Physicists have brought silicon quantum processors closer to one of the main goals of the industry: they learned to manage five qubits at the same time, without allowing a sharp increase in the number of errors. In parallel work, the accuracy of single-qubit operations was maintained near the practical milestone of 99.9%, which is often considered as a reference point for systems with quantum error correction. The results of the experiment are published in Nature Physics.
The team at RIKEN, National University of Taiwan, ETH Zurich and Delft Technical University used five spin qubits in quantum dots from isotope purified silicon-28 and silicon-germanium. With separate control, the accuracy of elementary operations X and Y exceeded 99.99% for each qubit, and the best results approached 99.999%. The simultaneous operation of two or three qubits did not worsen the indicators: accuracy also remained above 99.99%.
The main problem arose when trying to control several qubits through one common microwave line. The signal, designed for a single array element, slightly shifted the resonant frequencies of neighboring qubits due to the variable effect of Stark. Small shifts gradually turned into phase errors and reduced the accuracy of calculations. In previous experiments on similar silicon devices, parallel control of only two qubits could reduce accuracy from about 99.95% to 99.5%.
The researchers compensated for the interference with the help of pairwise phase calibration. The system measures the effect of each controlled qubit on the neighboring one, after which it adjusts the microwave pulses. The approach does not require separately to customize all possible combinations of qubits, so the number of necessary calibrations grows much slower when the array increases. Additional suppression of cross-interference provided specially formed pulses with the Kaiser window.
After calibration, all five qubits were able to run in parallel through one line of control. Four qubits exceeded the accuracy of 99.9%, the fifth kept at about 99.9%. The researchers associate the remaining errors mainly with the decoherence that occurs with an increase in the power of the microwave signal. Engineers believe that the refinement of the design can reduce the impact of this mechanism.
The overall control line is important not only for the sake of accuracy. If each qubit requires separate wiring and its own control channel, the complexity of the quantum chip will quickly become a problem when scaling. The authors estimate that the scheme applied at the current power constraints can cover about ten qubits, and several such blocks in the future can be combined into larger arrays.
The work does not yet mean the appearance of a ready-made failover quantum computer. The experiment primarily demonstrates the high-precision parallel operation of single-qubit valves. The accuracy of the two-qubit CZ valves used in the device remained below 97% due to the mixing of spin and valley states. For a full correction of errors, reliable two-qubit operations, preparation and reading of conditions and preservation of coherence during waiting are also necessary. The new result solves only one of the scaling problems, but shows that silicon qubits can be massively controlled through common lines without the inevitable loss of accuracy.
The team at RIKEN, National University of Taiwan, ETH Zurich and Delft Technical University used five spin qubits in quantum dots from isotope purified silicon-28 and silicon-germanium. With separate control, the accuracy of elementary operations X and Y exceeded 99.99% for each qubit, and the best results approached 99.999%. The simultaneous operation of two or three qubits did not worsen the indicators: accuracy also remained above 99.99%.
The main problem arose when trying to control several qubits through one common microwave line. The signal, designed for a single array element, slightly shifted the resonant frequencies of neighboring qubits due to the variable effect of Stark. Small shifts gradually turned into phase errors and reduced the accuracy of calculations. In previous experiments on similar silicon devices, parallel control of only two qubits could reduce accuracy from about 99.95% to 99.5%.
The researchers compensated for the interference with the help of pairwise phase calibration. The system measures the effect of each controlled qubit on the neighboring one, after which it adjusts the microwave pulses. The approach does not require separately to customize all possible combinations of qubits, so the number of necessary calibrations grows much slower when the array increases. Additional suppression of cross-interference provided specially formed pulses with the Kaiser window.
After calibration, all five qubits were able to run in parallel through one line of control. Four qubits exceeded the accuracy of 99.9%, the fifth kept at about 99.9%. The researchers associate the remaining errors mainly with the decoherence that occurs with an increase in the power of the microwave signal. Engineers believe that the refinement of the design can reduce the impact of this mechanism.
The overall control line is important not only for the sake of accuracy. If each qubit requires separate wiring and its own control channel, the complexity of the quantum chip will quickly become a problem when scaling. The authors estimate that the scheme applied at the current power constraints can cover about ten qubits, and several such blocks in the future can be combined into larger arrays.
The work does not yet mean the appearance of a ready-made failover quantum computer. The experiment primarily demonstrates the high-precision parallel operation of single-qubit valves. The accuracy of the two-qubit CZ valves used in the device remained below 97% due to the mixing of spin and valley states. For a full correction of errors, reliable two-qubit operations, preparation and reading of conditions and preservation of coherence during waiting are also necessary. The new result solves only one of the scaling problems, but shows that silicon qubits can be massively controlled through common lines without the inevitable loss of accuracy.
