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Dissipation in quantum gases

Final Report Summary - DIBEC (Dissipation in quantum gases)

The intriguing transition from quantum to classical lies in the mechanisms of interaction of a quantum system with its environment. This interaction, usually characterised by a very large number of degrees of freedom, is responsible for dissipation and decoherence. Indeed, any coupling with the environment produces an exchange of energy or particles and therefore represents a source of dissipation. The main objective of the project was to implement a controlled and localised source of dissipation for a macroscopic quantum object. To do this we proposed to shine a focused electron beam (EB) in the centre of a trapped atomic Bose-Einstein condensate (BEC) recording the number of produced ions (vie electron-impact ionisation) as a function of time. The most interesting parameter of the EB that can be varied is the current, i.e. the number of electrons that are sent into the BEC per unit of time. We can therefore consider the current of the EB as an indicator for the strength of the dissipative potential.

Our proposal was to probe some peculiar quantum properties of the BEC looking at its behaviour as the strength of the dissipation is varied. It comes directly from the basis of quantum mechanics that, with a single quantum system, as soon as the wave function is measured, it suddenly changes. This avoid to perform exclusive measurements on the same system (Bohr's complementarity) and also to determine the wave function of a single system perfectly well. Conversely, in case the system is composed by many particles with the same wave function (quantum state), these limitations do not apply any more. The wave function becomes similar to a classical field and we can use some particles to make one measurements while the others to make a complementary (exclusive) measurement. A BEC is a perfect example of this second kind of system indeed it is a many-body quantum state that can be described by a macroscopic classical field. If we shine the EB on the BEC, the position of some atoms will be measured (their wave function will collapse) via electron impact ionisation. This allow a local measurement of the macroscopic wave function of the BEC (that indeed is the wave function that all the atoms share) without completely destroy it.

The project clearly demonstrated the implementation of an open many-body quantum system whose Hamiltonian and dissipative dynamics can be independently and accurately controlled as described above. In the case of extremely strong and localised dissipation, this can lead to the creation of dissipation-resistant states and quantum Zeno dynamics. The possibility to create such states in a controlled fashion can give new insights for engineering generalised environmental dark states. These kinds of states are of fundamental interest and can possibly have practical applications in quantum computation schemes. And in as much as our technique exploits the demonstrated link between dissipation and measurement, it can be used to address fundamental issues in quantum mechanics, like the definition of the time of arrival. The dissipation mechanism studied in this project is also particularly suited for lattice systems, thanks to its localised character and hence to the ability to selectively control the dissipation in a single lattice site. Indeed the use of the EB offers the unique possibility to create and study long-living exotic states in optical lattices and to characterise the interplay between dissipation and interactions and so would give access to the engineering of quantum phases in open quantum systems.