Tunable polarization-entangled near-infrared photons from orthogonal GaAs nanowires

TL;DR

This work tackles the scalability challenge of polarization-entangled photon sources by introducing a nanoscale platform based on orthogonal GaAs nanowires that operate at telecom wavelengths. By combining projection-based characterization of the effective $\,\chi^{(2)}$ tensor with quantum-state tomography, the authors predict and experimentally validate tunable entanglement generated from two orthogonal nanowires, transitioning from separable to near-maximally entangled states as the pump polarization is rotated. The results show a dominant type-0 SPDC channel via $\,\chi^{(2)}_{xxx}$, with single-NW tomography revealing high-fidelity entangled states for diagonal/antidiagonal pumping and two-NW configurations delivering fidelities above $F>0.88$ and concurrences up to $C\approx0.9$ at $\lambda \approx 1550$ nm. This nanoscale source offers a pathway toward chip-scale integration and satellite-ready quantum networks due to its bottom-up fabrication, telecom operation, and tunable entanglement without post-selection.

Abstract

Quantum entanglement is a fundamental resource for emerging quantum technologies, enabling secure communication and enhanced sensing. For decades, generating polarization entangled states has been mainly achieved using bulk crystals with spontaneous parametric down conversion (SPDC), preventing scalability and on-chip integration. Miniaturizing the quantum source provides access to more versatility and tunability while enabling an easier integration to other devices, notably necessary for satellite-based quantum communication, and eventually reducing fabrication costs. This challenging task can be achieved with Zinc Blende GaAs nanowires. They already have shown an efficient photon pairs generation via SPDC at 1550 nm. Here we demonstrate that a pair of orthogonal GaAs nanowires constitutes a new nanoscale platform to control the quantum state at telecommunication wavelength, enabling a transition from polarization entangled to separable states as a function of the pump polarization, with fidelities reaching 90%

Figures (15)

• Figure 1: GaAs nanowires for the generation of tunable polarization entangled photon pairs. a) Sketch of the entangled idler and signal photons emitted from two orthogonal GaAs nanowires when pumped by a diagonal polarization (represented by the blue arrow). The pump polarization can also be rotated, as shown with the pink arrows, to probe selectively one single NW with a horizontal (H) or vertical (V) pump polarization, generating separable states. b) SEM image of a single GaAs used to characterize the susceptibility tensor of the nanowire. c) SEM image of the tunable nanosource consisting of two orthogonal nanowires labelled $\mathrm{NW_{H}}$ and $\mathrm{NW_{V}}$.
• Figure 2: Characterization of a single NW. A lens (f = 8 mm) focuses a continuous wave (CW) pump beam (at 778 nm) on the nanowire. The SPDC generated photons are collected with an identical lens, filtered by longpass filters. The half wave plate $\mathrm{HWP_{0}}$ controls the pump polarization, $\mathrm{HWP_{1}}$ rotates the photons emitted along the long axis of the nanowire on the horizontal x-axis while $\mathrm{HWP_{2}}$ rotates the polarization of the quantum state. The polarized beam splitter (PBS) transmits only the horizontal polarization. The idler and signal photons are then coupled to a fiber, separated by a fiber splitter and detected by two superconducting nanowire single-photon detectors (SNSPDs). (More details in the Supplementary Information.)
• Figure 3: Characterization of a single GaAs nanowire. a)-d) Normalized experimental coincidence counts as a function of the angle of rotation of $\mathrm{HWP_{2}}$ (blue stars), and normalized theoretical coincidence counts obtained with Eq. \ref{['Eq1']} after minimization of the cost function (see Methods), in red lines. Unnormalized experimental coincidence counts, shown in the Supplemenray Information, reveal that photon coincidences are about an order of magnitude higher under H pump polarization compared to V ones. e-h) Real part of the theoretical density matrix $\rho_{\mathrm{1}}^{\mathrm{th,P}} =\ket{\Psi_{\mathrm{1}}^{\mathrm{th,P}}}\bra{\Psi_{\mathrm{1}}^{\mathrm{th,P}}}$, with P the pump polarization. i-l) Real part of the experimental density matrix $\rho_{\mathrm{1}}^{\mathrm{exp,P}}$. Each column correspond to a given P pump polarization being horizontal (H) for a),e),i); vertical (V) for b),f),j); diagonal (D) for c),g),k); and antidiagonal (A) for d),h),l).
• Figure 4: Schematic illustration of the quantum state tomography setup. A lens (f = 8 mm) focuses a CW pump (at 778 nm) on the nanowire(s). The SPDC generated photons are collected with an identical lens, filtered by longpass filters. The signal and idler photons are separated by a dichroic mirror and quarter wave plates (QWP), half wave plates (HWP) and polarized beam splitters (PBSs) are inserted in the two paths. The photons are then coupled to fibers and detected by SNSPDs. The half wave plate $\mathrm{HWP_{0}}$ controls the pump polarization, $\mathrm{HWP_{1}}$ projects the photons emitted along the long axis of the nanowire on the horizontal axis. (More details in the Supplementary Information.)
• Figure 5: Polarization-entangled Bell state generation from the GaAs nanowires. a-d) Real part of the theoretical density matrix $\rho_{\mathrm{2}}^{\mathrm{th,P}} =\ket{\Psi_{\mathrm{2}}^{\mathrm{th,P}}}\bra{\Psi_{\mathrm{2}}^{\mathrm{th,P}}}$ obtained with the fitted $\chi^{(2)}$ (Eq. \ref{['Eq3']}), with P the pump polarization. e-h) Real part of the experimental density matrix $\rho_{\mathrm{2}}^{\mathrm{exp,P}} =\ket{\Psi_{\mathrm{2}}^{\mathrm{th,P}}}\bra{\Psi_{\mathrm{2}}^{\mathrm{th,P}}}$, with P the pump polarization. Each column correspond to a given P pump polarization being horizontal (H) for a),e); vertical (V) for b),f); diagonal (D) for c),g); and antidiagonal (A) for d),h). The fidelities $\mathcal{F}$ and concurrences $\mathcal{C}$ are computed for the comparison with ideal density matrices where the NW would emit pure type-0 SPDC.