QUROPE Quantum Information Processing and Communication in Europe

The ability to sensitively detect individual charges under ambient conditions would benefit a wide range of applications across disciplines. However, most current techniques are limited to low-temperature methods such as single-electron transistors, single-electron electrostatic force microscopy and scanning tunnelling microscopy. Here we introduce a quantum-metrology technique demonstrating precision three-dimensional electric-field measurement using a single nitrogen-vacancy defect centre spin in diamond. An a.c.

Diamond spins are an ideal test bed for exploring quantum physics of few well controllable qubit systems. Defect center electron spins show strong coupling to a light field and at the same time interact with few surrounding nuclei in the lattice. As a result the system usually constitutes a few qubit system with excellent coherence and controllability even at room temperature. It fulfills all characteristics of a quantum register including single shot read-out capability.

Diamond defects allow for precise measurement of single electron and nuclear spin quantum states. The excellent controllability of these spins as well as efficient decoupling from environment make them an ideal playground for engineering complex quantum states and development of elaborate control schemes. The talk will describe how nuclear spin states can be efficiently read-out and used as Qbits in spin clusters. Routes towards the controlled engineering of extended spin arrays as well as coupling to control structures will be discussed.

Interaction of defect centers in diamond and molecules with thin metal wires, bow tie, and other antenna structures: By studying the coupling efficiency and mechanism we achieve an improved insight into the quantum plasmon interaction.

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.