Spontaneous Parametric Down-conversion

The reversible transfer of quantum states of light in and out of matter constitutes an important building block for future applications of quantum communication: it allows synchronizing quantum information 1 , and enables one to build quantum repeaters 2 and quantum networks 3 . Much effort has been devoted worldwide over the past years to develop memories suitable for the storage of quantum states 1 . Of central importance to this task is the preservation of entanglement, a quantum mechanical phenomenon whose counter-intuitive properties have occupied philosophers, physicists and computer scientists since the early days of quantum physics 4 . Here we report, for the first time, the reversible transfer of photonphoton entanglement into entanglement between a photon and collective atomic excitation in a solid-state device. Towards this end, we employ a thulium-doped lithium niobate waveguide in conjunction with a photon-echo quantum memory protocol 5 , and increase the spectral acceptance from the current maximum of 100 MHz 6 to 5 GHz. The entanglement-preserving nature of our storage device is assessed by comparing the amount of entanglement contained in the detected photon pairs before and after the reversible transfer, showing, within statistical error, a perfect mapping process. Our integrated, broadband quantum memory complements the family of robust, integrated lithium niobate devices 7 . It renders frequency matching of light with matter interfaces in advanced applications of quantum communication trivial and institutes several key properties in the quest to unleash the full potential of quantum communication. Quantum communication is founded on the encoding of information, generally referred to as quantum information, into quantum states of light 8 . The resulting applications of quantum physics at its fundamental level offer cryptographic security through quantum key distribution without relying on unproven mathematical assumptions 9 and allow for the disembodied transfer of quantum states between distant places by means of quantum teleportation 8, . Reversible mapping of quantum states between light and matter is central to advanced applications of quantum communication, e.g. quantum repeaters 2 and quantum networks 3 , in which matter

Applying a multiphoton-subtraction technique to two-color macroscopic squeezed vacuum state of light generated via high-gain parametric down conversion we conditionally prepare a new state of light: bright multi-mode low-noise twin beams. The obtained results demonstrate up to 8-fold suppression of noise in each beam while preserving and even moderately improving the nonclassical photon number correlations between the beams. The prepared low-noise macroscopic state, containing up to 2000 photons per mode, is not among the states achievable through nonlinear optical processes. The proposed technique substantially improves the usefulness of twin beams for quantum technologies.

We present an analytical formulation of the recent one-shot decoupling model of Bràdler and Adami [arXiv:1505.0284] and compute the resulting "Page Information" curves, for the reduced density matrices for the evaporating black hole internal degrees of freedom, and emitted Hawking radiation pairs entangled across the horizon. We argue that black hole evaporation/particle production has a very close analogy to the laboratory process of spontaneous parametric down conversion, when the pump is allowed to deplete.

We outline a proposal for a method of preparing a single logically encoded two-state system (qubit) that is immune to collective noise acting on the Hilbert space of the particles supporting it. The logical qubit is comprised of three photonic 3-state systems (qutrits) and is generated by the process of spontaneous parametric down-conversion. The states are constructed using linear optical elements along with three down-conversion sources, and are deemed successful by the simultaneous detection of six events. We also show how to select a maximally entangled state of two qutrits by similar methods. For this maximally entangled state we describe conditions for the state to be decoherence-free which do not correspond to collective errors, but which have a precisely defined relationship between them.

Single photon first order interferences of spatially separated regions from the cone structure of spontaneous parametric down conversion allow for analyzing the role of the mode function in quantum optics regarding the complementarity principle. In earlier experiments the role of the vacuum fields could be demonstrated as the source of complementarity with respect to the temporal properties . Here the spatial coherence properties of these vacuum fields are demonstrated as the physical reason for complementarity in these single photon quantum optical experiments. These results are directly connected to the mode picture in classical optics.

Quantum entanglement between two or more bipartite entities is a core concept in quantum information areas limited to microscopic regimes directly governed by Heisenberg's uncertainty principle via quantum superposition, resulting in nondeterministic and probabilistic quantum features. Such quantum features cannot be generated by classical means. Here, a pure classical method of on-demand entangled light-pair generation is presented in a macroscopic regime via basis randomness. This conflicting idea of conventional quantum mechanics invokes a fundamental question about both classicality and quantumness, where superposition is key to its resolution.

We consider a process of parametric down conversion where the input state is a bosonic thermofield vacuum. This state leads to a parametric down conversion, generating an output of two excited photons. Following a thermofield dynamics scheme, the input state, initially in a bosonic thermofield vacuum, and the output states, initially in vacuum states, evolve under a Liouville-von Neumann equation.

