Heralded generation of ultrafast single photons in pure quantum States

We present an experimental demonstration of heralded single photons prepared in pure quantum states from a parametric down-conversion source. It is shown that, through controlling the modal structure of the photon pair emission, one can generate pairs in factorable states and thence eliminate the need for spectral filters in multiple-source interference schemes. Indistinguishable heralded photons were generated in two independent spectrally engineered sources and Hong-Ou-Mandel interference observed between them without spectral filters. The measured visibility of 94.4% sets a minimum bound on the mean photon purity.     

The Maxwell-Bloch equations for three-photon coherent absorption

The equations of ultra-short-pulse propagation by stimulated absorption and emission of three photons are presented, It is found that three photon self-induced transparency is impossible in principle.     

Interference of two photons of different color

We consider the interference of two photons with different colors in the context of a Hong-Ou-Mandel experiment, in which single photons enter each of the input ports of a beam splitter, and exit together in the same, albeit undetermined, output port. Such interference is possible if one uses an active (energy-non-conserving) beam splitter. We find scenarios in which one “red” and one “blue” photon enter the beam splitter, and either two red or two blue photons exit, but never one of each color. We show how the precise form of the active beam-splitter transformation determines in what way the spectral degrees of freedom of the input photons should be related to each other for perfect destructive interference of the different-color components in the output. We discuss two examples of active beam splitters: one is a gedanken experiment involving a moving mirror and the other is a more realistic example involving four-wave mixing in an optical fiber.     

Squeezing and photon statistical effects in spontaneous and stimulated eight-wave mixing process

Eight-wave mixing is a process which involves absorption of four pump photons and emission of three probe photons of same frequency and a signal photon of different frequency. Squeezing is investigated in the fundamental mode in spontaneous and stimulated eight-wave mixing process using quantum mechanical approach. The presence of squeezing in field amplitude, amplitude-squared and amplitude-cubed in the fundamental mode has been probed. The dependence of squeezing on photon number is established with investigations into degree of squeezing in the first and higher order of field amplitude. The photon statistics of the field in the fundamental mode has also been investigated. It has been found that the field shows sub-Poissonian behaviour in the fundamental mode.     

Observation of antinormally ordered Hanbury Brown-Twiss correlations

We have measured antinormally ordered Hanbury Brown-Twiss correlations for coherent states of the electromagnetic field by using a stimulated parametric down-conversion process. Photons were detected by stimulated emission, rather than by absorption, so that the detection responded not only to actual photons but also to zero-point fluctuations via spontaneous emission. The observed correlations were distinct from normally ordered ones as they showed excess positive correlations, i.e., photon bunching effects, which arose from the thermal nature of zero-point fluctuations.     

Creating single time-bin-entangled photon pairs

When a single emitter is excited by two phase-coherent pulses with a time delay, each of the pulses can lead to the emission of a photon pair, thus creating a "time-bin-entangled" state. Double pair emission can be avoided by initially preparing the emitter in a metastable state. We show how photons from separate emissions can be made indistinguishable, permitting their use for multiphoton interference. Possible realizations are discussed. The method might also allow the direct creation of n-photon entangled states (n > 2).     

Photon Correlations from the Mollow Triplet

Photon correlations between the photoluminescence peaks of the Mollow triplet have been known for a long time, and recently hailed as a resource for heralded single‐photon sources. Here, we provide the full picture of photon‐correlations at all orders (we deal explicitly with up to four photons) and with no restriction to the peculiar frequency windows that enclose the peaks. We show that a rich multi‐photon physics lies between the peaks, due to transitions involving virtual photons, and thereby much more strongly correlated than those transiting through the real states. Specifically, we show that such emissions occur in bundles of photons rather than as successive, albeit correlated, photons. We provide the recipe to frequency‐filter the emission of the Mollow triplet to turn it into a versatile and tunable photon source, allowing in principle all scenarios of photon emission, with advantages already at the one‐photon level, i.e., providing more strongly correlated heralded single‐photon sources than those already known.     

