Quantum computation and quantum information processing have emerged as exciting new fields of physics in the last decade.  While offering the promise of radical new technologies in the future, these fields are driving our basic understanding of quantum mechanics and its implications.  These fields have become increasingly interdisciplinary in recent years, drawing in physicists from a variety of disciplines, ranging from quantum optics to condensed matter physics.  Early work was dominated by results in natural quantum systems, such as photons and atoms.  Recently, work on engineered quantum systems, i.e. solid-state devices, has become increasingly important.

A particularly important contribution to this work has come from the field of superconducting quantum circuits.  From the first experiments a decade ago, the performance of superconducting quantum bits (qubits) has improved as much as 3 orders of magnitude.  Early advances were gained through improved designs of the qubits and measurement systems.  While this approach may continue to be fruitful, it is also general recognized in the field that there is a lack of understanding of the microscopic mechanisms that limit the performance of superconducting qubits.  There is now a major international effort to understand these mechanisms and eliminate them through advancements in our physical understanding, but also through technological advancements in materials and fabrication methods.

Decoherence processes in superconducting circuits

At the heart of all superconducting qubits is the Josephson junction. Studied for many decades as a macroscopic object, it is only in the last 15 years that a microscopic understanding of the Josephson effect has developed in terms of Andreev bound states. In mesoscopic Josephson links, the theory of Andreev states has been confirmed with impressive accuracy. It is also generally accepted that the macroscopic Josephson current in a Josephson junction is actually carried by a multitude of ABSs. Despite its success, this microscopic picture of Josephson junctions has not yet been integrated into our understanding of superconducting qubits. While conventional theory generally treats the Josephson junction as “dissipationless,” Andreev theory tells us that this is not the case, even for an ideal device. In the presence of imperfections, there are many paths by which ABSs can lead to decoherence. In particular, nonequilibrium quasiparticles, which are ubiquitous ly observed in superconducting qubits, interact very strongly with ABSs in ways that can produce both relaxation and dephasing. Inhomogeneities and defects in the tunnel barrier can also transform ABSs into parasitic two level systems, which are known to degrade the performance of qubits in a variety of ways. It is critical to understand the intrinsic and fundamental limits that Andreev physics place on the performance of superconducting qubits.

The position is available to start immediately. The actually starting time is negotiable. The project has long term funding, renewable for up to 4-5 years. However, standard postdoc contracts at Chalmers are for 1 year at a time.

We will start to interview qualified applicants after January 18, 2010. A candidate maybe chosen at anytime after that, but applications will be accepted until the position is filled.

Goals for the project include:

Elucidate and clarify the ways in which Andreev bound states, by themselves and in conjunction with imperfections, limit the performance of superconducting qubits.

Quantify the decoherence arising from ABS interacting with nonequilibrium quasiparticles and imperfections in the tunnel barrier.

Develop and implement strategies for eliminating nonequilibrium quasiparticles.
Develop and implement advanced electrical measurements, derived from Andreev physics, that will provide information on the microscopic functionality of Josephson junctions.

Use this information to develop an advanced fabrication process for deep-submicron Josephson junctions that reduces or eliminates sources of decoherence.

Integrate all of this into advanced qubits and demonstrate enhanced coherence times.

Christopher Wilson
chris.wilson@chalmers.se