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

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. 11 023021 (2009)

Quantum Information Processing Qubits Coherence Switchable interactions Scalability

MAGCHIPS

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)

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

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)

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

Rydberg Excitation |ns |nd 480nm (blue) Toptica TA-SHG 110 frequency doubled diode laser, tunable from 488-479nm (n=18-ionization threshold) (300mW) |5s |5p 780nm (infrared) Toptica DL-100 diode laser (30mW)

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)

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)

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

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

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

EIT Imaging optical fiber

EIT Imaging optical fiber

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

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.

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…

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…

THANK YOU Questions? Rutger M. T. Thijssen rmeijert@science.uva.nl

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

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