Quantum frequency combs (QFCs) are versatile resources for multi-mode entanglement, such as cluster states, crucial for quantum communication and computation. On-chip whispering gallery mode resonators (WGMRs) can generate these states at ultra-low threshold power. This work demonstrates the simultaneous generation of multiple QFCs using a single on-chip silicon nitride WGMR across distinct modal families. It presents a micro-ring resonator with a radius of 240 $\mathrm{μm}$, capable of supporting four modal families within the 130 to 260 $\mathrm{THz}$ frequency range for consistency regulation. The results indicate that, by carefully designing the structure of silicon nitride WGMRs, it is possible to generate quantum entangled frequency combs across distinct modal families simultaneously using monochromatic pump light. It is achieved by modulating the pump mode profiles with a spatial light modulator (SLM) or an on-chip inverse-designed mode converter. This approach offers a simple and low-cost method to achieve higher-density entanglement integration on-chip.
Simultaneous Generation of Quantum Frequency Combs across Distinct Modal Families in a Single Si3N4 Whispering Gallery Mode Resonator
Quantum frequency combs (QFCs) are versatile resources for multi-mode entanglement, such as cluster states, crucial for quantum communication and computation. On-chip whispering gallery mode resonators (WGMRs) can generate these states at ultra-low threshold power. This work demonstrates the simultaneous generation of multiple QFCs using a single on-chip silicon nitride WGMR across distinct modal families. It presents a micro-ring resonator with a radius of 240 μm, capable of supporting four modal families within the 130 to 260 THz frequency range for consistency regulation. The results indicate that, by carefully designing the structure of silicon nitride WGMRs, it is possible to generate quantum entangled frequency combs across distinct modal families simultaneously using monochromatic pump light. It is achieved by modulating the pump mode profiles with a spatial light modulator (SLM) or an on-chip inverse-designed mode converter. This approach offers a simple and low-cost method to achieve higher-density entanglement integration on-chip.
Integrated Kerr quantum frequency combs (QFCs), with the potential to generate multiple and scalable quantum states, have emerged as a compact, stable, and fundamental resource for broadband entanglement. Here, we build a platform for designing bipartite entangled QFCs via an on-chip silicon nitride microring resonator. By establishing the system quantum dynamics of Kerr nonlinear microresonators, our platform can support up to 12 continuous-variable quantum modes, in the form of six simultaneously two-mode squeezed pairs under the influence of hysteresis. The entanglement degree of frequency-mode pairs is dependent on the resonator structure and ambient temperature. By adjusting the cavity temperature, we can optimize the performance of entanglement under certain injected pump power and pump detuning. Our comprehensive QFC designing process and entanglement distribution control may ameliorate the generation and optimization of entanglement.
Quantum frequency combs (QFCs) are essential to quantum photonic systems, enabling high dimensional entanglement in both temporal and frequency domains. Silicon photonics offers a promising platform for scalable and monolithic quantum photonic integrated circuits. However, its performance in QFC generation has been hindered by nonlinear losses and restricted phase-matching bandwidth. To overcome these challenges, we introduce a novel design strategy that combines high-order spatial mode engineering with an optimized free-carrier loss suppression structure in a silicon micro-ring resonator. This approach enables a broadband QFC spanning 150 nm, along with a high single pair photon brightness of 6.43 × 109 Hz/mW2/GHz and photon efficiency of 19.9 GHz/mW2 with the collection of 11 frequency bins. The generated QFCs produce heralded single photons with low heralded second-order correlation gh20=0.0024 and energy-time entangled photon pairs with high visibility of 98.19%. Consistent performance across wafer-scale multiple dies confirms the robustness and manufacturability of the designed device. This work establishes a scalable and high-yield pathway toward monolithic quantum photonic integrated circuits.
Squeezed light constitutes a key resource for quantum optical technologies including quantum sensing, computing, communication and metrology. For many applications the generation of squeezed light typically requires at least two nonlinear optical stages involving careful phase and frequency matching to achieve the required mixing of squeezed and coherent light. In our work, we introduce an on-chip system that simplifies the generation of coherent-squeezed light, utilizing only a single squeezing stage. We achieve this by pumping a silicon nitride (Si3N4) microring resonator to produce single-mode squeezed light through four-wave mixing at the same frequency as the pump mode, leveraging the inherent χ(3)-nonlinearity of the Si3N4 resonator. Our on-chip system demonstrates a squeezing of -4.7 dB with a clear perspective towards -10 dB squeezing. We also provide a theoretical model that describes the straightforward yet robust generation of single-mode squeezing at the injection locking point of the ring resonator. In fact, we show that a design with a normal dispersion can be used for robust generation of bright squeezed light without the need for careful suppression of unwanted nonlinear processes. Overall, our findings highlight an approach which drastically simplifies the generation of coherent-squeezed light in photonic integrated circuits.
