Precision measurement of the quantum vacuum with petawatt lasers

Professor Wendell T. Hill, III (B.S.'74)
Joint Quantum Institute for Physical Science & Technology, Department of Physics, University of Maryland, College Park
Thursday, November 3, 2022
3:30 pm
ISEB 1200 (Zoom connection will be available as well)
Multi-petawatt laser pulses of short duration have placed us at the threshold of a new era where novel experimental investigations of nonlinear aspects of electrodynamics – quantum electrodynamics (QED) – will be possible.  Never before feasible tests of QED from the photon side, and the intimate coupling between QED and the quantum vacuum are on the horizon.  The very essence of the vacuum is entangled with a fundamental tenet of quantum physics – quantum fluctuation – virtual particles and antiparticles (e.g., electron-positron pairs) fluctuating into and out of existence.  Quantitative measurements of virtual particles not only will challenge calculations from the 1930s, they will set strenuous limits for add-ons to the Standard Model.  
Photons are unique probes in that they are uncharged and their Bosonic nature allows unlimited numbers of them to be co-located within an arbitrarily-small volume, at least classically.  Experimentally, this is realized by raising the peak intensity of a focused laser pulse through a combination of increasing the pulse energy and decreasing its duration.  Quantum mechanics makes a different prediction as the intensity increases.  The linear response of light propagating even in a physical vacuum, as Maxwell equations demand, gives way to a nonlinear response.  Post-Maxwellian theories, such as QED and Born-Infeld, allow virtual pairs to mediate an interaction between photons that can be viewed, to some extent, as light propagating through material.  At high enough intensity the quantum vacuum will break down, inducing real pairs to emerge.  The critical intensity (Icr) for breakdown, the so-called Schwinger limit, is ≃ 2×1029 W/cm2.  Even though Icr is beyond current technology, there are fundamental features of the quantum vacuum that can be explored at substantially lower intensities.  The intensity at which quantum nonlinear effects become unambiguously discernible in photon-only experiments is estimated to occur between 1023 and 1025 W/cm2, and perhaps below 1023 W/cm2.  In this talk we will explore some of these ideas, focusing on the new physics that can be learned, and the tools and conditions required.
Mu-Chun Chen