# Theoretical Physics

**27 February 2013**

**Time:**15:00 to 16:00

**Location:**Room 8.90

Eran Ginossar (Guildford, Surrey)

Quantum state sensitivity and strong nonlinearities in circuit-QED

The field of circuit quantum electrodynamics describes the physics of artificial solid state based qubits strongly coupled to electromagnetic modes in superconducting resonators. These systems are fabricated on a chip and designed to perform basic quantum information processing. These devices behave as a few-body interacting systems out of equilibrium whose transient dynamics needs to be accurately controlled on short time scales compared to the relaxation and decoherence times. In addition, subtle quantum optical effects and interactions have been demonstrated in these systems such as number splitting and synthesis of arbitrary photonic Fock states.

The high power transient response of superconducting qubit-cavity systems has recently become a method to perform high fidelity readout of transmon qubits. We will survey several types of high power responses which are instrumental for state-detection, and focus in particular on Autoresonance.

Autoresonance is a unique effect in classical dynamics where a frequency chirped drive can efficiently excite a nonlinear oscillator when it is continuously kept in sync with the shifting resonance of the oscillator, provided the drive strength exceeds the critical threshold. This effect has been recently demonstrated for superconducting resonators with one quantized degree of freedom with ensuing studies of the effect of thermal and quantum noise. We demonstrated experimentally and theoretically [Phys. Rev. B, 86, 220503(R)] that when such an oscillator is strongly coupled to a quantized superconducting qubit, both the effective nonlinearity and the threshold become a non-trivial function of the qubit-oscillator detuning. Moreover, the autoresonant threshold is sensitive to the quantum state of the qubit and may be used to realize a high fidelity, latching readout whose speed is not limited by the oscillator Q.

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