Two-photon Coincidence Research Articles

Competition-induced shifts of spectral peaks in photon coincidence spectroscopy

We show that shifts in locations of two-photon coincidence spectral peaks, for a bichromatically driven two-level atom passing through a single-mode cavity, are due to competition between excitation pathways for a Jaynes-Cummings system. We also discuss an analogous shift of (single-photon) spectral peaks for a driven three-level V system, which demonstrates that competition between excitation pathways is also important in this simple system.

Measurement of the ββ-decay rate of 100Mo to the first excited 0+ state in 100Ru

A new and independent confirmation of ββ(2ν) decay of 100Mo to the first 0+ excited state in 100Ru has been made. This was accomplished using a two-photon coincidence counting technique in which two HPGe detectors were used to observe the two emitted γrays (Eγ1 = 590.8 keV and Eγ2 = 539.6 keV) from the daughter nucleus as it deexcites to the ground state (0+ → 2+ → 0+). The half-life of this decay has been estimated as ∼(5–8)×1020 yr.

Multiphoton coincidence spectroscopy

We extend the analysis of photon coincidence spectroscopy beyond bichromatic excitation and two-photon coincidence detection to include multichromatic excitation and multiphoton coincidence detection. Trichromatic excitation and three-photon coincidence spectroscopy are studied in detail, and we identify an observable signature of a triple resonance in an atom-cavity system.

Search for correlated two-photon emission from e +( 82Sr)+Th interactions

A previously observed electron line at INS at 330.3±0.4 keV in e ++Th interactions has been implied by these authors to originate from the decay of an unknown neutral object X 0. QED predicts a two-photon decay mode of X 0 leading to a correlated two-photon coincidence peak at 841.3±0.4 keV. To search for such a peak γ γ-coincidence experiments were performed. From an observed peak-like structure in the spectrum at 841.7±0.3 keV and an associated cross section of 10.6±3.1 (stat) ± 6.9 (syst) μb an upper limit for the cross section of 15.2±1.5 (syst) μb for the production and 2 γ decay of the X 0 was deduced with a statistical confidence level of 95%.

Analysis of the two-photon radiation of metastable atomic hydrogen

The Stirling two-photon coincidence experiment based upon the decay radiation of metastable atomic hydrogen has been applied to the measurements of angular and polarization correlations, an experimental confirmation of the Breit-Teuer hypothesis, Einstein-Podolsky-Rosen (EPR) experiments, and measurements of the two-photon coherence length and time. Based upon angular conservation and parity arguments, a two-photon state vector can be defined and used for polarization correlations of the two-photon radiation. The results of the Einstein-Podolsky-Rosen experiments with two-photon radiation from metastable atomic hydrogen clearly favour quantum mechanics over local realistic theories in common with all other published studies. However, by inserting an additional linear polarizer in one of the two detection arms (a three-polarizer method) it was possible experimentally to confirm the difference of almost 40% between the quantum-mechanical prediction and Bell’s limit for local realistic theories, thereby demonstrating the largest such difference in EPR experiments so far. The coherence time and length of the two-photon emission of metastable atomic hydrogen was measured by inserting retardation (“multiwave”) quartz plates orientated with their fast axis at 45° with respect to the linear polarization, which in turn is determined by the polarization direction of the linear polarizer in the other detection arm. A Stokes parameter analysis of the coincident two photons proves the coherent nature of the two-photon radiation emitted from metastable atomic hydrogen. The coherence length (lcoh = 350 nm) found in this way is extremely short, only about 1.5 times the medium wavelength of 243 nm of the two-photon radiation. The corresponding coherence time (lcoh = lcoh/c) is approximately 10−15s, which confirms that it is not determined by the long lifetime of the metastable 2S-state (≅ 1/8s). Reference will be made to a new and recent theoretical prediction of the preceding coherence parameters.

