Quantum properties of electrical circuits
As the size of electronic circuits gets smaller and smaller, quantum effects such as energy quantisation and tunneling effects are becoming major problems for the electronics industry. The fundamental limits of miniaturisation are expected to be reached in the near future. Quantum effects are not, however, just a nuisance. Richard Feynman suggested in 1982 that they could be used as a very powerful resource, for instance for solving problems which could never be solved on classical data processing systems.
To benefit and exploit these quantum resources two paths are possible, roughly speaking. The first is to use microscopic systems such as atoms, ions or electrons, which naturally display quantum effects because of their small number of degrees of freedom and their small coupling to their environment, but on the other hand are not easily scalable. “Scalable” means that the components may be easily assembled into a larger device, say in the manner that semiconductor transistors are the building blocks of present day computers. The other route is to use macroscopic systems such as superconducting circuits, which are fabricated devices that are easily scalable using lithography techniques borrowed from the electronics industry, but are not easily made quantum because of their strong coupling to their environment and low energy scales.
In the Quantum Devices group, we have chosen to study superconducting circuits, since they provide a new and extremely interesting system for exploring and utilising quantum effects. The superconducting phase transition provides the opportunity to use a collective degree of freedom, the ‘superconducting phase’, as a macroscopic and robust quantum degree of freedom. Since this quantity can be directly coupled to the voltages and currents in an ordinary electrical circuit, one can try to engineer a quantum electrical circuit by assembling three main building blocks; ordinary capacitors and inductors, which are used to create an oscillating degree of freedom, and the Josephson junction, made by weakly coupling two superconducting electrodes through the tunnel effect. The Josephson junction introduces non-linear effects into the circuit, which allows the engineering of the energy spectrum into the one of an artificial atom.
It is remarkable that, despite the circuit containing thousands of atoms in an artificial arrangement, its behaviour is similar to that of a single natural atom, where transitions between discrete energy levels can be isolated. Quantum circuits might be used as model systems for studying the interaction between radiation and matter in new and broader ranges of parameters, they might also form the foundation of quantum computers, as qubits, or they might form the basis of new, ultrasensitive detectors of electromagnetic radiation.
Our goal in the Quantum Devices group is to develop the "quantum engineering" of electrical circuits in order to design and fabricate systems exploiting quantum effects for the realization of a given task. We have currently 3 projects:
Single microwave photon detection.
Electric current metrology.