Conference

Gravitational waves (GWs) are probes of the cosmological model since, as electromagnetic waves, they travel across the largest distances in our Universe. These signals can also track generic modifications of General Relativity on large scales, especially when combined with other cosmological probes. There are several methods for using such fascinating signals to learn more about cosmology. For instance, GWs are distance indicators, and similarly to supernovae, they allow reconstructions of a distance-redshift relation when the latter can be inferred from electromagnetic counterparts emitted by the same source or known indirectly via binary population studies or galaxy catalogs. In this colloquium I will discuss some of the current methods for doing cosmological parameter estimation and tests of gravity with GWs, focusing on studies of future multi-messenger observations and on combinations of GW data with large-scale structure surveys.

Quantum position verification (QPV) was first introduced, under the name quantum tagging, in a patent filed in 2004. It was first discussed in the academic literature in 2009-10. The schemes proposed to that point were shown to be breakable by teleportation attacks in 2010, and a general no-go theorem showing all schemes in this class are breakable was subsequently proved. However, in an alternative standard cryptographic security scenario, in which the tag is assumed to be able to keep classical data secret, unconditionally secure schemes were presented in 2010. The various terminologies and security scenarios highlight important points whose theoretical and practical implications still remain underexplored. In practice, one normally wants to verify the location of a person or valuable object, not of an easily replaceable tagging device, for at least two reasons: (i) the device itself is not so valuable, (ii) adversaries can easily construct a replacement device and thereby potentially spoof the scheme. This requires physical assumptions about the integrity of the tag and its attachment, and bounds on the speed with which the tag may be displaced or destroyed and replaced. QPV schemes that do not rely on such assumptions are breakable without teleportation or non-local computation attacks. In the other direction, when such significant physical assumptions are necessary, it may generally be reasonable to include tag security among them. In this overview I review the early history of QPV and describe various security scenarios and their potential applications. I give versions of the secure 2010 scheme designed for efficient practical implementation and discuss the frequency, accuracy and security of position verification attainable for these schemes with present technology. I also discuss the implied constraints on what may be attainable for QPV schemes involving real-time quantum measurement and/or quantum information processing.