  The first part of the paper reviews applications of 2-spinor methods to
relativistic qubits (analogies between tetrads in Minkowski space and 2-qubit
states, qubits defined by means of null directions and their role for
elimination of the Peres-Scudo-Terno phenomenon, advantages and disadvantages
of relativistic polarization operators defined by the Pauli-Lubanski vector,
manifestly covariant approach to unitary representations of inhomogeneous
SL(2,C)). The second part deals with electromagnetic fields quantized by means
of harmonic oscillator Lie algebras (not necessarily taken in irreducible
representations). As opposed to non-relativistic singlets one has to
distinguish between maximally symmetric and EPR states. The distinction is one
of the sources of `strange' relativistic properties of EPR correlations. As an
example, EPR averages are explicitly computed for linear polarizations in
states that are antisymmetric in both helicities and momenta. The result takes
the familiar form $\pm p\cos 2(\alpha-\beta)$ independently of the choice of
representation of harmonic oscillator algebra. Parameter $p$ is determined by
spectral properties of detectors and the choice of EPR state, but is unrelated
to detector efficiencies. Brief analysis of entanglement with vacuum and vacuum
violation of Bell's inequality is given. The effects are related to
inequivalent notions of vacuum states. Technical appendices discuss details of
the representation I employ in field quantization. In particular, M-shaped
delta-sequences are used to define Dirac deltas regular at zero.