A single trapped ion is converted into a pseudo-two-photon source by splitting its resonance fluorescence, delaying part of it and by recombining both parts on a beam splitter. A destructive twophoton interference is observed with a contrast reaching 83(5)%. The spectral brightness of our twophoton source is quantified and shown to be comparable to parametric down-conversion devices.

In the framework of the paraxial and of the slowly varying envelope approximations, with reference to a normally dispersive medium or to vacuum, the electromagnetic field is given as a continuous quantum superposition of non-dispersive and non-diffracting wave-packets (namely X-waves). Entangled states as pairs of elementary excitations traveling at (approximately) the same velocity are found in optical parametric amplification.

Entangled photons are crucial for the development of quantum technologies and especially important in quantum communications. Hence it is paramount to have a reliable, high-fidelity source of entangled photons. Here we describe the construction and characterization of a polarization-entangled photon source. We generate maximallyentangled Bell states using a type-II PPKTP nonlinear crystal inside a Sagnac interferometer. We characterize the source in terms of brightness, visibility, Bell-CHSH test and we perform quantum-state tomography of the density matrix. Our source violates Bell-CHSH inequality |S| ≤ 2 by n = 22 standard deviations. With visibilities up to V = 98.9% and fidelity F = 97%, our source is highly competitive, with state-of-the-art performance. To the best of our knowledge, this is the first source of entangled-photons designed and build in Romania and as such, represents an important step for the development of quantum technologies in our country. We envisage that our results will stimulate the progress of quantum technologies in Romania and will educate the future generation of quantum engineers.

Photon modes have an important role in characterizing the quantum sources of light. Proper coupling of various photon modes obtained in spontaneous parametric down conversion (SPDC) process in optical fibers is essential to generate an effective source of entangled photons. The two main pre-detection factors affecting the biphoton mode coupling in SPDC are the pump beam focusing parameter and the crystal thickness. We present the numerical and experimental results on the effect of pump focusing on conditional down-converted photon modes for a Type-I BBO crystal. We experimentally verify that biphoton coupling efficiency decreases asymptotically with pump beam focusing parameter. We attribute this behaviour to (a) the asymmetry in the spatial distribution of down-converted photons with the pump beam focusing parameter and (b) the ellipticity of biphoton modes introduced due to the focusing of the pump beam. We also show experimentally the ellipticity as well as quantify the asymmetry with the pump focusing parameter. These results may be useful in selecting optimum conditions in the down-conversion process for generating efficient sources of entangled photons for quantum information applications.

All Bell experiments carried out so far have had ten or more times fewer coincidence counts than singles counts and this, in effect, means a detection efficiency under 10%. Therefore, all these experiments relied only on coincidence counts and herewith on additional assumptions. Recently, however, Santos devised hidden variable models which do not obey the assumptions and thus made the experiments inconclusive. This, as well as recent improvements in detectors efficiencies, prompted an increasing interest in the loophole-free Bell experiments which do not rely on additional assumptions and which originally stem from the idea of the event-ready detectors (introduced by J.S. Bell) which would preselect Bell pairs ready for detection. Till recently it was assumed that such detectors would distort the pairs. Here we devise those that would not do so and propose an experiment which can realistically improve the detection efficiency and visibility up to over 80%. The setup uses two nonlinear crystals of type-II both of which simultaneously downconvert a singlet-like pair. We combine one photon from the first singlet with one from the second singlet at a beam splitter and consider their coincidence detections. Detectors determine optimally narrow solid angles for the downconverted photons. However, for their two companions (from each singlet) we use five times wider solid angles or even drop pinholes altogether and resort to frequency filters. So, we are able to realistically collect close to 100% of them. The latter pairs-preselected by coincidence detection at the beam splitter-appear entangled in (non)maximal singlet-like states, i.e., detectors at the beam splitter act as event-ready detectors for such Bell pairs.

We report the first measurement of the joint photon-number probability distribution for a two-mode quantum state created by a nondegenerate optical parametric amplifier. The measured distributions exhibit up to 1.9 dB of quantum correlation between the signal and idler photon numbers, whereas the marginal distributions are thermal as expected for parametric fluorescence.

Photon-number distributions for parametric f luorescence from a nondegenerate optical parametric amplifier are measured with a novel self-homodyne technique. These distributions exhibit the thermal-state character predicted by theory. However, a difference between the f luorescence gain and the signal gain of the parametric amplifier is observed. We attribute this difference to a change in the signal-beam profile during the travelingwave pulsed amplification process.

