arXiv subjects

P. L. Gould

Publications and source records attributed to P. L. Gould.

28 records · Page 2Linked to original sources

arXiv2008

Observation of Electric Quadrupole Transitions to Rydberg nd States of Ultracold Rubidium Atoms

We report the observation of dipole-forbidden, but quadrupole-allowed, one-photon transitions to high Rydberg states in Rb. Using pulsed UV excitation of ultracold atoms in a magneto-optical trap, we excite $5s \to nd$ transitions over a range of principal quantum numbers $n=27-59$. Compared to dipole-allowed (E1) transitions from $5s \to np$, these E2 transitions are weaker by a factor of approximately 2000. We also report measurements of the anomalous $np_{3/2} : np_{1/2}$ fine-structure transition strength ratio for $n=28-75$. Both results are in agreement with theoretical predictions.

physics.atom-ph↗

arXiv2008

Long-range potentials and $(n-1)d+ns$ molecular resonances in an ultracold rydberg gas

We have calculated long-range molecular potentials of the $0_g^{+}$, $0_u^{-}$ and $1_u$ symmetries between highly-excited rubidium atoms. Strong $np+np$ potentials characterized by these symmetries are important in describing interaction-induced phenomena in the excitation spectra of high $np$ Rydberg states. Long-range molecular resonances are such phenomena and they were first reported in S.M. Farooqi {\it et al.}, Phys. Rev. Lett. {\bf 91} 183002. One class of these resonances occurs at energies corresponding to excited atom pairs $(n-1)d+ns$. Such resonances are attributed to $\ell$-mixing due to Rydberg-Rydberg interactions so that otherwise forbidden molecular transitions become allowed. We calculate molecular potentials in Hund's case (c), use them to find the resonance lineshape and compare to experimental results.

physics.atom-ph↗

arXiv2007

Enhancement of the formation of ultracold $^{85}$Rb$_2$ molecules due to resonant coupling

We have studied the effect of resonant electronic state coupling on the formation of ultracold ground-state $^{85}$Rb$_2$. Ultracold Rb$_2$ molecules are formed by photoassociation (PA) to a coupled pair of $0_u^+$ states, $0_u^+(P_{1/2})$ and $0_u^+(P_{3/2})$, in the region below the $5S+5P_{1/2}$ limit. Subsequent radiative decay produces high vibrational levels of the ground state, $X ^1Σ_g^+$. The population distribution of these $X$ state vibrational levels is monitored by resonance-enhanced two-photon ionization through the $2 ^1Σ_u^+$ state. We find that the populations of vibrational levels $v''$=112$-$116 are far larger than can be accounted for by the Franck-Condon factors for $0_u^+(P_{1/2}) \to X ^1Σ_g^+$ transitions with the $0_u^+(P_{1/2})$ state treated as a single channel. Further, the ground-state molecule population exhibits oscillatory behavior as the PA laser is tuned through a succession of $0_u^+$ state vibrational levels. Both of these effects are explained by a new calculation of transition amplitudes that includes the resonant character of the spin-orbit coupling of the two $0_u^+$ states. The resulting enhancement of more deeply bound ground-state molecule formation will be useful for future experiments on ultracold molecules.

physics.atom-ph↗

arXiv2006

Coherent Control of Ultracold Collisions with Chirped Light: Direction Matters

We demonstrate the ability to coherently control ultracold atomic Rb collisions using frequency-chirped light on the nanosecond time scale. For certain center frequencies of the chirp, the rate of inelastic trap-loss collisions induced by negatively chirped light is dramatically suppressed compared to the case of a positive chirp. We attribute this to a fundamental asymmetry in the system: an excited wavepacket always moves inward on the attractive molecular potential. For a positive chirp, the resonance condition moves outward in time, while for a negative chirp, it moves inward, in the same direction as the excited wavepacket; this allows multiple interactions between the wavepacket and the light, enabling the wavepacket to be returned coherently to the ground state. Classical and quantum calculations support this interpretation.

physics.atom-ph↗

arXiv2006

Generation of Arbitrary Frequency Chirps with a Fiber-Based Phase Modulator and Self-Injection-Locked Diode Laser

