Ultrabright Entangled-Photon-Pair Generation from an $\mathrm{Al}\mathrm{Ga}\mathrm{As}$-On-Insulator Microring Resonator

Entangled-photon pairs are an essential resource for quantum-information technologies. Chip-scale sources of entangled pairs have been integrated with various photonic platforms, including silicon, nitrides, indium phosphide, and lithium niobate, but each has fundamental limitations that restrict the photon-pair brightness and quality, including weak optical nonlinearity or high waveguide loss. Here, we demonstrate a novel ultralow-loss $\mathrm{Al}\mathrm{Ga}\mathrm{As}$-on-insulator platform capable of generating time-energy entangled photons in a $Q>1$ million microring resonator with nearly 1000-fold improvement in brightness compared to existing sources. The waveguide-integrated source exhibits an internal generation rate greater than $20\times {10}^9$ pairs ${\mathrm{s}}^{-1}{\mathrm{mW}}^{-2}$, emits near 1550 nm, produces heralded single photons with $>99\mathrm{\%}$ purity, and violates Bell’s inequality by more than 40 standard deviations with visibility $>97\mathrm{\%}$. Combined with the high optical nonlinearity and optical gain of $\mathrm{Al}\mathrm{Ga}\mathrm{As}$ for active component integration, these are all essential features for a scalable quantum photonic platform.

Popular Summary

Quantum science and technologies rely on entangled states. Entanglement has been demonstrated using various quantum systems, such as electronic spins, superconducting circuits, trapped ions, and atoms and photons. Photons offer the unique ability to maintain entanglement over long distances in free space or fiber-optic networks. Here, we report a new platform for generating entangled photons via a spontaneous nonlinear process in an aluminum gallium arsenide ($\mathrm{Al}\mathrm{Ga}\mathrm{As}$) microring resonator on silica. The microring-resonator structure creates a cavity where photons can interact with each other as they travel around the ring. Occasionally (and randomly), a set of two photons can annihilate and create two photons at different wavelengths that are entangled. This process allows for the creation of an entangled state that can be used for quantum experiments.

Not only does the $\mathrm{Al}\mathrm{Ga}\mathrm{As}$-on-insulator ($\mathrm{Al}\mathrm{Ga}\mathrm{As}\mathrm{OI}$) material platform offer a higher nonlinearity than other photonic materials but we demonstrate that recent improvements to the nanofabrication of $\mathrm{Al}\mathrm{Ga}\mathrm{As}\mathrm{OI}$ waveguides enable ultralow-loss transmission of light through the material. These improvements result in increased interactions between photons in the resonator, allowing for a higher generation rate of entangled-photon pairs. We demonstrate a nearly 1000-fold improvement over the state-of-the-art brightness of on-chip entangled-photon-pair generation while maintaining $g>99\mathrm{\%}$ single-photon purity, suggesting the $\mathrm{Al}\mathrm{Ga}\mathrm{As}\mathrm{OI}$ platform as a viable source of entangled photons.

In addition to the ultrabright sources demonstrated in this work, the $\mathrm{Al}\mathrm{Ga}\mathrm{As}\mathrm{OI}$ platform is compatible with common laser designs and essential photonic circuit elements for quantum applications, pointing to the exciting prospects of this platform for scalable all-on-chip integrated quantum devices for optical computing, communications, and sensing.

Figure 1

The on-chip entangled-photon-pair brightness plotted as a function of the ${\chi }^{(3)}$ nonlinear coefficient of the material. All brightness values are normalized to 1 mW on-chip power for comparison. $\mathrm{Li}\mathrm{Nb}\mathrm{O}$${}_3$ and $\mathrm{Al}\mathrm{N}$ generate entangled-photon pairs via ${\chi }^{(2)}$ processes (as indicated by an asterisk) but they are plotted here using their ${\chi }^{(3)}$ coefficients for simplicity. Isolines of constant $Q^3{R}^{-2}$ are illustrated by the dashed lines. The data for the materials are estimated from the references shown in Table 1.

