# POSTPONED: Einstein’s Light Quanta : From Millikan to Circuit QED

Einstein is well known for his rejection of quantum mechanics in the form it emerged from the work of Heisenberg, Born and Schrodinger in 1926. Much less appreciated are the many seminal contributions he made to quantum theory prior to his final scientific verdict: that the theory was at best incomplete. His many key conceptual innovations leading to the emergence of modern quantum theory place him as arguably its central figure [1]. In this talk I will focus on his introduction of the idea of quanta of light in 1905, the beginning of the photon concept in physics. Einstein recognized these quanta as “fundamental waves”, i.e. entities with wave-like properties which do not require a supporting medium, and he connected them closely with his argument for energy-mass equivalence. He spent much of his research effort between 1908 and 1911 on a failed attempt to generalize Maxwell’s wave equation to include light quanta, but in 1909 he was able to derive rigorously a fluctuation formula for blackbody radiation which implied wave-particle duality. His 1916-17 works on the quantum theory of radiation derived the Planck blackbody formula by means of the detailed balance condition and introduced the concepts of spontaneous and stimulated emission, while also strengthening the argument for considering light quanta to be “real” and not a heuristic construct. The observation of the Compton effect in 1923-1925 finally led to general acceptance of this idea, and soon afterwards they acquired the label “photons”. In modern physics photons are one of the fundamental force-carrying bosons in the Standard Model. Modern research in quantum optics, and more recently in circuit quantum electrodynamics, has enabled us for the first time to completely control the states of one or a few photons, and create arbitrary non-classical states in their Hilbert space. Because of this new level of control, few-photon states of microwave cavities are becoming leading candidates for quantum information storage and processing, due to the possibility of efficient error detection and correction [2]. I will review both the history described above and some highlights of these very recent developments in this talk. [1] “Einstein and the Quantum: The Quest of the Valiant Swabian”, A. Douglas Stone, Princeton University Press (2013). [2] Extending the lifetime of a quantum bit with error correction in superconducting circuits, N. Ofek, R.J. Schoelkopf et al. Nature , 536, 441–445 (2016).