We present a novel technique for producing pulses of laser light whose frequency is arbitrarily chirped. The output from a diode laser is sent through a fiber-optical delay line containing a fiber-based electro-optical phase modulator. Upon emerging from the fiber, the phase-modulated pulse is used to injection-lock the laser and the process is repeated. Large phase modulations are realized by multiple passes through the loop while the high optical power is maintained by self-injection-locking after each pass. Arbitrary chirps are produced by driving the modulator with an arbitrary waveform generator.

physics.atom-ph↗

arXiv2006

Probing Ultracold Collisional Dynamics with Frequency-Chirped Pulses

We report on the dynamics of ultracold collisions induced by near-resonant frequency-chirped light. A series of identical chirped pulses, separated by a variable delay, is applied to an ultracold sample of 85Rb, and the rate of inelastic trap-loss collisions is measured. For small detunings of the chirped light below the atomic resonance, we observe that the rate of collisions induced by a given pulse can be increased by the presence of an earlier pulse. We attribute this to the enhancement of short-range collisional flux by the long-range excitation of atom pairs to an attractive molecular potential. For larger detunings and short delays, we find that a leading pulse can suppress the rate of collisions caused by a following pulse. This is due to a depletion of short-range atom pairs by the earlier pulse. Comparison of our data to classical Monte-Carlo simulations of the collisions yields reasonable agreement.

physics.atom-ph↗

arXiv2006

Superradiance in ultracold Rydberg gases

Experiments in dense, ultracold gases of rubidium Rydberg atoms show a considerable decrease of the radiative excited state lifetimes compared to dilute gases. This accelerated decay is explained by collective and cooperative effects, leading to superradiance. A novel formalism to calculate effective decay times in a dense Rydberg gas shows that for these atoms the decay into nearby levels increases by up to three orders of magnitude. Excellent agreement between theory and experiment follows from this treatment of Rydberg decay behavior.

quant-ph↗

arXiv2005

State-Selective Detection of Near-Dissociation Ultracold KRb $X^1Σ^+$ and $a^3Σ^+$ Molecules

We report on the state-selective detection of near-dissociation ultracold KRb molecules in the ground $X^1Σ^+$ state and the metastable $a^3Σ^+$ state. The molecules are produced by photoassociation of ultracold atoms followed by radiative decay into high vibrational levels of the $X$ and $a$ states. Detection utilizes resonance-enhanced one-color two-photon ionization, followed by time-of-flight mass spectroscopy. Scanning the detection laser frequency over the range 582-625 nm, we observe transitions from the $v''$=86-92 levels of the $X$ state, which are bound by up to 30 cm$^{-1}$, and the $v''$=17-23 levels of the $a$ state, which are also bound by up to 30 cm$^{-1}$. The measured vibrational spacings are in excellent agreement with those previously measured and those calculated from the relevant potential curves. Relative vibrational populations are also consistent with Franck-Condon factors for decay from the photoassociated levels.

physics.atom-ph↗

arXiv2004

Photoassociative Production and Trapping of Ultracold KRb Molecules

We have produced ultracold heteronuclear KRb molecules by the process of photoassociation in a two-species magneto-optical trap. Following decay of the photoassociated KRb*, the molecules are detected using two-photon ionization and time-of-flight mass spectroscopy of KRb$^+$. A portion of the metastable triplet molecules thus formed are magnetically trapped. Photoassociative spectra down to 91 cm$^{-1}$ below the K(4$s$) + Rb (5$p_{1/2}$) asymptote have been obtained. We have made assignments to all eight of the attractive Hund's case (c) KRb* potential curves in this spectral region.

physics.atom-ph↗

arXiv2004

Local Blockade of Rydberg Excitation in an Ultracold Gas

In the laser excitation of ultracold atoms to Rydberg states, we observe a dramatic suppression caused by van der Waals interactions. This behavior is interpreted as a local excitation blockade: Rydberg atoms strongly inhibit excitation of their neighbors. We measure suppression, relative to isolated atom excitation, by up to a factor of 6.4. The dependence of this suppression on both laser irradiance and atomic density are in good agreement with a mean-field model. These results are an important step towards using ultracold Rydberg atoms in quantum information processing.

physics.atom-ph↗