The transmon superconducting qubit is being intensely investigated as a promising approach for the physical implementation of quantum information processing, and high quality factorsof order 106 have been achieved both in two- and three-dimensional architectures. These high quality factors enable detailed investigations of decoherence mechanisms. An intrinsic decoherence process originates from the coupling between the qubit degree of freedom and the quasiparticles that tunnel across Josephson junctions. In a transmon, tunneling of a single quasiparticle is associated with a change in parity. Here we present the theory of the parity-switching rates in single-junction transmons and compare it with recent measurements. We also show that parity switching can have an important role in limiting the coherence time.
A promising quantum computing architecture couples superconducting qubits to
microwave resonators (circuit QED), a system in which three-dimensional
microwave cavities have become avaluable resource. Such cavities have
surface-to-volume ratios, or participation ratios a thousandfold smaller than
in planar devices, deemphasizing potentially lossy surface elements by an equal
amount. Motivated by this principle, we have tested aluminum superconducting
cavity resonators with internal quality factors greater than 0.5 billion and
intrinsic lifetimes reaching 0.01 seconds at single photon power and
millikelvin temperatures. These results are the first to explore the use of
superconducting aluminum, a ubiquitous material in circuit QED, as the basis of
highly coherent (Q~10^7-10^9) cavity resonators. Measurements confirm the
cavities‘ predicted insensitivity to quasiparticles (kinetic inductance
fraction-5ppm) and an absence of two level dielectric fluctuations.
In superconducting qubits, the interaction of the qubit degree of freedom
with quasiparticles defines a fundamental limitation for the qubit coherence.
We develop a theory of the puredephasing rate Gamma_{phi} caused by
quasiparticles tunneling through a Josephson junction and of the inhomogeneous
broadening due to changes in the occupations of Andreev states in the junction.
To estimate Gamma_{phi}, we derive a master equation for the qubit dynamics.
The tunneling rate of free quasiparticles is enhanced by their large density of
states at energies close to the superconducting gap. Nevertheless, we find that
Gamma_{phi} is small compared to the rates determined by extrinsic factors in
most of the current qubit designs (phase and flux qubits, transmon, fluxonium).
The split transmon, in which a single junction is replaced by a SQUID loop,
represents an exception that could make possible the measurement of
Gamma_{phi}. Fluctuations of the qubit frequency leading to inhomogeneous
broadening may be caused by the fluctuations in the occupation numbers of the
Andreev states associated with a phase-biased Josephson junction. This
mechanism may be revealed in qubits with small-area junctions, since the
smallest relative change in frequency it causes is of the order of the inverse
number of transmission channels in the junction.
We study the photon shot noise dephasing of a superconducting transmon qubit
in the strong-dispersive limit, due to the coupling of the qubit to its readout
cavity. As each random arrivalor departure of a photon is expected to
completely dephase the qubit, we can control the rate at which the qubit
experiences dephasing events by varying textit{in situ} the cavity mode
population and decay rate. This allows us to verify a pure dephasing mechanism
that matches theoretical predictions, and in fact explains the increased
dephasing seen in recent transmon experiments as a function of cryostat
temperature. We investigate photon dynamics in this limit and observe large
increases in coherence times as the cavity is decoupled from the environment.
Our experiments suggest that the intrinsic coherence of small Josephson
junctions, when corrected with a single Hahn echo, is greater than several
hundred microseconds.