Non-degenerate Photon Pairs Research Articles

Ultrabright source of non-degenerate polarization-entangled photon pairs based on off-the-shelf polarization optics

Ultrabright source of non-degenerate polarization-entangled photon pairs based on off-the-shelf polarization optics

Distributing Polarization-Entangled Photon Pairs with High Rate Over Long Distances through Standard Telecommunication Fiber

Entanglement distribution over long distances is essential for many quantum communication schemes like quantum teleportation, some variants of quantum key distribution, or implementations of a quantum internet. Distributing entanglement through standard telecommunication fiber is particularly important for quantum key distribution protocols with low vulnerability over metropolitan distances. However, entanglement distribution over long distance through optical fiber so far could only be accomplished with moderate photon pair rates. In this work, we present entanglement distribution over 50km of standard telecommunication fiber with pair rate more than 10,000 s$^{-1}$ using a bright non-degenerate photon pair source. Signal and idler wavelengths of this source are optimized for low dispersion in optical fiber and high efficiency for single-photon avalanche diode detectors, respectively. The resulting modest hardware requirement and high rate of detected entangled photon pairs could significantly enhance practical entanglement-based quantum key distribution in existing metropolitan fiber networks.

Mid-infrared single-photon upconversion spectroscopy based on temporal-spectral quantum correlation

Promoting the sensitivity of mid-infrared (MIR) spectroscopy to the single-photon level is a critical need for investigating photosensitive biological samples and chemical reactions. MIR spectroscopy based on frequency upconversion is a compelling pioneer allowing high-efficiency MIR spectral measurement with well-developed single-photon detectors, which overcomes the main limitations of high thermal noise of current MIR detectors. However, noise from other nonlinear processes caused by strong pump fields hinders the development of the upconversion-based MIR spectroscopy to reach the single-photon level. Here, a broadband MIR single-photon frequency upconversion spectroscopy is demonstrated based on the temporal-spectral quantum correlation of non-degenerate photon pairs, which is well preserved in the frequency upconversion process and is fully used in extracting the signals from tremendous noise caused by the strong pump. A correlation spectrum broader than 660 nm is achieved and applied for the demonstration of sample identification under a low incident photon flux of 0.09 average photons per pulse. The system is featured with non-destructive and robust operation, which makes single-photon-level MIR spectroscopy an appealing option in biochemical applications.

Non-classical correlations over 1250 modes between telecom photons and 979-nm photons stored in 171Yb3+:Y2SiO5

Quantum repeaters based on heralded entanglement require quantum nodes that are able to generate multimode quantum correlations between memories and telecommunication photons. The communication rate scales linearly with the number of modes, yet highly multimode quantum storage remains challenging. In this work, we demonstrate an atomic frequency comb quantum memory with a time-domain mode capacity of 1250 modes and a bandwidth of 100 MHz. The memory is based on a Y2SiO5 crystal doped with 171Yb3+ ions, with a memory wavelength of 979 nm. The memory is interfaced with a source of non-degenerate photon pairs at 979 and 1550 nm, bandwidth-matched to the quantum memory. We obtain strong non-classical second-order cross correlations over all modes, for storage times of up to 25 μs. The telecommunication photons propagated through 5 km of fiber before the release of the memory photons, a key capability for quantum repeaters based on heralded entanglement and feed-forward operations. Building on this experiment should allow distribution of entanglement between remote quantum nodes, with enhanced rates owing to the high multimode capacity.

Generation of counterpropagating and spectrally uncorrelated photon-pair states by spontaneous four-wave mixing in photonic crystal waveguides

In this work, we propose and theoretically analyze a new scheme for generation of counterpropagating photon pairs in photonic crystal waveguides through the process of spontaneous four-wave mixing. Using the fundamental properties of periodic Bloch modes in a standard photonic crystal waveguide, we demonstrate how modal phase-matching can be reached between forward-propagating pump modes and counterpropagating signal and idler modes, for generation of degenerate and non-degenerate photon pairs. We then show how this scheme can be used for generation of photon pairs that are nearly uncorrelated in the spectral degree of freedom. Such a source will be highly interesting as a heralded source of single photons, especially as the spectrally separable signal and idler photons are also spatially separated directly at the source. We conduct our investigation based on a design in silicon, yet our design concept is general and can in principle be applied to any nanostructured material platform.

