Infrared scaling for a graviton condensate

The coupling between gravity and matter provides an intriguing length scale in the infrared for theories of gravity within Einstein-Hilbert action and beyond. In particular, we will show that such an infrared length scale is determined by the number of gravitons Ng 1 associated to a given mass in the non-relativistic limit. After tracing out the matter degrees of freedom, the graviton vacuum is found to be in a displaced vacuum with an occupation number of gravitons Ng 1. In the infrared, the length scale appears to be L = Ngp, where L is the new infrared length scale, and p is the Planck length. In a specific example, we have found that the infrared length scale is greater than the Schwarzschild radius for a slowly moving in-falling thin shell of matter. We will argue that the appearance of such an infrared length scale in higher curvature theories of gravity, such as in quadratic and cubic curvature theories of gravity, is also expected. Furthermore, we will show that gravity is fundamentally different from the electromagnetic interaction where the number of photons, Np, is the fine structure constant after tracing out an electron wave function

Photon bunching in a rotating reference frame

Although quantum physics is well understood in inertial reference frames (flat spacetime), a current challenge is the search for experimental evidence of nontrivial or unexpected behavior of quantum systems in noninertial frames. Here, we present a novel test of quantum mechanics in a noninertial reference frame: we consider Hong-Ou-Mandel (HOM) interference on a rotating platform and study the effect of uniform rotation on the distinguishability of the photons. Both theory and experiments show that the rotational motion induces a relative delay in the photon arrival times at the exit beam splitter and that this delay is observed as a shift in the position of the HOM dip. This experiment can be extended to a full general relativistic test of quantum physics using satellites in Earth’s orbit and indicates a new route toward the use of photonic technologies for investigating quantum mechanics at the interface with relativity

Gravitational optomechanics: photon-matter entanglement via graviton exchange

The deflection of light in the gravitational field of the Sun is one of the most fundamental consequences for general relativity as well as one of its classical tests first performed by Eddington a century ago. However, despite its center stage role in modern physics, no experiment has tested it in an ostensibly quantum regime where both matter and light exhibit nonclassical features. This paper shows that the interaction which gives rise to the light-bending also induces photon-matter entanglement as long as gravity and matter are treated at par with quantum mechanics. The quantum light-bending interaction within the framework of perturbative quantum gravity highlights this point by showing that the entangled states can be generated already with coherent states of light and matter exploiting the nonlinear coupling induced by graviton exchange. Furthermore, the quantum light-bending interaction is capable of discerning between the spin-2 and spin-0 gravitons thus also providing a test for alternative theories of gravity at short distances and at the quantum level. We will conclude by estimating the order of magnitude of the entanglement generated by employing the linear entropy. In particular, we find that a half-ring cavity of radius 0.25 m placed around a 10 kg mechanical oscillator operating at 150 Hz, could be used to generate linear entropy of order unity using a petawatt laser source at optical wavelengths. While the proposed scheme is beyond the current experimental realities it nonetheless initiates the discussion about testing the spin of the gravitational interaction at the quantum level

Coherent-scattering two-dimensional cooling in levitated cavity optomechanics

The strong light-matter optomechanical coupling offered by coherent scattering set-ups have allowed the experimental realization of quantum ground-state cavity cooling of the axial motion of a levitated nanoparticle [U. Delić et al., Science 367, 892 (2020)]. An appealing milestone is now quantum two-dimensional (2D) cooling of the full in-plane motion, in any direction in the transverse plane. By a simple adjustment of the trap polarization, one obtains two nearly equivalent modes, with similar frequencies ω x ∼ ω y and optomechanical couplings g x ≃ g y —in this experimental configuration we identify an optimal trap ellipticity, nanosphere size, and cavity linewidth which allows for efficient 2D cooling. Moreover, we find that 2D cooling to occupancies n x + n y ≲ 1 at moderate vacuum ( 10 − 6 mbar) is possible in a “Goldilocks” zone bounded by √ κ Γ / 4 ≲ g x , g y ≲ ∣ ∣ ω x − ω y ∣ ∣ ≲ κ , where one balances the need to suppress dark modes while avoiding far-detuning of either mode or low cooperativities, and κ ( Γ ) is the cavity decay rate (motional heating rate). With strong-coupling regimes g x , g y ≳ κ in view one must consider the genuine three-way hybridization between x , y and the cavity light mode resulting in hybridized bright/dark modes. Finally, we show that bright/dark modes in the levitated set-up have a simple geometrical interpretation, related by rotations in the transverse plane, with implications for directional sensing

