Quantum optics and information with trapped ionsqiqfgba.df.uba.ar/giambiagi2019/slides/BA school FSK...
Transcript of Quantum optics and information with trapped ionsqiqfgba.df.uba.ar/giambiagi2019/slides/BA school FSK...
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• Introduction to ion trapping and cooling
• Trapped ions as qubits for quantum computing and simulation
• Rydberg excitations for fast entangling operations
• Quantum thermodynamics, Kibble Zureck law, and heat
engines
• Implanting single ions for a solid state quantum device
www.quantenbit.de
Quantum optics and information
with trapped ions
F. Schmidt-Kaler
Mainz, Germany: 40Ca+
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Ion Gallery
Boulder, USA: Hg+
Aarhus, Denmark: 40Ca+ (red) and 24Mg+ (blue)
Oxford, England: 40Ca+ coherent breathing motion of a 7-ion linear crystal
Innsbruck, Austria: 40Ca+
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Why using ions?• Ions in Paul traps were the first sample with which laser cooling was
demonstrated and quite some Nobel prizes involve laser cooling…
• A single laser cooled ion still represents one of the best understood objects for
fundamental investigations of the interaction between matter and radiation
• Experiments with single ions spurred the development of similar methods with
neutral atoms
• Particular advantages of ions are that they are
- confined to a very small spatial region (dx<l)
- controlled and measured at will for experimental times of days
• Ideal test ground for fundamental quantum optical experiments
• Further applications for
- precision measurements
- cavity QED
- optical clocks
- quantum computing
- thermodynamics with small systems
- quantum phase transitions
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Paul trap in 3D
Linear Paul trap
micro traps:
segmented linear trap
planar segmented trap
Eigenmodes of a linear ion crystal
Stability of a linear crystal
planar ion crystals
non-harmonic contributions
Micromotion
Introduction to ion trapping
Traditional Paul trap
Modern segmented micro Paul trap
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Dynamic confinement in Paul trap
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Invention of the
Paul trap
Wolfgang Paul (Nobel prize 1989)
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Binding in three dimensions
Electrical quadrupole potential
Binding force for charge Q
leads to a harmonic binding:
no static trapping in 3 dimensions
Laplace equation requires
Ion confinement requires a focusing force in 3 dimensions, but
such that at least one of the coefficients is negative,
e.g. binding in x- and y-direction but anti-binding in z-direction !
trap size:
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Dynamical trapping: Paul‘s idea
time depending potential
with
leads to the equation of motion for a particle with charge Q and mass m
takes the standard form of the Mathieu equation
(linear differential equ. with time depending cofficients)
with substitutions
radial and axial trap radius
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Mechanical
Paul trap
Rotating saddle
Stable confinement
of a ball in the
rotating potential
X-direction
Y-direction
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Regions of stability
time-periodic diff. equation leads to Floquet Ansatz
If the exponent µ is purely real, the motion is bound,
if µ has some imaginary part x is exponantially growing and the motion is unstable.
The parameters a and q determine if the motion is stable or not.
Find solution analytically (complicated) or numerically:
a=0, q =0.1 a=0, q =0.2
time time
excu
rsio
n
excu
rsio
n
a=0, q =0.3 a=0, q =0.4
time time
excu
rsio
n
excu
rsio
n
a=0, q =0.5 a=0, q =0.6
time time
excu
rsio
n
excu
rsio
n
a=0, q =0.7 a=0, q =0.8
time time
excu
rsio
n
excu
rsio
n
a=0, q =0.9 a=0, q =1.0
time time
excu
rsio
n
excu
rsio
n
6 1019
-3 1019
unstable
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time
po
sitio
n in t
rap
micromotion
1D-solution of Mathieu equation single Aluminium dust particle in trap
Two oscillation frequencies
slow frequency: Harmonic secular motion, frequency w increases with increasing q
fast frequency: Micromotion with frequency W
Ion is shaken with the RF drive frequency (disappears at trap center)
Lissajous figure
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3-Dim. Paul trap stability diagram
for a << q << 1 exist approximate solutions
The 3D harmonic motion with frequency wi can
be interpreted, approximated, as being caused by
a pseudo-potential Y
leads to a quantized harmonic oscillator
PP approx. : RMP 75, 281 (2003), NJP 14, 093023 (2012), PRL 109, 263003 (2012)
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Real 3-Dim. Paul traps
ideal 3-Dim. Paul trap with equi-potental
surfaces formed by copper electrodes
endcap electrodes at distance
ideal surfaces:
but non-ideal surfaces do trap also well:
rring ~ 1.2mm
A. Mundt, Innsbruck
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ideal 3 dim. Paul trap with equi-potental
surfaces formed by copper electrodes
non-ideal surfaces
rring ~ 1.2mm
numerical calculation
of equipotental lines
similar potential
near the center
Real 3-Dim. Paul traps
RMP 82, 2609 (2010)
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x
y
2-Dim. Paul mass filter stability diagram
time depending potential
with
dynamical confinement in the x- y-plane
with substitutions
radial trap radius
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2-Dim. Paul mass filter stability diagram
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x
y
A Linear Paul trap
plug the ends of a mass filter by positive electrodes:
mass filter blade design side view
RF
RF 0V
0V Uend Uend
numerically calculate the axial electric potential,
fit parabula into the potential
and get the axial trap frequency
with k geometry factor
z0
Numerical tools: RMP 82, 2609 (2010)
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Innsbruck design of linear ion trap
1.0mm
5mm
MHz5radialwMHz27.0 axialw
Blade design
eVdepthtrap F. Schmidt-Kaler, et al.,
Appl. Phys. B 77, 789 (2003).
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Ion crystals:
Equilibrium positions and eigenmodes
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Equilibrium positions in the axial potential
z-axis
mutual ion repulsiontrap potential
find equilibrium positions x0: ions oscillate with q(t) arround
condition for equilibrium:
dimensionless positions with length scale
D. James, Appl. Phys.
B 66, 181 (1998)
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Equilibrium positions in the axial potential
numerical solution (Mathematica),
e.g. N=5 ions
equilibrium positions
set of N equations for um
-1.74 -0.82 0 +0.82 +1.74
force of the
trap potentialCoulomb force
of all ions from left side Coulomb force
of all ions from left side
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-40 -30 -20 -10 0 10 20 30 400
1
2
3
4
5
6
7
8
9
10
Nu
mb
er
of
Ions
z-position (µm)
Linear crystal equilibrium positions
equilibrium positions
are not equally spaced
H. C. Nägerl et al.,
Appl. Phys. B 66, 603 (1998)
theory
experiment
minimum inter-ion distance:
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Eigenmodes and Eigenfrequencies
Lagrangian of the axial ion motion:
m,n=1m=1
N N
describes small excursions
arround equilibrium positions
with
and
N
m,n=1m=1
N ND. James, Appl. Phys.
B 66, 181 (1998)
linearized Coulomb interaction leads to Eigenmodes, but the
next term in Tailor expansion leads to mode coupling, which is
however very small.
C. Marquet, et al.,
Appl. Phys. B 76, 199
(2003)
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Eigenmodes and Eigenfrequencies
Matrix, to diagonize
numerical solution (Mathematica),
e.g. N=4 ions
Eigenvectors
Eigenvalues
for the radial modes:
Market et al., Appl. Phys.
B76, (2003) 199depends on N
pictorial
does not
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time
po
sitio
n
Center of
mass mode
breathing
mode
Common mode excitations
H. C. Nägerl, Optics
Express / Vol. 3, No. 2 /
89 (1998).
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Breathing mode excitation
H. C. Nägerl, Optics
Express / Vol. 3, No. 2 /
89 (1998).
