Evolution of Entanglement Between Distinguishable Light States

Abstract

We investigate the evolution of quantum correlations over the lifetime of a multiphoton state. Measurements reveal time-dependent oscillations of the entanglement fidelity for photon pairs created by a single semiconductor quantum dot. The oscillations are attributed to the phase acquired in the intermediate, nondegenerate, exciton-photon state and are consistent with simulations. We conclude that emission of photon pairs by a typical quantum dot with finite polarization splitting is in fact entangled in a time-evolving state, and not classically correlated as previously regarded.

Coherent dynamics of a telecom-wavelength entangled photon source

“…For the rectilinear basis shown in red, C begins close to unity and decays smoothly and slowly predominantly due to reexcitation. For the circular basis, shown in blue, C oscillates with the predicted cosine form 23,27 . The degree of correlation is also shown measured in the diagonal (green) and two elliptical bases (magenta and orange) that complete an equally spaced set around the equator of the Poincaré sphere.…”

“…For finite fine-structure splitting, d, the two eigenstate components of the single exciton state will then evolve, picking up a phase difference dt/h over time t until the exciton photon is emitted 23 . The corresponding two-photon evolving Bell state is as follows.…”

“…The fine-structure splitting of the orthogonally polarized exciton photons 21 is caused by asymmetries of the quantum-dot crystal structure, and must be small compared with the spectral bandwidth of the photons 22 , or frequency bandwidth of the detectors 23 , to allow entanglement to be discerned. Minimizing the splitting is typically achieved by growing quantum dots at a particular wavelength, B885 nm, due to a relationship between splitting and emission energy 24 .…”

Single pairs of time-bin-entangled photons

“…We measured a fine-structure splitting of S = 0.6 ± 0.2 μeV [ Fig. 2(e)], corresponding to a precession period of the X spin state of h/S = 7.3 ± 1.9 ns, where h is Planck's constant [18]. Since this period is much longer than the X lifetime, spin precession has only a small influence on the correlations between the polarization of the XX and X photons.…”

“…Of course, the data transmission rate is then reduced as well. A more rigorous solution, which has been shown to work for polarization entanglement (but not yet for time-bin entanglement), is to generate single pairs of entangled photons by exciting a biexciton (XX) in a semiconductor quantum dot [14][15][16][17][18][19][20][21][22]. A XX is an excited state consisting of two electrons and two holes.…”