Comment on “Measurement of the space-time extent of the hard-photon emitting source in heavy-ion collisions at 100 MeV/nucleon”

In a recent paper [Phys. Rev. C 55, 2521 (1997)] two-photon coincidences were studied for three nuclear reactions. From a correlation analysis, the authors evaluated the size and lifetime of the emitting source. In this Comment we correct some arguments used in the comparison of the extracted and estimated sizes, and show that the parametrization of the relative-momentum correlations leads to values which should not be assigned to the space-time extent of the source.

Intensity interference of ultrashort pulsed fluorescence.

We extend the intensity interference experiments to a pulse envelope measurement of ultrashort incoherent optical pulses. The interfering fields can be mutually incoherent light pulses from independent sources. We show that the pulse envelope or width of ultrashort incoherent light pulses can be measured independently from the coherence time by using a combination of interferometer and two-photon coincidence counter. With this method we have measured the envelope autocorrelation of mutually incoherent femtosecond fluorescence from a solution of Nile Blue in dimethylaniline.

Comment on “Experimental demonstration of the violation of local realism without Bell inequalities” by Torgerson et al.

We exhibit a local-hidden-variable model in agreement with the results of the two-photon coincidence experiment made by Torgerson et al. [Phys. Lett. A 204 (1995) 323]. The existence of any such model shows that the experiment does not exclude local realism.

Experimental demonstration of the violation of local realism without Bell inequalities

We report on a two-photon coincidence experiment that demonstrates the violation of local realism, as defined by Einstein, Podolsky and Rosen, by about 45 standard deviations without explicit use of Bell inequalities. The experiment is an implementation of ideas put forward by Hardy and Jordan; it depends on showing that certain coincidence rates are zero while another rate is non-zero.

A method for demonstrating violation of local realism with a two-photon downconverter without use of Bell inequalities

We show that the recent proposal by Hardy and Jordan for a test of local realism without the use of Bell inequalities can be implemented in two-photon coincidence measurements with linear polarizers, when the photon pairs are produced by parametric downconversion. If the probabilities measured with real detectors are proportional to the corresponding probabilities determined with ideal detectors, this method does not depend on the use of detectors with high or even known quantum efficiencies.

Two-photon correlation of squeezed pulse train

A two-photon coincidence counting measurement is reported of a squeezed pulse train from a parametric amplifier pumped by a mode-locked laser. The intensity correlation function at zero time delay can be determined by measuring the ratio of two types of the coincidences, the one within the same mode-locked pulse and the one among different pulses. A single mode analysis of the intensity correlation function g( 2) is made to explain the experimental results. A discrepancy between the theory and the experiments due to a geometrical problem of detecting the paired photons is discussed.

Two-photon decay of the 2 (1)S0 state in He-like bromine.

We report a measurement of the lifetime of the 1s2s $^{1}$${\mathit{S}}_{0}$ state in two-electron ${\mathrm{Br}}^{33+}$. This state decays to the 1${\mathit{s}}^{2}$ $^{1}$${\mathit{S}}_{0}$ ground state only by the simultaneous emission of two photons. Ions in the 1s2s $^{1}$${\mathit{S}}_{0}$ level are produced by excitation of energetic bromine ions in a thin carbon foil. The lifetime is determined by measuring the change in the rate of two-photon coincidences as a function of the foil-detector separation. The lifetime measurement yields 39.32(32) ps in agreement with a theoretical value of 39.63(16) ps.

An accelerator based electron-photon coincidence spectrometer for the study of inelastic electron-atom collisions

Inelastic electron-photon collisions can be studied with our two-photon coincidence spectrometer by replacing one of the Ge detectors with a Si(Li) detector with a 0.4 μm polymer window coupled to the target vacuum chamber. The present geometry has the detectors at ± 45° to the incident electron beam. The counting rate in the Si(Li) detector is dominated by the elastic scattered electrons and the backscattering tail of the elastic peak and is several orders of magnitude larger than the photon rate. Nevertheless, the coincidence electronics is able to separate real coincidences as a function of both photon and electron energy from the large accidental coincidence background. This permits the study of such processes as the doubly differential cross section for inner-shell ionisation and the triply differential cross section for single bremsstrahlung. This setup has a much more rapid data collection rate and yields a statistical error comparable to or better than has been previously achieved in the few experiments that have been done using a magnetic spectrometer on the electron. Preliminary examples of two dimensional energy spectra with 70 kev electrons on targets of Al, Cu, Ag, Tb and Au are discussed.

