Your email address will not be published. The reduced density matrix of the block is proportional to a projector to a degenerate ground state of another Hamiltonian. For example, by properties of the Borel functional calculus , we see that for any unitary operator U ,. Recent Comments. The UQ research team was able to verify this in a physical experiment by writing information in the shape of single photons — the particles that make up light. Archived from the original PDF on 12 April US: Prometheus Books.