Biphoton Generation Research Articles

Optimizing biphoton generation via reconfigurable nonlinear waveguide arrays based on scattering tensor

Optimizing biphoton generation via reconfigurable nonlinear waveguide arrays based on scattering tensor

Observation of highly correlated ultrabright biphotons through increased atomic ensemble density in spontaneous four-wave mixing

The pairing ratio, a crucial metric assessing a biphoton source's ability to generate correlated photon pairs, remains underexplored despite theoretical predictions. This study presents experimental findings on the pairing ratio, utilizing a double-Λ spontaneous four-wave mixing biphoton source in cold atoms. At an optical depth (OD) of 20, we achieved an ultrahigh biphoton generation rate of up to 1.3×107 per second, with a successful pairing ratio of 61%. Increasing the OD to 120 significantly improved the pairing ratio to 89%, while maintaining a consistent biphoton generation rate. This achievement, marked by high generation rates and robust biphoton pairing, holds great promise for advancing efficiency in quantum communication and information processing. Additionally, in a scenario with a lower biphoton generation rate of 5.0×104 per second, we attained an impressive signal-to-background ratio of 241 for the biphoton wavepacket, surpassing the Cauchy-Schwarz criterion by approximately 1.5×104 times. Published by the American Physical Society 2024

Nonlocal Rydberg enhancement for four-wave-mixing biphoton generation

Nonlocal Rydberg enhancement for four-wave-mixing biphoton generation

Maximally Efficient Biphoton Generation by Single Photon Decay in Nonlinear Quantum Photonic Circuits.

We develop a general nonperturbative formalism and propose a specific scheme for maximally efficient generation of biphoton states by parametric decay of single photons. We show that the well-known critical coupling concept of integrated optics can be generalized to the nonlinear coupling of quantized photon modes to describe the nonperturbative optimal regime of a single-photon nonlinearity and establish a fundamental upper limit on the nonlinear generation efficiency of quantum-correlated photons, which approaches unity for low enough absorption losses.

Integrated electro-optic control of biphoton generation using hybrid photonics

Parametric sources in quantum optics usually require some form of spectro-temporal control for conditional generation of high-purity single-photon states, but their properties have not yet been optimized using integrated microchips. Using external short-pulse lasers and separate devices for pump preparation and for photon generation, as is traditional, incurs many impediments such as reduced performance, increased loss, high cost, and limited scalability. To overcome these limitations, here we demonstrate a circuit including high-bandwidth, high-extinction ratio electro-optic modulators for pump-pulse preparation fully integrated with high-quality factor (Q) microresonators for efficient parametric nonlinearity, together with seamlessly interconnecting waveguides. The microchip uses different optical materials on a common platform and a multi-layer integrated photonic architecture. Using this, we control the joint spectrum of room-temperature biphoton generation for the first time on a single integrated microchip, and demonstrate that the theoretical purity bound can be achieved.

Biphoton generation enhanced by nonlocal nonlinearity via Rydberg interactions.

Strongly correlated Stokes and anti-Stokes photon pairs (biphotons) exhibiting very large generation rates and spectral brightnesses could be attained at extremely low pump powers and optical depths. This is realized via spontaneous four-wave mixing in cold atoms with enhanced nonlocal (Rydberg) optical nonlinearities and prepared into a dark state with a large population imbalance. The scheme works with all light fields on resonance yet with negligible linear absorption and Raman gain.

High-flux, high-visibility entangled photon source obtained with a non-collinear type-II PPKTP crystal pumped by a broadband continuous-wave diode laser

We demonstrated a high-brightness, polarization-entangled bi-photon source generated in a type-II phase-matched non-collinear PPKTP crystal pumped by a 405 nm continuous-wave (cw) broadband diode laser. Basing on the single-pass configuration, we investigated the distribution characteristics of the bi-photon cone and the wavelength tunability with crystal temperatures. Overcoming the degradation of the bi-photon generation rate and the spectral correlation induced by the broadband pump laser, we obtained a bi-photon generation rate of 7.48×103/s/mW/nm and an interference visibility over 95% via a Hong–Ou–Mandel (HOM) configuration. We established a theoretical model to address the bi-photon time-delayed HOM dip pumped by the broadband cw diode laser in spontaneous parametric down-conversion and obtained the complete analytic solution of the bi-photon coincidence counting rates.