Two-photon coincident emission from thick targets for 70-keV incident electrons.

Two-photon coincidence yields have been measured in thick targets of C, Al, Ag, and Ta for 70 keV incident electrons and photons radiated at ±45 o to the incident beam. A theoretical model, which is more rigorous, has been developed to simulate the two-photon processes of coherent thick-target double bremsstrahlung (TTDB) and the incoherent emission of two single-bremsstrahlung (SBSB) photons in a thick-target environment. The model is based on an integration of the thin-target cross sections over the target thickness taking into account electron energy loss, electron backscattering, and photon attenuation

Calibration of a two-photon coincidence experiment using 133Ba

The measurement of the cross section for electron or photon-induced two-photon emission processes using a coincidence technique requires the determination of the efficiency-solid angle product for the two detectors. Since geometries for accelerator setups are usually quite complicated, the most accurate way to determine this product is by measuring a well-known two-photon process. A calibrated 133Ba radioactive source is well suited for calibrating a coincidence system because it produces a high rate of coincident photons over a wide energy range from 31 to 384 keV. In this paper (K X-ray, γ-ray) coincidence probabilities for 133Ba decay are computed from the available nuclear and atomic data.

Observation of nonlocal interference in separated photon channels.

A two-photon coincidence experiment of the kind recently proposed by J. D. Franson [phys. Rev. Lett. 62, 2205 (1989)] has been carried out with signal and idler photons produced in the process of parametric down-conversion. The coincidence rate registered by the two detectors is found to exhibit a cosine variation with the optical path difference, with periodicity equal to the wavelength. A number of fourth-order optical interference experiments have been carried out in recent years. [1-8] Unlike conventional second-order interference experiments, these depend on the detection of photon pairs and the interference of two two-photon probability amplitudes.[9] It The two-photon probability amplitude for the shorter is an interesting feature of those experiments that quantum mechanics allows the visibility of the interference to be larger for a two-photon state than is allowed by classical electromagnetic theory. A new and particularly simple form of fourth-order interference experiment with two photons has recently been proposed by Franson [10] as a test for locality violations. The outline of the experiment is shown in Fig. 1. Figure 1: Outline of the experiment proposed by Franson. Two photons emitted together by some common source travel along arms A and B to two detectors DA and DB, either directly along the shortest path or via a longer path involving reflections from two beam splitters and two mirrors, as shown. Franson supposed that the two photons might be produced by the cascade decay of an atom in which the initial excited state is very long lived. But we may also suppose that the two photons could arise from the down-conversion of a highly monochromatic laser beam, of long coherence time, in a nonlinear crystal.[11] In both cases the two photons are highly correlated in time and their state is an entangled quantum state. Let us suppose that the difference in propagation time between the longer and the shorter paths is the same in both channels and is much greater than the coherence time (reciprocal bandwidth 1/∆ω) of the light, or the length of each photon wave packet. Then one might naively suppose that there would be no interference. Indeed the mean detection rate registered by DA or DB would not show any dependence on path difference. However, we arrive at a different conclusion if we look for simultaneous detections by both DA and DB. The two-photon probability amplitude for the shorter paths, A to DA and B to DB, then interferes with the two-photon probability amplitude for the longer paths involving the two mirrors. After forming the sum of the two probability amplitudes and squaring we find that the coincidence rate exhibits a cosine variation with a path difference. This is so despite the fact that the two detectors are widely separated and the trajectories of the two photons never mix. We wish to report the results of an experiment in which this nonlocal interference effect, which has no direct classical counterpart, has been observed. The experiment is shown in Fig. 2. The source of the two photons is the process of spontaneous parametric down-conversion[11] in a crystal of LiIO3 that is optically pumped by the 351.1-nm line of an argon-ion laser. The signal (s) and idler (i) photons produced have wavelengths close to 700 nm but substantial bandwidth, which is restricted by interference filters to about 1012Hz. The main difference between the optical arrangement in our experiment and that proposed by Franson is that the variable delay is introduced via an unbalanced Michelson-type interferometer, rather than with the MachZehnder interferometer arrangement shown in Fig. 1. This requires only one beam splitter in each arm instead of two. One of the mirrors M1, of the Michelson interferometer is mounted on a motor-driven micrometer and can also be moved piezoelectrically, and this allows the optical path difference 2(BS−M1)k−2(BS−M2)k ≡ cTk (where k = s, i) to be nearly equalized in the two arms, and for Ti to be varied in submicron steps in a controlled manner about the fixed value cTi ≈ cTs ≈ 3 cm. The time difference Ts, Ti ∼ 10−10sec therefore greatly exceeds the coherence time 1/∆ω ∼ 10−12sec of the light. In order to make Ts and Ti equal to within 10−12sec in the signal and idler

