Noise in quantum // get rid of or adapt?
Researchers have found a way to use (adapt) noise to process quantum information
“We suggest a completely different approach. Instead of getting rid of noise, we use continuous real-time noise surveillance and adapt the system as changes in the environment happen,” says Ph.D. Researcher at NBI Fabrizio Berritta, lead author on the study.
Optimal control of qubits requires the ability to adapt continuously to their ever-changing environment. We demonstrate a real-time control protocol for a two-electron singlet-triplet qubit with two fluctuating Hamiltonian parameters. Our approach leverages single-shot readout classification and dynamic waveform generation, allowing full Hamiltonian estimation to dynamically stabilize and optimize the qubit performance. Powered by a field-programmable gate array (FPGA), the quantum control electronics estimates the Overhauser field gradient between the two electrons in real time, enabling controlled Overhauser-driven spin rotations and thus bypassing the need for micromagnets or nuclear polarization protocols. It also estimates the exchange interaction between the two electrons and adjusts their detuning, resulting in extended coherence of Hadamard rotations when correcting for fluctuations of both qubit axes. Our study highlights the role of feedback in enhancing the performance and stability of quantum devices affected by quasistatic noise.
This paper demonstrates real-time control and stabilization of a singlet-triplet (ST0) spin qubit in a GaAs double quantum dot system. The key aspects are:
1. The qubit has two fluctuating Hamiltonian parameters — the Overhauser field gradient (ΔBz) between the two electrons and the exchange interaction (J) between them. These fluctuations lead to dephasing and loss of coherence.
2. An FPGA-based control system performs rapid, repeated Bayesian estimation of ΔBz and J by measuring the qubit frequency at different operating points (detunings).
3. Using the real-time estimates of ΔBz and J, the control pulses are adaptively adjusted to counteract the fluctuations and perform controlled coherent rotations of the qubit around the two axes.
4. This enables extended coherence of Overhauser-driven rotations without needing micromagnets or nuclear polarization protocols.
5. The real-time two-axis control allowed stabilized execution of the Hadamard gate by dynamically tuning the detuning such that J = |ΔBz|, enhancing the quality factor.
6. The feedback protocols harness the uncontrolled environmental fluctuations to stabilize and optimize qubit performance, representing a versatile technique for coherent spin qubit control.
“The next step for us will be to apply our protocol to systems of different materials and with more than one qubit,” says Berritta. “I cannot say when we will see the first truly useful quantum computer. Maybe 10 years from now.
The key points from the article are:
- Researchers at the Niels Bohr Institute and University of Copenhagen have developed a new method that uses noise to improve the performance of qubits in quantum computers by 700%.
- The method involves continuous real-time noise surveillance and adapting to changes in the environment using FPGA technology and machine learning for faster analysis.
- It was applied to a singlet-triplet spin qubit made of gallium arsenide in a double quantum dot, which is sensitive to magnetic and electric noise.
- The study was a collaboration between multiple research groups from different universities and companies, funded by the EU.
- The researchers claim this new protocol represents a significant advance towards practical quantum computers, but more steps need to be taken.
- The plan is to further implement the protocol across multiple qubits and materials, with the goal of eliminating noise and constructing superior qubits.
- While the completion of the first practical quantum computer is uncertain, the researchers believe their noise-adaptation strategy is promising and could be relevant to other qubit types.
