Atomic Photon Research Articles

Maltese cross coupling to individual cold atoms in free space.

We report on the simultaneous observation from four directions of the fluorescence of single 87Rb atoms trapped at the common focus of four high numerical aperture (NA=0.5) aspheric lenses. We use an interferometrically-guided pick-and-place technique to precisely and stably position the lenses along the four cardinal directions with their foci at a single central point. The geometry gives right angle access to a single quantum emitter, and will enable new trapping, excitation, and collection methods. The fluorescence signals indicate both sub-Poissonian atom number statistics and photon anti-bunching, showing suitability for cold atom quantum optics.

Coherently driving a single quantum two-level system with dichromatic laser pulses

The excitation of individual two-level quantum systems using an electromagnetic field is an elementary tool of quantum optics, with widespread applications across quantum technologies. The efficient excitation of a single two-level system usually requires the driving field to be at the same frequency as the transition between the two quantum levels. However, in solid-state implementations, the scattered laser light can dominate over the single photons emitted by the two-level system, imposing a challenge for single-photon sources. Here, we propose a background-free method for the coherent excitation and control of a two-level quantum system using a phase-locked dichromatic electromagnetic field with no spectral overlap with the optical transition. We demonstrate this method experimentally by stimulating single-photon emission from a single quantum dot embedded in a micropillar, reaching single-photon purity of 0.988(1) and indistinguishability of 0.962(6). The phase-coherent nature of our two-colour excitation scheme is demonstrated by the dependence of the resonance fluorescence intensity on the relative phase between the two pulses. Our two-colour excitation method represents an additional and useful tool for the study of atom–photon interaction, and the generation of spectrally isolated indistinguishable single photons. A quantum two-level system can be coherently excited by a phase-locked dichromatic electromagnetic field. This technique can make single-photon generation more efficient as the pump light does not overlap in frequency with the emitted single photons.

The amplitude of the cavity pump field and dissipation effects on the entanglement dynamics and statistical properties of an optomechanical system

In this paper we consider a cavity optomechanics consists of a two-level atom that interacts with a single-mode quantized field in the presence of a strong driving laser. By evaluating the effective Hamiltonian of the whole system which contains the field dissipation, we derive the time-dependent state vector of the system corresponding to particular initial conditions for the cavity field and oscillatory mirror. In the continuation, we pay our attention to the dynamical properties of the considered system by choosing different initial conditions for the atom. In particular, the effects of amplitude of the pump field, various initial atomic states and cavity loss rate on the entanglement, atomic population inversion and photon and phonon statistical properties are numerically analysed. According to the obtained results, maximal entanglement for all chosen initial states of the system is visible as time develops. Moreover, the presence of dissipative field leads to decrease the entanglement and finally tends to a fixed positive value, while the Mandel parameter as well as the population inversion reduce and tend to a fixed value in the negative region. Also, the appearance of non-classicality feature such as typical collapse–revival phenomenon in the (negative values of) Mandel parameter is noticeable.

Cavity Dark Mode of Distant Coupled Atom-Cavity Systems.

We report on a combined experimental and theoretical investigation into the normal modes of an all-fiber coupled cavity-quantum-electrodynamics system. The interaction between atomic ensembles and photons in the same cavities, and that between the photons in these cavities and the photons in the fiber connecting these cavities, generates five nondegenerate normal modes. We demonstrate our ability to excite each normal mode individually. We study particularly the "cavity dark mode," in which the two cavities coupled directly to the atoms do not exhibit photonic excitation. Through the observation of this mode, we demonstrate remote excitation and nonlocal saturation of atoms.

Studies on equivalent atomic number and photon buildup factors for some tissues and phantom materials.

Geometric progression fitting method was used for the computation of the gamma-ray EABF of eleven tissue equivalent materials and nine human organs and tissues up to energy 3 MeV. Quantitative comparison of photon absorption and scattering properties of tissue-equivalent materials was performed with that of human organs and tissues. The variations of equivalent atomic number and the buildup factors of the tissue/equivalent materials indicate which material can be used to substitute for different human tissues in phantom construction.