Figure 2

(a) A schematic illustration of entangled-photon-pair generation from a photonic microring resonator. Pump photons (${\lambda }_p$) are coupled into the waveguide bus and ring and are converted into signal (${\lambda }_s$) and idler (${\lambda }_i$) photons via spontaneous four-wave mixing (SFWM). (b) An optical-microscope image of a representative $\mathrm{Al}\mathrm{Ga}\mathrm{As}$OI ring-resonator pulley with a 30-$\mu \mathrm{m}$ radius. (c) The resonator transmission spectrum with signal (1572-nm) and idler (1542-nm) wavelengths two free-spectral ranges away from the pump (1557-nm) resonance. (d) The resonator transmission spectrum of the pump resonance (blue trace). A $Q=1.24\times {10}^6$ is determined from the superimposed Lorentzian fit (red trace).

Figure 3

(a) Corrected detected singles rates versus the on-chip pump power. The difference in the generated singles rates is due to the difference in the filter loss in the two channels (19.4 dB for the idler and 13.6 dB for the signal). (b) The on-chip pair generation rate versus the on-chip pump power. The dashed line is a fit to the data, yielding a pair generation rate of $20\times {10}^9$ pairs ${\mathrm{s}}^{-1}{\mathrm{mW}}^{-2}$ (c) The coincidence-to-accidental ratio (CAR) versus the on-chip pair generation rate.

Figure 4

(a) A schematic illustration of the setup for the two-photon interference experiment using a fiber-based folded Franson interferometer: BPF, band-pass filter; [S] ([L]), short (long) interferometer path; SNSPD, superconducting-nanowire single-photon detector. (b) Coincidence histograms for different interferometric phase ${\varphi }_{s+i}=0.4\pi$ and $\pi$, respectively.(c) Singles counts (top) measured with the interferometer setup simultaneously with the two-photon interference (bottom). The visibility of the raw (fitted) data yields $95\mathrm{\%}$ ($97.1\mathrm{\%}$).

Figure 5

(a) The tunable $\mathrm{Al}\mathrm{Ga}\mathrm{As}$-on-insulator ($\mathrm{Al}\mathrm{Ga}\mathrm{As}$OI) microring-resonator entangled-photon-pair source. The $\mathrm{Al}\mathrm{Ga}\mathrm{As}$OI platform enables the large-scale integration of active and passive quantum photonic components, including tunable lasers, nonlinear quantum light sources, filters and wavelength-division multiplexing (WDM), superconducting-nanowire single-photon detectors (SNSPDs), and microcontrollers. These components can be monolithically integrated for all-on-chip quantum photonic circuits, including (b) quantum gates for optical computing, (c) m-mode unitary operations for boson sampling, and (d) Bell-state measurements for chip-to-chip teleportation of quantum states.

Figure 6

A schematic of the experimental setup for a fiber-based folded Franson-type interferometer. The tunable cw laser diode is swept and held at the resonance wavelength of the microring resonator. The laser is sent through amplified-spontaneous-emission (ASE) rejection filters and coupled via lensed fiber onto and off of the photonic chip. The light is split into a short and a long arm of an interferometer. A piezo-based phase shifter is used to modify the phase of the photons that travel through the long arm. The pump photons are removed via an FBG and filters on the signal and idler channels. The signal and idler channels are coupled to SNSPDs to determine the count rates.

Figure 7

An example singles scan for an on-chip power of $-15.14$ dBm ($30.6\mu \mathrm{W}$). The scan begins with the laser set to an off-resonance wavelength. The laser is then swept to the resonance wavelength, reaching the resonance wavelength at approximately 200 s. The laser is held at this wavelength for 10 min to allow for the average singles rate to be determined.

Figure 8

An example histogram showing the normalized coincidence counts (in counts per minute) as a function of the time delay. The slight offset from 0 is due to unequal filter path lengths between the signal and idler channels.

Figure 9

The maximum quality factor for each microring-resonator sample.