Final Report Summary - ULTRAPHASE (Ultrafast Quantum Physics in Amplitude and Phase)
This project has explored the quantum physics of both light and matter operating out of a time-domain perspective. In the applied part of the activities, novel laser technologies and metrology tools have been developed to induce and study quantum phenomena which occur on molecular time scales, i.e. in the few-femtosecond range. Based on this advanced instrumentation, it became possible to investigate condensed matter under extreme conditions of high electric fields which can only be applied in a transient way. We were able to contribute fundamental knowledge to our understanding of complex electronic systems. Examples include the discovery of collective spin-phonon interactions in iron-pnictide high-temperature superconductors where the pairing mechanism is not yet understood. We were also able to demonstrate ultrafast switching from an insulating to metallic state in the strongly correlated compound vanadium dioxide by purely off-resonant interband tunneling of electrons. Also, it is possible to completely remove the chemical bonding in the standard semiconductor gallium arsenide in the direction of a transient mid-infrared field which induces Wannier-Stark localization - thus implementing a transient state of matter with very exotic but potentially useful properties. The most spectacular results of the project, however, are related to the first direct detection of the vacuum flucturations of the electric field. These are pure quantum fluctuations which exist at completely vanishing intensity due to the uncertainty principle. Using the bound electrons in a semiconductor as a test charge, we were able to investigate this fundamental quantum phenomenon without amplification. This feat is enabled by the fact that at extremely fast timescales, no energy conservation holds locally and purely virtual photons therefore become directly accessible. Going one step further, we have generated squeezed states of the light field where the fluctuation level can fall below the one of the bare vacuum at certain times. In order to still obey the uncertainty principle, the noise then exceeds the vacuum level at other places. Whether less noise than the vacuum level, i.e. squeezing, are found at a certain point in space-time or anti-squeezing with excess noise is determined by the deceleration or acceleration of the local reference frame co-propagating with the quantum vacuum inside an emitter crystal. Nonliner refractive index changes induced by intense few-femtosecond light fields are exploited to achieve these conditions. This technology represents the first access to quantum physics which operates out of a time-domain perspective and also the first quantum measurements at mid-infrared and terahertz frequencies. In the future, these tools might become very useful for science and technology because this frequency range hosts all the collective excitations of condensed matter which are key to the function of many complex and important systems as diverse as e.g. high-temperature superconductors or large biomolecules in a liquid environment.