1. Coherent excitation of Rydberg atoms on an atom chip
Rutger M. T. Thijssen
Van der Waals - Zeeman Instituut voor Experimentele Natuurkunde

2. Abstract
In Amsterdam We have recently produced the first two-dimensional lattice of magnetic microtraps for ultracold atoms based on patterned magnetic films [1]. Ultracold rubidium atoms are transferred to hundreds of individual microtraps, each cloud hovering 10 micrometers above the chip surface and separated by ~20 micrometers. We are currently investigating highly excited Rydberg states of the atoms, used to mediate long-range interactions between neighbouring microtraps. This could allow entanglement of mesoscopic ensembles of atoms and paves the road toward quantum information processing with neutral atoms. We have built a dedicated laser system using 780 nm and 480 nm narrow-band diode lasers stabilised to a two-photon electromagnetically induced transparency resonance in a Rubidium vapour cell. We can excite Rydberg states from n=19 up to n~100. We have used this system to excite and image Rydberg atoms in ultracold rubidium gas confined in a surface magneto-optical trap. We are now studying the influence of the nearby (magnetic and conducting) chip surface on the Rydberg excited atoms. [1] S. Whitlock, R. Gerritsma, T. Fernholz and R. J. C. Spreeuw, New J. Phys (2009)

3. Quantum Information Processing
Qubits
Coherence
Switchable interactions
Scalability

4. MAGCHIPS

5. MAGCHIPS Permanent magnetic lattice atom chip
Gold-coated for laser cooling
500 populated magnetic microtraps
Prospective qubits
87Rb, T~mK
10 µm
22 µm
Magnetised
film
“Atom chip”
(room temperature)

6. Quantum information on MAGCHIPS
Neutral atoms: intrinsically weak interaction with environment
Exquisite control & manipulation
Scalability
Stable qubits

7. Quantum information on MAGCHIPS
Neutral atoms: intrinsically weak interaction with environment
Exquisite control & manipulation
Scalability
Stable qubits
Intrinsically weak interaction with environment
Good: long coherence times (~sec.)
Challenge: quantum information requires interaction: we have to work to add an interaction between qubits (i.e. traps)

8. Rydberg atoms Hydrogen-like atom High principal (n) quantum number
Large dipole-dipole interaction between Rydberg atoms
Dipole blockade

9. Rydberg Excitation |ns |nd 480nm (blue)
Toptica TA-SHG 110 frequency doubled diode laser, tunable from nm (n=18-ionization threshold) (300mW)
|5s
|5p
780nm (infrared)
Toptica DL-100 diode laser (30mW)

10. Electromagnetically Induced Transparency
Large Ωc couples 5p and Rydberg state. Modifies absorption in probe as seen below
(Ωc)2 in equation
Dephasing rate between |Rydb and |5s
|5s becomes dark state: atoms in this state cannot be excited
|5p
δωp Ωp
γ12
|5s
Detuning (δωp)

11. Electromagnetically Induced Transparency
Large Ωc couples 5p and Rydberg state. Modifies absorption in probe as seen below
(Ωc)2 in equation
Dephasing rate between |Rydb and |5s
|5s becomes dark state: atoms in this state cannot be excited
|5p
δωp Ωp
γ12
|5s
Detuning (δωp)

12. Electromagnetically Induced Transparency – dressed states
Rediagonalise interaction Hamiltonian
Interference between |a+ and |a- dressed states: reduced probe absorption on two-photon resonance
|a+
780nm (infrared)
|a0 (5s)
|a-
Autler – Townes splitting + Fano interference

13. EIT – interfering pathways
|nd Ωc Ωp
|5s
|5p
|nd Ωp
|nd
Fano interference
(Ωc)2 +
|5p Ωp
|5s

14. EIT – frequency stabilisation in a vapour cell
480 nm diode laser
fast photodiode
Coupling laser detuning (MHz)
vapour cell EIT, |39d
dichroic mirror
Rubidium vapour cell
dichroic mirror
780 nm diode laser

15. EIT Imaging
optical fiber

16. EIT Imaging
optical fiber

17. EIT Imaging Blue laser frequency locked to vapour cell EIT
Red laser scanned over resonance
Position (px)
Detuning (MHz)
Optical density

18. Surface effects Near-field blackbody radiation from chip
“mirror” effect: Rydberg atom interacting with itself
Photoelectric effect on surface: adsorbed Rb, Au
Patch potentials
Crystal defects in FePt
Adsorbed Rb ions
-remind about lattice trapping setup
-|5s> more or less insensitive (except magnetically), not true for Rydb atoms.

19. Summary MAGCHIPS experiment
Rydberg / EIT for interactions between qubits
Built laser system
Built frequency locking setup for probe and coupling laser
Imaged Rydberg / EIT in surface magneto-optical trap
Investigating effects of surface on Rydberg levels
Build a quantum computer…

20. Summary MAGCHIPS experiment
Rydberg / EIT for interactions between qubits
Built laser system
Built frequency locking setup for probe and coupling laser
Imaged Rydberg / EIT in surface magneto-optical trap
Investigating effects of surface on Rydberg levels
Build a quantum computer…

21. THANK YOU
Questions?
Rutger M. T. Thijssen

23. 2-photon gates Zoller Mesoscopic Rydberg gates using EIT Focused
lasers
|0> |1>
Microwave/Raman
6.8 GHz
|0> |1>
Rydberg
interaction
Ensemble A
Ensemble B

25. Rydberg Atoms One highly excited electron (n=20-100) Rydberg formula:
Size ~ n^2
Lifetime ~ n^3
Polarisability ~n^7
Van der Waals interaction ~ n^11
Dipole blockade shifts nearby Rydberg levels