Glass Fiber Quantum Optics - ifraf

Trapping and Interfacing Cold Neutral Atoms
Using Optical Nanofibers
Wokshop on Cold Atoms and Quantum Engineering,
Paris, France,
May 30–31, 2013
Arno Rauschenbeutel
Vienna Center for Quantum Science and Technology,
Atominstitut, TU Wien, Austria

Introduction: Glass Fibers: “Backbone” of
the Modern Communication Society
Image: PTB

Introduction: Tapered Fibers
• Problem: Light field in standard optical fibers
cannot be accessed from outside.
• Trick: Narrow down optical fiber until light field
gets to the surface.

Overview
• Optical nanofibers
– Properties and fabrication
• Nanofiber-based atom trap
– Trapping potential
– Experimental realization
– Coherence properties of fiber-trapped atoms
• Summary / Outlook

Optical Nanofibers
• Significant part of the optical power propagates outside of
optical nanofiber in form of evanescent wave:

Tapered Optical Fibers
• Tapered optical fibers allow one to couple light in and out
of nanofiber waist:
125 µm
taper 500 nm
transition
taper
transition
• Adiabatic mode transformation up to 99% transmission!

Tapered Fibers of Predetermined Shape
A. Stiebeiner et al., Opt. Express, 18, 22677 (2010)

Single Atom Absorption
Single atom should have significant effect on transmission!

Overview
• Optical nanofibers
– Properties and fabrication
• Nanofiber-based atom trap
– Trapping potential
– Experimental realization
– Coherence properties of fiber-trapped atoms
• Summary / Outlook

Radial Trapping Potential
Evanescent field around fiber exerts dipole force on atoms.
Blue light is more tightly bound to nanofiber than red light.
Fam Le Kien, V. I. Balykin, and K. Hakuta, PRA 70, 063403 (2004)

Axial Trapping Potential
Two counterpropagating red beams create standing wave
axial confinement:

Azimuthal Trapping Potential
• HE 11 mode, quasi linearly polarized along x.
• Parameters: a = 250 nm, n 1 = 1.46 (silica), n 2 = 1
(vacuum / air), and λ = 852 nm.

Azimuthal Trapping Potential
Linear polarization breaks cylindrical symmetry
azimuthal confinement:

1d Optical Lattice
Resulting potential:
Array of trapping sites on both sides of the fiber!

Overview
• Optical nanofibers
– Properties and fabrication
• Nanofiber-based atom trap
– Trapping potential
– Experimental realization
– Coherence properties of fiber-trapped atoms
• Summary / Outlook

Experimental Setup
1064 nm
852 nm
780 nm
APD
IF
DM
Cs atoms
DM
• Trapping lasers:
- Nd:YAG 1064 nm, 2 x 2.2 mW, standing wave
- Diode laser 780 nm, 25 mW, running wave
• Fiber diameter: 500 nm trap depth ~ 0.4 mK

Experimental Setup
1064 nm
852 nm
780 nm
APD
IF
DM
Cs atoms
DM
• Trapping lasers:
- Nd:YAG 1064 nm, 2 x 2.2 mW, standing wave
- Diode laser 780 nm, 25 mW, running wave
• Fiber diameter: 500 nm trap depth ~ 0.4 mK

Experimental Setup
1064 nm
852 nm
780 nm
APD
IF
DM
Cs atoms
DM
• Trapping lasers:
- Nd:YAG 1064 nm, 2 x 2.2 mW, standing wave
- Diode laser 780 nm, 25 mW, running wave
• Fiber diameter: 500 nm trap depth ~ 0.4 mK

Experimental Setup
1064 nm
852 nm
780 nm
APD
IF
DM
Cs atoms
DM
Fluorescence of ~ 2000 atoms trapped 200 nm above fiber!
• Trapping lasers:
At most 1 atom per potential well (collisional blockade)!
- Nd:YAG 1064 nm, 2 x 2.2 mW, standing wave
- Diode laser 780 nm, 25 mW, running wave
• Fiber diameter: 500 nm trap depth ~ 0.4 mK

Spectroscopy of Trapped Atoms
• Inhomogeneous line width ↔ state dependent light shifts.
• Optically dense (OD 32) ensemble of fiber coupled atoms!
• 1.6 % absorption per atom!
E. Vetsch et al., PRL 104, 203603 (2010)

Overview
• Optical nanofibers
– Properties and fabrication
• Nanofiber-based atom trap
– Trapping potential
– Experimental realization
– Coherence properties of fiber-trapped atoms
• Summary / Outlook

Fiber-Coupled Quantum Memory
Major experimental advantages:
• No thermal motion and collisions (atoms are trapped in
optical lattice with at most one atom per lattice site).
• Quantum fields are intrinsically mode matched and
coupled into single mode optical fiber.

