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Collective excitation and emission phenomena in a strongly interacting ultracold gas

D. Ciampini

Published Jan 1, 2013• 0 citations
OpenAlex

Abstract

Atoms excited to high lying g quantum states, so-called Rydberg atoms, are highly polarizable and, therefore interact strongly with each other at large distances. One result of these interactions is the Rydberg dipole blockade effect, in which strongly interacting atoms excited by the same driving pulse may prevent other atoms from getting excited. This blockade offers several applications, for example the realization of quantum logic gates [1] and the possibility \nof creating entangled states with two atoms [2].

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Atoms excited to high lying g quantum states, so-called Rydberg atoms, are highly polarizable and, therefore interact strongly with each other at large distances. One result of these interactions is the Rydberg dipole blockade effect, in which strongly interacting atoms excited by the same driving pulse may prevent other atoms from getting excited. This blockade offers several applications, for example the realization of quantum logic gates [1] and the possibility \nof creating entangled states with two atoms [2].
Summary form only given. Interactions between neutral atoms are generally weak, but exciting these atoms in a Rydberg state considerably enhances their interaction through a long-range dipole-dipole potential. An interesting application of this dipole-dipole interaction is the dipole blockade in Rydberg excitation. This effect offers exciting possibilities for manipulating quantum bits stored in a single collective excitations in mesoscopic ensembles, or for realizing scalable quantum logic gate. The dipole blockade effect has been demonstrated in atomic ensembles by different groups, and recently also with two independently trapped atoms, which shows the possibility of creating entangle states and quantum logic gates. To extend and continue these results with more atoms, a promising method is to use atoms trapped in an optical lattice in the Mott insulator phase. To excite atoms into a Rydberg state, and to control the interactions and the dipole blockade, a high resolution laser scheme is needed. We use a two-photon excitation scheme 5s1/2rarr 6p3/2rarr ns or nd. The first step, at 420 nm, is realized by doubling a MOPA laser, with the second step at ~ 1000 nm. Both lasers are locked to a Fabry-Perot cavity, and the total excitation bandwidth is less than 3 MHz. After the excitation into the Rydberg state, we apply a pulsed electric field to ionize and detect the Rydberg atoms with an ion detector. To characterize and improve the efficiency of our ion detection, we have explored the direct ionization of ultracold atoms in a magneto-optical trap, or in a Bose-Einstein condensate. This calibration will allows us to characterize precisely the Rydberg excitation and to monitor its time evolution. First results on the Rydberg excitation of ultracold atoms in dipole traps and optical lattices will be presented. The prospect for creating Rydberg excitations inside a Mott insulator will also be discussed.
Owing to the unique physical characteristics of Rydberg atoms, which play an important role in quantum information and quantum computation, the theoretical and applied research of Rydberg atoms have become the hot spots of scientific research in recent years. With the large polarizability of Rydberg atoms, even a small electric field could cause a considerable electric dipole moment, resulting in a strong dipole-dipole interaction between Rydberg atoms. The multiple excitations of the Rydberg states are strongly inhibited because of the strong dipole interaction between atoms within a mesoscopic interaction (blockade) region. We call this phenomenon the dipole blockade effect. The dipole blockade effect makes it possible to build single-photon quantum devices, implement quantum gates, generate quantum entanglement, and simulate many-body quantum problems, etc. A Rydberg atomic ensemble in the same blockade region can be regarded as a superatom. In the same way, if these atoms trapped in two optical dipole traps, each sub-ensemble can be considered as a sub-superatom which is closely related to the superatom. According to the fact that two Rydberg sub-superatoms can be strongly correlated due to sharing no more than one excited Rydberg atom, we study correlated collective excitation and quantum entanglement between two Rydberg sub-superatoms in a steady state. With the superatom model, the problem of exponentially increasing system size with the number of atoms can be circumvented to a certain extent in studying many-body physics. By solving the two-body Lindblad’s master equation accurately, we obtain the analytical expressions for the collective excitation probabilities of the two sub-superatoms, and the concurrence measuring the bipartite entanglement between them. Our results show that they are all sensitive to the number of atoms in each Rydberg superatom: the bigger (including more atoms) the Rydberg superatom, the higher the collective Rydberg excitation probability is. And that the maximally entangled state can only be obtained with two equal-sized Rydberg superatoms. When this condition is fulfilled, the mesoscopic entanglement can be generated by adding the number of atoms in each Rydberg superatom. This may provide an attractive platform for studying the quantum-classical correspondence and have potential promising applications in quantum information processing.
We investigate the excitation of a collection of cold atoms to Rydberg states. By a direct numerical solution of Schrödinger's equation, we are able to compute various interesting properties of the many-body wavefunction. The high polarizability of Rydberg atoms allows them to support large dipole moments which in turn can interact with each other over long ranges. If the interaction energy between excited atoms is large enough, the resultant energy shift will move the two excitation states out of resonance, thus effectively blocking a two excitation state from occurring. One particular topic investigated is the quantum phase gate, where both groups of atoms are within a blockade radius R b and subjected to a π − 2π − π sequence of pulses. We examine the regime where the groups are neither totally within nor totally outside the blockade radius. Our results explore the tolerance in variation of intergroup lattice distances for a series of quantum gates.