A monolithically integrated polarization entangled photon pair source on a silicon chip

Integrated photonic circuits are one of the most promising platforms for large-scale photonic quantum information systems due to their small physical size and stable interferometers with near-perfect lateral-mode overlaps. Since many quantum information protocols are based on qubits defined by the polarization of photons, we must develop integrated building blocks to generate, manipulate, and measure the polarization-encoded quantum state on a chip. The generation unit is particularly important. Here we show the first integrated polarization-entangled photon pair source on a chip. We have implemented the source as a simple and stable silicon-on-insulator photonic circuit that generates an entangled state with 91 ± 2% fidelity. The source is equipped with versatile interfaces for silica-on-silicon or other types of waveguide platforms that accommodate the polarization manipulation and projection devices as well as pump light sources. Therefore, we are ready for the full-scale implementation of photonic quantum information systems on a chip.

In Fig. S1, we see an almost straight line consisting of created idler peaks, which appear almost symmetric with the signal line with respect to the pump wavelength. This is explained by the energy conservation of the FWM. The clear idler peak was observed only for the TE-polarized FWM, due to a strong field confinement in the core (Fig. S2) and the near-zero-dispersion property of the TE mode in the telecom band (Fig. S3). The data in Fig. 2a was extracted from the data in Fig. S1.
-Imperfection in obtained fully entangled fraction F( ent ) First, to verify the reproducibility of the experiment we measured  ent four times, and on each occasion we reset the input pump polarization manually using the input half-wave plate. The average value and the standard error were F( ent ) = 0.91 ± 0.02, which means that the angular settings of the wave plates were sufficiently reproducible.
Second, the value of F( ent ) without accidental coincidence counts was obtained as 0.92 ± 0.02, which indicates that statistical background coincidence events contribute little to the degraded fidelity.
The density matrix  ref in Fig. 4a exhibits |TE,TM> s,i and |TM,TE> s,i components, implying that the generated |TE,TE> s,i pairs in the reference silicon-wire waveguide suffered slight polarization rotation. We investigated this for the signal and idler wavelength modes using a classical amplified-spontaneous-emission source. The light Intensity ( source passed through the WDM filter before being coupled to the reference silicon-wire waveguide so that its spectrum was identical to that of the filtered photon pairs. Then, the polarization rotation property of the device was investigated (Fig. S4) in the same way as for the pump wavelength. As can be seen, there is a horizontal offset in the fringes for the signal and idler modes, even without the silicon polarization rotator.
The unexpected polarization rotation might be due to a fabrication error, which is presumably a slight horizontal offset between the axis of the inverse-taper silicon wire and the second core at the SSCs. The amplified-spontaneous-emission source centred at the signal and the idler wavelengths had polarization rotations of − 11.0 ± 0.2° and − 9.9 ± 0.2°, respectively. If these values are contributed by the SSCs at the input and the output equally, the created |TE,TE> s,i pairs in the reference SWW can be expected to suffer a polarization rotation of approximately − 6° at the output SSC. We plot the estimated density matrix for this case in Fig. S5, which well describes the feature of the experimentally obtained  ref (Fig. 4a). In addition we can also see the wavelength-dependent polarization rotation at the silicon polarization rotator.
These insufficient polarization rotations cause the propagating optical field to have a slightly rotated linearly polarized state from either the TE or TM mode that suffers from the strong birefringence of the following silicon-wire waveguides. As a result, the propagating field exhibits a rapid oscillation of the polarization state in the spectral region [32]. Such polarization oscillation is averaged out in the WDM filter window for the signal and idler modes, causing the degree of polarization to degrade. Therefore, we obtained the degraded fringe visibility seen in Fig. S4 and also an imperfect F( ent ) value. We obtained a fringe visibility of 0.92. F( ent ) could degrade to the same degree.