Fine-grained all-fiber nonlocal dispersion compensation in the telecommunications O-band.

Nonlocal dispersion compensation between broadband nondegenerate photon pairs propagated over fiber corresponding to the ITU-T G.652D telecommunications standard was studied extensively via fine-grained measurements of the temporal correlation between them. We demonstrated near-ideal levels of nonlocal dispersion compensation by adjusting the propagation distance of the photon pairs to preserve photon timing correlations close to the effective instrument resolution of our detection apparatus (41.0±0.1ps). Experimental data indicates that this degree of compensation can be achieved with relatively large fiber increments (1km), compatible with real-world deployment. Ultimately, photon timing correlations were preserved down to 51ps±21ps over two multi-segmented 10km spans of deployed metropolitan fiber.

Mid-infrared photon pair generation in AgGaS2

We demonstrate nondegenerate photon pair generation by spontaneous parametric downconversion in a silver gallium sulfide AgGaS2 crystal. By tuning the pump wavelength, we achieve phase matching over a large spectral range. This allows to generate idler photons in the mid-infrared spectral range above a 6 μm wavelength with corresponding signal photons in the visible. Also, we show photon pair generation with broad spectral bandwidth. These results are a valuable step toward the development of quantum imaging and sensing techniques in the mid-infrared.

Autoheterodyne Characterization of Narrow-Band Photon Pairs.

We describe a technique to measure photon pair joint spectra by detecting the time-correlation beat note when nondegenerate photon pairs interfere at a beam splitter. The technique implements a temporal analog of the Ghosh-Mandel effect with one photon counter and a time-resolved Hong-Ou-Mandel interference with two. It is well suited to characterize pairs of photons, each of which can interact with a single atomic species, as required to study recently predicted photon-photon interaction in subwavelength atomic arrays. With this technique, we characterize photon pairs from cavity-enhanced parametric down-conversion with a bandwidth ≈ 5 MHz and frequency separation of ∼200 MHz near the D_{1} line of atomic Rb.

SPDC photon pairs using a spatially anti-symmetric pump beam in a ppLN ridge waveguide

In this paper, we study the possible parametric down-conversion processes in a type II phase-matched, Lithium Niobate ridge waveguide, designed to generate photon pairs in the telecommunication range. A quantum analysis of spontaneous parametric down-conversion (SPDC), first, with a pulsed Gaussian pump beam and second, with a pulsed, spatially anti-symmetric Hermite-Gaussian HG (1,0) pump beam predict the possible down conversion processes in each case. In case of the former, degenerate photon pairs are emitted at 1550 nm with the highest efficiency in the fundamental waveguide mode. While, in case of the latter, non-degenerate photon pairs in different higher-order spatial modes are generated. The joint spectral amplitude (JSA) analysis of these processes, prove that the generated photons pairs having orthogonal polarizations are negatively correlated. With multiple degrees of freedom, like polarization and spatial modes, such photons can be further harnessed towards modal-entangled and hyper-entangled photons for quantum information applications. This study involving the JSA is one of the first kinds, especially, to show the possibility of photon pairs generated in different spatial modes and polarization, after being incident with a spatially anti-symmetric pump beam in a ridge waveguide scenario.

Heralded spectroscopy with a fiber photon-pair source

The correlations between photons generated by nonlinear optical processes offer advantages for many quantum technology applications, including spectroscopy, imaging, and metrology. Here, we use spontaneous four-wave mixing in a birefringent single-mode fiber pumped by a tunable pulsed laser as a broadly tunable source of phase-matched non-degenerate photon pairs for spectroscopy. The pairs are tunable such that the idler beam measures the transmittance spectrum of a sample in the near infrared, while the visible signal beam independently reports correlation information. By the time-resolved counting of both signal and idler photons, we use photon-number correlations to remove uncorrelated noise from the probe beam. Here, we have used heralded spectroscopy to measure the absorption spectrum of gallium arsenide near its band edge, despite the idler photon spectrum being dominated by a large background from spontaneous Raman scattering.