We experimentally demonstrate the first remote state preparation of arbitrary single-qubit states, encoded in the polarization of photons generated by spontaneous parametric downconversion. Utilizing degenerate and nondegenerate wavelength entangled sources, we remotely prepare arbitrary states at two wavelengths. Further, we derive theoretical bounds on the states that may be remotely prepared for given two-qubit resources.

We report the experimental realization of heralded distribution of single-photon path entanglement at telecommunication wavelengths in a repeater-like architecture. The entanglement is established upon detection of a single photon, originating from one of two spontaneous parametric down-conversion photon pair sources, after erasing the photon's which-path information. In order to certify the entanglement, we use an entanglement witness which does not rely on postselection. We herald entanglement between two locations, separated by a total distance of 2 km of optical fiber, at a rate of 1.6 kHz. This work paves the way towards high-rate and practical quantum repeater architectures.

We display the results of the numerical simulations of a set of Langevin equations, which describe the dynamics of a degenerate optical parametric oscillator in the Wigner representation. The scan of the threshold region shows the gradual transformation of a quantum image into a classical roll pattern. An experiment on parametric downconversion in lithium triborate shows strikingly similar results in both the near and the far field, displaying qualitatively the classical features of quantum images.

We show that the quantum disentanglement eraser implemented on a two-photon system from parametric down-conversion is a general method to create hybrid photonic entanglement, namely the entanglement between different degrees of freedom of the photon pair. To demonstrate this, we generate and characterize a source with tunable degree of hybrid entanglement between two qubits, one encoded in the transverse momentum and position of a photon, and the other in the polarization of its partner. In addition, we show that a simple extension of our setup enables the generation of two-photon qubit-qudit hybrid entangled states. Finally, we discuss the advantages that this type of entanglement can bring for an optical quantum network.

We present our approach for sharing photons and assessing resultant four-photon visibility between two distant parties using concatenated entanglement swapping. In addition we determine the corresponding key generation rate and the quantum bit-error rate. Our model is based on practical limitations of resources, including multipair parametric down-conversion sources, inefficient detectors with dark counts and lossy channels. Through this approach, we have found that a trade-off is needed between experimental run-time, pair-production rate and detector efficiency. Concatenated entanglement swapping enables huge distances for quantum key-distribution but at the expense of low key generation rate.

We propose a new scheme for quantum key distribution using macroscopic non-classical pulses of light having of the order 10 6 photons per pulse. Sub-shot-noise quantum correlation between the two polarization modes in a pulse gives the necessary sensitivity to eavesdropping that ensures the security of the protocol. We consider pulses of two-mode squeezed light generated by a type-II seeded parametric amplification process. We analyze the security of the system in terms of the effect of an eavesdropper on the bit error rates for the legitimate parties in the key distribution system. We also consider the effects of imperfect detectors and lossy channels on the security of the scheme.

Quantum lithography achieves phase super-resolution using fragile, experimentally challenging entangled states of light. We propose a scalable scheme for creating features narrower than classically achievable, with reduced use of quantum resources and consequently enhanced resistance to loss. The scheme is an implementation of interferometric lithography using a mixture of an SPDC entangled state with intense classical coherent light. We measure coincidences of up to four photons mimicking multiphoton absorption. The results show a narrowing of the interference fringes of up to 30% with respect to the best analogous classical scheme using only 10% of the non-classical light required for creating NOON states.

Spin-entaglement has been proposed and extensively used in the case of correlated triplet pairs which are intermediate states in singlet fission process in select organic semiconductors. Here, we employ quantum process tomography of polarization entangled photon-pairs resonant with the excited state absorption of these states to investigate the nature of the inherent quantum correlations and to explore for an unambiguous proof for the existence of exciton entanglement.

Characteristics of the propagation of Li's flattened Mathieu-Gaussian (FTMG) beams traveling a free paraxial ABCD optical system with: annular aperture, circular aperture and black screen are studied theoretically in this work, respectively. By means of expanding of a hard aperture function into a finite sum of complex Gaussian functions, and based on the Collins integral formula, a main approximate analytical expression of propagation of FTMG beams through an annular apertured ABCD optical system is developed. From this general result, we investigate the propagation of FTMG beams through an ABCD optical system with a circular aperture system, a circular black screen and unapertured optical system as particular cases. The actual work generalizes also the above studies concerning the Mathieu, Mathieu-Gauss, flattened Bessel, Bessel-Gauss, pure Bessel, flattened Gaussian, and pure Gaussian beams passing through the considered optical systems. Some numerical calculations of the cases of propagation of FTMG through an annular apertured ABCD optical system and with a circular black are given and discussed in the present study.