Angular distribution of multiple Compton scattered gamma rays of 662 keV in tin

In multiple Compton scattering, the incident gamma photon is scattered more than once due to finite dimensions of the sample. It serves as a noise for the exact evaluation of electron momentum distribution in Compton profile, Compton cross section and non-destructive testing of in-assessable large samples. To overcome these difficulties, there is a need to characterise the contribution of multiple scattering as a function of atomic number, scattering angle and incident gamma photon energy. In the present work, measurements are carried out to investigate the multiple scattering of 662 keV photons with tin sample of different thickness in the forward and backward hemispheres. The scattered photon flux is recorded by a NaI(Tl) scintillation detector and with an HPGe solid-state detector to check for the trend of saturation thickness as a function of resolution of the detector. The number of multiply scattered photons is found to be increasing with the sample thickness up to a particular thickness value called saturation thickness beyond which it remains constant with increase in target thickness. The effect of detector collimator size on multiply scattered photons is also studied. Both the detectors show the same trend of saturation thickness as a function of thickness. Angular distribution of multiply scattered photons shows symmetry around 90° scattering angle. The low energy side of multiple scattered photon spectrums is also characterised as a function of sample thickness and order of scattering. Monte Carlo calculations support the present experimental results.

Effective atomic number estimation by energy-resolved X-ray computed tomography with a current-mode detector system

We have developed a current-mode detector system, called a “transXend” detector, consisting of several segmented detectors aligned along the direction of X-ray incidence, for energy-resolved computed tomography (CT). The transXend detector measures X-rays as electric currents and after analysis gives the energy spectrum of the incident X-ray. In previous work, we measured the linear attenuation coefficients of resins as a function of X-ray energy with the energy-resolved CT using the transXend detector. This paper shows a method to estimate the effective atomic number and electron density of nine resins from the linear attenuation coefficients using a photon–atom cross-section lookup table. The estimated and agreed well with calibration lines with accuracies of about 2%.

Ground state and thermal entanglement between two two-level atoms interacting with a nondegenerate parametric amplifier: Different sub-spaces

In this paper, a Hamiltonian model that includes interaction of two coupled two-level atoms with a nondegenerate parametric amplifier in a cavity is introduced. By using the two-mode squeezing operator and under a certain condition, the introduced Hamiltonian is reduced to a generalized Jaynes–Cummings Hamiltonian. The constants of motion of system imply the existence of a decomposition of the system’s Hilbert space [Formula: see text] into a direct sum of three infinite dimensional sub-spaces, as [Formula: see text]. This decomposition enables us to study ground and thermally induced entanglement between the atoms in each of the sub-spaces as well as whole Hilbert space. The effect of atom–atom and atom–photon couplings on the degree of ground state and thermal entanglement are also investigated using the concurrence measure. It is found that in the subspaces [Formula: see text] and [Formula: see text] for all experimental values of parameters of the system, the atoms are disentangled. It is also observed that in the total space [Formula: see text] the ground state entanglement is always zero, while in the subspace [Formula: see text] it is at its maximum value. Moreover, it is found that in the subspace [Formula: see text] thermal entanglement between the atoms is robust against temperature.

Optimization of myco-synthesized silver nanoparticles by response surface methodology employing Box-Behnken design

The objective of current study was to optimize the average diameter size of biosynthesized Silver Nanoparticles (AgNPs) induced by Penicillium citrinum. The AgNPs were prepared with a spherical shape and good monodispersity, determined by scanning electron microscopy (SEM), atomic force microscopy (AFM) and photon correlation spectroscopy (PCS). Besides, response surface methodology (RSM) was conducted to optimize the average diameter size of AgNPs by employing Box-Behnken design (BBD). The effective factors in this process were concentration of AgNO3 (mM) (X1), pH of solution (X2), temperature of shaker incubator (°C) (X3), rate of shaker incubator (rpm) (X4) and time of incubation (hour) (X5). The R2 (0.8894) value of the obtained model indicated a good correlation between the observed and predicted values. Overall, it could be concluded that multivariate analysis facilitated to find out the optimum conditions for the biosynthesis of AgNPs induced by P. citrinum with the reduced time and cost.

