Observation of Stueckelberg oscillations in dipole-dipole interactions

We have observed Stueckelberg oscillations in the dipole-dipole interaction between Rydberg atoms with an externally applied radio-frequency field. The oscillating RF field brings the interaction between cold Rydberg atoms in two separated volumes into resonance. We observe multi-photon transitions when varying the amplitude of the RF-field and the static electric field offset. The angular momentum states we use show a quadratic Stark shift, which leads to a fundamentally different behavior than linearly shifting states. Both cases are studied theoretically using the Floquet approach and are compared. The amplitude of the sidebands, related to the interaction strength, is given by the Bessel function in the linearly shifting case and by the generalized Bessel function in the quadratically shifting case. The oscillatory behavior of both functions corresponds to Stueckelberg oscillations, an interference effect described by the semi-classical Landau-Zener-Stueckelberg model. The measurements prove coherent dipole-dipole interaction during at least 0.6 micro-seconds

Radio-frequency driven dipole-dipole interactions in spatially separated volumes

Radio-frequency (rf) fields in the MHz range are used to induce resonant energy transfer between cold Rydberg atoms in spatially separated volumes. After laser preparation of the Rydberg atoms, dipole-dipole coupling excites the 49s atoms in one cylinder to the 49p state while the 41d atoms in the second cylinder are transferred down to the 42p state. The energy exchanged between the atoms in this process is 33 GHz. An external rf-field brings this energy transfer into resonance. The strength of the interaction has been investigated as a function of amplitude (0-1 V/cm) and frequency (1-30 MHz) of the rf-field and as a function of a static field offset. Multi-photon transitions up to fifth order as well as selection rules prohibiting the process at certain fields have been observed. The width of the resonances has been reduced compared to earlier results by switching off external magnetic fields of the magneto-optical trap, making sub-MHz spectroscopy possible. All features are well reproduced by theoretical calculations taking the strong ac-Stark shift due to the rf-field into account

Numerical study of two-body correlation in a 1D lattice with perfect blockade

We compute the dynamics of excitation and two-body correlation for two-level "pseudoatoms" in a 1D lattice. We adopt a simplified model where pair excitation within a finite range is perfectly blocked. Each superatom is initially in the ground state, and then subjected to an external driving laser with Rabi frequency satisfying a Poissonian distribution, mimicking the scenario as in Rydberg gases. We find that two-body quantum correlation drops very fast with the distance between pseudoatoms. However, the total correlation decays slowly even at large distance. Our results may be useful to the understanding of Rydberg gases in the strong blockade regime

Simultaneous position and state measurement of Rydberg atoms

We present a technique for state-selective position detection of cold Rydberg atoms. Ground state Rb atoms in a magneto-optical trap are excited to a Rydberg state and are subsequently ionized with a tailored electric field pulse. This pulse selectively ionizes only atoms in e.g. the 54d state and not in the 53d state. The released electrons are detected after a slow flight towards a micro channel plate. From the time of flight of the electrons the position of the atoms is deduced. The state selectivity is about 20:1 when comparing 54d with 53d and the one-dimensional position resolution ranges from 6 to 40 $\mu$m over a range of 300 $\mu$m. This state selectivity and position resolution are sufficient to allow for the observation of coherent quantum excitation transport

The impact of Fourier-Domain optical coherence tomography catheter induced motion artefacts on quantitative measurements of a PLLA-based bioresorbable scaffold

Intracoronary Fourier-Domain optical coherence tomography (FD-OCT) enables imaging of the coronary artery within 2-4 seconds, a so far unparalleled speed. Despite such fast data acquisition, cardiac and respiratory motion can cause artefacts due to longitudinal displacement of the catheter within the artery. We studied the influence of longitudinal FD-OCT catheter displacement on serial global lumen and scaffold area measurements in coronary arteries of swine that received PLLA-based bioresorbable scaffolds. In 10 swine, 20 scaffolds (18 x 3.0 mm) were randomly implanted in two epicardial coronary arteries. Serial FD-OCT imaging was performed immediately after implantation (T1) and at 3 (T2) and 6 months (T3) follow-up. Two methods for the selection of OCT cross-sections were compared. Method A did not take into account longitudinal displacement of the FD-OCT catheter. Method B accounted for longitudinal displacement of the FD-OCT catheter. Fifty-one OCT pullbacks of 17 scaffolds were serially analyzed. The measured scaffold length differed between time points, up to one fourth of the total scaffold length, indicating the presence of longitudinal catheter displacement. Between method A and B, low error was demonstrated for mean area measurements. Correlations between measurements were high: R-2 ranged from 0.91 to 0.99 for all mean area measurements at all time points. Considerable longitudinal displacement of the FD-OCT catheter was observed, diminishing the number of truly anatomically matching cross-sections in serial investigations. Global OCT dimensions such as mean lumen and scaffold area were not significantly affected by this displacement. Accurate co-registration of cross-sections, however, is mandatory when specific regions, e.g. jailed side branch ostia, are analyzed