Spin-Phonon Interfaces in Coupled Nanomechanical Cantilevers.

Coupled micro- and nanomechanical oscillators are of fundamental and technical interest for emerging quantum technologies. Upon interfacing with long-lived solid-state spins, the coherent manipulation of the quantum hybrid system becomes possible even at ambient conditions. Although the ability of these systems to act as a quantum bus inducing long-range spin-spin interactions has been known, the possibility to coherently couple electron/nuclear spins to the common modes of multiple oscillators and map their mechanical motion to spin-polarization has not been experimentally demonstrated. We here report experiments on interfacing spins to the common modes of a coupled cantilever system and show their correlation by translating ultralow forces induced by radiation from one oscillator to a distant spin. Further, we analyze the coherent spin-spin coupling induced by the common modes and estimate the entanglement generation among distant spins.

Controlling the fluorescence properties of nitrogen vacancy centers in nanodiamonds.

Control over the formation and fluorescence properties of nitrogen vacancy (NV) centers in nanodiamonds (NDs) is an important factor for their use in medical and sensor applications. However, reports providing a deep understanding of the potential factors influencing these properties are rare and focus only on a few influencing factors. The current contribution targets this issue and we report a comprehensive study of the fluorescence properties of NVs in nanodiamonds as a function of electron irradiation fluence and surface termination. Here we show that process parameters such as defect center interactions, in particular, different nitrogen defects and radiation induced lattice defects, as well as surface functionalities have a strong influence on the fluorescence intensity, fluorescence lifetime and the charge state ratio of the NV centers. By employing a time-correlated single photon counting approach we also established a method for fast macroscopic monitoring of the fluorescence properties of ND samples. We found that the fluorescence properties of NV centers may be controlled or even tuned depending upon the radiation treatment, annealing, and surface termination.

A molecular quantum spin network controlled by a single qubit.

Scalable quantum technologies require an unprecedented combination of precision and complexity for designing stable structures of well-controllable quantum systems on the nanoscale. It is a challenging task to find a suitable elementary building block, of which a quantum network can be comprised in a scalable way. We present the working principle of such a basic unit, engineered using molecular chemistry, whose collective control and readout are executed using a nitrogen vacancy (NV) center in diamond. The basic unit we investigate is a synthetic polyproline with electron spins localized on attached molecular side groups separated by a few nanometers. We demonstrate the collective readout and coherent manipulation of very few (≤ 6) of these S = 1/2 electronic spin systems and access their direct dipolar coupling tensor. Our results show that it is feasible to use spin-labeled peptides as a resource for a molecular qubit-based network, while at the same time providing simple optical readout of single quantum states through NV magnetometry. This work lays the foundation for building arbitrary quantum networks using well-established chemistry methods, which has many applications ranging from mapping distances in single molecules to quantum information processing.

Nuclear quantum-assisted magnetometer.

Magnetic sensing and imaging instruments are important tools in biological and material sciences. There is an increasing demand for attaining higher sensitivity and spatial resolution, with implementations using a single qubit offering potential improvements in both directions. In this article we describe a scanning magnetometer based on the nitrogen-vacancy center in diamond as the sensor. By means of a quantum-assisted readout scheme together with advances in photon collection efficiency, our device exhibits an enhancement in signal to noise ratio of close to an order of magnitude compared to the standard fluorescence readout of the nitrogen-vacancy center. This is demonstrated by comparing non-assisted and assisted methods in a T1 relaxation time measurement.

A compact, diode laser based excitation system for microscopy of NV centers.

We demonstrate that a recently introduced family of direct-emitting green laser diodes is a simple yet efficient light source for excitation of NV centers in diamond. Thanks to their fast (sub-ns) response time, these sources are suitable for a broad variety of measurements, including pulsed optically detected magnetic resonance (ODMR) and fluorescence lifetime imaging. This feature, together with a drastically simplified design, is a significant advantage over the traditional excitation system comprising an Nd: YAG laser switched by an acousto-optic modulator. We introduce a simple design for such a compact laser system and experimentally verify that it enables simultaneous lifetime and ODMR measurements on NV centers. In particular, we find that the NV(-) charge state remains stable in spite of the short excitation wavelength of the new source.

Single defect center scanning near-field optical microscopy on graphene.

We present a scanning-probe microscope based on an atomic-size emitter, a single nitrogen-vacancy center in a nanodiamond. We employ this tool to quantitatively map the near-field coupling between the NV center and a flake of graphene in three dimensions with nanoscale resolution. Further we demonstrate universal energy transfer distance scaling between a point-like atomic emitter and a two-dimensional acceptor. Our study paves the way toward a versatile single emitter scanning microscope, which could image and excite molecular-scale light fields in photonic nanostructures or single fluorescent molecules.