QUROPE Quantum Information Processing and Communication in Europe

Teleportation of a quantum state may be used for distributing entanglement between distant qubits in quantum communication and for realizing universal and fault-tolerant quantum computation. Here we demonstrate the implementation of a teleportation protocol, up to the single-shot measurement step, with superconducting qubits coupled to a microwave resonator. Using full quantum state tomography and evaluating an entanglement witness, we show that the protocol generates a genuine tripartite entangled state of all three-qubits.

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.

Quantum computers, should they be realized one day, will inevitably make errors. Therefore, they need special error correcting mechanisms. The most important part of it, a so-called Toffoli gate, has now been realized by ETH scientists with superconducting circuits.