubits/