QUROPE Quantum Information Processing and Communication in Europe

We have embedded an artificial atom, a superconducting "transmon" qubit, in an open transmission line and investigated the strong scattering of incident microwave photons (~GHz). When an input coherent state, with an average photon number N<<1 is on resonance with the artificial atom, we observe extinction of up to 90% in the forward propagating field. We use two-tone spectroscopy to study scattering from excited states and we observe electromagnetically induced transparency (EIT).

A quantum coherent interface between optical and microwave photons can be used as a basic building block within a future quantum information network. The interface is envisioned as an ensemble of rare-earth ions coupled to a superconducting resonator allowing for coherent transfer between optical and microwave photons. Towards this end, we have realized a hybrid device coupling a Er^(3+) doped Y_2 SiO_5 crystal in a superconducting coplanar waveguide cavity. We observe a collective spin coupling of 4 MHz and a spin linewdith of down to 70 MHz. 

We have embedded an artificial atom, a superconducting transmon qubit, in a 1D open space and investigated the scattering properties of an incident microwave coherent state. By studying the statistics of the reflected and transmitted fields, we demonstrate that the scattered states can be nonclassical. In particular, by measuring the second-order correlation function, g((2)), we show photon antibunching in the reflected field and superbunching in the transmitted field.

Measurements of the temperature and bias dependence of Single Electron Transistors (SETs) show that charge noise increases linearly with temperature above a voltage-dependent threshold temperature, and that its low temperature saturation is due to self-heating. We show further that the two-level fluctuators responsible for charge noise are in strong thermal contact with hot electrons on the SET island, and at a temperature significantly higher than that of the substrate. We suggest that the noise is caused by electrons tunneling between the island and nearby potential wells. 

In highly resistive superconducting tunnel junctions, excess subgap current is usually observed and is often attributed to microscopic pinholes in the tunnel barrier. We have studied the subgap current in superconductor–insulator–superconductor (SIS) and superconductor–insulator–normal-metal (SIN) junctions. In Al/AlOx/Al junctions, we observed a decrease of 2 orders of magnitude in the current upon the transition from the SIS to the SIN regime, where it then matched theory.

We have embedded an artificial atom, a superconducting transmon qubit, in an open transmission line and investigated the strong scattering of incident microwave photons (∼6  GHz). When an input coherent state, with an average photon number N≪1 is on resonance with the artificial atom, we observe extinction of up to 99.6% in the forward propagating field. We use two-tone spectroscopy to study scattering from excited states and we observe electromagnetically induced transparency (EIT).

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.