Photon formation length in Hanbury-Brown and Twiss interferometry of bremsstrahlung photons

We study the effect of the photon formation length on the interference of two bremsstrahlung photons in the simplest case of radiation emitted by only two charged particles. Most of interference patterns obtained can be interpreted in terms of the photon formation length. After having analyzed the simplest situations we discuss the interference of bremsstrahlung photons in more realistic and more complicated situations. A possibility of studying the effect in synchrotron radiation is briefly sketched.     

Reprint of : Photon correlations generated by inelastic scattering in a one-dimensional waveguide coupled to three-level systems

We study photon correlations generated by scattering from three-level systems (3LS) in one dimension. The two systems studied are a 3LS in a semi-infinite waveguide (3LS plus a mirror) and two 3LS in an infinite waveguide (double 3LS). Our two-photon scattering approach naturally connects photon correlation effects with inelastically scattered photons; it corresponds to input–output theory in the weak-probe limit. At the resonance where electromagnetically induced transparency (EIT) occurs, we find that no photons are scattered inelastically and hence there are no induced correlations. Slightly away from EIT, the total inelastically scattered flux is large, being substantially enhanced due to the additional interference paths. This enhancement carries over to the two-photon correlation function, which exhibits non-classical behavior such as strong bunching with a very long time-scale. The long time scale originates from the slow-light effect associated with EIT.     

Photon correlations generated by inelastic scattering in a one-dimensional waveguide coupled to three-level systems

We study photon correlations generated by scattering from three-level systems (3LS) in one dimension. The two systems studied are a 3LS in a semi-infinite waveguide (3LS plus a mirror) and two 3LS in an infinite waveguide (double 3LS). Our two-photon scattering approach naturally connects photon correlation effects with inelastically scattered photons; it corresponds to input–output theory in the weak-probe limit. At the resonance where electromagnetically induced transparency (EIT) occurs, we find that no photons are scattered inelastically and hence there are no induced correlations. Slightly away from EIT, the total inelastically scattered flux is large, being substantially enhanced due to the additional interference paths. This enhancement carries over to the two-photon correlation function, which exhibits non-classical behavior such as strong bunching with a very long time-scale. The long time scale originates from the slow-light effect associated with EIT.     

Evaluation of polarization entanglement generated by spontaneous parametric downconversion using photon number counting

Spontaneous parametric downconversion (SPDC) is widely used to generate entangled photon pairs; however, multi-pair emissions degrade the quality of the entanglement. We numerically evaluate polarization-entangled photon pairs created by SPDC. The effects of multi-pair emission events on the visibility of two-photon interference and on the fidelity (the probability overlap for ideal and real states) are analyzed using single-photon detectors that can count the number of incoming photons and discard multiphoton events. Compared with conventional threshold single-photon detectors, photon-number resolving single-photon detectors have higher fidelity for the same or lower visibility.     

Plasmid DNA damage induced by helium atmospheric pressure plasma jet

Recently demonstrated ghost interference using correlated photons of different frequencies, has been theoretically analyzed. The calculation predicts an interesting nonlocal effect: the fringe width of the ghost interference depends not only on the wave-length of the photon involved, but also on the wavelength of the other photon with which it is entangled. This feature, arising because of different frequencies of the entangled photons, was hidden in the original ghost interference experiment. This prediction can be experimentally tested in a slightly modified version of the experiment.     

Fast path and polarization manipulation of telecom wavelength single photons in lithium niobate waveguide devices

We demonstrate fast polarization and path control of photons at 1550?nm in lithium niobate waveguide devices using the electro-optic effect. We show heralded single photon state engineering, quantum interference, fast state preparation of two entangled photons, and feedback control of quantum interference. These results point the way to a single platform that will enable the integration of nonlinear single photon sources and fast reconfigurable circuits for future photonic quantum information science and technology.     

