Experimental Observation of Current Reversal in a Rocking Brownian Motor.

A reversal of the particle current in overdamped rocking Brownian motors was predicted more than 20 years ago; however, an experimental verification and a deeper insight into this noise-driven mechanism remained elusive. Here, we investigate the high-frequency behavior of a rocking Brownian motor for 60 nm gold spheres based on electrostatic interaction in a 3D-shaped nanofluidic slit and electro-osmotic forcing of the particles. We measure the particle probability density in situ with 10 nm spatial and 250 µs temporal resolution and compare it with theory. At a driving frequency of 250 Hz, we observe a current reversal that can be traced to the asymmetric and increasingly static probability density at high frequencies.

Fast turnaround fabrication of silicon point-contact quantum-dot transistors using combined thermal scanning probe lithography and laser writing.

The fabrication of high-performance solid-state silicon quantum-devices requires high resolution patterning with minimal substrate damage. We have fabricated room temperature (RT) single-electron transistors (SETs) based on point-contact tunnel junctions using a hybrid lithography tool capable of both high resolution thermal scanning probe lithography and high throughput direct laser writing. The best focal z-position and the offset of the tip- and the laser-writing positions were determined in situ with the scanning probe. We demonstrate <100 nm precision in the registration between the high resolution and high throughput lithographies. The SET devices were fabricated on degenerately doped n-type >1020/cm3 silicon on insulator chips using a CMOS compatible geometric oxidation process. The characteristics of the three devices investigated were dominated by the presence of Si nanocrystals or phosphorous atoms embedded within the SiO2, forming quantum dots (QDs). The small size and strong localisation of electrons on the QDs facilitated SET operation even at RT. Temperature measurements showed that in the range 300 K > T > âˆ¼100 K, the current flow was thermally activated but at <100 K, it was dominated by tunnelling.

Publisher's Note: Multipartite Entanglement Detection with Minimal Effort [Phys. Rev. Lett. 117, 210504 (2016)].

This corrects the article DOI: 10.1103/PhysRevLett.117.210504.

Multipartite Entanglement Detection with Minimal Effort.

Certifying entanglement of a multipartite state is generally considered a demanding task. Since an N qubit state is parametrized by 4^{N}-1 real numbers, one might naively expect that the measurement effort of generic entanglement detection also scales exponentially with N. Here, we introduce a general scheme to construct efficient witnesses requiring a constant number of measurements independent of the number of qubits for states like, e.g., Greenberger-Horne-Zeilinger states, cluster states, and Dicke states. For four qubits, we apply this novel method to experimental realizations of the aforementioned states and prove genuine four-partite entanglement with two measurement settings only.

Genuine Multipartite Entanglement without Multipartite Correlations.

Nonclassical correlations between measurement results make entanglement the essence of quantum physics and the main resource for quantum information applications. Surprisingly, there are n-particle states which do not exhibit n-partite correlations at all but still are genuinely n-partite entangled. We introduce a general construction principle for such states, implement them in a multiphoton experiment and analyze their properties in detail. Remarkably, even without multipartite correlations, these states do violate Bell inequalities showing that there is no classical, i.e., local realistic model describing their properties.

Systematic errors in current quantum state tomography tools.

Common tools for obtaining physical density matrices in experimental quantum state tomography are shown here to cause systematic errors. For example, using maximum likelihood or least squares optimization to obtain physical estimates for the quantum state, we observe a systematic underestimation of the fidelity and an overestimation of entanglement. Such strongly biased estimates can be avoided using linear evaluation of the data or by linearizing measurement operators yielding reliable and computational simple error bounds.

Experimental comparison of efficient tomography schemes for a six-qubit state.

Quantum state tomography suffers from the measurement effort increasing exponentially with the number of qubits. Here, we demonstrate permutationally invariant tomography for which, contrary to conventional tomography, all resources scale polynomially with the number of qubits both in terms of the measurement effort as well as the computational power needed to process and store the recorded data. We demonstrate the benefits of combining permutationally invariant tomography with compressed sensing by studying the influence of the pump power on the noise present in a six-qubit symmetric Dicke state, a case where full tomography is possible only for very high pump powers.

Experimental Schmidt decomposition and state independent entanglement detection.

We introduce an experimental procedure for the detection of quantum entanglement of an unknown quantum state with a small number of measurements. The method requires neither a priori knowledge of the state nor a shared reference frame between the observers and can thus be regarded as a perfectly state-independent entanglement witness. The scheme starts with local measurements, possibly supplemented with suitable filtering, which essentially establishes the Schmidt decomposition for pure states. Alternatively we develop a decision tree that reveals entanglement within few steps. These methods are illustrated and verified experimentally for various entangled states of two and three qubits.

Useful multiparticle entanglement and sub-shot-noise sensitivity in experimental phase estimation.

We experimentally demonstrate a general criterion to identify entangled states useful for the estimation of an unknown phase shift with a sensitivity higher than the shot-noise limit. We show how to exploit this entanglement on the examples of a maximum likelihood as well as of a Bayesian phase estimation protocol. Using an entangled four-photon state we achieve a phase sensitivity clearly beyond the shot-noise limit. Our detailed comparison of methods and quantum states for entanglement enhanced metrology reveals the connection between multiparticle entanglement and sub-shot-noise uncertainty, both in a frequentist and in a Bayesian phase estimation setting.

Nanofluidic rocking Brownian motors.

Control and transport of nanoscale objects in fluids is challenging because of the unfavorable scaling of most interaction mechanisms to small length scales. We designed energy landscapes for nanoparticles by accurately shaping the geometry of a nanofluidic slit and exploiting the electrostatic interaction between like-charged particles and walls. Directed transport was performed by combining asymmetric potentials with an oscillating electric field to achieve a rocking Brownian motor. Using gold spheres 60 nanometers in diameter, we investigated the physics of the motor with high spatiotemporal resolution, enabling a parameter-free comparison with theory. We fabricated a sorting device that separates 60- and 100-nanometer particles in opposing directions within seconds. Modeling suggests that the device separates particles with a radial difference of 1 nanometer.