Frequency-time is a degree of freedom suitable for photonic high-dimensional entanglement, with advantages such as compatibility with single-mode devices and insensitivity to dispersion. The engineering control of the frequency-time amplitude of a photon's electric field has been demonstrated on platforms with second-order optical nonlinearity. For integrated photonic platforms with only third-order optical nonlinearity, the engineered generation of the state remains unexplored. Here, we demonstrate a cavity-enhanced photon-pair source on the silicon-on-insulator (SOI) platform that can generate both separable states and controllable entangled states in the frequency domain without post-manipulation. By choosing different resonance combinations and employing on-chip optical field differentiation, we achieve independent control over two functions that affect the joint spectral intensity (JSI) of the state. A semi-analytical model is derived to simulate the biphoton spectral wavefunction in the presence of resonance splitting and pump differentiation, and its parameters can be fully determined through fitting-based parameter extraction from the resonator's measured linear response. The measured spectral purity for the separable state is $95.5\pm 1.2\%$, while the measured JSIs for the entangled states show two- or four-peaked functions in two-dimensional frequency space. The experiments and simulations demonstrate the capacity to manipulate the frequency-domain wavefunction in a silicon-based device, which is promising for applications like quantum information processing using pulsed temporal-mode encoding or long-distance quantum key distribution.
Frequency-time is a degree of freedom suitable for photonic high-dimensional entanglement, with advantages such as compatibility with single-mode devices and insensitivity to dispersion. The engineering control of the frequency-time amplitude of a photon's electric field has been demonstrated on platforms with second-order optical nonlinearity. For integrated photonic platforms with only third-order optical nonlinearity, the engineered generation of the state remains unexplored. Here, we demonstrate a cavity-enhanced photon-pair source on the silicon-on-insulator (SOI) platform that can generate both separable states and controllable entangled states in the frequency domain without post-manipulation. By choosing different resonance combinations and employing on-chip optical field differentiation, we achieve independent control over two functions that affect the joint spectral intensity (JSI) of the state. A semi-analytical model is derived to simulate the biphoton spectral wavefunction in the presence of resonance splitting and pump differentiation, and its parameters can be fully determined through fitting-based parameter extraction from the resonator's measured linear response. The measured spectral purity for the separable state is 95.5±1.2%, while the measured JSIs for the entangled states show two- or four-peaked functions in two-dimensional frequency space. The experiments and simulations demonstrate the capacity to manipulate the frequency-domain wavefunction in a silicon-based device, which is promising for applications like quantum information processing using pulsed temporal-mode encoding or long-distance quantum key distribution.
The Silicon On Insulator (SOI) is a promising platform for the generation of correlated and entangled photon pairs, as silicon photonics inherits the precise, mature, and accurate technologies from Silicon micro-electronics and allows the large-scale production of integrated photonic chips. In particular, high-quality-factor micro-resonators are well suited for producing telecom-wavelength bi-photon frequency combs, key elements for quantum information, as telecom photons are good candidates for the future links between quantum communication nodes or quantum computers [1]. Using the frequency domain for encoding information is an elegant way to access a high-dimensional Hilbert space, in one spatial mode. Recent progress in the manipulation using electro-optics devices leveraged versatile applications [2] and Hadamard gates have been implemented with classical light [3]
Frequency-bin encoding has recently emerged as a powerful approach for photonic quantum information processing, offering high dimensionality, gate-parallelization, and compatibility with existing telecommunication infrastructure. However, its scalable deployment has so far been hindered by the lack of an integrated platform capable of unifying quantum state generation, coherent frequency mixing, and programmable spectral control.\\ Here, we report the first fully integrated quantum frequency processor, monolithically integrating on the same silicon photonic chip a microresonator-based biphoton quantum frequency comb source, a pump-rejection filter, high-speed phase modulators, and a four-channel, line-by-line pulse shaper. We demonstrate key functionalities, such as tunable frequency beamsplitters with success probabilities exceeding 94% and fidelities above 99.9%, as well as the ability to synthesize more general single-qubit gates. Finally, we generate and coherently manipulate high-dimensional frequency-bin entangled states entirely on chip, showcasing control over two-photon quantum walks and performing the first on-chip frequency-bin quantum state tomography of a Bell-state with a fidelity of 95.7(3)%. By integrating all key functional elements on the same 4×7mm2 chip, with the possibility of scaling to a larger number of modes, our work marks an important step toward large-scale frequency-domain photonic processors for both classical and quantum applications.
