Faster, lower-hardware quantum bit reading method advances quantum technology

Quantum computers hold the promise of solving problems that resist today’s machines by using qubits—systems that can occupy multiple states at once. Yet this advantage only becomes real if qubit readout is both fast and trustworthy. In practice, measurement is still a major bottleneck: the act of checking a qubit can disturb it, limiting accuracy and slowing down computations.

Many leading quantum processors rely on superconducting circuits cooled to near absolute zero. In these devices, qubits are typically measured by coupling them to microwave resonators. But the conventional connection often uses a capacitor between the qubit and its resonator, and that same capacitive link can partially mix the two systems. This mixing can increase the risk of information loss or unintended state changes during measurement.

A team led by Pasquale Scarlino at EPFL has now demonstrated an alternative readout architecture that targets these limitations while using fewer added components. Working with Alexander Blais’s group at the University of Sherbrooke, the researchers reported results in PRX Quantum, presenting a strategy designed for transmon qubits—superconducting qubits engineered to reduce sensitivity to charge noise.

Rather than relying solely on a capacitive coupling, the new design adds a Josephson junction alongside the capacitor. A Josephson junction is formed by sandwiching a thin non-superconducting barrier between two superconductors, enabling quantum tunneling of current. This produces a nonlinear element that reshapes how the qubit interacts with the readout resonator.

Crucially, the Josephson junction introduces an interaction that provides intrinsic protection against one major measurement-induced mechanism: effective Purcell-related decay. By altering the coupling so that the qubit-resonator dynamics become less perturbative, the architecture allows stronger measurement signals without sacrificing the qubit’s state.

In experiments, the team identified the qubit state with 99.4% fidelity using only 68 nanoseconds of integration time. They also achieved a quantum non-demolition fidelity of 98.4%, meaning the measurement almost always leaves the qubit unchanged. The results closely matched theoretical predictions developed in collaboration with the Sherbrooke group.

Beyond performance, the approach simplifies hardware. It removes the need for Purcell filters and near-quantum-limited amplifiers that are common in state-of-the-art superconducting readout chains. Fewer elements can mean easier fabrication, less calibration overhead, and a more compact system that still supports multiplexed readout.

Finally, the researchers note the design can be adapted for more conventional linear readout schemes by tuning resonator properties, setting the stage for future generations. If this architecture scales, it could make qubit measurement faster, cleaner, and more practical for large quantum processors.