Stimulated emission tomography for entangled photon pairs with different detection spectral ranges

Frequency nondegenerate entangled photon pairs have been employed in quantum communication, imaging, and sensing. To characterize quantum entangled states with long-wavelength [infrared (IR) or even terahertz (THz)] photons, one needs to either develop the single-photon detectors at the corresponding wavelengths or use a novel tomography technique that does not rely on single-photon detections, such as stimulated emission tomography (SET). We use standard quantum state tomography and SET to measure the density matrix of entangled photon pairs, with one photon at 1550 nm and the other one at 810 nm, and obtain highly consistent results, showing the reliability of SET. Our work paves the way for efficient measurement of entangled photons with highly dissimilar frequencies, even to the frequencies where single-photon detections are not available.

Characterization of opening angle correlations of a biphoton state decomposed in Bessel modes

The spontaneous parametric down-conversion of photons has been widely applied for generating entangled photon pairs. We theoretically explore the entangled down-converted state of the photon pair, also known as biphoton state, for both degenerate and nondegenerate photon pairs. In particular, the spatial structure of the biphoton state has been expressed in Bessel modes to better understand the correlation with regard to the opening angle ${\ensuremath{\vartheta}}_{k}$ of Bessel modes. In fact, the opening angles of the down-converted photon pair are not independent of each other, but rather are correlated. Furthermore, we confirm the experimentally observed conditions concerning the optimization of generating high-degree spatial entanglement by controlling the beam waist of the pump beam. We also introduce a new experimental setup for efficient measurement of the spatial entanglement of the biphoton state using geometrical optics arguments.

Generation of two different polarization-entangled biphoton states in a potassium titanyl phosphate waveguide device with multiple periodic poling

In this paper, we propose a single integrated waveguide device to generate two different polarization-entangled Bell states involving nondegenerate photon pairs (with the signal photons being emitted in the telecom $C$ band) by utilizing type-0, type-I, and type-II quasi-phase-matching (QPM). In this context, we have studied ppKTP (periodically-poled potassium titanyl phosphate) in detail. To obtain polarization entanglement of the emitted photons, we have used two different situations, two cascaded QPM gratings (of the same length) to obtain $|{\ensuremath{\psi}}^{+}\ensuremath{\rangle}$, and two separate QPM gratings (of different lengths) to obtain $|{\ensuremath{\phi}}^{+}\ensuremath{\rangle}$. In such a scenario, maximal entanglement can be quantified in terms of overlap bandwidths of the two phase-matched processes.

Ultra-broadband SPDC for spectrally far separated photon pairs.

Photon pairs from spontaneous parametric down-conversion (SPDC) are important for a wide range of quantum optics experiments with spectral properties such as their bandwidths often being a crucial concern. Here we show the generic existence of particular phase-matching conditions in quasi-phase-matched KTP, MgO:LN, and SLT crystals that lead to ultra-broadband, widely nondegenerate photon pairs. It is based on the existence of group-velocity-matched, far apart wavelength pairs and, for 2mm long crystals, results in SPDC bandwidths between 15 and 25THz for photon pairs with the idler photon in the technologically relevant mid-IR range of 3-5μm and the signal photon in the near-IR below 1100nm. We experimentally demonstrate this type of broadband phase matching in ppKTP crystals for photon pairs centered at 800 and 3800nm and measure a bandwidth of 15THz. This novel method of generating broadband photon pairs will be highly beneficial for SPDC-based imaging, spectroscopy, refractometry, and optical coherence tomography with undetected mid-IR photons.

