Low-loss polarization-maintaining router for single and entangled photons at a telecom wavelength

TL;DR

This work addresses the need for a low-loss, polarization-maintaining router capable of handling arbitrarily polarized single photons and polarization-entangled photons in the telecom domain. It introduces a semi-common-path Mach-Zehnder interferometer with cross-aligned RTP-based EOMs operating in push-pull mode to preserve polarization with a minimal optical footprint. The router achieves a measured insertion loss of $0.057$ dB, a switching extinction ratio of $>22$ dB, and a polarization-process fidelity of $>99\%$, while quantum-process tomography confirms near-identity routing with $F_{ij} > 99.3\%$ across port combinations and two-photon N00N entanglement routing with $V \approx 97\%$. These results enable polarization-encoded photonic quantum networks and multi-photon entanglement synthesis via time and spatial multiplexing, with compatibility to telecom fiber infrastructure and high-performance detectors.

Abstract

Photon polarization serves as an essential quantum information carrier in quantum information and measurement applications. Routing of arbitrarily polarized single photons and polarization-entangled photons is a crucial technology for scaling up quantum information applications. Here, we demonstrate a low-loss, noiseless, polarization-maintaining routing of arbitrarily polarized single photons and, crucially, multi-photon entangled states where the entanglement is encoded in orthogonal polarization bases, at the telecom L-band. Our interferometer-based router is constructed by optics with a low angle of incidence and cross-aligned electro-optic crystals, achieving the polarization-maintaining operation with a minimal number of optical components. We demonstrate the routing of arbitrarily-polarized heralded single photons with a 0.057 dB (1.3%) loss, a $>$ 22 dB switching extinction ratio, and $>$ 99% polarization process fidelity to ideal identity operation. Moreover, the high-quality router achieves the routing of two-photon N00N-type entangled states with a highly maintained interference visibility of $\approx$ 97%. The demonstrated router scheme preserving multi-photon polarization state paves the way toward polarization-encoded photonic quantum networks as well as multi-photon entanglement synthesis via spatial- and time-multiplexing techniques.

Figures (6)

• Figure 1: Schematic diagram of our polarization-maintaining photonic router. For stable routing operation, we employed the semi-common-path configuration of an MZI with the transverse distance between the two arms of 15 mm. The push-pull operation enabled by an EOM in each arm reduces the applied voltage by half and also improves the thermal stability of the MZI. An angle of incidence to optics component is nearly normal ($5^{\circ}$) so that they are operated independently of polarization. Note that the figure is not to scale. Inset: Polarization-maintaining EOM. Two rubidium titanyl phosphate (RTP) crystals with orthogonally oriented crystallographic axes are placed in series. An electric field is applied to each RTP crystal along the crystallographic $Z$-axis direction. Thus, the static and EO birefringence is compensated for each other, enabling an identical phase shift to arbitrary input polarization states. QWP, quarter-wave plate; HWP, half-wave plate; PBS, polarizing beam splitter; SMF, single-mode optical fiber; NPBS, non-polarizing beam splitter; DM, dichroic mirror; cpKTP, custom-poled potassium titanyl phosphate crystal; SNSPD, superconducting nanowire single-photon detector.
• Figure 2: Normalized heralded single photon count rate versus delay time of trigger signal to the EOMs. Rise and fall times (10% to 90% of signal intensity transition time) of 3.3 and 3.1 ns are observed. The uncertainty is estimated from Poissonian photon counting statistics.
• Figure 3: Relative photon detection probability at the two output ports ($P_1$ and $P_2$) versus applied voltage to the EOMs $U$ for horizontal ($H$, circles), diagonal ($D$, triangles), and right-circular ($R$, squares) input polarization states. The blank and solid symbols show the results at the output ports 1 and 2, respectively; for example, the result with horizontal input and the output port 1 ($H1$) is shown as blank circles. The solid, dashed, and dotted curves are sinusoidal data fittings for $H$, $D$, and $R$ input polarization states, respectively. The error bars are estimated by Poissonian photon counting statistics.
• Figure 4: Reconstructed process matrix $\chi_\mathrm{R}$ in Pauli basis for the input port $i$ and the output port $j$ ($i,j = 1,2$). $F_{ij}$ denotes the process fidelity to ideal identity operation. The uncertainties of $F_{ij}$ are obtained by the standard deviations of ten independent tomographic measurement datasets.
• Figure 5: Observed two-photon N00N-type interference at the input port 1 and the output ports 1 and 2. The coincidence count rate between two output ports of PBS is measured while changing the measurement basis with an HWP. The curve shows the sinusoidal fitting for the data points of the input state. The interference visibilities at the input and output ports are $V_2 \approx 97\%$ and closely matched with each other. The uncertainties are estimated by Poissonian photon counting statistics.