Emission Of Photon Pairs Research Articles

Single-photon and photon pair emission from MOVPE-grown In(Ga)As quantum dots: shifting the emission wavelength from 1.0 to 1.3 μm

InAs quantum dots grown on a GaAs substrate have been one of the most successful semiconductor material systems to demonstrate single-photon-based quantum optical phenomena. In this context, we present the feasibility to extend the low-temperature photoluminescence emission range of In(Ga)As/GaAs quantum dots grown by metal-organic vapor-phase epitaxy from the typical window between 880 and 960 nm to wavelengths above 1.3 μm. A low quantum dot density can be obtained throughout this range, enabling the demonstration of single- and cascaded photon emission. We further analyze polarization-resolved micro-photoluminescence from a large number of individual quantum dots with respect to anisotropy and size of the underlying fine-structure splittings in the emission spectra. For samples with elevated emission wavelengths, we observe an increasing tendency of the emitted photons to be polarized along the main crystal axes.

Phase matching for the optical frequency conversion processes in whispering gallery mode resonators

Optical nonlinear processes in whispering gallery mode resonators require the phase matching that is profoundly different from that in bulk crystals or waveguides. We analyze the phase matching conditions for frequency doubling and parametric down conversion in the resonators, and find the configurations allowing for simultaneous occurrence of these two processes. Such double phase matching gives us access to a variety of interesting quantum nonlinear optical phenomena. For example, it can lead to antibunched emission of photon pairs through the quantum Zeno photonic blockade.

Visible Spectrum Quantum Light Sources Based on InxGa1–xN/GaN Quantum Dots

We present a method for designing quantum light sources, emitting in the visible band, using wurtzite InxGa1−xN quantum dots (QDs) in a GaN matrix. This system is significantly more versatile than previously proposed arsenide- and phosphide-based QDs, having a tuning range exceeding 1 eV. The quantum mechanical configuration interaction method, capturing the fermionic nature of electrons and associated quantum effects explicitly, is used to find shapes and compositions of dots to maximize the excitonic dipole matrix element and optimize the biexciton binding energy. These results provide QD morphologies tailored for either bright single-photon emission or entangled- photon-pair emission at any given wavelength in the visible spectrum.

Plasmonics: Hybrid Graphene–Giant Nanocrystal Quantum Dot Assemblies with Highly Efficient Biexciton Emission (Advanced Optical Materials 1/2015)

When giant nanocrystal quantum dots (g-NQDs) are coupled to graphene, pairs of excitons optically excited in the g-NQDs experience efficient recombination and subsequent simultaneous emission of photon pairs. This enhancement is provided by a localized plasmon supported by a charge puddle that forms underneath the photocharged g-NQD. On page 39, H. Htoon and co-workers reveal the tremendous potential of graphene-g-NQD hybrids for lasing and entangled photon source applications.

Dynamical Casimir effect for surface plasmon polaritons

Abstract The emission of photon pairs by a metal–dielectric interface placed between the mirrors of the resonator and excited by a plane wave is considered. The excitation causes oscillations in time of the optical length of surface plasmon polaritons in the interface. This leads to the dynamical Casimir effect – the generation of pairs of surface plasmon polariton quanta, which transfer to photons outside the interface. In the case of a properly chosen interface, the yield of two-photon emission may exceed that of the usual spontaneous parametric down-conversion.

Photon-pair generation in nonlinear metal-dielectric one-dimensional photonic structures

Nonlinear metal-dielectric layered structures are shown to be able to efficiently generate entangled photon pairs using spontaneous parametric down-conversion. Increase of electric-field amplitudes in these structures enhanced by the presence of metal layers is sufficient to compensate for losses inside thin metal layers. As an example, photon pairs emitted from a structure composed of alternating nonlinear dielectric GaN layers and metal Ag layers are analyzed in spectral, temporal, as well as spatial domains. Also, correlations and entanglement between two photons in a pair are determined. Very narrow photon-pair spectra together with strong directionality of photon-pair emission are observed making the photons suitable for photon-atom interactions. Highly enhanced electric-field amplitudes provide high photon-pair generation efficiencies.

Proposal for the generation of photon pairs with nonzero orbital angular momentum in a ring fiber.

We present a method for the generation of correlated photon pairs in desired orbital-angular-momentum states using a non-linear silica ring fiber and spontaneous parametric down-conversion. Photon-pair emission under quasi-phase-matching conditions with quantum conversion efficiency 6 × 10(-11) is found in a 1-m long fiber with a thermally induced χ(2) nonlinearity in a ring-shaped core.

Emission of microwave photon pairs by a tunnel junction.

