The device physics area of condensed matter physics deals with exploration of the fundamental physics of structures with potential for applications. The quantum device physics programme is a collaboration between Dr. C. M. Muirhead and Dr. Mark Colclough, who helped design the world's first commercial HTS SQUID for Conductus in the USA.
In 1987 following the first demonstration of flux quantisation in HTS, Birmingham were the first to realise a rudimentary HTS SQUID, making use of the naturally occurring weak-links between grains in a ceramic sample. Subsequently the group has worked with a number of groups world-wide investigating the latest state-of-art structures with a view to understanding the physics underpinning device performance. In 1996 a new clean room facility was established, enabling the group to make its own device structures. In the light of new international developments in superconductivity and the interests of new staff, this work was broadened to include studies of the microscopic processes which limit the performance of quantum devices and, more recently the interaction of microwave resonators with nanobar mechanical resonators.
At Birmingham we instrumented one of our dilution refrigerators for the ultralow noise measurements of devices called phase-qubits (see below), and performed a range of studies on these devices. All such devices are limited by a process called 'quantum decoherence'. A particular interest here is the role played in quantum decoherence by atomic scale defects in the structure, called two-level systems.
Another exciting area we have worked in is the coupling of low-loss microwave resonators to nanometre-sized mechanical resonators, in collaboration with the University of Nottingham. Reduction of the mechanical vibration (cooling) can be obtained by careful tuning of the microwave resonant frequency. Again, these experiments require not only very low temperatures, but also ultralow noise electronic measurement. A second dilution refrigerator has been instrumented for these experiments.
Topics that we have investigated include:
- Decoherence mechanisms in Josephson Junction devices
- Classical to quantum transition in Josephson devices
- π-Josephson junctions with Ferromagnetic barriers
- Cavity Quantum Electro-Dynamics (QED) and the interaction of superconducting microwave resonators with submicron low loss mechanical resonators
Images above. Left: a Josephson junction sample ready for measurement in an RF shielded box. Middle: the cavity and resonator for QED experiments. Right:a niobium nanobar. It is 10 µm in length and 100 nm across, and it is free of the substrate, so it can vibrate like a string.
Intrinsic Josephson Junctions
Initially it was believed that High Temperature Superconductors had no possible applications in quantum electronics. This belief was mostly due to the prescence of 'quasi-particle' (Electron-like and hole-like excitations) down to very low temperatures, characteristic of the d-wave nature of the materials. These excitations would normally destroy the delicate quantum coherence. Recently however, there has been a renewed interest following several experiments apparently demonstrating quantum effects at relatively high temperatures.
BSCCO is an important HTS material as its properties are highly anisotropic. This means that the conductivity in the a-b planes is much greater than along the c-axis. The layered nature of BSCCO gives rise to Josephson Junctions between the layers, these are known as Intrisic Josephson Junctions (IJJ) and are of very high quality in single crystal superconductors. Some of our research focusses on the Josephson junctions in such environments.
We have patterened samples of high-Tc YBCO films to tracks as narrow at 50 nm. We have shown evidence for phase-slip centres in these tracks.
Josephson junction and Qubit devices
We have investigated the design and application of qubit structures, and their potential for combination with other quantum devices. It is important to understand the physics behind the operation of devices in the quantum regime; superconductors allow us to probe this regime with relative ease due to the macroscopic nature of the superconducting wavefunction. We have mainly investigated 'π-junction' flux qubits. Our designs of π-qubits utilise both High Temperature and Low Temperature Superconductors in their operation and have many advantages over qubit designs based on a single material type.
A superconducting qubit in a microwave resonant cavity provides the opportunity to construct a solid state analogue of an atom in an optical trap, but because the qubit is a circuit, we now have a quantum system whose properties can be dramatically altered using control lines, weak magnetic fields, etc. This not only gives rise to new physical phenomena, but also opens the way to a range of novel microwave quantum devices, such as sources of single photons on demand, a new class of microwave laser and quantum encryption. The development of such quantum microwave systems presents a considerable challenge, because the quantum states are fragile. We have investigated ways of coupling persistent current qubits with microwave cavities in collaboration with colleagues at the National Physical Laboratory. The project combines the very low temperature and low noise device expertise in the department of Physics with the microwave design expertise at Birmingham and Cardiff. This will put us into a very strong position internationally to construct a range of sophisticated microwave circuits to drive and manipulate qubits and microwave photons, and to investigate the exciting physics of such systems. Support from our colleagues at NPL will continue.
The resonators are made from a submicron film of the superconductor Niobium (TC = 9.2K) on sapphire or silicon substrates. These are patterned into resonant structures in our class 100 cleanroom, using very similar methods to those used for making integrated circuits. The use of superconducting niobium means that resistive losses are very low (but not zero) and this leads to a very sharply defined resonance (Q=resonant frequency/width of resonance). We can obtained Q's up to 106 at the lowest temperatures.
These very high Qs are limited at the lowest temperatures by the very same two-level systems (TLS) that cause decoherence in qubits, so studies of microwave resonators alone provide a powerful tool for studies of TLS in substrates and interfaces with superconductors.
Graph description: resonator response at low temperatures. The Q of the resonator (blue data) increases as the temperature is reduced and the number density of Cooper pairs approaches a maximum. The maximum in the resonant frequency (red data) is not predicted by superconductivity theory and is due to two-level-system effects.
An interesting spin-off of this work has been the observation that the resonant frequency of a resonator can be changed by up to 200 linewidths by the application of magnetic fields only a few times larger than that from the earth. We are pursuing this phenomenon because of its potential application to the generation of single photons 'on-demand'.
Coupling of mechanical nanobars to microwave resonators
It has been known for a few years that the vibrational modes of a mechnical resonator can be coupled to the oscillation of the electromagnetic field in a microwave resonator. This coupling relies on the force which the electric field component of the EM wave in the resonator applies to the electrically conducting mechanical resonance. This is very closely analogous to a phenomena well known to the optics community, where there is an interaction between the photon field in an optical cavity and the vibrational modes of the end mirrors. This interaction places a serious limitation on the use of such cavities as gravitational wave detectors. The sort of experiments that we are engaged in are, in some ways, much easier than the optical counterpart, particularly because we can cool the thermal vibrations of the nanobar down to mK temperatures.
The temperature of such bars can be further cooled by driving the system at a microwave frequency slightly lower than the electromagnetic resonance frequency. If the bars can be cooled to the quantum mechanical ground state of the mechanical resonance (E = 0.5hf where h is Planck's constant), then they may be incorporated into qubit structures.
These studies have been done in collaboration with the University of Nottingham who made the structure shown above using advanced e-beam lithography techniques.