Second generation quantum detectors

Nowadays a large number of the most sensitive detectors are based on quantum phenomena, so as for example the SQUIDs. Despite the quantum nature of a SQUID one can still find a classical analogue to describe its behaviour, as the macroscopic wave function of the Cooper pair condensate plays a similar role as the vector potential of photons in an optical interferometer.

Beyond modern physics has demonstrated that artificial solid state systems can exhibit a superposition between macroscopically distinguished quantum states. There is clear and convincing proof that there are quantum coherent oscillations of the level occupation in such systems. Since the relaxation and dephasing of these oscillations are extremely sensitive to the external signals, these effects can be used for detectors with quantum limited sensitivity. These detectors called quantum detectors of the second generation.

At IPHT the group around Evgeni Ilichev has worked on the field of quantum physics in solid state systems for several years. The group has established a novel technique for reading out fragile quantum states without destruction, using a non-dissipative impedance measuring technique. With that they have successfully demonstrated the superposition of two quantum states in macroscopic superconductor devices, in particular in nano-scaled superconducting rings containing three Josephson junctions, so called persistent current qubits (Mooij-type). Such rings can be treated as artificial atoms, with energy levels separation which can be tuned by external parameters. Recent successes have been the first proof of an entanglement of three and four solid-state qubits. Further, together with Professor Grajcar from the University of Bratislava/Slovakia a new approach for quantum computation based on adiabatic evaluation has been proposed and preliminary results have been reported.

With the profound knowledge on macroscopic and mesoscopic quantum systems and the sophisticated measurement equipment for low temperature (down to 10 mK) the group is now heading towards new concepts for future detector systems. In principle, with the control of quantum states, it will be possible to construct sensors based on customized artificial quantum systems. They will exceed the limits of modern detectors, touching nature’s limits.