QUROPE Quantum Information Processing and Communication in Europe

In circuit quantum electrodynamics (QED) the cavity-mediated dispersive interaction is the dominant inter-qubit coupling mechanism when the qubits are detuned from the resonator. This mechanism can be used to realize two-qubit gates. Here, we investigate the strength of this interaction explicitly considering the Fabry-Perot like multi-mode structure of the microwave frequency transmission line resonator. We observe the formation of dark states when the qubits are driven jointly by the same resonator microwave field and tuned into resonance with each other.

Microwave cavities with high quality factors enable coherent coupling of distant quantum systems. Virtual photons lead to a transverse exchange interaction between qubits, when they are non-resonant with the cavity but resonant with each other. We experimentally probe the inverse scaling of the inter-qubit coupling with the detuning from a cavity mode and its proportionality to the qubit-cavity interaction strength. We demonstrate that the enhanced coupling at higher frequencies is mediated by multiple higher-harmonic cavity modes.

One of the most surprising predictions of modern quantum theory is that the vacuum of space is not empty. In fact, quantum theory predicts that it teems with virtual particles flitting in and out of existence. While initially a curiosity, it was quickly realized that these vacuum fluctuations had measurable consequences, for instance producing the Lamb shift of atomic spectra and modifying the magnetic moment for the electron. This type of renormalization due to vacuum fluctuations is now central to our understanding of nature.

Modern quantum theory predicts that the vacuum of space is not empty, but instead teeming with virtual particles flitting in and out of existence. While initially a curiosity, it was quickly realized that these vacuum fluctuations had measurable consequences, for instance producing the Lamb shift of atomic spectra and modifying the magnetic moment for the electron. This type of renormalization due to vacuum fluctuations is now central to our understanding of nature.

We report the observation of photon generation in a microwave cavity with a time-dependent boundary condition. Our system is a microfabricated quarter-wave coplanar waveguide cavity. The electrical length of the cavity is varied by using the tunable inductance of a superconducting quantum interference device. It is measured at a temperature significantly less than the resonance frequency. When the length is modulated at approximately twice the static resonance frequency, spontaneous parametric oscillations of the cavity field are observed.

We present an experiment performed on two coupled Transmon qubits forming a universal iSWAP ^(1/2) quantum logic gate. Each of the qubits is equipped with its own circuit for driving and single-shot readout and a fast flux line for frequency tuning. We perform quantum process tomography to characterize the operation of the iSWAP^(1/2) gate and obtain a fidelity of 88 %. Furthermore we run Grover’s search algorithm on the system and obtain single-shot fidelities above the classical bound of 50 %, demonstrating quantum speed-up for this particular search problem.