A monopole antenna at optical frequencies: single-molecule near-field measurements

We present a monopole antenna for optical frequencies (~600 THz) and discuss near-field measurements with single fluorescent molecules as a technique to characterize such antennas. The similarities and differences between near-field antenna measurements at optical and radio frequencies are discussed in detail. Two typical antenna properties, polarization selectivity and resonances, are studied for the optical monopole by direct near-field measurements and finite integration technique calculations. The antenna is driven by the local field of a sub-wavelength aperture. This gives rise to a dependence of the antenna response on the orientation of the local field vector, in an analogous way to the polarization selectivity of linear wire antennas. The antenna resonances are studied by varying the antenna length. Typical monopole resonances are demonstrated. The finite conductivity of metals at optical frequencies (in combination with the antenna radius) causes the wavelength of the surface charge density oscillation (surface plasmon polariton) along the antenna to be shortened in comparison to the free space wavelength. As a result, resonances for the optical monopole antenna occur at much shorter relative lengths than for conventional radio monopole antennas\ud \u

Nanorod optical antennas for dipolar transitions

Optical antennas link objects to light. Here, we analyze metal nanorod antennas as cavities with variable reflection coefficients to derive the interaction of dipolar transitions with radiation through the antenna modes. The presented analytical model accurately describes the complete emission process, and is summarized in a phase-matching equation. We show how antenna modes evolve as they become increasingly more bound, i.e. plasmonic. The results illustrate why efficient antennas should not be too plasmonic, and how subradiant even modes can evolve into weakly-interacting dark modes. Our description is valid for the interaction of nanorods with light in general, and is thus widely applicable.Comment: 10 pages, 4 figures, submitte

One-second coherence for a single electron spin coupled to a multi-qubit nuclear-spin environment

Single electron spins coupled to multiple nuclear spins provide promising multi-qubit registers for quantum sensing and quantum networks. The obtainable level of control is determined by how well the electron spin can be selectively coupled to, and decoupled from, the surrounding nuclear spins. Here we realize a coherence time exceeding a second for a single electron spin through decoupling sequences tailored to its microscopic nuclear-spin environment. We first use the electron spin to probe the environment, which is accurately described by seven individual and six pairs of coupled carbon-13 spins. We develop initialization, control and readout of the carbon-13 pairs in order to directly reveal their atomic structure. We then exploit this knowledge to store quantum states for over a second by carefully avoiding unwanted interactions. These results provide a proof-of-principle for quantum sensing of complex multi-spin systems and an opportunity for multi-qubit quantum registers with long coherence times

Robust quantum-network memory using decoherence-protected subspaces of nuclear spins

The realization of a network of quantum registers is an outstanding challenge in quantum science and technology. We experimentally investigate a network node that consists of a single nitrogen-vacancy (NV) center electronic spin hyperfine-coupled to nearby nuclear spins. We demonstrate individual control and readout of five nuclear spin qubits within one node. We then characterize the storage of quantum superpositions in individual nuclear spins under repeated application of a probabilistic optical inter-node entangling protocol. We find that the storage fidelity is limited by dephasing during the electronic spin reset after failed attempts. By encoding quantum states into a decoherence-protected subspace of two nuclear spins we show that quantum coherence can be maintained for over 1000 repetitions of the remote entangling protocol. These results and insights pave the way towards remote entanglement purification and the realisation of a quantum repeater using NV center quantum network nodes

Demonstration of entanglement-by-measurement of solid state qubits

Projective measurements are a powerful tool for manipulating quantum states. In particular, a set of qubits can be entangled by measurement of a joint property such as qubit parity. These joint measurements do not require a direct interaction between qubits and therefore provide a unique resource for quantum information processing with well-isolated qubits. Numerous schemes for entanglement-by-measurement of solid-state qubits have been proposed, but the demanding experimental requirements have so far hindered implementations. Here we realize a two-qubit parity measurement on nuclear spins in diamond by exploiting the electron spin of a nitrogen-vacancy center as readout ancilla. The measurement enables us to project the initially uncorrelated nuclear spins into maximally entangled states. By combining this entanglement with high-fidelity single-shot readout we demonstrate the first violation of Bells inequality with solid-state spins. These results open the door to a new class of experiments in which projective measurements are used to create, protect and manipulate entanglement between solid-state qubits.Comment: 6 pages, 4 figure

Atomic-scale confinement of optical fields

In the presence of matter there is no fundamental limit preventing confinement of visible light even down to atomic scales. Achieving such confinement and the corresponding intensity enhancement inevitably requires simultaneous control over atomic-scale details of material structures and over the optical modes that such structures support. By means of self-assembly we have obtained side-by-side aligned gold nanorod dimers with robust atomically-defined gaps reaching below 0.5 nm. The existence of atomically-confined light fields in these gaps is demonstrated by observing extreme Coulomb splitting of corresponding symmetric and anti-symmetric dimer eigenmodes of more than 800 meV in white-light scattering experiments. Our results open new perspectives for atomically-resolved spectroscopic imaging, deeply nonlinear optics, ultra-sensing, cavity optomechanics as well as for the realization of novel quantum-optical devices

Thresholds for the distributed surface code in the presence of memory decoherence

In the search for scalable, fault-tolerant quantum computing, distributed quantum computers are promising candidates. These systems can be realized in large-scale quantum networks or condensed onto a single chip with closely situated nodes. We present a framework for numerical simulations of a memory channel using the distributed toric surface code, where each data qubit of the code is part of a separate node, and the error-detection performance depends on the quality of four-qubit Greenberger–Horne–Zeilinger (GHZ) states generated between the nodes. We quantitatively investigate the effect of memory decoherence and evaluate the advantage of GHZ creation protocols tailored to the level of decoherence. We do this by applying our framework for the particular case of color centers in diamond, employing models developed from experimental characterization of nitrogen-vacancy centers. For diamond color centers, coherence times during entanglement generation are orders of magnitude lower than coherence times of idling qubits. These coherence times represent a limiting factor for applications, but previous surface code simulations did not treat them as such. Introducing limiting coherence times as a prominent noise factor makes it imperative to integrate realistic operation times into simulations and incorporate strategies for operation scheduling. Our model predicts error probability thresholds for gate and measurement reduced by at least a factor of three compared to prior work with more idealized noise models. We also find a threshold of 4 X 102 in the ratio between the entanglement generation and the decoherence rates, setting a benchmark for experimental progress