Quantum computers are expected to take on highly complex tasks in the near future. However, with superconducting quantum computers, it has been difficult to read the results of an experiment. This is because the measurements can lead to disruptive quantum transitions. Researchers at the Karlsruhe Institute of Technology (KIT) and the Université de Sherbrooke in Québec have now deepened their understanding of these processes through experiments and shown that calibrating the charge on the qubits helps to prevent errors. The findings have now been published in the journal Physical Review Letters.

Qubits from transmons

Quantum computers undoubtedly have great potential to meet the challenges of the future. These include, for example, the development of new materials with precisely defined properties. Quantum processors work with qubits, which can assume not only the states zero or one, but both at the same time. In addition, qubits can be entangled with each other. This enables previously unknown computing power. This makes quantum computers particularly well suited for highly complex tasks such as cryptography or simulations for the natural sciences and engineering. Qubits can be built from transmons, among other things. Transmons are artificial atoms consisting of tiny circuits. They are superconducting, which means they have no electrical resistance at low temperatures. Transmons are currently the most stable superconducting qubits. They are easy to manufacture and control.

Qubits can break out into undesirable states during measurements

However, when scaling quantum computers based on superconducting qubits, especially transmons, it has been difficult to reliably read out the results of an experiment without disturbing the quantum state. During readout, many microwave photons are sent into a resonator. This can cause the qubit to jump to higher energy states. This process, which can be compared to the ionization of an atom under strong light, makes the measurement unreliable. “If we understand at which photon numbers in the resonator and at which charge on the transmon the qubit breaks out into undesirable states, we can optimize the measurement procedure, for example, by specifically selecting the operating parameters or stabilizing the charge,” explains Professor Ioan M. Pop, who heads quantum computing research at the Institute for Quantum Materials and Technologies (IQMT) at KIT.

Practical strategies for more reliable quantum readout

Researchers at the IQMT and the Institute of Physics (PHI) at KIT, as well as at the Université de Sherbrooke in Québec, Canada, have now conducted a joint study to deepen their understanding of measurement backaction in superconducting qubits through experiments and to develop practical strategies for more reliable quantum readout. “A major difficulty in studying measurement-induced quantum transitions is the presence of charge fluctuations in the circuit, a ubiquitous problem for all solid-state platforms,” explains Dr. Mathieu Féchant, who conducts research on quantum computing at the IQMT. “In this work, we repeatedly monitor and recalibrate this parameter while varying the readout strength.”

Experimental results consistent with theoretical models

The experimental results are consistent with recently proposed theoretical models and confirm the understanding of the underlying physics. The researchers also show that active calibration of the charge on the transmons allows readout to be performed in photon number ranges where disruptive quantum transitions are reduced. In the long term, the study should help to avoid readout errors and thus make superconducting quantum computers more reliable.

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