Rydberg excitation in ultracold gases

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The Crew

Jonathan Balewski	(Diploma student)
Vera Bendkowsky	(PhD student)
Björn Butscher	(PhD student)
Robert Löw	(Senior Scientist)
Johannes Nipper	(PhD student)
Tilman Pfau	(head of institute)

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Abstract

Rydberg atoms are highly excited atoms with one valence electron of principal quantum number n >>1. Because of their huge size of the order of n²a₀ they are very susceptible to external electric fields. Therefore Rydberg atoms provide the opportunity to study effects that are hardly noticeable in ground state atoms.
The main goal of this project is the investigation of interactions between Rydberg atoms (dipole-dipole and van der Waals interaction) which lead to line shifts and blockade of excitation in the so called blockade radius of the Rydberg atom. Moreover, we are interested in the effect of  Rydberg atoms on an ultracold sample of atoms in the ground state - especially on a Bose-Einstein condensate.

 

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Research

In our experiments we use ⁸⁷Rb: it belongs to the alkali metals and thus has only one electron in the outermost shell, which we excite to the Rydberg state. We start with a cloud of cold rubidium atoms in the ground state that we can produce in two different experimental setups:

Figure 1: Schematic setup of the experiments: a) In the reflection-MOT setup, a grid and a gold-covered copper plate are used to apply a homogeneous electric field to the Rydberg atoms and for field ionization. The Rydberg atoms are detected as ions by a MCP [1]. b) Eight electric field plates inside the BEC-vacuum chamber allow us to create various electric field configurations at the position of the atoms.

In the Rb-MOT-experiment (Fig. 1a), we cool and trap ⁸⁷Rb atoms in a magneto-optical trap (MOT) with atom numbers in the order of 10⁷ and temperatures of about 300 µK. For a detailed description of the setup see [1].
In the Rb-BEC-experiment (Fig. 1b) atoms from a magnetic trap or a BEC can be excited to Rydberg states. In this setup, the temperature and density of the atomic cloud can be tuned over several orders of magnitude. Additionally, a one-dimensional optical lattice is already set up, which will be used to generate spacially structured samples.

Rydberg excitation and detection

The excitation energies to Rydberg states are hardly reached in a one-photon process, e.g. the excitation to the n = 40 state in ⁸⁷Rb requires a laser at 297 nm. Therefore we use a two-photon excitation scheme (Fig. 2a): The red laser (780 nm) is nearly resonant (detuning Δ) with the transition to the 5P_3/2 state and together with the blue laser resonant with the Rydberg level. The principal quantum number n can be varied between n = 30...80 by tuning the blue laser in the range of 479-488 nm. Both lasers are cw-lasers with a total linewidth below 1 MHz. In our current experiments, we excite to nS_1/2 states with n between 34 and 43, which have no electric dipole moment but can be polarized in an electric field.
After excitation the Rydberg atoms are field ionized by an electric field pulse of 200 V/cm (Fig. 2b) and the ions are subsequently detected by a microchanel plate (MCP) [2].

Figure 2: a) Excitation scheme for ⁸⁷Rb. b) Experimental sequence.

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News from the lab

	22.04.2009	Observation of ultra-long range Rydberg molecules more...
	20.12.2007	Exciting Rydberg atoms in Bose-Einstein condensates more...
	14.11.2007	Coherent excitation of a strongly interacting Rydberg gas more...
	25.09.2006	Rydberg excitation of magnetically trapped atoms more...
	04.08.2005	Observation of Autler-Townes splitting via Rydberg-excitation of ultracold 87Rb atoms more ...
	04.08.2005	Rydberg excitation in a gas of ultracold Rubidium atoms more ...

Outlook

The interaction between Rydberg atoms is state dependent. While the S states have no electric dipole moment but are polarized in electric fields (quadratic Stark effect), which results in a van der Waals interaction between these atoms, the hydrogen-like states show a linear Stark effect (Fig. 3). The manifold consists of n - | m | states which are degenerate in zero-field and split up as the electric field is increased, where  | m | is the absolute value of the magnetic moment. In contrast to the van der Waals interaction, the sign and the magnitude of the dipole-dipole interaction depends on the relative orientation of the electric dipoles, which means the interaction between two Rydberg atoms can be tuned from repulsive to attractive.
In conclusion, the type and strength of the interaction between Rydberg atoms can be adjusted by the choice of the Rydberg state and the applied electric field. This makes it possible to study these interactions isolated, but a state-selective excitation of the different Stark states, i.e. a small laser bandwidth, is required.
Moreover the Rydberg excitation can take place in a spacially structured cloud of ground state atoms as it is available by use of an optical lattice. This provides the opportunity of defined distances between single Rydberg atoms and thereby of adressibility by e.g. electric fields.

Figure 3: Calculatetd Stark map of Rubidium for | m | = 1 in the vicinity of the n = 40 manifold (see PhD-thesis of A. Grabowski).

 

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Refs & Publications

[1] Axel Grabowski, Rolf Heidemann, Robert Löw, Jürgen Stuhler, and Tilman Pfau
"High Resolution Rydberg Spectroscopy of ultracold Rubidium Atoms"
Fortschr. Phys. 54, 765 (2006) (arXiv-pdf)

[2] Robert Löw, Ulrich Raitzsch, Rolf Heidemann, Vera Bendkowsky, Björn Butscher, Axel Grabowski, and Tilman Pfau
"Apparatus for excitation and detection of Rydberg atoms in quantum gases"
arXiv:0706.2639v1 [quant-ph] (pdf)

[3] Rolf Heidemann, Ulrich Raitzsch, Vera Bendkowsky, Björn Butscher, Robert Löw, Luis Santos, and Tilman Pfau
" Evidence for Coherent Collective Rydberg Excitation in the Strong Blockade Regime"
Phys. Rev. Lett. 99, 163601 (2007) (abstract) (arXiv)

[4] Ulrich Raitzsch, Rolf Heidemann, Vera Bendkowsky, Björn Butscher, Robert Löw, and Tilman Pfau
"Echo experiments in a strongly interacting Rydberg gas"
Phys. Rev. Lett. 100, 013002 (2008) (abstract)

[5] R. Heidemann, U. Raitzsch, V. Bendkowsky, B. Butscher, R. Löw, T. Pfau,
"Rydberg excitation of Bose-Einstein condensates"(arXiv)
Phys. Rev. Let. 100, 033601 (2008)(abstract)

[6] V. Bendkowsky, B. Butscher, J. Nipper, R. Löw, J. P. Shaffer, T. Pfau,
"Observation of ultralong-range Rydberg molecules"(arXiv)
Nature 458, 1005(2009)(abstract)

[7] Old webpage: Rb-MOT-experiment

 

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The Rubidium-BEC and Rydberg team:

From left to right: Hendrik Weimer, Robert Löw, Tilman Pfau, Björn Butscher, Vera Bendkowsky, Jonathan Balewski, Johannes Nipper

 

Former Crew

Ulrich Raitzsch	PhD student	thesis
Rolf Heidemann	PhD student	thesis
Peter Kollmann	Diploma student	thesis
Livio Romano	PhD student
Eva Kuhnle	Diploma student	thesis
Jürgen Stuhler	Senior Scientist
Axel Grabowski	PhD student	thesis
Christian Kuke	Bachelor student	thesis
Jörg Bauer	Diploma student	thesis
Alexander Benner	Diploma student	thesis

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