J. von Zanthier

Quantum imaging with incoherent photons

We propose a technique to obtain sub-wavelength resolution in imaging with 100% visibility using incoherent light. We can in principle obtain a resolution of ¿/N using coincidence detection of N photons emitted from N excited atoms.

Frequency Comparison and Absolute Frequency Measurement of I2-stabilized Lasers at 532 nm

We present a frequency comparison and an absolute frequency measurement of two independent I2-stabilized frequency-doubled Nd:YAG lasers at 532 nm, one set up at the Institute of Laser Physics, No vosibirsk, Russia, the other at the Physikalisch-Technische Bundesanstalt, Braunschweig, Germany. The absolute frequency of the I2-stabilized lasers was determined using a CH4-stabilized He-Ne laser as a reference. This laser had been calibrated prior to the measurement by an atomic cesium fountain clock. The frequency chain linking phase-coherently the two frequencies made use of the frequency comb of a Kerr-lens mode-locked Ti:sapphire femtosecond laser where the comb mode separation was controlled by a local cesium atomic clock. A new value for the R(56)32-0:a10 component, recommended by the Comite International des Poids et Mesures (CIPM) for the realization of the metre [1], was obtained with reduced uncertainty. Absolute frequencies of the R(56)32-0 and P(54)32-0 iodine absorp tion lines together with the hyperfine line separations were measured.

Absolute frequency measurement of the 115In+ 5S2 1S0-5S5p 3P0 transition

Abstract We have measured the absolute frequency of the 115 In+ 5s 2 1 S0–5s5p 3 P0 clock transition at 236.5 nm with an accuracy of 3.3 parts in 1011. For this measurement, a frequency synthesis chain was used which links the indium clock transition to a methane-stabilized He–Ne laser at 3.39 μm and a Nd:YAG laser at 1064 nm whose second harmonic was locked to a hyperfine component in molecular iodine. A frequency gap in the chain of 1.43 THz at 850 nm was bridged with the help of an optical frequency comb generator. The frequency of the 115 In+ clock transition was determined to 1 267 402 452 914 (41) kHz, where the accuracy is limited by the uncertainty of the iodine reference. This measurement represents an improvement of more than three orders of magnitude in accuracy compared to previous measurements of the line.

Generation of symmetric Dicke states of remote qubits with linear optics.

We propose a method for generating all symmetric Dicke states, either in the long-lived internal levels of N massive particles or in the polarization degrees of freedom of photonic qubits, using linear optical tools only. By means of a suitable multiphoton detection technique, erasing Welcher-Weg information, our proposed scheme allows the generation and measurement of an important class of entangled multiqubit states.

Nonclassical interference effects in the radiation from coherently driven uncorrelated atoms

We demonstrate the existence of nonclassical correlations in the radiation of two atoms, that are coherently driven by a continuous laser source. The photon-photon correlations of the fluorescence light show a spatial interference pattern not present in a classical treatment. A feature of this phenomenon is that bunched and antibunched light is emitted in different spatial directions. The calculations are performed analytically. It is pointed out that the correlations are induced by state reduction due to the measurement process when the detection of the photons does not distinguish between the atoms. It is interesting to note that the phenomena show up even without any interatomic interaction.

Quantum-interference-initiated superradiant and subradiant emission from entangled atoms

We calculate the radiative characteristics of emission from a system of entangled atoms which can have a relative distance larger than the emission wavelength. We develop a quantum multipath interference approach which explains both super- and subradiance though the entangled states have zero dipole moment. We derive a formula for the radiated intensity in terms of different interfering pathways. We further show how the interferences lead to directional emission from atoms prepared in symmetric W states. As a byproduct of our work we show how Dickes classic result can be understood in terms of interfering pathways. In contrast to the previous works on ensembles of atoms, we focus on finite numbers of atoms prepared in well characterized states.

Towards an indium single-ion optical frequency standard

A narrow transition of a single laser-cooled ion, stored in a radio-frequency trap, can serve as a reference for a frequency standard of very high accuracy and stability. For the implementation of such a system, we study the 5s2 1S0–5s5p 3P0 transition in In+, at a wavelength of 237 nm. This resonance has a linewidth of 0.8 Hz, with systematic frequency shifts expected to be at the mHz level. A fractional resolution of 1.3 × 10−13 has been so far achieved, limited by the frequency instability of the clock laser used to excite the line. The absolute frequency of the 1S 0–3P 0 transition was measured using the frequency comb of a mode-locked fs laser, and as a reference a methane-stabilized He–Ne laser at 3.4 μm, which was calibrated against an atomic caesium fountain clock. The transition frequency was determined as 1267 402 452 899.92 (0.23) kHz, the uncertainty being limited by the uncertainty of the He–Ne standard. The short-term frequency stability of the clock laser was recently greatly improved. With a new laser set-up a linewidth < 4 Hz (FWHM) for integration times up to 26 s was achieved, using laser platforms which actively isolate from external mechanical vibrations.

Operational determination of multiqubit entanglement classes via tuning of local operations.

We present a physical setup with which it is possible to produce arbitrary symmetric long-lived multiqubit entangled states in the internal ground levels of photon emitters, including the paradigmatic Greenberger-Horne-Zeilinger and W states. In the case of three emitters, where each tripartite entangled state belongs to one of two well-defined entanglement classes, we prove a one-to-one correspondence between well-defined sets of experimental parameters, i.e., locally tunable polarizer orientations, and multiqubit entanglement classes inside the symmetric subspace.

Intensity-intensity correlations as a probe of interferences under conditions of noninterference in the intensity

The different behavior of first-order interferences and second-order correlations are investigated for the case of two coherently excited atoms. For intensity measurements this problem is in many respects equivalent to Youngs double-slit experiment and was investigated in an experiment by Eichmann et al. [Phys. Rev. Lett. 70, 2359 (1993)] and later analyzed in detail by Itano et al. [Phys. Rev. A 57, 4176 (1998)]. Our results show that in cases where the intensity interferences disappear the intensity-intensity correlations can display an interference pattern with a visibility of up to 100%. The contrast depends on the polarization selected for the detection and is independent of the strength of the driving field. The nonclassical nature of the calculated intensity-intensity correlations is also discussed.

Directional superradiant emission from statistically independent incoherent nonclassical and classical sources.

Superradiance has been an outstanding problem in quantum optics since Dicke introduced the concept of enhanced directional spontaneous emission by an ensemble of identical two-level atoms. The effect is based on the correlated collective Dicke states which turn out to be highly entangled. Here we show that enhanced directional emission of spontaneous radiation can be produced also with statistically independent incoherent sources, via the measurement of higher-order correlation functions of the emitted radiation. Our analysis is applicable to a wide variety of quantum emitters, like trapped atoms, ions, quantum dots, or nitrogen-vacancy centers, and is also valid for incoherent classical emitters. This is experimentally confirmed with up to eight statistically independent thermal light sources. The arrangement to measure the higher-order correlation functions corresponds to a generalized Hanbury Brown-Twiss setup, demonstrating that the two phenomena, superradiance and the Hanbury Brown-Twiss effect, stem from the same interference phenomenon.