Two superconducting qubits on a single chip sound as clean as soloists in an orchestra. Each is a transmon, a qubit with reduced sensitivity to charge noise. They are connected through a tunable coupler, an auxiliary element that adjusts the interaction strength between them. A team from IQM Quantum Computers made this duo play without a single false note. The average fidelity of single-qubit gates reached 99.98%, the two-qubit CZ gate reached 99.93%, and readout reached 99.94%. Quantum error correction requires the whole set: fast single-qubit gates, precise two-qubit gates, and readout that does not destroy the quantum state. Here, this set is assembled on a single device for the first time.

The main challenge is leakage. This is the transition of the qubit beyond the computational subspace, into the state |02⟩. Leakage is like a musician who, in the middle of a piece, starts playing the wrong note and throws off the entire ensemble. Standard calibration methods poorly isolate this error from others. The new PALEA protocol acts like a conductor with perfect pitch: after each gate, it adds a layer of dynamical decoupling and averages the signal over random phases. This amplifies only the target leakage error, allowing the gate to be tuned with jewel-like precision. As a result, leakage is reduced by at least half compared to the standard error amplification method.

Optimizing the qubit–coupler coupling strength was another key. First, they numerically simulated gate errors as a function of coupling strength and found an optimum around 75 MHz. Then they fabricated a chip with an actual coupling strength of 66 MHz. This balance allowed simultaneous minimization of single-qubit and two-qubit operation errors. Readout was optimized using shelving, transferring the excited state to a higher level before measurement to prolong its lifetime. This technique improves fidelity but slightly increases leakage. Finding a compromise between these requirements remains an open problem.

Before this work, it was unclear whether a single superconducting chip could simultaneously achieve fidelities above 99.9% for single-qubit gates, two-qubit gates, and readout. Optimizing one operation usually degrades others. Now it has been shown that careful selection of coupling strength and the new PALEA calibration protocol allow all three fidelities to exceed 99.9% on a single device. The method can be immediately used in laboratories working with superconducting qubits to speed up calibration and improve gate fidelity as systems scale up.

The quantum orchestra is preparing for bigger stages. The next step could be transferring this architecture to a two-dimensional qubit lattice with four connections per qubit. Improving coherence times would allow further reduction of the optimal coupling strength and reduce crosstalk. For readout, shelving to the third excited state could be used to further enhance fidelity. When every instrument sounds clean, an ensemble of thousands of qubits will be able to perform the most complex quantum algorithms without a false note.