Title: Decoherence in superconducting transmon qubits: quasiparticles and photons
Luyan Sun, Yale University
Date: 11:10 -- 12:10 Jul 2, 2012
Venue: FIT 1-222
Luyan Sun is currently a Postdoctoral Associate at Yale University, working on superconducting qubits in circuit quantum electrodynamics architecture for quantum information processing. His research interests include high-fidelity qubit readout, multiple-qubit entanglement, and quantum error correction. He has led projects investigating the decoherence mechanisms for superconducting transmon qubits: the limitation of energy relaxation time due to quasiparticles and how reducing photon shot noise might improve the phase coherence time. Luyan Sun got his B.S. degree from Zhejiang University in 2001. After his undergraduate studies, he attended the University of Maryland at College Park, USA, where he got his Ph.D. in 2008 under the supervision of Dr. Bruce Kane in the silicon-based quantum computing group. Luyan Sun’s Ph.D. research included measurements of nanostructures using a scanning force microscope at millikelvin temperatures and the ionization of single phosphorus atoms in silicon using an Al single-electron transistor (SET) to demonstrate a readout scheme for silicon-based quantum computation. His Ph.D. thesis was on the detection of a single-charge defect in a metal-oxide-semiconductor structure using self-aligned and vertically coupled Al and Si SETs.
Quantum information processing based on superconducting qubits has made tremendous progress towards realizing a practical quantum computer in the last few years. However, the coherence times of superconducting qubits still need to improve to reach the fault-tolerant threshold and to reduce the circuit complexity for error correction. One intrinsic energy relaxation channel is through quasiparticle tunneling across the qubit Josephson junction. In this talk, I will describe our method of adopting a bandgap engineering technique to study the energy relaxation of transmon qubits due to non-equilibrium quasiparticles. We set an upper bound on the energy relaxation time based on our measurement of quasiparticle tunneling dynamics in real time. This upper bound indicates that reducing the quasiparticle-induced relaxation rate will soon be necessary to achieve a qubit lifetime much longer than 100 microseconds.
In the second half of the talk, I will describe our understanding of qubit dephasing from photon shot noise. In the strong-dispersive limit, the passage of a single photon through the readout cavity performs a complete measurement of the qubit state, limiting coherence times in a way that depends directly upon the cavity decay rate. This understanding has led to our recent experimental progress towards qubit coherence times of nearly 100 microseconds by proper filtering and thermalization over a wide band of frequencies.