Levitated optomechanical systems, and particularly particles trapped in vacuum, provide unique platforms for studying the mechanical behavior of objects well-isolated from their environment. Ultimately, such systems may enable the study of fundamental questions in quantum mechanics, gravity, and other weak forces. While the optical trapping of nanoparticles has emerged as the prototypical levitated optomechanical system, it is not without problems due to the heating from the high optical intensity required, particularly when combined with a high vacuum environment. Here we investigate a magneto-gravitational trap in ultra-high vacuum. In contrast to optical trapping, we create an entirely passive trap for diamagnetic particles by utilizing the magnetic field generated by permanent magnets and the gravitational interaction. We demonstrate cooling the center of mass motion of a trapped silica microsphere from ambient temperature to an effective temperature near or below one milliKelvin in two degrees of freedom by optical feedback damping.

Levitated diamond nanocrystals with nitrogen-vacancy (NV) centres in high vacuum have been proposed as a unique system for experiments in fundamental quantum mechanics, including the generation of large quantum superposition states and tests of quantum gravity. This system promises extreme isolation from its environment while providing quantum control and sensing through the NV centre spin. While optical trapping has been the most explored method of levitation, recent results indicate that excessive optical heating of the nanodiamonds under vacuum may make the method impractical with currently available materials. Here, we study an alternative magneto-gravitational trap for diamagnetic particles, such as diamond nanocrystals, with stable levitation from atmospheric pressure to high vacuum. Magnetic field gradients from permanent magnets confine the particle in two dimensions, while confinement in the third dimension is gravitational. We demonstrate that feedback cooling of the centre-of-mass motion of a trapped nanodiamond cluster results in cooling of one degree of freedom to less than 1 K.

No abstract

We study transport mediated by Andreev bound states formed in InSb nanowire quantum dots. Two kinds of superconducting source and drain contacts are used: epitaxial Al/InSb devices exhibit a doubling of tunneling resonances, while in NbTiN/InSb devices Andreev spectra of the dot appear to be replicated multiple times at increasing source-drain bias voltages. In both devices, a mirage of a crowded spectrum is created. To describe the observations a model is developed that combines the effects of a soft induced gap and of additional Andreev bound states both in the quantum dot and in the finite regions of the nanowire adjacent to the quantum dot. Understanding of Andreev spectroscopy is important for the correct interpretation of Majorana experiments done on the same structures.The superconductor-semiconductor hybrid structures are of recent interest due to the possibility of inducing topological superconductivity accompanied by Majorana bound states (MBS) [1][2][3][4]. More generally, when a semiconductor is of finite size, proximity to a superconductor gives rise to subgap quasiparticle excitations, the so-called Andreev bound states (ABS), that appear due to successive Andreev reflections at the interfaces. Single ABS have been demonstrated in a variety of structures including self-assembled quantum dots, semiconductor nanowires, atomic break junctions, carbon nanotubes and graphene [5][6][7][8][9][10][11]. ABS exhibit many similarities to MBS, and therefore ABS can be served as a prototypical system for Majorana studies [12,13]. A powerful experimental method for investigating both MBS and ABS is via tunneling, either from a nanofabricated probe or by scanning tunneling spectroscopy. The latter is typically performed on Yu-Shiba-Rusinov states which are closely analogous to ABS but originate from magnetic impurities in superconductors [14,15].In this paper, we focus on the mesoscopic effects within the tunneling probes. We show that the non-trivial densities of states (DOS) in the probes can drastically affect tunneling characteristics by generating multiple replicas of ABS. To experimentally investigate these effects, we use semiconductor nanowires coupled to superconductors. ABS are induced in a quantum dot by strongly coupling the dot to one superconducting contact. A second superconducting contact and a nanowire segment ad-jacent to it act as a tunneling probe. To explain our observations, we consider the effects of soft induced superconducting gap in the nanowire, and of additional ABS induced in nanowire segments adjacent to the dot. The surprising observation of sub-gap negative differential conductance (NDC) is found to be consistent with a peak in the density of states of the probe at zero chemical potential, which is present even at zero magnetic field. The exact origin of this anomalous density of states remains an open question. Our findings emphasize the importance of understanding the spectral structure of the measuring contacts to interpret tunneling experiments in mesoscopic systems. We expect t...

No abstract