Theoretical investigation of pure-state single-photon and large-bandwidth-correlation biphoton generation from micro/nanofiber.

Pure-state single photons and large-bandwidth-correlation biphotons are fundamental resources for quantum information processing. The dispersion properties of micro/nanofiber (MNF) can be tailored by carefully choosing its diameter, resulting in a flexibly tailored biphoton spectrum. We theoretically investigate pure-state single photons and large-bandwidth-correlation biphotons produced by degenerate spontaneous four-wave mixing in MNF. In our simulation, a single-photon state with a purity of 99% will be theoretically attained by choosing the appropriate pump bandwidth and center wavelength with respect to the diameter and length of the MNF. Further, when an appropriate diameter with a negligible curvature at the zero dispersion wavelength is chosen, even a narrow pump bandwidth is capable of motivating remarkably broadband correlation biphotons, e.g., for a MNF diameter of 0.7 μm, the theoretical full width at half maximum is 473 nm. In practice, the application of a MNF-based quantum light source is dependent on the technologies that precisely control and measure the diameter. Our theoretical investigation will guide the experimental realization of high-quality quantum light sources based on MNF.

Quantum Langevin theory for two coupled phase-conjugated electromagnetic waves

While loss-gain-induced Langevin noises have been intensively studied in quantum optics, the effect of a complex-valued nonlinear coupling coefficient on the noises of two coupled phase-conjugated optical fields has never been questioned before. Here, we provide a general macroscopic phenomenological formula of quantum Langevin equations for two coupled phase-conjugated fields with linear loss (gain) and complex nonlinear coupling coefficient. The macroscopic phenomenological formula is obtained from the coupling matrix to preserve the field commutation relations and correlations, which does not require knowing the microscopic details of light-matter interaction and internal atomic structures. To validate this phenomenological formula, we take spontaneous four-wave mixing in a double-$\Lambda$ four-level atomic system as an example to numerically confirm that our macroscopic phenomenological result is consistent with that obtained from the microscopic Heisenberg-Langevin theory. Finally, we apply the quantum Langevin equations to study the effects of linear gain and loss, complex phase mismatching, as well as complex nonlinear coupling coefficient in entangled photon pair (biphoton) generation, particularly to their temporal quantum correlations.

Background-Free Near-Infrared Biphoton Emission from Single GaAs Nanowires.

The generation of photon pairs from nanoscale structures with high rates is still a challenge for the integration of quantum devices, as it suffers from parasitic signals from the substrate. In this work, we report type-0 spontaneous parametric down-conversion at 1550 nm from individual bottom-up grown zinc-blende GaAs nanowires with lengths of up to 5 μm and diameters of up to 450 nm. The nanowires were deposited on a transparent ITO substrate, and we measured a background-free coincidence rate of 0.05 Hz in a Hanbury-Brown-Twiss setup. Taking into account transmission losses, the pump fluence, and the nanowire volume, we achieved a biphoton generation of 60 GHz/Wm, which is at least 3 times higher than that of previously reported single nonlinear micro- and nanostructures. We also studied the correlations between the second-harmonic generation and the spontaneous parametric down-conversion intensities with respect to the pump polarization and in different individual nanowires.

Broadband Biphoton Generation and Polarization Splitting in a Monolithic AlGaAs Chip

Broadband Biphoton Generation and Polarization Splitting in a Monolithic AlGaAs Chip

Broadband Biphoton Generation and Polarization Splitting in a Monolithic AlGaAs Chip

Integrated quantum photonics is a key tool towards large scale quantum technologies. In this work we present an AlGaAs-based photonic circuit for the on-chip generation of broadband orthogonally polarized photons and the deterministic separation of the photons into separate spatial modes, facilitating their further use in protocols. We demonstrate that 85% of the pairs are deterministically separated by the chip over a full 60 nm bandwidth and we assess the chip operation in the quantum regime via a Hong-Ou-Mandel experiment displaying a raw visibility of 75.5% over the same full bandwidth.