Entanglement of single-photons and chiral phonons in atomically thin WSe2

Quantum entanglement is a fundamental phenomenon that, on the one hand, reveals deep connections between quantum mechanics, gravity and spacetime1,2, and on the other hand, has practical applications as a key resource in quantum information processing3. Although it is routinely achieved in photon–atom ensembles4, entanglement involving solid-state5–7 or macroscopic objects8 remains challenging albeit promising for both fundamental physics and technological applications. Here, we report entanglement between collective, chiral vibrations in a two-dimensional WSe2 host—chiral phonons (CPs)—and single-photons emitted from quantum dots9–13 (QDs) present in it. CPs that carry angular momentum were recently observed in WSe2 and are a distinguishing feature of the underlying honeycomb lattice14,15. The entanglement results from a ‘which-way’ scattering process, involving an optical excitation in a QD and doubly-degenerate CPs, which takes place via two indistinguishable paths. Our unveiling of entanglement involving a macroscopic, collective excitation together with strong interactions between CPs and QDs in two-dimensional materials opens up ways for phonon-driven entanglement of QDs and engineering chiral or non-reciprocal interactions at the single-photon level. A ‘which-way’ scattering process can generate entanglement between single photons and collective chiral vibrations in two-dimensional tungsten diselenide. The result opens up ways for engineering non-reciprocal interactions at the quantum level.

Chemically assisted physical sputtering of Tungsten: Identification via the [formula omitted] transition of WD in TEXTOR and ASDEX Upgrade plasmas

Tungsten (W) is a preferred plasma-facing material for areas of high particle and power impact in present day and future fusion devices. The lifetime of W plasma-facing components (PFCs) under steady-state conditions is limited by erosion processes induced by energetic impinging particles such as impurities (carbon C, oxygen O, nitrogen N, etc.) and hydrogen isotopes above the threshold energy for Physical Sputtering (PS). We discovered for the first time, in addition to the bare physical sputtering process of W, a second W erosion mechanism at PFCs in TEXTOR (limiter surfaces) and ASDEX Upgrade (divertor target plates) during deuterium plasma bombardment. The tungsten deuteride molecule WD has been identified spectroscopically via the 6Π→6Σ+ transition in parallel to ordinary neutral W line emission of the WI transition (5d5(6S)6s 7S3 → 5d5(6S)6p 7P4) at λ=400.9nm used as measure to quantify the bare PS process or, more precisely, the gross erosion of W. We identified the underlying process for the molecular release as Chemically Assisted Physical Sputtering (CAPS) as observed also in the case of beryllium or lanthanum. Measurements in TEXTOR and ASDEX Upgrade showed a dependence of the WD band emission on the surface temperature - connected to the deuterium content in the near W surface - as well as on the flux and energy of impinging energetic particles. A quantification of the released WD is not yet possible due to lack of appropriate molecular data for the conversion of photons into particles, but the spatially resolved ratio of atomic (WI) and molecular photon flux (WD) indicates that the release takes place at the same time, but differently distributed along the target plates and limiters reflecting the variation in the described critical parameters (impinging ion flux, ion impact energy, and material temperature) along the surface. The plasma conditions in front of the interaction zone are in both cases fully ionising with an electron temperature Te > 10eV, thus, recombination into WD can be excluded.

Optical resonance near the edge of a photonic band gap (PBG): turning the effects of a PBG on/off using a resonant driving field

ABSTRACTWe studied the effects of a coherent monochromatic resonant laser field driving the transition of a two-level atom embedded in a photonic band gap (PBG) material on the emission dynamics of the atom. When the transition frequency of the atom lies outside a PBG and sufficiently far from the band edge that the emission dynamics of the atom is not normally affected by the gap, the dynamic stark splitting of the upper level of the atom into two dressed states by a sufficiently strong resonant driving field can bring the lower dressed state close enough to the band edge or even inside the gap so as to be affected by the gap, providing a switchable means of extending the novel effects of atom–photon interaction near the edge of a PBG for atomic transitions outside the gap and far from the band edge. The net effect is as if a sufficiently strong resonant field driving a transition of an atom embedded in a PBG actively shifts the PBG close enough to the transition so that the gap affects the emission dynamics associated with the transition.