We report the observation of photon pairs in the photoassisted shot noise of a tunnel junction in the quantum regime at very high frequency and very low temperature. We have measured the fluctuations of the noise power generated by the junction at two different frequencies, f(1) = 4.4 and f(2) = 7.2 GHz, while driving the junction with a microwave excitation of frequency f(0) = f(1) + f(2). We observe clear correlations between the fluctuations of the two noise powers even when the mean photon number per measurement is much smaller than one. This is strong evidence for photons being emitted in pairs. We also demonstrate that the electromagnetic field generated by the junction exhibits two-mode amplitude squeezing, a proof of its nonclassicality. The data agree very well with predictions based on the fourth cumulant of the current fluctuations generated by the junction.

Spatio-spectral characteristics of parametric down-conversion in waveguide arrays

High dimensional quantum states are of fundamental interest for quantum information processing. They give access to large Hilbert spaces and, in turn, enable the encoding of quantum information on multiple modes. One method to create such quantum states is parametric down-conversion (PDC) in waveguide arrays (WGAs) which allows for the creation of highly entangled photon pairs in controlled, easily accessible spatial modes, with unique spectral properties.In this paper we examine both theoretically and experimentally the PDC process in a lithium niobate WGA. We measure the spatial and spectral properties of the emitted photon pairs, revealing correlations between spectral and spatial degrees of freedom of the created photons. Our measurements show that, in contrast to prior theoretical approaches, spectrally dependent coupling effects have to be taken into account in the theory of PDC in WGAs. To interpret the results, we developed a theoretical model specifically taking into account spectrally dependent coupling effects, which further enables us to explore the capabilities and limitations for engineering the spatial correlations of the generated quantum states.

Parametrization of the angular correlation and degree of linear polarization in two-photon decays of hydrogenlike ions

The spontaneous two-photon emission in hydrogenlike ions is investigated within the framework of second-order perturbation theory and Dirac's equation. Special attention is paid to the angular correlation of the emitted photons as well as to the degree of linear polarization of one of the two photons, if the second is just observed under arbitrary angles. Expressions for the angular correlation and the degree of linear polarization are expanded in powers of cosine functions of the two-photon opening angle, whose coefficients depend on the atomic number and the energy sharing of the emitted photons. The effects of including higher (electric and magnetic) multipoles upon the emitted photon pairs beyond the electric-dipole approximation are also discussed. Calculations of the coefficients are performed for the transitions $2{s}_{1/2}\ensuremath{\rightarrow}1{s}_{1/2}$, $3{d}_{3/2}\ensuremath{\rightarrow}1{s}_{1/2}$, and $3{d}_{5/2}\ensuremath{\rightarrow}1{s}_{1/2}$, along the entire hydrogen isoelectronic sequence ($1\ensuremath{\le}Z\ensuremath{\le}100$).

Emission of correlated photon pairs from superluminal perturbations in dispersive media

We develop a perturbative theory that describes a superluminal refractive perturbation propagating in a dispersive medium and the subsequent excitation of the quantum vacuum zero-point fluctuations. We find a pro-cess similar to the anomalous Doppler effect: Photons are emitted in correlated pairs and mainly within a Cerenkov-like cone, one on the forward and the other in backward directions. The number of photon pairs emitted from the perturbation increases strongly with the degree of superluminality and under realizable experimental conditions; it can reach up to $\ensuremath{\sim}$10${}^{\ensuremath{-}2}$ photons per pulse. Moreover, it is, in principle, possible to engineer the host medium so as to modify the effective group refractive index. In the presence of ``fast-light'' media, e.g., a with group index smaller than unity, a further $\ensuremath{\sim}$10$\ifmmode\times\else\texttimes\fi{}$ enhancement may be achieved and the photon emission spectrum is characterized by two sharp peaks that, in future experiments, would clearly identify the correlated emission of photon pairs.

Antibunched Emission of Photon Pairs via Quantum Zeno Blockade

We propose a new methodology, namely, the "quantum Zeno blockade," for managing light scattering at a few-photon level in general nonlinear-optical media, such as crystals, fibers, silicon microrings, and atomic vapors. Using this tool, antibunched emission of photon pairs can be achieved, leading to potent quantum-optics applications such as deterministic entanglement generation without the need for heralding. In a practical implementation using an on-chip toroidal microcavity immersed in rubidium vapor, we estimate that high-fidelity entangled photons can be produced on-demand at MHz rates or higher, corresponding to an improvement of ≳10(7) times from the state-of-the-art.

Indistinguishable Photon Pairs from Independent True Chaotic Sources

Indistinguishability of events in quantum mechanics is manifested by interference between their probability amplitudes. We report a unique kind of interference occurring between indistinguishable events of photon-pair emission, where each photon of the pair is emitted from a distinct true chaotic light source and has a different energy. The indistinguishability results in an interference which is observed as an ultrafast modulation of the second-order coherence function, measured on a femtosecond time scale by two-photon absorption in a semiconductor photomultiplier tube.

