Understanding gravity in the framework of quantum mechanics is one of the great challenges in modern physics. However, the lack of empirical evidence has lead to a debate on whether gravity is a quantum entity. Despite varied proposed probes for quantum gravity, it is fair to say that there are no feasible ideas yet to test its quantum coherent behavior directly in a laboratory experiment. Here, we introduce an idea for such a test based on the principle that two objects cannot be entangled without a quantum mediator. We show that despite the weakness of gravity, the phase evolution induced by the gravitational interaction of two micron size test masses in adjacent matter-wave interferometers can detectably entangle them even when they are placed far apart enough to keep Casimir-Polder forces at bay. We provide a prescription for witnessing this entanglement, which certifies gravity as a quantum coherent mediator, through simple spin correlation measurements.

We describe a method based on precision magnetometry that can extend the search for axionmediated spin-dependent forces by several orders of magnitude. By combining techniques used in nuclear magnetic resonance and short-distance tests of gravity, our approach can substantially improve upon current experimental limits set by astrophysics, and probe deep into the theoretically interesting regime for the Peccei-Quinn (PQ) axion. Our method is sensitive to PQ axion decay constants between 10 9 and 10 12 GeV or axion masses between 10 −6 and 10 −3 eV, independent of the cosmic axion abundance.

We propose an experiment using optically trapped and cooled dielectric microspheres for the detection of short-range forces. The center-of-mass motion of a microsphere trapped in vacuum can experience extremely low dissipation and quality factors of 10 12 , leading to yoctonewton force sensitivity. Trapping the sphere in an optical field enables positioning at less than 1 µm from a surface, a regime where exotic new forces may exist. We expect that the proposed system could advance the search for non-Newtonian gravity forces via an enhanced sensitivity of 10 5 − 10 7 over current experiments at the 1 µm length scale. Moreover, our system may be useful for characterizing other short-range physics such as Casimir forces.PACS numbers: 04.80. Cc,07.10.Pz,42.50.Pq Since the pioneering work of Ashkin and coworkers in the 1970s [1], optical trapping of dielectric objects has become an extraordinarily rich area of research. Optical tweezers are used extensively in biophysics to study and manipulate the dynamics of single molecules, and in soft condensed-matter physics to study macromolecular interactions [2,3]. Much recent work has focused on trapping in solution where strong viscous damping dominates particle motion. There has also been interest in extending the regime that Ashkin and coworkers opened, namely, trapping sub-wavelength particles in vacuum where particle motion is strongly decoupled from a room-temperature environment [1,4].Recent theoretical studies have suggested that nanoscale dielectric objects trapped in ultrahigh vacuum might be cooled to their ground state of (center-of-mass) motion via radiation pressure forces of an optical cavity [5,6]. This remarkable result is made possible by isolation from the thermal bath, robust decoupling from internal vibrations, and lack of a clamping mechanism. In fact, a trapped dielectric nanosphere has been predicted to attain an ultrahigh mechanical quality factor Q exceeding 10 12 for the center-of-mass mode, limited by background gas collisions. Such large Q factors enable cavity cooling, for which the lowest possible phonon occupation of the mechanical oscillator is n T /Q, where n T is the number of room-temperature thermal phonons. Although such Q factors have yet to be observed in experiment, optically levitated microspheres have been trapped in vacuum for lifetimes exceeding 1000 s [1] and electrically levitated microspheres have exhibited pressurelimited damping that is consistent with theoretical predictions down to ∼ 10 −6 Torr [7].In addition to being beneficial for ground-state cooling and studies of quantum coherence in mesoscopic systems, mechanical oscillators with high quality factors also enable sensitive resonant force detection [8,9]. Optically levitated microspheres in vacuum have been studied theoretically in the context of reaching and exceeding the standard quantum limit of position measurement [10]. In this paper, we discuss the force sensing capability of a microsphere trapped inside a medium-finesse optical cavity at ultra-high vacuum, ...

Optically trapped nanospheres in high-vaccum experience little friction and hence are promising for ultra-sensitive force detection. Here we demonstrate measurement times exceeding $10^5$ seconds and zeptonewton force sensitivity with laser-cooled silica nanospheres trapped in an optical lattice. The sensitivity achieved exceeds that of conventional room-temperature solid-state force sensors, and enables a variety of applications including electric field sensing, inertial sensing, and gravimetry. The optical potential allows the particle to be confined in a number of possible trapping sites, with precise localization at the anti-nodes of the optical standing wave. By studying the motion of a particle which has been moved to an adjacent trapping site, the known spacing of the lattice anti-nodes can be used to calibrate the displacement spectrum of the particle. Finally, we study the dependence of the trap stability and lifetime on the laser intensity and gas pressure, and examine the heating rate of the particle in high vacuum in the absence of optical feedback cooling.Comment: 5 pages, 4 figures, minor changes, typos corrected, references adde

We propose a tunable resonant sensor to detect gravitational waves in the frequency range of 50 -300 kHz using optically trapped and cooled dielectric microspheres or micro-discs. The technique we describe can exceed the sensitivity of laser-based gravitational wave observatories in this frequency range, using an instrument of only a few percent of their size. Such a device extends the search volume for gravitational wave sources above 100 kHz by 1 to 3 orders of magnitude, and could detect monochromatic gravitational radiation from the annihilation of QCD axions in the cloud they form around stellar mass black holes within our galaxy due to the superradiance effect.PACS numbers: 04.80. Nn,95.55.Ym,14.80.Va Introduction. Over the past 40 years optical trapping of dielectric objects, both macroscopic and atomic, has made a profound impact in a wide range of fields ranging from fundamental physics to the life sciences. First studied by Ashkin and coworkers [1], optically trapped dielectrics in ultra-high vacuum become well decoupled from their room temperature environment [2][3][4][5][6]. Recent work suggests that the center of mass motion of such levitated objects can attain mechanical quality factors in excess of 10 12 , while internal vibrational modes are completely decoupled. This remarkable decoupling can be harnessed for cooling the center of mass motion of such objects to the quantum ground state [5][6][7][8]. These systems also have been considered in the context of reaching and exceeding the standard quantum limit of position measurement [9]. In addition, these techniques enable ultra-sensitive force detection [10][11][12] and extend the study of quantum coherence to the mesoscopic regime.