Image-based monitoring of femtosecond laser machining via a neural network

Femtosecond laser machining offers the potential for high-precision materials processing. However, due to the nonlinear processes inherent when using femtosecond pulses, experimental random noise can result in large variations in the machined quality, and hence methods for closed loop feedback are of interest. Here we demonstrate the application of a neural network (NN), acting as a pattern recognition algorithm, for visual monitoring of the target substrate via a camera that observes the sample during machining. This approach has the advantage that it requires zero knowledge of the underlying physical processes, and hence avoids the need for modelling the complex photon–atom interactions that occur with femtosecond laser machining. The NN was shown to accurately determine the type of material, the laser fluence and the number of pulses, directly from a single image of the sample and within ten milliseconds. This approach provides the potential for real-time feedback for femtosecond laser materials processing.

Analysis of atom–photon quantum interface with intracavity Rydberg-blocked atomic ensemble via two-photon transition

We study the atom-photon quantum interface with intracavity Rydberg-blocked atomic ensemble where the ground-Rydberg transition is realized by two-photon transition. Via theoretical analysis, we report our recent findings of the Jaynes-Cummings model on optical domain and robust atom-photon quantum gate enabled by this platform. The requirement on the implementation is mild which includes an optical cavity of moderately high finesse, typical alkali atoms such as Rb or Cs and the condition that cold atomic ensemble is well within the Rydberg blockade radius. The analysis focuses on the atomic ensemble's collective coupling to the quantized optical field in the cavity mode. We demonstrate its capability to serve as a controlled-PHASE gate between photonic qubits and matter qubits. The detrimental effects associated with several major decoherence factors of this system are also considered in the analysis.

Analysis of stabilization of photon beam softening with off-axis distance for filtration system enhancement to increase dosimetry in radiotherapy

Beam softening is depending on the material atomic number and photon beam energy for photon beam filtration; the filtration quality is essential to producing a clinical beam in external radiotherapy. Beam softening analysis is done using Monte Carlo method. The Monte Carlo model was performed for 6 MV photon beam produced by Varian Clinac 2100 with flattening filter thereafter the flattening filter was removed and replaced by slab of aluminum and copper with different thickness. The purpose of this study is to analyze the photon beam softening with off-axis distance for the reference field size of 10 × 10 cm2; the beam softening is studied in terms of tow coefficients a1 and a2.Inside the irradiation field, for aluminum slab, the coefficient a1 varied in interval from −1.2 cm−1 to 1.7 cm−1 and it has big fluctuations with off-axis distance. For copper, a1 varied from 0 cm−1 to 1.5 cm−1 and it has small fluctuations with off-axis distance. The coefficient a2, for aluminum slab, varied from −1 cm−2 to 0.4 cm−2 with big fluctuations. For copper slab, a2 varied from −0.9 cm−2 to 0 cm−2 with small fluctuations. We conclude that the beam softening is very stabilized for copper than aluminum in despite of copper attenuate more photons than aluminum for same slab thickness when they are used in photon beam filtration. To increase dosimetry in radiotherapy, the flattening filter should be constructed based on low atomic number materials but with high stabilization of beam softening with off-axis distance.

Nonclassical properties of asymmetric two two-level atoms interacting with multi-photon quantized field

A quantum model for the interaction between asymmetric two two-level atoms and an electromagnetic field is presented. The [Formula: see text]-photon processes and atom–atom interaction are considered in this quantum model. The wavefunction for asymmetric case of the proposed model is obtained analytically. While, the explicit analytical formula of the wavefunction for symmetric case was calculated in previous works. Initially, the electromagnetic field is in the coherent state and two two-level atoms are identical in the excited states. For the proposed model, some statistical aspects are obtained, such as the linear entropy, atomic population inversion, entropy squeezing, atomic variance and von Neumann entropy. The evaluations of these statistical properties are discussed for the variation in the detuning parameters. Moreover, the nonclassical effects for atom–atom interaction are observed.

