Recent advances in design and fabrication have increased the energy-relaxation times of isolated superconducting transmon qubits to the hundreds-of-microseconds regime, with reportedvalues exceeding 500 μs. However, the same progress has not automatically translated to multiqubit processors, where qubits are embedded in connected qubit-coupler lattices and often exhibit much shorter lifetimes than isolated qubits. To identify possible sources of this discrepancy, here we use finite-element simulation to investigate how surface participation ratios and the resulting surface dielectric loss change when a qubit is embedded in a flip-chip qubit-coupler lattice. Controlled comparisons show that higher connectivity can indeed lead to larger surface loss: in the simulated lattice, connecting a qubit to two and four couplers increases the surface loss by factors of 1.3 and 1.8, respectively. We attribute this change to the combined effects of added edge fields from coupling claws, field redistribution over the larger connected metal network, and hybridization with coupler modes. We further examine how this connectivity-induced surface-loss penalty depends on the geometric design parameters of both the qubit electrodes and the coupling claws, and derive guidelines for designing low-loss multiqubit processors.
Qubit readout and reset are critical components for the practical realization of quantum computing systems, as outlined by the DiVincenzo criteria. Here, we present a scalable architectureemploying frequency-tunable nonlinear Purcell filters designed specifically for superconducting qubits. This architecture enables flexible readout and unconditional reset functionalities. Our readout protocol dynamically adjusts the effective linewidth of the readout resonator through a tunable filter, optimizing the signal-to-noise ratio during measurement while suppressing photon noise during idle periods. Achieving a readout fidelity of 99.3% without using Josephson parametric amplifiers or traveling-wave parametric amplifiers, even with a small dispersive shift, demonstrates its effectiveness. For reset operations, our protocol utilizes the tunable coupler adjacent to the target qubit as an intermediary to channel qubit excitations into the Purcell filter, enabling rapid dissipation. We demonstrate unconditional reset of both leakage-induced |2⟩ and |1⟩ states within 200 ns (error rate ≤1%), and reset of the |1⟩ state alone in just 75 ns. Repeated reset cycles (≤600 ns) further reduce the error rate below 0.1%. Furthermore, the filter suppresses both photon noise and the Purcell effect, thereby reducing qubit decoherence. This scalable Purcell filter architecture shows exceptional performance in qubit readout, reset, and protection, marking it as a promising hardware component for advancing fault-tolerant quantum computing systems.
Efficient qubit reset and leakage reduction are essential for scalable superconducting quantum computing, particularly in the context of quantum error correction. However, such operationsoften require additional on-chip components. Here, we propose and experimentally demonstrate a mode-efficient approach to qubit reset and readout using a multi-mode Purcell filter in a superconducting quantum circuit. We exploit the inherent multi-mode structure of a coplanar waveguide resonator, using its fundamental and second-order modes for qubit reset and readout, respectively, thereby avoiding additional circuit elements. Implemented in a flip-chip architecture, our device achieves unconditional reset with residual excitation below 1% in 220 ns, and a leakage reduction unit that selectively resets the second excited state within 62 ns. Simulations predict Purcell-limited relaxation times exceeding 1 ms over an 800 MHz bandwidth. To our knowledge, this is the first experimental trial that exploits different-order modes of a microwave resonator for distinct qubit operations, representing a new direction toward scalable, mode-efficient quantum processor design.