Non-local and nonclassical effects in two-photon down-conversion

Some past and some possible future experiments that make use of the photon pairs generated in the process of parametric down-conversion are reviewed and briefly analysed. The experiments demonstrate that the average position in time of an optical photon can be determined with accuracy better than one period, and that two-photon coincidence measurements can carry useful information about the phase of the electro-magnetic field. Several different experiments for exhibiting violations of Einstein locality with down-converted photon pairs are discussed.

Double atomic-field bremsstrahlung from thick targets

Measurements of the two-photon coincidence rate from 84 keV electrons from a 109Cd source on thick targets of Ag, Au, and Pb are compared with a thick-target double atomic-field bremsstrahlung (TBD) model. Each photon energy range was 32–52 keV. The target thickness ranged from 3 to 30 mg/cm 2. Results for the thick-target single-bremsstrahlung (TSB) energy spectrum for 64 keV electrons in coincidence with the 22 keV X-ray from the source will also be compared with the TSB theory.

A multiparameter analysis system for low coincidence rate experiments

Multiparameter analysis (MPA) is especially attractive in a low coincidence rate experiment. However, conventional MPA methods have been expensive and frequently limited to two parameters. We will discuss a three parameter system used in a two-photon coincidence experiment with high purity germanium detectors which uses an ND 62 multichannel analyzer (MCA) and a PC computer. Standard NIM electronics are used to provide energy signals, make logic decisions for acceptable events and prepare a waveform consisting of three sequentially delayed signals (one time and two energies) that are digitized by the MCA and sent to the PC by serial readout. The PC separates real and accidental events to create two 2-D energy arrays. This is important if the accidental rate is high. Extension of this system to four or more parameters is primarily limited by the dead time associated with sequential analysis of the signals, which may not be a problem in very low rate experiments. Alternative MPA approaches are also discussed.

The 458 nm diffuse band of the lithium dimer

Laser induced fluorescence studies of the lithium diffuse band at 458 nm were performed using single-and double-photon excitations. Single-photon excitation was performed using UV lines of Ar++ and Kr++ lasers, and the diffuse band fluorescence was induced by collisions in 7Li2 and 6Li2. The double-photon excitation process populated relevant upper states through several accidental coincidences for two-step excitation in 7Li2. In two cases, in addition to structured continuum emission, an adjacent line spectrum was observed. In five other cases, two-photon accidental coincidences produced collision induced fluorescence of the 7Li2 diffuse band. We also found (in a few cases) that the (two-photon) excitation energy of the 2 3Πg state (with respect to the 1 3Σ+u state) was collisionally transferred to simultaneously excited lithium 2p atoms, thus creating a lithium atom in the 4d state.