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Manipulating Rydberg Atoms with Microwaves
P. Pillet H. Maeda D. Norum J. Nunkaew J. Han
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Why is this interesting?
Energy level tuning Population transport Making new states with controllable properties
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Floquet description applied to two level system
AC Stark shifts for resonant energy transfer Population transport by chirped pulses New states in combined dc and microwave fields
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Floquet eigenstates in a periodically varying field are periodic (2π/ω) .
Consider an s and a p state. p s+1φ p-1φ s p Ecosωt ω μ s We add or subtract integral multiples of the frequency ω to construct the Floquet states. All pairs of states are equivalent.
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s and p Floquet (time average) energies vs ω
Ws+ω p Wp Energy s+1φ ω The degeneracy at resonance is removed by the dipole coupling μE, producing an avoided crossing
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The same levels plotted with a non zero field,
creating an avoided crossing Wp Ws+ω Ω ω
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Exploration of this idea using a second microwave field
as a probe. p’ EcosωAt s’ μE s s
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Excitation and Timing 53p 53s microwaves 50s field ramp laser 480 nm
5p t (μs) 53s/53p 50s 780 nm 5s
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Observing the dressed states of Rb in a MOT.
53p 53s Scan 50s-53s frequency at different fixed 53s-53p transition frequencies. 50s resonance laser
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With the dressing field below resonance the 53s state shifts down;
With the dressing field above resonance the 53s state shifts up. 53s 53p 53s Scan 50s-53s frequency at different fixed 53s-53p transition frequencies. 50s resonance
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The AC Stark shift has been used by Martin et al to observe otherwise
inaccessible Forster energy transfer resonances. Bohloulli-Zanjani et al PRL 2006
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43d Forster resonances observed by microwave AC Stark shifts
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Population transfer using chirped microwave fields
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Adiabatic Rapid Passage
(through a one photon avoided level crossing) Chirp ω from below to above ω0 to transfer population from s to p. p Ecosωt ω0 μ ω s Wp Ω Floquet energies Ws+ω ω ARP condition: dω/dt > Ω2 Ω=μE
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The Floquet energies are time average energies.
Floquet eigenstates are periodic (2π/ω). p s+1φ p-1φ s p Ecosωt ω s (s+1φ) + p μ aligned opposite E W ΩR (s+1φ) – p μ aligned with E E In both states the dipoles are phase locked to the field!
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In adiabaic rapid passage the dipole is phaselocked.
Chirp ω from below to above ω0 to transfer population from s to p. p Ecosωt ω0 μ ω s μ↓ Wp Ω Floquet energies μ↑ Ws+ω ω ARP condition: dω/dt > Ω2 Ω=μE
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Suggested by Kalinski and Eberly
If we apply a microwave field at the Kepler frequency the dipole moment, and the orbital motion of the electron becomes phase locked to the microwave field. E E Can we speed up or slow the electron’s orbital motion by chirping the microwave frequency ω? Suggested by Kalinski and Eberly Since ω =1/n 3, this amounts to changing n or the binding energy.
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Moving n=70 atoms to n=80 with a chirped microwave pulse
+ + EMW time + + x The orbital frequency decreases, n increases, and the orbit becomes larger and more weakly bound. The electron’s motion remains phase locked.
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Lasers mw Field ramp Time n=79 n=70 19 GHz 13 GHz 500 ns long pulse
np Li n ~ 70 3s 2p 2s Lasers mw Field ramp Time n=79 n=70 19 GHz GHz ns long pulse n= n= GHz/μs Maeda et al, Science
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Time resolved field ionization signals
field ramp time excite n=70 low mw power high power
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n=75, 76, and 77 Floquet levels in no microwave field
W75, W76-ω, W77-2ω 1ω Energy (GHz) 2ω n (GHz) Chirping ω from 17 to 14 GHz transfers population from n = 75 to n =77 by two one photon resonances. Chirping from 14 to 17GHz does it by a two photon resonance Oreg, Hioe, and Eberly; Noordam et al, Bergmann
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N=68 to 84 levels no microwave field
Energy (GHz) Microwave frequency (GHz)
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With a 0.1V/cm microwave field the single photon avoided
crossings overlap and become a smooth curve. A B Energy (GHz) ν (GHz) Chirping the frequency from 19 to 13 GHz moves population from n = 71(A) to n=82 (B).
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Population transfer from n=72 to 82 by chirping the frequency from 18 to 12 GHz
The Floquet energies suggest a different approach. Maeda et al.
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0 V/cm 4 V/cm Starting in n=72 at 13 GHz and Chirping to 19 GHz leads to n=82 Only a chirp of 600 MHz is required. finish start
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N=72 to n=82 0 V/cm 2 V/cm 3 V/cm
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Energy transfer from dressed states in combined static and microwave fields
Pillet et al For simplicity we consider one of the energy transfer resonances.
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In the treatment of resonant collisions we introduced the
direct product states ss and pp’ At R=∞ the levels cross at the resonance field E0. At finite R there is an avoided crossing due to the dipole-dipole coupling Wpp’ Wss E W The energy transfer cross section is given by
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Adding a microwave field Emwcosωt to the static field Es modulates
the energy of the pp’ state Wpp’ Energy modulation of the pp’ state Wss W E Es Field modulation
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Just as modulating the frequency of laser beam breaks it into a carrier
and sidebands, the addition of the microwave field does the same to the pp’ state. The pp’ wavefunction without microwaves becomes The original pp’ state has been replaced by a carrier and sideband states The sidebands have appreciable amplitude for
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In terms of the energy level picture
Microwave field Wpp’ Wss E W Wpp’ Wss E W Each of the pp’ sideband states crosses the ss state, and has an avoided crossing at finite R.
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The dipole dipole coupling between
the mth sideband state and the ss state is Wpp’ Wss E W Since the cross section is proportional to the avoided crossing (at any R), the cross section for the m photon assisted collision Is given by At finite R there are avoided crossings where σ is the cross section in the absence of a microwave field.
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Plot of the expected cross sections for 0, 1, 2, 3 photon resonances
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Resonant Dipole-dipole Collisions of two Na atoms
Populate 17s in an atomic beam Collisions (fast atoms hit slow ones) Field ramp to ionize 17p Sweep field over many laser shots
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There should be sets of four transitions,
corresponding to the emission of 0, 1, 2, and 3 microwave photons
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18s+18s→18p+17p collisions in the presence of a 15.4 GHz microwave field. 0 V/cm 13.5 V/cm 50 V/cm 105 V/cm 165 V/cm
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0 photons ○ 1 photon ● 2 photons ▲ 3 photons ■
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Adding the microwave field creates new states, the sideband states.
The strength of the dipole-dipole interaction can be tuned at will. The strength of the coupling between the ss state and the mth sideband state is
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Microwaves make it possible to make and manipulate Rydberg states.
The frequency characteristics of microwave sources are MUCH better than those of lasers. Due to the large, ~n2, dipole matrix elements very low powers are required.
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