  Classical structured light with controlled polarization and orbital angular
momentum (OAM) of electromagnetic waves has varied applications in optical
trapping, bio-sensing, optical communications and quantum simulations. However,
the definition of quantum density of spin and OAM at the single-photon level
remains elusive. Here, we develop a theoretical framework and put forth the
concept of quantum structured light for space-time wavepackets at the single
photon level. Our work marks a paradigm shift beyond scalar-field theory as
well as the paraxial approximation and can be utilized to study the quantum
properties of the spin and OAM of all classes of twisted quantum light pulses.
Our framework captures the uncertainty in projections of vector spin and OAM
demonstrating their quantum behavior beyond the conventional concept of
classical polarization. Specifically, we show that the spin density generates
modulated helical texture beyond paraxial limit and exhibits distinct
statistics for Fock-state vs. coherent-state twisted pulses. We introduce the
quantum correlation of the photon spin density to characterize the nonlocal
spin noise providing a rigorous parallel with fermionic spin noise operators.
Our work paves the way for quantum spin-OAM physics in twisted single photon
pulses and can also opens explorations for new phases of light with long-range
spin order.
