Entanglement of cold molecules using strong optical pulses

Guangjiong Dong, Weiping Lu, P. F. Barker +1 more

Felipe Herrera, Sabre Kais, K. Birgitta Whaley

While many-particle entanglement can be found in natural solids and strongly interacting atomic and molecular gases, generating highly entangled states between weakly interacting particles in a controlled and scalable way presents a significant challenge. We describe here a one-step method to generate entanglement in a dilute gas of cold polar molecules. For molecules in optical traps separated by a few micrometers, we show that maximally entangled states can be created using the strong off-resonant pulses that are routinely used in molecular alignment experiments. We show that the resulting alignment-mediated entanglement can be detected by measuring laser-induced fluorescence with single-site resolution and that signatures of this molecular entanglement also appear in the microwave absorption spectra of the molecular ensemble. We analyze the robustness of these entangled molecular states with respect to intensity fluctuations of the trapping laser and discuss possible applications of the system for quantum information processing.

D. Meiser, T. Miyakawa, Hermann Uys +1 more

Felipe Herrera

Ultracold molecules are expected to find applications in cold chemistry, quantum phases, precision measurements and quantum information. In this thesis three novel applications of cold molecules are studied. First the thesis presents a general method for coherent control of collisions between non-identical particles. It is shown that by preparing two alkali-metal atoms in a superposition of hyperfine states, the elastic-to-inelastic cross section ratio can be manipulated at ultracold temperatures by tuning laser parameters in the presence of a magnetic field. The static field is needed to induce quantum interference between scattering states. Extensions of this scheme for ultracold molecular reactive scattering are discussed. Second, the thesis describes rotational excitons and polarons in molecular ensembles trapped in optical lattices. Rotational excitons can be manipulated using static electric and magnetic fields. For a one-dimensional molecular array with substitutional impurities any localized exciton state can be delocalized by applying a suitable electric field. The electric field induces correlations between diagonal and off-diagonal disorder. It is also shown that the translational motion of polar molecules in an optical lattice can lead to phonons. The lattice dynamics and the phonon spectrum depend on the strength and orientation of a static electric field. An array of polar molecules in an optical lattice can be described by generalized polaron model with tunable parameters including diagonal and off-diagonal exciton-phonon interactions. It is shown that in a strong electric field the system is described by a generalized Holstein model, and at weak electric fields by the Su-Schrieffer-Heeger (SSH) model. The possibility of observing a sharp polaron transition in the SSH model using polar alkali-metal dimers is discussed. Finally, the thesis presents a method to generate entanglement of polar molecules using strong off-resonant laser pulses. Bipartite entanglement between alkali-metal dimers separated by hundreds of nanometers can be generated. Maximally entangled states can be prepared by tuning the pulse intensity and duration. A scheme is proposed to observe the violation of Bell’s inequality based on molecular orientation correlation measurements. It is shown that using a combination of microwave and off-resonant optical pulses, arbitrary tripartite and many-particle states can be prepared.

F. J. Duarte

Amichay Vardi, D.G. Abrashkevich, Einat Frishman +1 more

A time dependent theory for radiative recombination induced by strong pulses is presented. Analytic solutions in the adiabatic limit are derived and found to be in excellent agreement with exact numerical solutions. Both the pump-before-dump “intuitive” and dump-before-pump “counter-intuitive” schemes are considered. Resonantly enhanced two-photon recombination of ultracold atoms is shown to be an efficient mechanism for the production of ultracold molecules. We have performed detailed calculations on the radiative recombination of cold Na atoms by short laser pulses. Our calculations show that, per pulse, it is possible for up to 97% of all head-on Na-Na colliding pairs to end up as v=0, J=0 translationally cold Na2 molecules. We show that these findings, translated to thermally cooled ensemble conditions, mean that the fraction of Na atoms at μKelvin which can be recombined by a pulse of 20 ns duration and 108 W/cm2 peak intensity, to form J=0 molecules is 6×10−6 per pulse. With the above parameters, a laser operating at 100 Hz can convert half of an ensemble of cold atoms to cold molecules in ∼25 min. The efficiency of the process can be increased by going to longer pulses of lower intensity, by going to lower temperatures or by increasing the density of the ensemble. In particular, the “counter-intuitive” scheme which allows for use of longer pulses of lower intensities, with no spontaneous emission losses, considerably increases the yield.