Slowing 80-ns light pulses by four-wave mixing in potassium vapor

We experimentally and theoretically study propagation of 80-ns Gaussian-like probe pulses in hot potassium vapor under conditions of four-wave mixing (FWM). The atomic scheme for FWM is off-resonant, double-$\mathrm{\ensuremath{\Lambda}}$ atomic scheme, with pump and probe photons, mediated in the K vapor, generating new probe and conjugate photons. We define the subset of FWM parameters, one-photon pump detuning, two-photon pump-probe Raman detuning, vapor density, the pump Rabi frequencies, when slowed pulses exit the vapor are also Gaussian-like. When Gaussian-like pulses exit the cell we are able to compare theoretical and experimental results for fractional delays and broadening for the probe and conjugate. We have obtained fractional delays above 1. Results of the model are compared with the experiment, with and without the model of Doppler averaging, when the atom velocity distribution is divided into different number of groups. We analyze possible causes for pulse broadening and distortion of slowed probe pulses and show that they are the result of quite different behavior of the probe pulse in the FWM vapor. Besides presenting the first results of slowing 80-ns probe pulses, this work is a useful test of the numerical model and values of parameters taken in the model that are not known in experiments.

A passive photon–atom qubit swap operation

Deterministic quantum interactions between single photons and single quantum emitters are a vital building block towards the distribution of quantum information between remote systems1–4. Deterministic photon–atom state transfer has previously been demonstrated with protocols that include active feedback or synchronized control pulses5–10. Here we demonstrate a passive swap operation between the states of a single photon and a single atom. The underlying mechanism is single-photon Raman interaction11–15—an interference-based scheme that leads to deterministic interaction between two photonic modes and the two ground states of a Λ-system. Using a nanofibre-coupled microsphere resonator coupled to single Rb atoms, we swap a photonic qubit into the atom and back, demonstrating fidelities exceeding the classical threshold of 2/3 in both directions. In this simultaneous write and read process, the returning photon, which carries the readout of the atomic qubit, also heralds the successful arrival of the write photon. Requiring no control fields, this single-step gate takes place automatically at the timescale of the atom’s cavity-enhanced spontaneous emission. Applicable to any waveguide-coupled Λ-system, this mechanism, which can also be harnessed to construct universal gates16,17, provides a versatile building block for the modular scaling up of quantum information systems. Demonstration of a passive swap gate between the states of a fibre-guided photonic qubit and a single atom.

A Quantum Transistor Based on an Atom–Photon Molecule

A quantum transistor based on an atomic–photon molecule is proposed. It is a linear chain of three coupled microresonators, each of which contains one resonant atom. The circuit of the proposed quantum transistor is scalable and can be used to create a quantum logical CNOT gate.

Photonuclear Reactions in Astrophysics

Nucleosynthesis in stars and stellar explosions proceeds via nuclear reactions in thermalized plasmas. Nuclear reactions not only transmutate elements and their isotopes, and thus create all known elements from primordial hydrogen and helium, they also release energy to keep stars in hydrostatic equilibrium over astronomical timescales. A stellar plasma has to be hot enough to provide sufficient kinetic energy to the plasma components to overcome Coulomb barriers and to allow interactions between them. Plasma components in thermal equilibrium are bare atomic nuclei, free electrons, and photons (radiation). Typical temperatures of plasmas experiencing nuclear burning range from 107 K for hydrostatic hydrogen burning (mainly interactions among protons and He isotopes) to 1010 K or more in explosive events, such as supernovae or neutron star mergers. This still translates into low interaction energies by nuclear physics standards, as the most probable energy E between reaction partners in terms of temperature is derived from Maxwell-Boltzmann statistics and yields E = T9/11.6045 MeV, where T9 is the plasma temperature in GK.