QUROPE Quantum Information Processing and Communication in Europe

In a recent publication in Physical Review Letters the TU-Delft group led by SOLID partner Leo DiCarlo demonstrated an indirect partial measurement of a transmon qubit in circuit quantum electrodynamics.  This was don by interaction with an ancilla qubit and projective ancilla measurement with a dedicated readout resonator. Accurate control of the interaction and ancilla measurement basis allowed the group to show how it is possible to tailor the measurement strength and operator.

In a recent publication in Nature Communications the TU-Delft team including SOLID partner Leo Kouwenhoven demonstrated a semiconductor nanowire based single photon source with a near unity efficiency.  To do this, the group positioned single quantum dots on the axis of a tailored nanowire waveguide using bottom-up growth.

The Quantronics group at SOLID partner CEA have recently published a paper in Physical Review Letters in wihch they measure the quantum fluctuations of a pumped nonlinear resonator using a superconducting artificial atom as an in situ probe. The qubit excitation spectrum provided access to the frequency and amount of excitation of the intracavity field fluctuations, from which we infer its effective temperature.

The Chalmers team led by Chris Wilson recently published a paper in Physical Review Letters in which they investigated the effective interaction between two microwave fields, mediated by a transmon-type superconducting artificial atom that is strongly coupled to a coplanar transmission line. The interaction between the fields and atom produces an effective cross–Kerr coupling.

The Chalmers SOLID group recently published an article in Physical Review Letters in which they show, in the context of single-photon detection, that an atomic three-level model for a transmon in a transmission line does not support the predictions of the nonlinear polarizability model known as the cross-Kerr effect.

The group led by Lieven Vandersypen at TU-Delft recently published a paper in Nature Nanotechnology in which they show that two distant sites in an electrostatically defined quantum dot array can be tunnel-coupled directly. The coupling is mediated by virtual occupation of an intermediate site, with a strength that is controlled via the energy detuning of this site.

In a recent publication in Nature Physics the group led by SOLID partner Ronald Hanson realized a two-qubit parity measurement on nuclear spins localized near a nitrogen-vacancy centre in diamond by exploiting an electron spin as a readout ancilla. The measurement enabled the group to project the initially uncorrelated nuclear spins into maximally entangled states.

In a recent publication in Nature, the group led by Leo DiCarlo at SOLID partner TU-Delft have performed a time-resolved, continuous parity measurement of two superconducting qubits using the cavity in a three-dimensional circuit quantum electrodynamics architecture and phase-sensitive parametric amplification. Using postselection, the group managed to produce entanglement by parity measurement reaching 88 per cent fidelity to the closest Bell state.

In a recent publication in Nature Nanotechnology, the group led by Jörg Wrachtrup experimentally demonstrated entanglement between two engineered single solid-state spin quantum bits (qubits) at ambient conditions. Photon emission of defect pairs reveals ground-state spin correlation. Entanglement (fidelity = 0.67±0.04) was proved by quantum state tomography.