Co-Creating Community-Based Solutions through Social Media in Estonia during the COVID-19 Pandemic

In this study, we aimed to explore and describe the prosocial behaviour of the community during the COVID-19 crisis in Estonia on Facebook, using mixed-method content analysis.This article focuses on the role of social media in co-creation in the context of the COVID-19 pandemic and the use of Facebook (FB) as a modern communication technology in times of crisis. Our goal was to learn how Facebook as a social media channel can be a tool and accelerator that allows people to find solutions to social problems in communities experiencing crises. The focus of the research is on finding solutions in co-creation for vulnerable target groups, including the elderly, people with disabilities, and other people who need support. This research expands on the role and potential of using FB as a communication platform to enhance co-creation.We used Kaun and Uldam’s (2018) model as a theoretical framework for this study. The study is characterised by a descriptive and exploratory research design. We studied the prosocial behaviour of the community on Facebook through a three-stage mixed method content analysis of existing data, including posts and comments on FB pages, using both quantitative (descriptive statistics) and qualitative (thematic analysis) data analysis methods. Our findings suggest that Facebook as a social media channel could be successfully utilised as a tool for sharing calls to action, activating citizens to co-create solutions, and disseminating results.Keywords: prosocial behaviour; co-creation; community support; Facebook; COVID-19

Requirements on quantum superpositions of macro-objects for sensing neutrinos

We examine a macroscopic system in a quantum superposition of two spatially separated localized states as a detector for a stream of weakly interacting relativistic particles. We do this using the explicit example of neutrinos with MeV-scale energy scattering from a solid object via neutral-current neutrino-nucleus scattering. Presuming the (anti)neutrino source to be a nuclear fission reactor, we utilize the estimated flux and coherent elastic neutrino-nucleus cross section to constrain the spatial separation Δx and describe the temporal evolution of the sensing system. Particularly, we find that a potentially measurable relative phase between quantum superposed components is obtained for a single gram scale mass placed in a superposition of spatial components separated by 10−14 m under sufficient cooling and background suppression

Dynamical model selection near the quantum-classical boundary

We discuss a general method of model selection from experimentally recorded time-trace data. This method can be used to distinguish between quantum and classical dynamical models. It can be used in post-selection as well as for real-time analysis, and offers an alternative to statistical tests based on state-reconstruction methods. We examine the conditions that optimize quantum hypothesis testing, maximizing one's ability to discriminate between classical and quantum models. We set upper limits on the temperature and lower limits on the measurement efficiencies required to explore these differences, using a novel experiment in levitated optomechanical systems as an example.Comment: 9 pages, 1 figure. Accepted for publication in Physical Review A (Rapid Communication

Relative acceleration noise mitigation for nanocrystal matter-wave interferometry: Applications to entangling masses via quantum gravity

Matter-wave interferometers with large momentum transfers, irrespective of specific implementations, will face a universal dephasing due to relative accelerations between the interferometric mass and the associated apparatus. Here we propose a solution that works even without actively tracking the relative accelerations: putting both the interfering mass and its associated apparatus in a freely falling capsule, so that the strongest inertial noise components vanish due to the equivalence principle. In this setting, we investigate two of the most important remaining noise sources: (a) the noninertial jitter of the experimental setup and (b) the gravity-gradient noise. We show that the former can be reduced below desired values by appropriate pressures and temperatures, while the latter can be fully mitigated in a controlled environment. We finally apply the analysis to a recent proposal for testing the quantum nature of gravity [S. Bose et al., Phys. Rev. Lett. 119, 240401 (2017)] through the entanglement of two masses undergoing interferometry. We show that the relevant entanglement witnessing is feasible with achievable levels of relative acceleration noise

Gravitons in a box

Gravity and matter are universally coupled, and this unique universality provides us with an intriguing way to quantify quantum aspects of space-time in terms of the number of gravitons within a given box. In particular, we provide a limit on the number of gravitons if we trace out the matter degrees of freedom. We obtain the universal bound on the number of gravitons, which would be given by N g ≈ ( m / M p ) 2 . Since the number of gravitons also signifies the number of bosonic states they occupy, the number of gravitons indirectly constrain the system’s gravitational entropy. We show that it saturates the Bekenstein bound on the gravitational area law of entropy. Based on these observations, we ascertain that the gravitons permeating in the observable Universe are always N g ≫ 1