Circuit Quantum Electrodynamics

Electrical circuits can be made to behave quantum mechanically, when fabricated from superconductors and operated at microwave frequencies and very low temperatures. This allows them to be used to both study quantum mechanical effects in a uniquely controllable manner, and also to be explored for use in quantum information technology. A particular area of physics that can be accessed using quantum electrical circuits is cavity quantum electrodynamics. This is the study of light-matter interactions by confinement of single atoms and light inside a cavity. This enhances the interaction strength between the systems, and allows coherent exchange of quantum information between them. An example of an electrical circuit version of this concept is shown below:

The light cavity is here replaced by a transmission line resonator (a), and the atoms by superconducting qubits (b), based on Josephson Junctions. The concept of circuit QED was first realised in 2004 [1], and has revolutionized the field of superconducting circuits. The particular device shown here was used for two-qubit control and measurement, and creation of quantum states of the resonator microwave field in prior work at ETH Zurich [2]. In our group we are working on electrical circuit implementations of cavity QED in a variety of forms, with both 3D electromagnetic cavities and acoustic resonators coupled to artificial atoms in the form of Josephson Junction based superconducting qubits, and other novel solid state systems such as carbon nanotubes and spin ensembles.

  • [1] Wallraff et al., Nature 431, 162 (2004)
  • [2] Leek et al., Phys. Rev. Lett. 104,100504 (2010)

Surface Acoustic Wave Resonators at Low Temperature

Surface acoustic waves (SAWs) are mechanical modes that travel along the surface of a piezoelectric crystal. They have been used in commercial electronic devices in high quality resonators and filters. Since such devices can easily operate up to GHz frequencies, they can be operated in the quantum regime at typical dilution refrigerator temperature and can be piezoelectrically coupled to superconducting circuits [1], opening up exciting possibilities for quantum information and fundamental studies of mechanical quantum systems.

SAW devices consist of a suitably shaped metallic thin film deposited on the surface of a piezoelectric crystal, such as quartz or GaAs. Piezoelectricity allows an electrical signal to be converted into an acoustic wave. The component of a SAW device that allows this conversion is called InterDigital Transducer (IDT). A SAW resonator consists of an IDT placed between two highly reflective Bragg gratings, as shown in the figure below.

We investigate the geometric factors and loss mechanisms at low temperatures of SAW devices, working towards coupling them to superconducting qubits.

  • [1] M. Gustafsson et al, Local probing of propagating acoustic waves in a gigahertz echo chamber, Nat. Phys. 8, 338-343 (2012)
  • [2] El Habti et al, High frequency surface acoustic wave devices at very low temperature: Application to loss mechanisms evaluation J. Acoust. Soc. Am. (USA), 100, 272-277 (1996)

Circuit Quantum Electrodynamics with Hybrid Superconductor-Carbon Nanotube Devices

This project is aimed to probe exotic Andreev bound states (ABS), see figure below, in hybrid superconductor-carbon nanotube (CNT) devices via coupling a flux biased nanotube superconducting quantum interference device (SQUID) to the microwave field of an on-chip resonator. ABSs are entangled electron-hole states forming as a result of Andreev reflection of electrons and holes at superconductor-CNT interfaces and are localized within the superconducting energy gap. ABSs are the main carriers of a dissipationless Josephson current through the nanotube.

We aim to realize coupling between the resonator and CNT SQUID, whereby a strong coupling will provide with a good controlling and probing mechanism for Andreev physics in the CNT. Taking into account that in a single conduction channel only two such states exist, this Andreev system can be studied as a quantum bit.