Measuring the Polarization State of a Beam of Individual Photons

Yu.P. Peresun'ko, Valentin F. Boldyshev, E.A. Vinokurov

John Rarity, P. C. M. Owens, P. R. Tapster

When single photons are used to code for individual bits, security can be guaranteed by exploiting Heisenberg's uncertainty principle. Binary information can be coded into two orthogonal states, such as polarization or relative phase (in an interferometer). To receive this information we require an analyzer able to discriminate between the two states in an error-free way. In a linear-polarization system a polarizing beam splitter is used with its axes aligned to the two possible polarization directions. However, using a polarizer at 45° to these directions will ensure a random output uncorrelated to the single-photon-coded bit stream. The principle of secure key sharing, or quantum cryptography, is that a random bit stream is encoded onto individual photons while randomly switching the coding basis between the 0° and 45° polarization bases (in general, any non-orthogonal measurement bases). The receiver also randomly switches between these basis, and information is shared only when a photon is received and the same basis is used for coding and for measurement. An initial experiment of this type was carried out by using faint pulses of polarized visible light over a 40-cm transmission distance.1

Dong Li, Junxi Wang, Xiao‐Ming Xiu +3 more

We propose a scheme for preparing the distributed four-photon cluster-type polarization-entangled states associated with the integration of the controlled phase gate and two kinds of entanglement gates. Employing weak cross-Kerr nonlinear interactions and Homodyne measurement, two individual photons entangle together based on the bus function of coherent states. Assisted by optical elements mainly including polarization beam splitters, the entanglement of the signal photons is converted from the spatial mode to the polarization mode. The application of classical feed forward provides the high-efficiency to the preparation of Einstein–Podolsky–Rosen (EPR) states, Greenberger–Horne–Zeilinger (GHZ) states, and four-photon cluster-type polarization-entangled states. With regard to the four-photon cluster-type state, it can be generated in two distributed nodes with less experimental requirements, integrating the features of three logical gates.

M. Iinuma

Werner A. Hofer

We simulate correlation measurements of entangled photons numerically. The model employed is strictly local. In our model correlations arise from a phase, connecting the electromagnetic fields of the two photons at their separate points of measurement. We sum up coincidences for each pair individually and model the operation of a polarizer beam splitter numerically. The results thus obtained differ substantially from the classical results. In addition, we analyze the effects of decoherence and non-ideal beam splitters. It is shown that under realistic experimental conditions the Bell inequalities are violated by more than 30 standard deviations.

James M. Knight, Lev Vaidman A