Date of Award
Doctor of Philosophy (PhD)
Josephson junctions;Phonon downconversion;Quantum computing;Quantum error correction;Quasiparticles;Superconducting qubits
Physical Sciences and Mathematics | Physics | Quantum Physics
Quantum computers have the potential to solve critical problems that are intractable on conventional processors, with applications in a wide range of areas. The operation of a fault-tolerant quantum processor requires the use of quantum error correction, which involves encoding logical qubits across a large array of physical qubits. One promising system for implementing physical qubits is based on superconducting circuits, such as the transmon. For superconducting qubits, recent research have demonstrated that one source of decoherence comes from high-energy particles, such as gamma rays from background radioactivity and cosmic ray muons. When these particles hit the device substrate, they produce a burst of energetic phonons that travel throughout the chip. Upon hitting the device layer, these phonons generate dissipative excitations, which are quasiparticles, in the superconducting films that make up the qubit, thus causing an error in the qubit. Because the phonons from a single particle impact spread throughout the chip, a single impact event can cause correlated errors across the qubit array, which cannot be mitigated by conventional quantum error correction. In this thesis, we demonstrate a scheme to downconvert pair-breaking phonon energy by fabricating normal metal reservoirs on the back side of our qubit chip. We utilize voltage-biased Josephson junctions around the perimeter of our qubit chip to inject pair-breaking phonons into the substrate, allowing us to quantify our phonon downconversion efficiency. We investigate two devices, one with and one without normal metal reservoirs, that are measured in the same low-temperature environment. For the device with back-side metallization, we observe a reduction in the flux of injected pair-breaking phonons reaching the qubit by more than an order of magnitude. We also measured the quasiparticle charge parity switching rate on multiple qubits in the array, and observed a reduction in the two-fold and three-fold correlated switching rates by two orders of magnitude for the device with the normal metal reservoirs. This work thus provides a practical phonon downconversion technique that suppresses two-fold correlated errors in qubit arrays below the threshold required for running quantum error correction in the presence of background radioactivity.
Iaia, Vito, "Downconversion of Phonons to Suppress Correlated Errors in Superconducting Qubit Arrays" (2023). Dissertations - ALL. 1755.