October 2014

Abstracts of the QSIT Lunch Seminar, Thursday October 2, 2014

Measurement and control of a mechanical oscillator at its thermal decoherence rate

D.J. Wilson, Laboratoire de Photonique et de Mesure Quantique (LPQM), EPF Lausanne

co-authors:  V. Sudhir, N. Piro, R. Schilling, A. Ghadimi, and T.J. Kippenberg
In real-time quantum feedback protocols, the record of a continuous measurement is used to stabilize a desired quantum state. Recent years have seen highly successful applications in a variety of well-isolated, individual quantum systems, including microwave photons and superconducting qubits. Extending such protocols to mechanical oscillators remains an outstanding challenge, however: the main obstacle is rapid thermal decoherence, which places stringent requirements on the rate at which the oscillator’s position must be measured. We report on a sensor that approaches, for the first time, the requirements of real-time (Markovian) quantum feedback for a solid state mechanical oscillator — namely, the ability to resolve its zero-point motion in a timescale comparable to that of its thermal decoherence. The sensor is based on cavity optomechanical coupling, and achieves an imprecision nearly four orders of magnitude below that at the standard quantum limit (SQL) for a weak continuous position measurement — a more than 100-fold improvement over the most precise position measurements to date — while maintaining an imprecision-back-action product within a factor of √ 27 of the Heisenberg uncertainty limit. As a demonstration of its utility, we employ the measurement as an error signal in a feedback cooling experiment: Using radiation pressure as an actuator, the 4.3 MHz oscillator is efficiently cold damped from a cryogenic bath temperature of 4.4 K to an effective value of 1.1±0.1 mK, corresponding to a mean phonon number of 5.4±0.7 (i.e. a ground state population of 16%). Our results set a new benchmark for the performance of a linear position sensor, and signal the emergence of mechanical oscillators as practical subjects for measurement-based quantum control.

Diamond based nanoscale nuclear magnetic resonance

Michael Loretz, Spin Physics and Imaging - Christian Degen's Lab, ETH Zurich

Extension of magnetic resonance imaging to the atomic scale has been a longs standing aspiration driven by the prospect of directly mapping atomic positions in molecules with three-dimensional spatial resolution. The nitrogen vacancy center in diamond (NV center) is an optically controllable single spin system with long coherence times at room temperature. These features turn the NV center into a promising candidate to outperform conventional induction based measurement techniques in sensitivity and spatial resolution. In the first part of the talk, I will focus on the engineering of shallow NV centers in a diamond chip and explain its importance in terms of decreasing sample-to-sensor distance in order to increase sensitivity. In the second part, I will present the working principle of diamond based nuclear magnetic resonance on the example of small spin ensembles. Moreover I will demonstrate the determination of hyperfine couplings of single proton spins and show how to map their positions. This results demonstrate the potential of this technique as application towards MRI with single nuclear spin sensitivity.