Mitigating leakage errors due to cavity modes in a superconducting quantum computer

Abstract

A practical quantum computer requires quantum bit(qubit) operations with low error probabilities in extensible architectures. We study a packaging method that makes it possible to address hundreds of superconducting qubits by means of coaxial Pogo pins. A qubit chip is housed in a superconducting box, where both box and chip dimensions lead to unwanted modes that can interfere with qubit operations. We analyze these interference effects in the context of qubit coherent leakage and qubit decoherence induced by damped modes. We propose two methods, half-wave fencing and antinode pinning, to mitigate the resulting errors by detuning the resonance frequency of the modes from the qubit frequency. We perform electromagnetic field simulations indicating that the resonance frequency of the modes increases with the number of installed pins and can be engineered to be significantly higher than the highest qubit frequency. We estimate that the error probabilities and decoherence rates due to suitably shifted modes in realistic scenarios can be up to two orders of magnitude lower than the state-of-the-art superconducting qubit error and decoherence rates. Our methods can be extended to different types of packages that do not rely on Pogo pins. Conductive bump bonds, for example, can serve the same purpose in qubit architectures based on flip chip technology. Metalized vias, instead, can be used to mitigate modes due to the increasing size of the dielectric substrate on which qubit arrays are patterned.

A quantum engineer's guide to superconducting qubits

“…While dilution refrigerators are now readily available, off-the-shelf commercial products, the details of how to optimally do signal-routing and rapid data processing in a scalable fashion, is also a field in rapid development. However, with the recent demonstrations of enabling technologies such as 3D integration, packages for multi-layered devices and superconducting interconnects [407][408][409][410][411][412][413] , some of the immediate concerns for how to scale the number of qubits in the superconducting modality, have been addressed. On the control software side, there currently exist multiple commercial and free software packages for interfacing with quantum hardware, such as QCoDeS 414 , the related py-CQED 415 , qKIT 416 and Labber 417 .…”

Modeling Enclosures for Large-Scale Superconducting Quantum Circuits

“…The plasma model, in particular, predicts the fundamental enclosure frequency and the rate of cross-talk decay to be simple functions of the shunt radius and spacing [Eqs. (6) and ( 18)], providing a tool for the design of the shunt array. The predictions agree well with a FE simulation of a realistic device.…”

“…For r/a < 0.1, we attribute the difference to the vacuum regions in the simulation model introduced by the drive lines, which are not included in Eq. (6). Removal of the qubit pads results in small (< 1%) changes to simulated cavity-mode frequencies.…”

“…However, it also makes superconducting qubits prone to couple to spurious electromagnetic (EM) modes in their environment. This can cause deleterious effects such as radiative energy relaxation [5], coherent leakage of the qubit state [6], and mediation of undesired interqubit couplings [7]. It is therefore important to engineer their environment such that couplings to spurious EM modes are suppressed.…”

Resonant Coupling Parameter Estimation with Superconducting Qubits

“…Despite these successes, the high-fidelity operation of medium-and large-scale quantum computers is accompanied by the daunting task of calibrating numerous physical qubits. In particular, calibrating tunable qubits requires the estimation of resonant interaction parameters such as the resonance frequency and coupling coefficient between pairs of interacting qubits [27,28], a qubit and a resonator [18], two-level state (TLS) defects [29][30][31], or substrate and box modes [32]. These calibrations are necessary to implement two-qubit gates and avoid loss of quantum information due to spurious interactions leading to coherent or incoherent errors.…”