128 Identical Quantum Sources Integrated on a Single Silica Chip

Quantum technology is playing an increasingly important role due to the intrinsic parallel processing capabilities enabled by quantum superposition, exceeding the upper limits of classical performances in diverse fields. An integrated photonic chip offers an elegant way to construct large-scale quantum systems in a physically scalable fashion; however, nonuniformity of quantum sources prevents all the elements from being connected coherently for exponentially increasing Hilbert space. Here, we experimentally demonstrate 128 identical quantum sources integrated on a single silica chip. By actively controlling the light-matter interaction in femtosecond laser direct writing, we are able to unify the properties of waveguides comprehensively and therefore the spontaneous four-wave mixing process for quantum sources. We verify the indistinguishability of the on-chip sources by a series of heralded two-source Hong-Ou-Mandel interferences, with all the dip visibilities above 90%. In addition, the brightness of the sources is found easily reaching the megahertz level and being applicable to both discrete-variable and continuous-variable platforms, showing either clear antibunching feature or large squeezing parameter under different pumping regimes. The demonstrated scalability and uniformity of the quantum sources, together with integrated photonic network and detection, will enable large-scale all-on-chip quantum processors for real-life applications.

Figure 1

Schematic of nonlinear interaction locking and 128 identical SFWM integrated quantum sources. (a) Energy level diagram of the SFWM process. Nonidentical photon emission ensembles are induced by virtual energy level variances. (b) Identical photon emission without virtual energy level variance. (c) Schematic of femtosecond laser direct writing and birefringence engineering with comprehensive fabrication parameter locking. (d) Experimental setup for the characterization of the arrayed quantum sources. The testing apparatus can be easily switched between different groups. DM, dichroic mirror; MCCM, multichannel coincidence module.

Figure 2

Homogeneous spectra of 128 identical SFWM integrated sources. (a) Experimentally measured central wavelengths of 128 SFWM integrated sources. Signal photon is depicted by blue dots while idler photon by red dots. (b) Signal wavelength drifting with perturbation of birefringence $\mathrm{\Delta }n$. (c) Idler wavelength drifting with perturbation of birefringence $\mathrm{\Delta }n$. (d) Measured 128 signal full spectra in blue color. (e) Measured 128 idler full spectra in red color. The intensity of each spectral measurement is normalized to its maximum.

Figure 3

Characterization of spectral uniformity of 128 quantum sources. (a) Experimentally measured spectral overlap of 128 SFWM integrated sources. The spectral overlap of signal photons is depicted in blue color, with a minimum value of 0.943 and overall standard deviation of 0.007. (b) The spectral overlap of idler photons is depicted in red color, with a minimum value of 0.963 and overall standard deviation of 0.004. The intensity of spectrum measurement is normalized to its maximum.

Figure 4

Experimental characterization of quantum sources. (a) Heralded $g_H^2(0)$ in the low-pump-power regime. The results are given by the Hanbury-Brown and Twiss experiments, indicating a clear antibunching feature. (b) While scanning pump power range from 10 to 150 mW, both the squeezing parameter and coincidence counts are depicted with blue and purple colors. Some error bars are too small to be visible.

Figure 5

Interconnected test of Hong-Ou-Mandel interference. With the experimental apparatus shown in Fig. 1, a series of Hong-Ou-Mandel interferences are conducted with visibilities all beyond 0.9, showing a good indistinguishability and uniformity of the arrayed quantum sources. The experimental data of {01,10} and {50,60} are enlarged to show more details.

Figure 6

Progress of the n umber of quantum sources.