Free induction decay of a superposition stored in a quantum dot

We study the free evolution of a superposition initialized with high fidelity in the neutral-exciton state of a quantum dot. Readout of the state at later times is achieved by polarized photon detection, averaged over a large number of cycles. By controlling the fine-structure splitting (FSS) of the dot with a dc electric field, we show a reduction in the degree of polarization of the signal when the splitting is minimized. In analogy with the "free induction decay" observed in nuclear magnetic resonance, we attribute this to hyperfine interactions with nuclei in the semiconductor. We numerically model this effect and find good agreement with experimental studies. Our findings have implications for storage of superpositions in solid-state systems and for entangled photon pair emission protocols that require a small value of the FSS.

Optimized heralding schemes for single photons

A major obstacle to a practical, heralded source of single photons is the fundamental trade-off between high purity and high production rate. To overcome this difficulty, we propose applying sequential spectral and temporal filtering on the signal photons before they are detected for heralding. Based on a multimode theory that takes into account the effect of simultaneous multiple photon-pair emission, we find that these filters can be optimized to yield both high purity and a high production rate. While the optimization conditions vary depending on the underlying photon-pair spectral correlations, all correlation profiles can lead to similarly high performance levels when optimized filters are employed. This suggests that a better strategy for improving the performance of heralded single-photon sources is to adopt an appropriate measurement scheme for the signal photons, rather than tailoring the properties of the photon-pair generation medium.

Position controlled nanowires for infrared single photon emission

We report the experimental demonstration of single-photon and cascaded photon pair emission in the infrared, originating from a single InAsP quantum dot embedded in a standing InP nanowire. A regular array of nanowires is fabricated by epitaxial growth on an electron-beam patterned substrate. Photoluminescence spectra taken on single quantum dots show narrow emission lines. Superconducting single photon detectors, which have a higher sensitivity than avalanche photodiodes in the infrared, enable us to measure auto and cross correlations. Clear antibunching is observed [g(2)(0)=0.12] and we show a biexciton–exciton cascade, which can be used to create entangled photon pairs.

Polarization-correlated photon pairs from a single ion

In the fluorescence light of a single atom, the probability for emission of a photon with certain polarization depends on the polarization of the photon emitted immediately before it. Here correlations of such kind are investigated with a single trapped calcium ion by means of second order correlation functions. A theoretical model is developed and fitted to the experimental data, which show 91% probability for the emission of polarization-correlated photon pairs within 24 ns.

Quantum‐Dot‐Based Photon Emission and Media Conversion for Quantum Information Applications

Single‐photon as well as polarization‐correlated photon pair emission from a single semiconductor quantum dots is demonstrated. Single photon generation and single photon‐pair generation with little uncorrelated multiphoton emission and the feasibility of media conversion of the quantum states between photon polarization and electron spin are fundamental functions for quantum information applications. Mutual media conversion for the angular momentum between photon polarization and electron spin is also achieved with high fidelity via positively charged exciton state without external magnetic field. This is a clear indication that the coupling of photon polarizations and electron spins keeps secured during whole processes before photon emission. Possibility of a metal‐embedded structure is demonstrated with the observation of drastic enhancement of excitation and/or collection efficiency of luminescence as well as clear antibunching of photons generated from a quantum dot.

Entangled photons on demand: Erasing which-path information with sidebands

The biexciton cascade in a quantum dot can be used to generate entangled-photon pairs rapidly and deterministically (on demand). However, due to a large fine-structure splitting between intermediate exciton energy levels, which-path information encoded in the frequencies of emitted photon pairs leads to a small degree of entanglement. Here we show that this information can be efficiently erased by modulating the exciton and biexciton energy levels, giving rise to additional decay paths through sidebands. The resulting degree of entanglement is substantial and can be made maximal through spectral filtering, with only a nominal reduction in collection efficiency.

Emission of photon pairs at discontinuities of nonlinearity in spontaneous parametric down-conversion

In order to fulfill the continuity requirements for electric- and magnetic-field amplitudes at discontinuities of ${\ensuremath{\chi}}^{(2)}$ nonlinearity, additional photon pairs have to be emitted in the area of discontinuity. Generalized two-photon spectral amplitudes can be used to describe properties of photon pairs generated in this process that we call surface spontaneous parametric down-conversion. The spectral structure of such photon pairs is similar to that derived for photon pairs generated in the volume. Surface and volume contributions to spontaneous down-conversion can be comparable as an example of nonlinear layered structures shows.