Fiber-Coupled Quantum Memory
Central open question:
Spin wave (ground state) coherence properties
• two hundred nanometers from a dielectric surface?
• in optical near fields with complex polarization?

Coherence Properties?

HE 11 Mode: Field Components
• HE 11 mode, quasi linearly polarized along x.
• Parameters: a = 250 nm, n 1 = 1.46 (silica), n 2 = 1 (vacuum /
air), and λ = 852 nm.
along (x, y = 0)
along (x = 0, y)

Fictitious B-Field
Fam Le Kien, P. Schneeweiß & A.R., in preparation (2013)

trap potential (mK)
State-Dependent Azimuthal Potential
groud
state
wave
function
spread
azimuthal position φ

Experimental Setup
Typical experimental parameters
• microwave frequency: cesium clock transition @ 9.2 GHz
• Rabi frequency: Ω/2π ≈ 18 kHz @ 1 W
• Initial temperature of ensemble: ≈ 30 μK
D. Reitz et al., Phys. Rev. Lett. in press (2013).

Simplified Experimental Sequence
• Atoms are initially in F=3, m F =0
• Drive the clock transition via microwave pulses
• Probe F=4 on the Cesium D2 line, F=4 => F’=5
Cesium
level scheme
}

Rabi Oscillations
• Rabi frequency: Ω/2π ≈ 17 kHz
• Exponential decay time: τ Rabi = (3.2 ± 0.2) μs limited by
power fluctuations of MW
D. Reitz et al., Phys. Rev. Lett. in press (2013).

Ramsey & Spin-Echo Signals
• Reversible dephasing time: T 2 ∗ ≈ 0.6 ms
• Current limit: temperature ⇒ spread of differential light shift

Ramsey & Spin-Echo Signals
• Irreversible dephasing time: T 2 ′ ≈ 3.7 ms
• Current limit: heating rate of ≈ 3 mK/s ⇒ change of
differential light shift
D. Reitz et al., Phys. Rev. Lett. in press (2013).

Summary
• Nanofiber-based atom trap:
– Trapping of cold atoms in the evanescent field around
nanofibers.
– Realization of optically dense 1d arrays of individual
fiber-coupled atoms (collisional blockade).
– Coherence properties encouraging for realization of
coherent operations.

Fiber Coupled Atoms!

Outlook: Nanofiber-Based F.-P. Resonator
Write two fiber Bragg gratings (FBGs) into unprocessed part
of tapered optical fiber.
C. Wuttke et al., Opt. Lett. 37, 1949 (2012)

Outlook: Nanofiber-Based F.-P. Resonator
Write two fiber Bragg gratings (FBGs) into unprocessed part
of tapered optical fiber.
C. Wuttke et al., Opt. Lett. 37, 1949 (2012)
• Exp. finesse: F = 86 (T TOF = 98.3%)
• Single atom coop.: C = g 2 /2κγ = 30
• Cs, F = 4, m F = 0 → F ′ = 5, m F
′ = 0
g, κ, γ /2π = (33, 8.6, 2.6) MHz
Coherent strong coupling !
Combination with atom trapping scheme possible:
⇒ OD eff ≈ 1000 @ 2000 atoms !

Outlook: Double Helix Potential
s + -s - -polarized red-detuned standing wave combined with
s + -polarized blue-detuned running wave

Outlook: Double Helix Potential
D. Reitz & A.R., Opt. Commun. 285, 4705 (2012)

People
Students: Bernhard Albrecht, Christian Junge, Rudolf Mitsch,
David Papencordt, Jan Petersen, Daniel Reitz, Michael
Scheucher, Danny O‘Shea, Ariane Stiebeiner, Christian Wuttke
Group Technician: Thomas Hoinkes
Postdocs & Senior Scientist: Pham Le Kien, Clément Sayrin,
Philipp Schneeweiß, Jürgen Volz

Funding
: Lichtenberg Professorship
: European Young Investigator Award

Thank you…
… for your attention.