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Physical Review A

Generation of maximally-polarization-entangled photons on a chip

Sergei V. Zhukovsky, L. G. Helt, Dongpeng Kang, Payam Abolghasem, Amr Saher Helmy, John E. Sipe

Published Jan 25, 2012• 41 citations
Publisher

Abstract

We propose a method of generating maximally-polarization-entangled states by type-II spontaneous parametric down-conversion in Bragg reflection waveguides. Analytic expressions for the group velocities of down-converted modes are used to engineer zero group-velocity mismatch at the operating point, making the spectra of photons within a down-converted pair indistinguishable and thus leading to a maximally-polarization-entangled state. The results can be used for the creation and manipulation of polarization-entangled qubits entirely on a chip.

DOI
10.1103/physreva.85.013838

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We propose a method of generating maximally-polarization-entangled states by type-II spontaneous parametric down-conversion in Bragg reflection waveguides. Analytic expressions for the group velocities of down-converted modes are used to engineer zero group-velocity mismatch at the operating point, making the spectra of photons within a down-converted pair indistinguishable and thus leading to a maximally-polarization-entangled state. The results can be used for the creation and manipulation of polarization-entangled qubits entirely on a chip.
Quantum states of photons such as entangled and single photons are indispensable components for applications in optical quantum information processing. Being able to generate and manipulate quantum states of photons on a chip is a crucial step towards realizing real-world quantum information processing systems such as an optical quantum computer. In this thesis, theoretical and experimental research was carried out for on-chip generation of quantum states of photons via the nonlinear process of spontaneous parametric down-conversion (SPDC), including polarization entangled photons, hyperentangled photons and heralded pure single photons. A majority of the study was based on III-V semiconductor AlGaAs, which could lead to monolithic integration of pump lasers and enable electrically pump sources operating at room temperature. The generation of cross-polarized entangled photons via a type-II SPDC in an AlGaAs Bragg reflection waveguide (BRW) was first investigated. The lack of material birefringence enables the direct generation of wavelength-multiplexed polarization entangled photons with a concurrence of at least 0.77 in a bandwidth of 95 nm. Second, a technique using concurrent type-0 and type-I processes on an AlGaAs chip to generate polarization entangled photon was developed. This technique allows the generation of both co-polarized and cross-polarized entangled photons on the same chip without the need of off-chip compensation, interferometer and bandpass filter. Third, a technique of generating hyperentangled photons on an AlGaAs chip was developed by utilizing spatial modes of different guiding mechanisms. High quality entanglement could be achieved with bandpass filtering of a few nanometers without off-chip phase compensation. Four, a generic technique using nonlinear photonic nanowires to engineer two-photon quantum states was proposed. Via dispersion engineering which controls the group velocity of each mode, maximally entangled polarization entangled photons and pure heralded single photons can be generated on-chip. Design examples were applied to III-V semiconductor and ferroelectric materials. Last, a strategy to enhance the bandwidths of wavelength conversion and SPDC spectrum was developed by tapering the waveguides. Combined with further integration of pump lasers, photonic circuits and detectors, the techniques developed in this thesis provides a viable route to achieve fully integrated generation and manipulations of photonic quantum qubits.
Integrated quantum photonics is a key tool towards large scale quantum technologies. In this work we present an AlGaAs-based photonic circuit for on-chip generation and manipulation of broadband polarization-entangled photon pairs. The quantum state is generated by Type-II spontaneous parametric down conversion in AlGaAs Bragg reflection waveguides at telecom wavelengths and room temperature. Polarization entangled photons are manipulated over a frequency band of around 50 nm by an integrated polarizing beam splitter. We demonstrate Hong-Ou-Mandel visibility of 80% for a quantum device where the photon pairs generation and manipulation are implemented in a single chip.
We report the observation of polarization entangled photon pairs generated by means of type-II spontaneous parametric down-conversion (SPDC) within an Al x Ga 1-x As Bragg reflection waveguide (BRW).Even though SPDC in BRW had been observed before, the photons detected were not entangled in the polarization degree of freedom.As a necessary previous step, we also characterized the phasematching properties of the waveguides designed by means of the nonlinear process of second-harmonic generation (SHG).
We demonstrate experimentally that spontaneous parametric down-conversion in an AlxGa(1-x)As semiconductor Bragg reflection waveguide can make for paired photons highly entangled in the polarization degree of freedom at the telecommunication wavelength of 1550 nm. The pairs of photons show visibility higher than 90% in several polarization bases and violate a Clauser-Horne-Shimony-Holt Bell-like inequality by more than 3 standard deviations. This represents a significant step toward the realization of efficient and versatile self pumped sources of entangled photon pairs on-chip.