# CeNS Colloquium

Place: Kleiner Physik-HĂ¶rsaal, Geschwister-Scholl-Platz

Date: 03.12.10, Time: 15:30 h

#### Quantum physics with electrical circuits

**Prof. Daniel Esteve**

Quantronics, SPEC, CEA-Saclay, Gif-sur-Yvette

Solid state electrical circuits that behave as artificial quantum atoms are now fabricated for quantum information processing, and for addressing fundamental issues in quantum mechanics. The circuits operated in the Quantronics group are obtained by embedding a Cooper pair box, which is the simplest Josephson quantum bit, in a microwave resonator. This circuit is analogous to an atom in a resonant cavity, hence the name of circuit QED for this field.

In this system, the field in the resonator measures the quantum state of the artificial atom through a small frequency pull of the resonator that depends on the qubit state, and with a strength proportional to the number of photons in the cavity. With this system, we have observed the violation of an inequality demonstrated by Leggett and Garg for physical systems fulfilling the assumptions of macrorealism, and considered as plausible for macroscopic systems such as our electrical circuits [1].

We have enriched circuit QED by using a nonlinear resonator to achieve the single-shot readout of the Josephson qubit [2]. In this system, the bifurcation phenomenon that occurs between two different dynamical states of the driven nonlinear oscillator allows to discriminate the two qubit states with high fidelity. The spectroscopy of the qubit allows then to probe the back-action of the readout method and to determine if it is quantum limited [3].

Josephson qubits do not rival yet with real atoms or spins in terms of quantum coherence. In order to benefit nevertheless from their superior design flexibility, an appealing hybrid way that aims at picking the best of both worlds has been proposed. As a first step in this direction, we have coupled an assembly of nitrogen-vacancy (N-V) spins in a diamond crystal to a superconducting microwave resonator. The observation of a vacuum Rabi splitting in the transmission of the resonator, demonstrates the strong coupling regime between the resonator and the collective spin variable of the N-V spins, a first step towards hybrid structures[4].

[1] A. Palacios-Laloy, et al. Nature Phys. 6, 442447 (2010).

[2] F. Mallet et al., Nature Phys. 5, 791-795 (2009).

[3] F.R. Ong et al., arXiv:1010.6248.

[4] Y.Kubo et al, Phys. Rev. Lett. 105, 140502 (2010).