Abstract
Photonic-based qubits and integrated photonic circuits have enabled demonstrations of quantum information processing (QIP) that promises to transform the way in which we compute and communicate. To that end, sources of polarization-entangled photon pair states are an important enabling technology. However, such states are difficult to prepare in an integrated photonic circuit. Scalable semiconductor sources typically rely on nonlinear optical effects where polarization mode dispersion (PMD) degrades entanglement. Here, we directly generate polarization-entangled states in an AlGaAs waveguide, aided by the PMD and without any compensation steps. We perform quantum state tomography and report a raw concurrence as high as 0.91 ± 0.01 observed in a 1,100-nm-wide waveguide. The scheme allows direct Bell state generation with an observed maximum fidelity of 0.90 ± 0.01 from another (800-nm-wide) waveguide. Our demonstration paves the way for sources that allow for the implementation of polarization-encoded protocols in large-scale quantum photonic circuits.
Highlights
Integrated sources of polarization-entangled photon pairs are of great importance for the utilization of quantum information science in real world applications
We demonstrate a simple way of generating polarization-entangled photon pairs via orthogonally-polarized spontaneous four-wave mixing (SFWM) processes and show that an optimal, non-vanishing polarization mode dispersion is a necessity to generate such highly entangled states
SFWM processes with co-polarized interacting photons are preferentially phase matched in the spectral region of near degeneracy
Summary
Integrated sources of polarization-entangled photon pairs are of great importance for the utilization of quantum information science in real world applications. Most polarization-entanglement sources that are based on semiconductor waveguides rely on either the second-order χ(2) process or the third-order χ(3) process, in which pump photons (one photon for χ(2) and two for χ(3)) spontaneously decay into two correlated photons. The former process is commonly known as spontaneous parametric down-conversion (SPDC) and typically exploited in III-V materials such as AlGaAs12–16. Lv et al.[21] showed the possibility of utilizing both modes in parallel SFWM processes to generate polarization-entangled states in silicon waveguides; the PMD was still a major limitation and the demonstrated device is far from optimal. Through quantum state tomography measurements of different waveguides on the same chip, that the source exhibits a high raw concurrence of 0.91 ± 0.01 and is capable of directly producing maximally-entangled Bell states with a fidelity up to 0.90 ± 0.01
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