This project deals with phenomena where both quantum mechanics and gravitation play an important role and focuses on the following three main lines of research:
The most accurate clocks to date rely on atomic transitions in the optical regime. On the other hand, some of the best inertial sensors employ atom interferometers, which exploit the wave nature of quantum particles and where laser pulses acting as diffraction gratings split, redirect and recombine atomic wave packets. A new kind of atom interferometers combines both aspects and makes use of laser pulses driving the transition between the two clock states to diffract the atomic wave packets.
In this way, detectors consisting of a pair of simultaneously operated interferometers separated by a long baseline and interrogated by a common laser beam can be extremely sensitive to tiny changes of the gravitational field. With baselines ranging from hundreds of meters to a few kilometers on ground, and from thousands to millions of kilometers or more in space, such instruments can be applied to Earth observation and gravitational-wave detection. In addition, they can be exploited to search for ultralight dark-matter candidates that give rise to a small oscillation of the energy difference between the two clock states.
Existing methods applicable to conventional atom interferometers need to be adapted to this new kind of interferometers, and whenever this is not possible, suitable alternatives are being developed.
In order to fully exploit the unprecedented sensitivities afforded by atom interferometry with long interferometer times, it is essential to employ slowly expanding atom ensembles with very narrow velocity distributions that correspond to effective temperatures in the picokelvin regime. This can be achieved by combining the use of Bose-Einstein condensates with matter-wave lensing techniques, which can reduce the expansion rate of atomic wave packets analogously to the way regular lenses collimate the divergence of a light beam along its direction of propagation.
In practice, however, small imperfections of the lensing potential lead to distortions of the atomic wave packets analogous to lens aberrations in optical systems. It is therefore important to develop analytical and numerical tools for characterizing and minimizing their impact on precision measurements. Moreover, some of these aberrations are interesting in their own right because they reveal genuine quantum effects with no classical counterpart that arise in systems undergoing nonlinear dynamics, and their possible experimental observation is being investigated.
When dealing with ultracold mixtures, atomic lensing is particularly challenging because the lensing potential acts differently on the various atomic species and it is typically not possible to collimate all the species simultaneously. This difficulty can be overcome with a very special kind of optical potentials capable of achieving a simultaneous collimation that are currently under investigation.
This subproject involves the study of general relativistic effects on quantum states of light such as single-photon states and entangled photon pairs. Particular attention is paid to experiments involving laser links to spacecrafts in low Earth orbits, such as the ISS, but also in higher orbits or even to the future Lunar Gateway orbiting the Moon. Such quantum links over long baselines will enable Bell tests and quantum teleportation in regimes where the effects of spacetime curvature are non-negligible. In addition, we are devising schemes relying on two-photon interference with pairs of frequency-entangled photons and capable of measuring the gravitational redshift and testing the equivalence principle with interferometers that have no classical analog.