Nonlocal quantum correlation has been the main issue of quantum mechanics over the last century. The Hong-Ou-Mandel (HOM) effect relates to the two-photon intensity correlation on a beam splitter, resulting in a nonclassical photon-bunching phenomenon. The HOM effect has been used to verify the quantum feature via Bell measurements for quantum technologies such as quantum repeaters and photonics quantum computing. Here, a coherence version of the HOM effect is proposed and analyzed to understand the fundamental physics of the anticorrelation and entanglement. For this, frequency-correlated coherent photon pairs are prepared in an independent set of Mach-Zhender interferometers (MZI) using a synchronized pair of modulators from an attenuated laser. For the HOM effect, the phase relation between frequency-correlated photons plays an essential role. For the product-basis randomness, the symmetrically modulated two independent MZIs are combined together incoherently. A classical intensity product between two independent photodetectors is also discussed for the same HOM effect in a selective macroscopic measurement scheme.

The development of linear quantum computing within integrated circuits demands high quality semiconductor single photon sources. In particular, for a reliable single photon source it is not sufficient to have a low multi-photon component, but also to possess high efficiency. We investigate the photon statistics of the emission from a single quantum dot with a method that is able to sensitively detect the trade-off between the efficiency and the multi-photon contribution. Our measurements show, that the light emitted from the quantum dot when it is resonantly excited possess a very low multi-photon content. Additionally, we demonstrated, for the first time, the non-Gaussian nature of the quantum state emitted from a single quantum dot.

We report on a double slit experiment where indistinguishable correlated photons generated through spontaneous parametric downconversion, are each sent to a well-defined slit. An analysis of the results allows a test of de Broglie-Bohm mechanics against standard quantum mechanics.

Quantum optics in combination with integrated optical devices shows great promise for efficient manipulation of single photons. New physical concepts, however, can only be found when these fields truly merge and reciprocally enhance each other. Here we work at the merging point and investigate the physical concept behind a two-coupled-waveguide system with an integrated parametric down-conversion process. We use the eigenmode description of the linear system and the resulting modification in momentum conservation to derive the state generation protocol for this type of device. With this new concept of state engineering, we are able to effectively implement a two-in-one waveguide source that produces the useful two-photon NOON state without extra overhead such as phase stabilization or narrow-band filtering. Experimentally, we benchmark our device by measuring a two-photon NOON state fidelity of F = (84.2 ± 2.6) % and observe the characteristic interferometric pattern directly given by the doubled phase dependence with a visibility of VNOON = (93.3 ± 3.7) %.

We propose an optical parametric down conversion (PDC) scheme that does not suffer a trade-off between the state-purity of single-photon wave-packets and the rate of packet production. This is accomplished by modifying the PDC process by using a microcavity to engineer the density of states of the optical field at the PDC frequencies. The high-finesse cavity mode occupies a spectral interval much narrower than the bandwidth of the pulsed pump laser field, suppressing the spectral correlation, or entanglement, between signal and idler photons. Spectral filtering of the field occurs prior to photon creation rather than afterward as in most other schemes. Operator-Maxwell equations are solved to find the Schmidt-mode decomposition of the two-photon states produced. Greater than 99% pure-state packet production is predicted to be achievable.

We study theoretically the generation of photon pairs by spontaneous four-wave mixing (SFWM) in photonic crystal optical fiber. We show that it is possible to engineer two-photon states with specific spectral correlation ("entanglement") properties suitable for quantum information processing applications. We focus on the case exhibiting no spectral correlations in the two-photon component of the state, which we call factorability, and which allows heralding of single-photon pure-state wave packets without the need for spectral post filtering. We show that spontaneous four wave mixing exhibits a remarkable flexibility, permitting a wider class of two-photon states, including ultra-broadband, highlyanticorrelated states.

We show that the quantum disentanglement eraser implemented on a two-photon system from parametric down-conversion is a general method to create hybrid photonic entanglement, namely the entanglement between different degrees of freedom of the photon pair. To demonstrate this, we generate and characterize a source with tunable degree of hybrid entanglement between two qubits, one encoded in the transverse momentum and position of a photon, and the other in the polarization of its partner. In addition, we show that a simple extension of our setup enables the generation of two-photon qubit-qudit hybrid entangled states. Finally, we discuss the advantages that this type of entanglement can bring for an optical quantum network.

