X-ray precision polarimetry and the detection of vacuum birefringence
Vacuum isn’t just empty space. Rather there is a continuous creation and annihilation of virtual pairs. A strong electric field can align them to a certain degree such that vacuum becomes birefringent – according to quantum electrodynamics. The effect has been predicted almost 90 years ago, but never been directly verified to date.
We have been developing X-ray polarimetry over the past 12 years in order to detect vacuum birefringence. The current status is an extinction ratio of 11 orders of magnitude using channel-cut crystals. This is a figure not nearly matched by any other polarimeter in any spectral region. Besides the physics of X-ray polarimetry, I will also discuss the remaining issues for the detection of vacuum birefringence.
The Helium hydride ion in strong laser fields
The Helium hydride ion is considered to be the first molecule that has formed after the big bang, a fact already pointing to the fundamental importance of this ion. Nevertheless, its behavior in intense, ultrashort laser fields has not been addressed until recently. This is in strong contrast to another fundamentally important molecule, the hydrogen molecular ion, on which many thousands of papers have been published.
I will discuss a series of experiments using different isotopologues of the Helium hydride ion at different wavelengths. The dissociation and ionization dynamics turns out to be vastly different from the hydrogen molecular ion. Moreover, it changes dramatically when moving from the near- to the mid-infrared spectral region. Although Helium hydride and the hydrogen molecule are isoelectronic, they can be seen as opposing extremes.
