  A quantum spin liquid (QSL) arises from a highly entangled superposition of
many degenerate classical ground states in a frustrated magnet, and is
characterized by emergent gauge fields and deconfined fractionalized
excitations (spinons). Because such a novel phase of matter is relevant to
high-transition-temperature superconductivity and quantum computation, the
microscopic understanding of QSL states is a long-sought goal in condensed
matter physics. The 3D pyrochlore lattice of corner-sharing tetrahedra can host
a QSL with U(1) gauge fields called quantum spin ice (QSI), which is a quantum
(with effective $S=1/2$) analog of the classical (with large effective moment)
spin ice. A key difference between QSI and classical spin ice is the predicted
presence of the linearly dispersing collective excitations near zero energy,
dubbed the "photons", arising from emergent quantum electrodynamics, in
addition to the spinons at higher energies. Recently, 3D pyrochlore systems
Ce2M2O7 (M = Sn, Zr, Hf) have been suggested as effective $S=1/2$ QSI
candidates, but there has been no evidence of quasielastic magnetic scattering
signals from photons, a key signature for a QSI. Here, we use polarized neutron
scattering experiments on single crystals of Ce2Zr2O7 to conclusively
demonstrate the presence of magnetic excitations near zero energy at 50 mK in
addition to signatures of spinons at higher energies. By comparing the energy
(E), wave vector (Q), and polarization dependence of the magnetic excitations
with theoretical calculations, we conclude that Ce2Zr2O7 is the first example
of a dipolar-octupolar $\pi$ flux QSI with dominant dipolar Ising interactions,
therefore identifying a microscopic Hamiltonian responsible for a QSL.