Temporally ultralong biphotons with a linewidth of 50 kHz

We report the generation of biphotons, with a temporal full width at the half maximum (FWHM) of 13.4 ± 0.3 µs and a spectral FWHM of 50 ± 1 kHz, via the process of spontaneous four-wave mixing with laser-cooled atoms. The temporal width is the longest, and the spectral linewidth is the narrowest to date. This is also the first biphoton result that obtains a linewidth below 100 kHz, reaching a new milestone. The very long biphoton wave packet has a signal-to-background ratio of 3.4, which violates the Cauchy–Schwarz inequality for classical light by 4.8 folds. Furthermore, we demonstrated a highly tunable-linewidth biphoton source and showed that while the biphoton source’s temporal and spectral width were controllably varied by about 24 folds, its generation rate only changed by less than 15%. A spectral brightness or generation rate per pump power per linewidth of 1.2× 106 pairs/(s mW MHz) was achieved at the temporal width of 13.4 µs. The above results were made possible by the low decoherence rate and high optical depth of the experimental system, as well as a novel scheme of classical fields’ and biphotons’ propagation directions in the experiment. This work has demonstrated a high-efficiency ultranarrow-linewidth biphoton source and has made substantial advancements in quantum technology utilizing heralded single photons.

Direct Observation of Dynamically Localized Quantum Optical States.

Quantum-correlated biphoton states play an important role in quantum communication and processing, especially considering the recent advances in integrated photonics. However, it remains a challenge to flexibly transport quantum states on a chip, when dealing with large-scale sophisticated photonic designs. The equivalence between certain aspects of quantum optics and solid-state physics makes it possible to utilize a range of powerful approaches in photonics, including topologically protected boundary states, graphene edge states, and dynamic localization. Optical dynamic localization allows efficient protection of classical signals in photonic systems by implementing an analogue of an external alternating electric field. Here, we report on the observation of dynamic localization for quantum-correlated biphotons, including both the generation and the propagation aspects. As a platform, we use sinusoidal waveguide arrays with cubic nonlinearity. We record biphoton coincidence count rates as evidence of robust generation of biphotons and demonstrate the dynamic localization features in both spatial and temporal space by analyzing the quantum correlation of biphotons at the output of the waveguide array. Experimental results demonstrate that various dynamic modulation parameters are effective in protecting quantum states without introducing complex topologies. Our Letter opens new avenues for studying complex physical processes using photonic chips and provides an alternative mechanism of protecting communication channels and nonclassical quantum sources in large-scale integrated quantum optics.

Attosecond entangled photons from two-photon decay of metastable atoms: A source for attosecond experiments and beyond

We propose the generation of attosecond entangled bi-photons in the extreme-ultraviolet regime by two-photon decay of a metastable atomic state as a source similar to spontaneous parametric down-conversion photons. The 1s2s $^1S_0$ metastable state in helium decays to the ground state by emission of two energy-time entangled photons with a photon bandwidth equal to the total energy spacing of 20.62 eV. This results in a pair correlation time in the attosecond regime making these entangled photons a highly suitable source for attosecond pump-probe experiments. The bi-photon generation rate from a direct four photon excitation of helium at 240 nm is calculated and used to assess some feasible schemes to generate these bi-photons. Possible applications of entangled bi-photons in attosecond time scale experiments, and a discussion of their potential to reach the zeptosecond regime are presented.

Spontaneous parametric down-conversion in bottom-up grown lithium niobate microcubes

Nonclassical light sources are highly sought-after as they are an integral part of quantum communication and quantum computation devices. Typical sources use bulk nonlinear crystals that rely on stringent phase-matching conditions, limiting the operating wavelength and bandwidth. In this work, we demonstrate the generation of photon pairs from a free-standing lithium niobate microcube at the telecommunication wavelength of 1.56 µm through the spontaneous parametric down-conversion process. The maximum photon pair generation rate obtained from a single microcube with the size of 3.6 µm is 490 Hz, resulting in an efficiency of 20.6 GHz/Wm, which is three orders of magnitude larger than the efficiency of biphoton generation in bulk nonlinear crystals. The microcubes are synthesized through a solvothermal method, offering the possibility for scalable devices via bottom-up assembly on any substrates. Our work constitutes an important step forward in the realization of compact nonclassical light sources with a wide bandwidth for various quantum applications.

