QUROPE Quantum Information Processing and Communication in Europe

In their recent article in Science Magazine the SOLID group at KIT led by Alexey Ustinov demonstrated experimentally that minute deformation of the oxide barrier changes the energies of the atomic tunneling systems, and measured these changes by microwave spectroscopy of the superconducting qubit through coherent interactions between these two quantum systems. By measuring the dependence of the energy splitting of atomic tunneling states on external strain, the group could verify a central hypothesis of the two-level tunneling model for disordered solids.

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. The tradeoff between measurement strength and qubit backaction was shown to be characterized through the distortion of a qubit Rabi oscillation imposed by ancilla measurement in different bases. By combining partial and projective qubit measurements, the group made a solid-state demonstration of the correspondence between a nonclassical weak value and the violation of a Leggett-Garg inequality.

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. In comparison to quantum dots in nanowires without waveguides, the group could demonstrate a 24-fold enhancement in the single-photon flux, corresponding to a light-extraction efficiency of 42%. Such high efficiencies in one-dimensional nanowires are promising to transfer quantum information over large distances between remote stationary qubits using flying qubits within the same nanowire p–n junction.

IThe SOLID group of Lieven Vandersypen recently published a paper in Physical Review Letters in which they investigated phonon-induced spin and charge relaxation mediated by spin-orbit and hyperfine interactions for a single electron confined within a double quantum dot. They extract an electron spin relaxation rate that varies nonmonotonically with the detuning between the dots and confirm this model with experiments performed on a GaAs double dot.  The results obtained demonstrated that the relaxation rate exhibit the expected detuning dependence and can be electrically tuned over several orders of magnitude. Analysis suggests that spin-orbit mediated relaxation via phonons serves as the dominant mechanism through which the double-dot electron spin-flip rate varies with detuning.

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. These quantities were found to be in agreement with theoretical predictions; in particular, the group experimentally observed the phenomenon of quantum heating.

 

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 group demonstrated average cross–Kerr phase shifts of up to 20 degrees per photon with both coherent microwave fields at the single-photon level. Their results provide an important step toward quantum applications with propagating microwave photons.

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 managed to demonstrate that the induced displacement of a probe in the presence or absence of a single photon in the signal field, cannot be resolved above the quantum noise in the probe. This strongly suggests that cross-Kerr media are not suitable for photon counting or related single-photon applications, a conclusion that also applies to optical systems. 

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. It permits a single charge to oscillate coherently between the outer sites of a triple dot array without passing through the middle, as demonstrated through the observation of Landau–Zener–Stückelberg interference. The long-distance coupling significantly improves the prospects of fault-tolerant quantum computation using quantum dot arrays, and opens up new avenues for performing quantum simulations in nanoscale devices.