Integrated photonic microresonators have become an essential resource for generating photonic qubits for quantum information processing, entanglement distribution and networking, and quantum communications. The pair-generation rate is enhanced by reducing the microresonator radius, but this comes at the cost of increasing the frequency-mode spacing and reducing the quantum information spectral density. Here, we circumvent this rate-density trade-off in an Al Ga As -on-insulator photonic device by multiplexing an array of 20 small-radius microresonators, each producing a 650-GHz-spaced comb of time-energy entangled-photon pairs. The resonators can be independently tuned via integrated thermo-optic heaters, enabling control of the mode spacing from degeneracy up to a full free spectral range. We demonstrate simultaneous pumping of five resonators with up to 50 -GHz relative comb offsets, where each resonator produces pairs exhibiting time-energy entanglement visibilities up to 95 % , coincidence-to-accidental ratios exceeding 5000 , and an on-chip pair rate up to 2.6 G Hz / mW 2 per comb line—an improvement over prior work by more than a factor of 40. As a demonstration, we generate frequency-bin qubits in a maximally entangled two-qubit Bell state with fidelity exceeding 87 % ( 90 % with background correction) and detected frequency-bin entanglement rates up to 7 kHz (an approximately 70 MHz on-chip pair rate) using a pump power of approximately 250 μ W . Multiplexing small-radius microresonators combines the key capabilities required for programmable and dense photonic qubit encoding while retaining high pair-generation rates, heralded single-photon purity, and entanglement fidelity.
The on-chip generation of large and complex optical quantum states will enable low-cost and accessible advances for quantum technologies, such as secure communications and quantum computation. Integrated frequency combs are on-chip light sources with a broad spectrum of evenly-spaced frequency modes, commonly generated by four-wave mixing in optically-excited nonlinear micro-cavities, whose recent use for quantum state generation has provided a solution for scalable and multi-mode quantum light sources. Pulsed quantum frequency combs are of particular interest, since they allow the generation of single-frequency-mode photons, required for scaling state complexity towards, e.g., multi-photon states, and for quantum information applications. However, generation schemes for such pulsed combs have, to date, relied on micro-cavity excitation via lasers external to the sources, being neither versatile nor power-efficient, and impractical for scalable realizations of quantum technologies. Here, we introduce an actively-modulated, nested-cavity configuration that exploits the resonance pass-band characteristic of the micro-cavity to enable a mode-locked and energy-efficient excitation. We demonstrate that the scheme allows the generation of high-purity photons at large coincidence-to-accidental ratios (CAR). Furthermore, by increasing the repetition rate of the excitation field via harmonic mode-locking (i.e. driving the cavity modulation at harmonics of the fundamental repetition rate), we managed to increase the pair production rates (i.e. source efficiency), while maintaining a high CAR and photon purity. Our approach represents a significant step towards the realization of fully on-chip, stable, and versatile sources of pulsed quantum frequency combs, crucial for the development of accessible quantum technologies.
Integrated microresonator facilitates the realization of quantum frequency comb (QFC), which provides a large number of discrete frequency modes with broadband spectral range and narrow linewidth. However, all previous demonstrations have focused on the generation of energy-time or time-bin entangled photons from QFC. Realizing polarization-entangled quantum frequency comb, which is a useful resource for the fundamental study of quantum mechanics and quantum information applications, remains challenging. Here, we demonstrate a broadband polarization-entangled quantum frequency comb by combining an integrated silicon-nitride microresonator with a Sagnac interferometer. With a free spectral range of about 99 GHz and a narrow linewidth of about 190 MHz, our source provides 22 polarization-entangled photon pairs with frequency covering the whole telecom C-band. The entanglement fidelities for all 22 pairs are above 81%, including 17 pairs with fidelities higher than 90%. Our demonstration paves the way for employing the polarization-entangled quantum frequency comb in quantum network using CMOS technology as well as standard dense wavelength division-multiplexing technology.