Authors: Sahar Sahebdivan

In this paper, we are exploring the feasibility of observing non-classical features of gravity in a low-energy regime in a quantum optomechanical experiment. If gravity is to have an underlying quantum nature, it should hold the most fundamental quantum characteristics such as the superposition principle and entanglement. Despite the weakness of gravity, in principle there is a chance, to observe such a quantum signature of the gravity by exploiting the quantum optomechanical techniques, without direct observation of graviton. We are investigating a new dynamical scheme called, gravitational quantum regime, in which the source of gravity is a quantum particle, and its centre of mass is subject to the spatial superposition. In a Gedankenexperiment, a test particle is gravitationally interacting with a quantum nanoparticle in a double-slit setup. Possible entanglement or superposition of the fields is investigated. We are looking for the corresponding deviation of the classical description of gravity despite being far from the Planck scale. Any experimental interrogation which reveals that gravitational field obeys the quantum superposition principle would be the first recognition of quantumness of gravity. This study will show how feasible it is to search for a non-classical feature of gravity in such a regime of motion. Moreover, this proposal would be an attempt to test the objectivity of the quantum superposition principle and its contribution to the microstructure of space-time.

Authors: Joy Christian (Oxford)

Bell inequalities are usually derived by assuming locality and realism, and therefore experimental violations of Bell inequalities are usually taken to imply violations of either locality or realism, or both. But, after reviewing an oversight by Bell, here we derive the Bell-CHSH inequality by assuming only that Bob can measure along the directions b and b’ simultaneously while Alice measures along either a or a’, and likewise Alice can measure along the directions a and a’ simultaneously while Bob measures along either b or b’, without assuming locality. The observed violations of the Bell-CHSH inequality therefore simply verify the manifest impossibility of measuring along the directions b and b’ (or along the directions a and a’) simultaneously, in any realizable EPR-Bohm type experiment.

Authors: Morgan W. Mitchell, Silvana Palacios Alvarez

The energy resolution per bandwidth $E_R$ is a figure of merit that combines the field resolution, bandwidth or duration of the measurement, and size of the sensed region. Several very different dc magnetometer technologies approach $E_R = \hbar$, while to date none has surpassed this level. This suggests a technology-spanning quantum limit, a suggestion that is strengthened by model-based calculations for nitrogen-vacancy centres in diamond, for dc SQUID sensors, and for optically-pumped alkali-vapor magnetometers, all of which predict a quantum limit close to $E_R = \hbar$. Here we review what is known about energy resolution limits, with the aim to understand when and how $E_R$ is limited by quantum effects. We include a survey of reported sensitivity versus size of the sensed region for a dozen magnetometer technologies, review the known model-based quantum limits, and critically assess possible sources for a technology-spanning limit, including zero-point fluctuations, magnetic self-interaction, and quantum speed limits. Finally, we describe sensing approaches that appear to be unconstrained by any of the known limits, and thus are candidates to surpass $E_R = \hbar$.

How a particle racing through a vacuum leaves a trail of blue light

How a particle racing through a vacuum leaves a trail of blue light, Published online: 02 May 2019; doi:10.1038/d41586-019-01430-0

Blue-tinged Cherenkov radiation could help to illuminate quantum interactions between light and matter.