We derive and illustrate multimode joint photon-number distributions, integrated-intensity distributions, sum-and difference-distributions and conditional distributions for stimulated parametric downconversion on the basis of exact formulae. The stimulated process can increase or decrease nonclassical fluctuations from spontaneous process and it can be used to control quantum noise. The description is suitable for interpretation of experimental data.

We derive multimode generating functions, photon-number distributions, integrated-intensity distributions, moments and quadrature fluctuations for stimulated parametric down-conversion in relation to experiments measuring joint distributions. The border between classical and quantum behaviour is given.

We show that radiation generated in optical parametric down-conversion with losses and noise is entangled for all times if the coupling coefficient is higher than half of damping constant and the product of damping constant and mean number of noise photons. For the process stimulated by means of chaotic light there is a saturable bound of its intensity for the generation of nonclassical light. Otherwise the quantum behaviour and entanglement are fully reduced. Under some restrictions for noise nonclassical light can also be generated at and below the threshold.

The parametric fluorescence from a nonlinear crystal forms a conical radiation pattern. We measure the angular and spectral distributions of parametric fluorescence in a beta-barium borate crystal pumped by a 405-nm diode laser employing angle-resolved imaging spectroscopy. The experimental angle-resolved spectra and the generation efficiency of parametric down conversion are compared with a plane-wave theoretical analysis. The parametric fluorescence is used as a broadband light source for the calibration of the instrument spectral response function in the wavelength range from 450 to 1000 nm.

We have developed an entangled photon pair source based on a domain-engineered, type-II periodically poled lithium niobate crystal that produces signal and idler photons at 1533 nm and 1567 nm. We characterized the spectral correlations of the generated entangled photons using fiber-assisted signal-photon spectroscopy. We observed interference between the two downconversion paths after erasing polarization distinguishability of the down-converted photons. The observed interference signature is closely related to the spectral correlations between photons in a Hong-Ou-Mandel interferometer. These measurements suggest good indistinguishability between the two downconversion paths, which is required for high entanglement visibility.

We address parametric down-conversion seeded by multimode pseudothermal fields. We show that this process may be used to generate multimode pairwise correlated states with entanglement properties that can be tuned by controlling the seed intensities. Parametric down-conversion seeded by multimode pseudothermal fields represents a source of correlated states, which allows one to explore the classical-quantum transition in pairwise correlations and to realize ghost imaging and ghost diffraction in regimes not yet explored by experiments.

We analyze a frequency degenerate two-arm seeded downconversion process in which the seed fields are spatially multimode chaotic fields. We prove that the output state exhibits a transition from entanglement to separability as far as the intensities of the seeds increase, whereas it is suitable for a ghost imaging experiment irrespective of its entanglement properties. Furthermore we present two experiments of ghost imaging with good visibilty based on parametric downconversion seeded on a single arm.

In this work, we discuss a novel compact source that generates six pairs of entangled photons via spontaneous parametric down-conversion from a single pass of a pump beam through a crystal assembly. The experimental demonstrations reported are at 810 nm so as to utilize high quantum efficiency Si-APD detectors, but the design can be readily implemented in other wavelength regimes including the telecom bands near 1550 nm. An immediate application of this source enables particular multi-qubit cluster states to be generated in a highly compact unidirectional configuration. This can significantly simplify the interferometric stability, as well as feed-forward methods required in photon-based quantum logic circuitry.

The phenomenon of internal conical diffraction has been studied extensively for the case of laser beams with Gaussian intensity profiles incident along an optic axis of a biaxial material. This work presents experimental images for a top-hat input beam and offers a theoretical model which successfully describes the conically diffracted intensity profile, which is observed to differ qualitatively from the Gaussian case. The far-field evolution of the beam is predicted to be particularly interesting with a very intricate structure, and this is confirmed experimentally.

Multiphoton effects in down-conversion are investigated based on the full-quantum multimode formalism by considering a three-level system as a prototype nonlinear system. We analytically derive the three-photon output wave function for two input photons, where one of the two input photons is down-converted and the other one is not. Using this output wave function, we calculate the down-conversion probability, the purity, and the fidelity to evaluate the entanglement between a down-converted photon pair and a non-down-converted photon. It is shown that the saturation effect occurs by multiphoton input and that it affects both the down-conversion probability and the quantum correlation between the down-converted photon pair and the non-down-converted photon. We also reveal the necessary conditions for multiphoton effects to be strong.

The multiphoton wave function after Kerr interaction is obtained analytically for an arbitrary photon number. The wave function is composed of two fundamental functions: the input mode function and the linear response function. The nonlinear effects appearing in this wave function are evaluated quantitatively, revealing the limitations of nonlinear quantum optics theories based on single-mode approximations.