QUROPE Quantum Information Processing and Communication in Europe

We present a quantum algorithm to prepare injective PEPS on a quantum computer, a class of open tensor networks representing quantum states. The run-time of our algorithm scales polynomially with the inverse of the minimum condition number of the PEPS projectors and, essentially, with the inverse of the spectral gap of the PEPS' parent Hamiltonian.

SOLID members from the Kavli Institute at TU-Delft have shown how spin-orbit interaction provides a way to control spins electrically. A spin–orbit quantum bit (qubit) is electrostatically defined in an indium arsenide nanowire, where the spin–orbit interaction is so strong that spin and motion can no longer be separated. In this regime, the group has realized fast qubit rotations and universal single-qubit control using only electric fields; the qubits are hosted in single-electron quantum dots that are individually addressable.

Amongst all the microscopic quantum spin systems that can be coupled to superconducting circuits, negatively charged nitrogen- vacancy centers (N-V) in diamond are particularly attractive. One of the major reasons is that the spin coherence time has been shown to be as long as 2ms at room temperature. Compared to atoms, N-V centers are perfectly compatible with superconducting circuits, because they do not require challenging trapping techniques or large magnetic fields to bring them in resonance at GHz frequency with the circuit.

The Karlsruhe groups of A. Ustinov and A. Shnirman (KTA) have demonstrated a new method to directly manipulate the state of individual two-level systems (TLSs) in phase qubits. The method allows one to characterize the coherence properties of TLSs using standard microwave pulse sequences, while the qubit is used only for state readout. The group has applied this method to perform the first measurement of the temperature dependence of TLS coherence.

Here, the Mooij group at TU-Delft in close collaboration with group of SOLID theorist, E. Solano have measured the dispersive energy-level shift of an LC resonator magnetically coupled to a super- conducting qubit. The results clearly show that the system operates in the ultrastrong coupling regime of the light matter interaction. The large mutual kinetic inductance provides a coupling energy of ≈0.82GHz, requiring the addition of counter-rotating-wave terms in the description of the Jaynes-Cummings model.

Controlling the interaction of a single quantum system with its environment is a fundamental challenge in quantum science and technology. In this publication, the group of Ronald Hanson at TU-Delft managed to strongly suppress the coupling of a single spin in diamond with the surrounding spin bath by using double-axis dynamical decoupling. The coherence was preserved for arbitrary quantum states, as verified by quantum process tomography. The resulting coherence time enhancement followed a general scaling with the number of decoupling pulses.

In circuit quantum electrodynamics (QED), where superconducting artificial atoms are coupled to on-chip cavities, the strong-coupling regime has greatly evolved. In this regime, an atom and a cavity can exchange a photon frequently before coherence is lost. Nevertheless, all experiments so far are well described by the renowned Jaynes–Cummings model. In this work, the group at the Walter Meissner Institut in Garching, Germany collaborated closely with the SOLID theorist E.

In this paper SOLID researcher A. Imamoglu and colleagues at ETH-Zürich show how the spin-state-dependent resonance fluorescence from a charged quantum dot can be used to realize a classical single spin-photon interface. The measurement is background free and shows how detecting the scattered photon intensity with 300 ps time resolution projects the quantum dot spin into a definite spin eigenstate with a fidelity exceeding 99%. The bunching of resonantly scattered photons reveals information about electron spin dynamics.

Projective measurement of single electron and nuclear spins has evolved from a Gedanken experiment to a problem relevant for applications in atomic-scale technologies like quantum computing. Although several approaches allow for detection of a spin of single atoms and molecules, multiple repetitions of the experiment that are usually required for achieving a detectable signal obscure the intrinsic quantum nature of the spin’s behavior. In this paper the group of F. Jelezko and J.