Precursor states for charge carrier generation in conjugated polymers probed by ultrafast spectroscopy

Photocurrent experiments using two femtosecond laser pulses are performed on a photodiode using a ladder-type conjugated polymer as the active layer. With a photon energy of 3.1 eV the first pulse excites singlet excitons. A time-delayed second pulse with a photon energy of 2.49 eV leads to a decrease of the photocurrent by exciton depletion due to stimulated emission. S1 excitons being dissociated during their entire lifetime are identified as the only relevant channel for charge carrier generation. Intrachain polaron pairs are also formed on an ultrafast time scale with a yield of approximately 10%. They can be efficiently dissociated by reexcitation with photons of an energy of 1.9 eV.     

Interference effect in the photoproduction of electron-positron pairs on a nucleus in the field of two light waves

In the general relativistic case, the nonresonance photoproduction of electron-positron pairs on a nucleus in the field of two circularly polarized light waves propagating in one direction is theoretically investigated. It is shown that there are two essentially different kinematical regions: the noninterference region, where the Bunkin-Fedorov quantum parameters play the role of multiphoton parameters, and the interference region, where interference effects are important and where quantum interference parameters are treated as multiphoton parameters. The interference effect is found in the photoproduction of an electron-positron pair on a nucleus in the field of two light waves. This effect occurs in the interference region and is due to the production of an electron-positron pair in the plane spanned by the light wave vector and the incident-photon momentum and to stimulated correlated emission and absorption of photons of the two waves. The cross sections for pair photoproduction on a nucleus in the above kinematical regions are determined for moderately strong fields. It is shown that, in the interference region, the partial cross sections for the case where there is a correlated emission (absorption) of the same number of photons of the two waves can significantly exceed the corresponding cross sections in any other geometry.     

Photon echo without a free induction decay in a double-Λ system

We have characterized a novel photon-echo pulse sequence for a double-Λ-type energy level system where the input and rephasing transitions are different from the applied π pulses. We show that, despite having imperfect π-pulses associated with large coherent emission due to free induction decay (FID), the noise added in the echo mode is only 0.2 ± 0.1 photons per shot, compared to 4 × 10? photons in the FID modes. Using this echo pulse sequence in the "rephased amplified spontaneous emission" (RASE) scheme [Phys. Rev. A 81, 012301 (2010)] will allow for generation of entangled photon pairs that are in different frequency, temporal, and potentially spatial modes to any bright driving fields. The coherence and efficiency properties of this sequence were characterized in a Pr(3+):Y?SiO? crystal.     

Erasing distinguishability using quantum frequency up-conversion

The frequency distinguishability of two single photons was successfully erased using single photon frequency up-conversion. A frequency nondegenerate photon pair generated via spontaneous four-wave mixing in a dispersion shifted fiber was used to emulate two telecom-band single photons that were in the same temporal mode but in different frequency modes. The frequencies of these photons were converted to the same frequency by using the sum-frequency generation process in periodically poled lithium niobate waveguides, while maintaining their temporal indistinguishability. As a result, the two converted photons exhibited a nonclassical dip in a Hong-Ou-Mandel quantum interference experiment. The present scheme will add flexibility to networking quantum information systems that use photons with various wavelengths.     

Nonclassical interference and entanglement generation using a photonic crystal fiber pair photon source

We demonstrate two key components for optical quantum information processing: a bright source of heralded single photons; and a bright source of entangled photon pairs. A pair of pump photons produces a correlated pair of photons at widely spaced wavelengths (583 nm and 900 nm), via a chi((3)) four-wave mixing process. We demonstrate nonclassical interference between heralded photons from independent sources with a visibility of 95% (after correction for background), and an entangled photon pair source, with a fidelity of 89% with a Bell state.     

多光子相长干涉解释光子的广义聚束效应

光的聚束效应的发现是量子光学的重要里程碑.对其进行合理解释的努力,促进了量子光学理论与技术的大发展,使量子光学迅速发展和完善起来.现在一般认为,光场二阶相干所体现的聚束效应的物理本质是双光子干涉.本文发现,在高阶相干中有类似的广义聚束效应,这种广义聚束效应也可以用光子相长干涉解释.而且在一些典型的光路中,Ⅳ个光子参与的干涉,可以使这种广义聚束效应的增强因子达到N!.