High-dimensional entangled states of light provide novel possibilities for quantum information, from fundamental tests of quantum mechanics to enhanced computation and communication protocols. In this context, the frequency degree of freedom is particular attractive thanks to its robustness to propagation in optical fibers and its capability to convey large scale of quantum information into a single spatial mode. This provides a strong incentive for the development of efficient and scalable methods for the generation and the manipulation of frequency-encoded quantum states. Nonlinear parametric processes are powerful tools to generate such states, but up to now the manipulation of the generated frequency states has been carried out mostly by post-manipulation, which demands complex and bulk-like experimental setups. Direct production of on-demand frequency-states at the generation stage, and if possible using a chip-based source, is crucial in view of practical and scalable applications for quantum information technologies. Here we use an integrated semiconductor chip to engineer the wavefunction and exchange statistics of frequency-entangled photon pairs directly at the generation stage, without post-manipulation. Tuning the pump spatial intensity allows to produce frequency-anticorrelated, correlated and separable states, while tuning the spatial phase enables to switch between symmetric and antisymmetric spectral wavefunctions, leading respectively to bosonic and fermionic behaviors of the photons. We also demonstrate the generation of non-Gaussian entanglement in the continuous variables formed by the frequency and time degrees of freedom of the photon pairs. These results, obtained at room temperature and telecom wavelength, and with a chip-based source, open promising perspectives for the quantum simulation of fermionic problems with photons on an integrated platform, as well as for communication and computation protocols exploiting antisymmetric high-dimensional quantum states.
Integrated quantum photonics leverages the on-chip generation of nonclassical states of light to realize key functionalities of quantum devices. Typically, the generation of such nonclassical states relies on whispering gallery mode resonators, such as integrated optical micro-rings, which enhance the efficiency of the underlying spontaneous nonlinear processes. While these kinds of resonators excel in maximizing either the temporal confinement or the spatial overlap between different resonant modes, they are usually associated with large mode volumes, imposing an intrinsic limitation on the efficiency and footprint of the device. Here, we engineer a source of time-energy entangled photon pairs based on a silicon photonic crystal cavity, implemented in a fully CMOS-compatible platform. In this device, resonantly enhanced spontaneous four-wave mixing converts pump photon pairs into signal/idler photon pairs under the energy-conserving condition in the telecommunication C-band. The design of the resonator is based on an effective bichromatic confinement potential, allowing it to achieve up to nine close-to-equally spaced modes in frequency, while preserving small mode volumes, and the whole chip, including grating couplers and access waveguides, is fabricated in a single run on a silicon-on-insulator platform. Besides demonstrating efficient photon pair generation, we also implement a Franson-type interference experiment, demonstrating entanglement between signal and idler photons with a Bell inequality violation exceeding five standard deviations. The high generation efficiency combined with the small device footprint in a CMOS-compatible integrated structure opens a pathway toward the implementation of compact quantum light sources in all-silicon photonic platforms.
The intrinsic third-order nonlinearity in silicon has proven it to be quite useful in the field of quantum optics. Silicon is suitable for producing time-correlated photon pairs that are sources of heralded single-photon states for quantum integrated circuits. A quantum signal source in the form of single photons is an inherent requirement for the principles of quantum key distribution technology for secure communications. Here, we present numerical simulations of a silicon ring with a 6 μ m radius side-coupled with a bus waveguide as the source for the generation of single photons. The photon pairs are generated by exploring the process of degenerate spontaneous four-wave mixing (SFWM). The free spectral range (FSR) of the ring is quite large, simplifying the extraction of the signal/idler pairs. The phase-matching condition is considered by studying relevant parameters like the dispersion and nonlinearity. We optimize the ring for a high quality factor by varying the gap between the bus and the ring waveguide. This is the smallest ring studied for photon pair generation with a quality factor in the order of 10 5 . The width of the waveguides is chosen such that the phase-matching condition is satisfied, allowing for the propagation of fundamental modes only. The bus waveguide is pumped at one of the ring resonances with the minimum dispersion (1543.5 nm in our case) to satisfy the principle of energy conservation. The photon pair generation rate achieved is comparable to the state of the art. The photon pair sources exploiting nonlinear frequency conversion/generation processes is a promising alternative to atom-like single-photon emitters in the field of integrated photonics. Such miniaturized structures will benefit future on-chip architectures where multiple single-photon source devices are required on the same chip.
Fully integrated high spectral purity photon-pair sources represent critical core components for scalable photonic quantum information processing systems. While micro-ring resonator based single-photon sources demonstrate superior integration capabilities, their spectral purity typically suffers from a theoretical upper bound below 92% in the absence of narrow-band spectral filtering, rendering it inadequate for advanced quantum applications. This work successfully implements an on-chip dual-pulse controlled photon pair generation architecture on a complementary metal oxide semiconductor (CMOS) compatible silicon photonic platform, achieving high spectral purity exceeding 98% by optimizing the beam splitting ratio and adjusting the relative time delay between the two pulses. Furthermore, to mitigate operating point drift in the micro-ring photon-pair generator induced by micro-heater current variations during testing, we developed a power control algorithm that locks the micro-ring resonance to the corresponding frequency of the shaped optical pulse, significantly improving long-term system stability.