Quantum computers will be able to assume highly complex tasks in the future. With superconducting quantum processors, however, it has thus far been difficult to read out experimental results because measurements can cause interfering quantum state transitions. Researchers at Karlsruhe Institute of Technology (KIT) and Université de Sherbrooke in Québec have performed experiments that improve our understanding of these processes and have shown that calibrating the charge at the qubits contributes to fault avoidance. Their findings have been published in Physical Review Letters. (DOI: 10.1103/yljv-b4kj[SW1)
Quantum computers have great potential for meeting the challenges of the future. This includes, for example, the development of new materials with exactly defined properties. Quantum processors use qubits that can assume not only the states 0 or 1, but both at the same time. Moreover, qubits can be entangled. These properties make previously inconceivable computing performance possible; quantum computers will be particularly effective at highly complex tasks, such as cryptography or simulations in natural and engineering sciences. Qubits can be made from transmons, artificial atoms that consist of tiny circuits. They are superconducting, meaning they have no electrical resistance at low temperatures. Transmons are currently the most stable superconducting qubits. They are easy to manufacture and control.
During Measurements, Qubits Can Jump to Undesired States
Scaling quantum computers based on superconducting qubits, particularly transmons, has proven difficult so far in terms of achieving reliable readout fidelity of experimental results without affecting the quantum state. During the readout process, many microwave photons are sent into a resonator. This can cause the qubit to jump into higher energy states. This effect, which can be compared to the ionization of an atom under intense light, makes the measurement unreliable. “If we understand at which photon number in the resonator and at which charge level at the transmon the qubit jumps to undesired states, we can optimize the measuring procedure, for example with a purposeful choice of the operating parameters or by charge stabilization,” said Professor Ioan M. Pop, who heads research on quantum computing at KIT’s Institute for Quantum Materials and Technologies (IQMT).
Researchers at the IQMT, KIT’s Physikalisches Institut (PHI), and the Université de Sherbrooke in Québec, Canada, recently conducted a joint study in which they improved understanding of the measurement-induced transitions in superconducting qubits by means of experiments and in elaborating practical strategies for more reliable quantum readouts. “A key difficulty in the investigation of quantum transitions caused by measurements is the presence of charge fluctuations in the circuit, a ubiquitous problem for all solid-state platforms,” said Dr. Mathieu Féchant, a quantum computing researcher at the IQMT. “In our work, we monitor this parameter and recalibrate it repeatedly while varying the readout level.”
Experimental Results in Line with Theoretical Models
The results of the experiments are in line with recently proposed theoretical models and confirm the understanding of the underlying physics. The researchers also showed that actively calibrating the charge at the transmons allows them to take the readouts to photon number ranges in which the interfering quantum transitions are reduced. In the long term, the study contributes to the avoidance of readout faults, helping to make superconducting quantum computers more reliable.
Original publication:
Mathieu Féchant, Marie Frédérique Dumas, Denis Bénâtre, Nicolas Gosling, Philipp Lenhard, Martin Spiecker, Simon Geisert, Sören Ihssen, Wolfgang Wernsdorfer, Benjamin D’Anjou, Alexandre Blais, and Ioan M. Pop: Offset Charge Dependence of Measurement-Induced Transitions in Transmons. Physical Review Letters, 2025. DOI: 10.1103/yljv-b4kj
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