Storage of telecom-C-band heralded single photons with orbital-angular-momentum encoding in a crystal

A memory-based quantum repeater architecture provides a solution to distribute quantum information to an arbitrary long distance. Practical quantum repeaters are likely to be built in optical-fiber networks which take advantage of the low-loss transmission between quantum memory nodes. Most quantum memory platforms have characteristic atomic transitions away from the telecommunication band. A nondegenerate photon pair source is therefore useful for connection of a quantum memory to optical fibers. Here, we report a high-brightness narrowband photon-pair source which is compatible with a rare-earth-ion-doped crystal Pr3+:Y2SiO5. The photon-pair source is generated through a cavity-enhanced spontaneous parametric down-conversion process with the signal photon at 606 nm and the idler photon at 1540 nm. Moreover, using the telecom C-band idler photons for heralding, we demonstrate the reversible transfer of orbital-angular-momentum qubit between the signal photon and the quantum memory based on Pr3+:Y2SiO5.

Quantum Storage of Frequency-Multiplexed Heralded Single Photons.

We report on the quantum storage of a heralded frequency-multiplexed single photon in an integrated laser-written rare-earth doped waveguide. The single photon contains 15 discrete frequency modes separated by 261MHz and spanning across 4GHz. It is obtained from a nondegenerate photon pair created via cavity-enhanced spontaneous down-conversion, where the heralding photon is at telecom wavelength and the heralded photon is at 606nm. The frequency-multimode photon is stored in a praseodymium-doped waveguide using the atomic frequency comb (AFC) scheme, by creating multiple combs within the inhomogeneous broadening of the crystal. Thanks to the intrinsic temporal multimodality of the AFC scheme, each spectral bin includes 9 temporal modes, such that the total number of stored modes is about 130. We demonstrate that the storage preserves the nonclassical properties of the single photon, and its normalized frequency spectrum.

Universal Photonic Quantum Interface for a Quantum Network

A quantum network consisting of more than one physical system can combine the advantages and avoid the inherent drawbacks of those different systems. However, a compatible quantum interface is needed to connect them and form a larger quantum network. The authors use nondegenerate narrow-band polarization-entangled photon pairs to entangle different nodes, creating a universal photonic quantum interface that will significantly aid in the development of more complex networks, for quantum communication or distributed quantum computing.

Towards Non-Degenerate Quantum Lithography

The photonic de Broglie wavelength of a non-degenerate entangled photon pair is measured by using a Young’s double slit interferometer, which proves that the non-degenerate entangled photon pairs have the potential to be used in quantum lithography. Experimental results show that the de Broglie wavelength of non-degenerate biphotons is well defined and its wavelength is neither the wavelength of the signal photon, nor the wavelength of the idler photon. According to the de Broglie equation, its wavelength corresponds to the momentum of the biphoton, which equals the sum of the momenta of signal and idler photons. The non-degenerate ghost interference/diffraction is also observed in these experiments.

LiNbO3 waveguides for integrated SPDC spectroscopy

Spontaneous parametric down-conversion (SPDC) spectroscopy using photon pairs is a promising avenue towards affordable mid-infrared (MIR) spectroscopy. Here, we experimentally investigate the feasibility of using periodically poled waveguides in lithium niobate for SPDC spectroscopy applications. We find the waveguides suitable to generate wavelength non-degenerate photon pairs with one photon in the MIR spectral range with high fluence. We use this to determine the cutoff wavelengths of the waveguide mode in the MIR by performing only measurements in the near-infrared spectral range.

Two-photon interferences of nondegenerate photon pairs from Doppler-broadened atomic ensemble.

We report two-photon interference experiments performed with correlated photon pairs generated via spontaneous four-wave mixing in a Doppler-broadened atomic ensemble involving the 5S1/2-5P3/2-5D5/2 transition of 87Rb atoms. When two photons with different wavelengths are incident on a polarization-based Michelson interferometer, two kinds of two-photon superposition states, the frequency-entangled state and dichromatic path-entangled state depending on whether the two photons are in different paths or in the same path, are probabilistically generated within the interferometer arms. Hong-Ou-Mandel-type interference fringes resulting from the frequency-entangled state are observed over the range of the single-photon coherence length, following introduction of a coarse path-length difference between the two interferometer arms and employing phase randomization. When the interferometer is highly phase-sensitive without phase randomization, a phase super-resolved fringe arising from the dichromatic path-entangled state is observed, both with and without the accompanying one-photon interference fringes.