Telecom-band hyperentangled photon pairs from a fiber-based source

Hyperentanglement, the simultaneous and independent entanglement of quantum particles in multiple degrees of freedom, is a powerful resource that can be harnessed for efficient quantum information processing. In photonic systems, the two degrees of freedom (DoF) often used to carry quantum and classical information are polarization and frequency, thanks to their robustness in transmission, both in free space and in optical fibers. Telecom-band hyperentangled photons generated in optical fibers are of particular interest because they are compatible with existing fiber-optic infrastructure, and can be distributed over fiber networks with minimal loss. Here, we experimentally demonstrate the generation of telecom-band biphotons hyperentangled in both the polarization and frequency DoFs using a periodically-poled silica fiber and observe entanglement concurrences above 0.95 for both polarization and frequency DOFs. Furthermore, by concatenating a Hong-Ou-Mandel interference test for frequency entanglement and full state tomography for polarization entanglement in a single experiment, we can demonstrate simultaneous entanglement in both the polarization and frequency DOFs. The states produced by our hyperentanglement source can enable protocols such as dense coding and high-dimensional quantum key distribution.

Violation of high-dimensional Bell inequality using narrowband orbital-angular-momentum entanglement from warm atomic vapor

Narrowband biphotons from an atomic ensemble are an extraordinary source for quantum information science. The generation of biphotons in the warm atomic system has many applicable prospects for its simplicity, stability, and compactness. In this article we experimentally demonstrate the generation of biphotons from the spontaneous four-wave mixing (SFWM) process in a warm rubidium vapor cell. By performing high-dimensional Bell-type measurements and mutually unbiased measurements, we maximally obtain a three-dimensional orbital-angular-momentum discrete-variable (DV) entangled state. The violation of high-dimensional Bell inequalities shows the great potential of warm atomic systems in generating DV high-dimensional entanglement. Such narrowband photon pairs may find applications in quantum-memory-based quantum communication networks. Through integration with the continuous-variable (CV) entanglement already existing in the same SFWM platform, our results here pave the way to construct atomic-ensemble-based DV-CV hybrid quantum information protocols.

Generation of sub-MHz and spectrally-bright biphotons from hot atomic vapors with a phase mismatch-free scheme.

We utilized the all-copropagating scheme, which maintains the phase-match condition, in the spontaneous four-wave mixing (SFWM) process to generate biphotons from a hot atomic vapor. The linewidth and spectral brightness of our biphotons surpass those of the biphotons produced with the hot-atom SFWM in the previous works. Moreover, the generation rate of the sub-MHz biphoton source in this work can also compete with those of the sub-MHz biphoton sources of the cold-atom SFWM or cavity-assisted spontaneous parametric down conversion. Here, the biphoton linewidth is tunable for an order of magnitude. As we tuned the linewidth to 610 kHz, the generation rate per linewidth is 1,500 pairs/(s·MHz) and the maximum two-photon correlation function, gs,as(2), of the biphotons is 42. This gs,as(2) violates the Cauchy-Schwarz inequality for classical light by 440 folds, and demonstrates that the biphotons have a high purity. By increasing the pump power by 16 folds, we further enhanced the generation rate per linewidth to 2.3×104 pairs/(s·MHz), while the maximum gs,as(2) became 6.7. In addition, we are able to tune the linewidth down to 290±20 kHz. This is the narrowest linewidth to date among all single-mode biphoton sources of room-temperature and hot media.

Bright 547-dimensional Hilbert-space entangled resource in 28-pair modes biphoton frequency comb from a reconfigurable silicon microring resonator

High-dimensional entanglement provides valuable resources for quantum technologies, including quantum communication, quantum optical coherence tomography, and quantum computing. Obtaining a high brightness and dimensional entanglement source has significant value. Here we utilize a tunable asymmetric Mach–Zehnder interferometer coupled silicon microring resonator with 100 GHz free spectral range to achieve this goal. With the strategy of the tunable coupler, the dynamical and extensive tuning range of quality factors of the microring can be obtained, and then the biphoton pair generation rate can be optimized. By selecting and characterizing 28 pairs from a more than 30-pair modes biphoton frequency comb, we obtain a Schmidt number of at least 23.4 and on-chip pair generation rate of 19.9 MHz/mW2 under a low on-chip pump power, which corresponds to 547 dimensions Hilbert space in frequency freedom. These results will prompt the wide applications of quantum frequency comb and boost the further large density and scalable on-chip quantum information processing.