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Journal of Lightwave Technology

Fast Measurements of Entangled Photons

S. X. Wang, Paul M. Moraw, Daniel R. Reilly, Joseph B. Altepeter, Gregory S. Kanter

Published Dec 5, 2012• 11 citations
Publisher

Abstract

We report on a system for the generation and measurement of polarization entangled photons. High speed quantum state tomographies with a raw fidelity of >; 91 % with respect to an ideal entangled state are recorded in <; 2 seconds, with longer-term accidental-count subtracted fidelities exceeding 99%. Two-photon interference measurements are recorded using an automated alignment and measurement procedure with the signal distributed over 20 km of fiber. These high speed measurement methods are useful for high rate monitoring of entangled states.

DOI
10.1109/jlt.2012.2231854

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We report on a system for the generation and measurement of polarization entangled photons. High speed quantum state tomographies with a raw fidelity of >; 91 % with respect to an ideal entangled state are recorded in <; 2 seconds, with longer-term accidental-count subtracted fidelities exceeding 99%. Two-photon interference measurements are recorded using an automated alignment and measurement procedure with the signal distributed over 20 km of fiber. These high speed measurement methods are useful for high rate monitoring of entangled states.
We generate time-bin entangled photons and measure the resulting quantum state with a tomography system that uses asymmetric interferometers having $3\ifmmode\times\else\texttimes\fi{}3$ output couplers. This configuration allows for measurements to be made simultaneously in all bases that are required for tomographic reconstruction. By eliminating the burden of tuning interferometer phases and by measuring all the spatial and temporal modes, substantial improvements in measurement speed are observed. Raw fidelities of 84% with respect to an ideal entangled state are measured in about 2 s using a 12-MHz entangled state generator, with corresponding accidental count subtracted fidelities exceeding 90%.
We demonstrate the suitability of fiber-generated entangled photon pairs for practical quantum communications in the telecom band by measuring their properties with superconducting single-photon detectors that produce negligible dark counts. The photon pairs are created in approximately 5-ps duration windows at 50 MHz rate while the detectors are operated in ungated free running mode. We obtain a coincidence to accidental-coincidence ratio >80 with raw photon-counting data, i.e., without making any post-measurement corrections. Using a previously demonstrated counter-propagating scheme we also produce polarization-entangled photon pairs at 50-MHz rate, which in coincidence detection directly yield two-photon interference with a fringe visibility >98%.
We describe the generation of entangled and hyperentangled photon pairs using a microstructure-fiber Sagnac interferometer, which is formed by a polarizing beam splitter and a highly nonlinear microstructure fiber twisted by 90° from end to end. This interferometer allows two identical four-wave mixing processes to occur on the same fiber principal axis, ensuring perfect spatial and temporal mode matching of the two four-wave mixing outputs on the polarizing beam splitter to create entanglement over the entire four-wave mixing phase-matching spectral range. With an average pump power of 220 μW, we measure a two-photon coincidence rate of 1 kHz with ▵λ = 0.9 nm. Two-photon interference visibilities exceed 91% for polarization-entangled photon pairs generated from this source, and are > 84% for both time-bin and polarization degrees of freedom for hyperentangled photons, all without subtracting accidental coincidences.