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• Depends on a=(wax/wrad)2
• Depends on the number of ions acrit= cNb
• Generate a planar Zig-Zag when wax < wyrad << wx
rad
• Tune radial frequencies in y and x direction
1D, 2D, 3D ion crystals
Enzer et al., PRL85,
2466 (2000)
Wineland et al., J. Res. Natl. Inst.
Stand. Technol. 103, 259 (1998)
3D
1D
Kaufmann et al, PRL
109, 263003 (2012)
2D
dx~50nm
±0.25%
Planar crystal
equilibrium positions
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There are many structural phase transitions!
• Vary anisotropy and observe the critcal ai
• Agreement with expected values
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Structural phase transition in ion crystal
Upot,harm.Ekin UCoulomb
Phase transition @ CP:
• One mode frequency 0
• Large non-harmonic
contributions
• coupled Eigen-functions
• Eigen-vectors reorder to
generate new structures
6 ion
crystal
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Marquet, Schmidt-
Kaler, James, Appl.
Phys. B 76, 199 (2003)
Ion crystal beyond harmonic approximations
Upot,harm.Ekin UCoulomb
Z0 wavepaket size
lz ion distance
g,l ion frequencies
Dn,m,p coupling matrix
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Non-linear couplings in ion crystal Lemmer, Cormick, C.
Schmiegelow, Schmidt-
Kaler, Plenio, PRL 114,
073001 (2015)
Self-interaction
Cross
Kerr coupling
Resonant inter-mode coupling
…. remind yourself of non-
linear optics: frequency
doubling, Kerr effect, self-
phase modulation, ….
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Non-linear couplings in ion crystal
Cross
Kerr coupling
Resonant inter-mode coupling
Ding, et al, PRL119, 193602 (2017)
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Micro-motion
Problems due to micro-motion:
• relativistic Doppler shift in frequency measurements
• less scattered photons due to broader resonance line
• imperfect Doppler cooling due to line broadening
• AC Stark shift of the clock transition due to trap drive field W
• for larger # of ions: mutual coupling of ions can lead to
coupling of secular frequency w and drive frequency W.
• Heating of the ion motion
• for planar ion crystals non-equal excitation
• Shift of motional frequencies
• for atom-ion experiments, large collision energies
time
posi
tion
in tr
ap
micromotion
Feldker, et al, PRL 115,
173001 (2015)
Ewald et al, PRL122,
253401 (2019)
Kaufmann et al, PRL
109, 263003 (2012)
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Micro-motion
frequency W : Micro-motion
Ion is shaken with the RF
drive frequency
alters the optical spectrum of the trapped ion
due to Doppler shift, Bessel functions Jn(b) appear.
Electric field seen by the ion:
a) broadening of the ion‘s resonance b) appearing of micro-motion sidebands
time
posi
tion
in tr
ap
micromotion
Wide line
limitNarrow line
limit
PRL 81, 3631 (1998),
PRA 60, R3335 (1999)
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Compensate micro-motion
how to detect micro-motion:
a) detect the Doppler shift
and Doppler broadening
Fluorescence modulation technique:
apply voltages here
and shift the ion into the symetry center
of the linear quadrupole
frequency
ion oscillation leads a
modulation in # of scattered
photons. Synchron detection
via a START (photon)
STOP (WRF trigger) measurement
b) detect micro-motional sidebands
Sideband spectroscopy
J. Appl. Phys. 83, 5025
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Laser cooling
Laser-ion interaction
Lamb Dicke parameter
Strong and weak
confinement regime
Rate equation model
Cooling rate and cooling limit
Doppler cooling of ions
Resolved sideband spectroscopy
Temperature measurement techniques
Sideband Rabi oscillations
Red / blue sideband ratio
Carrier Rabi oscillations
dark resonances
observation of scatter light in far field
Reaching the
ground state of vibration
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Basics: Harmonic oscillator
Why? The trap confinement is leads to three
independend harmonic oscillators !
here only for the linear direction
of the linear trap no micro-motion
treat the oscillator quantum mechanically and
introduce a+ and a
and get Hamiltonian
Eigenstates |n> with:
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Harmonic oscillator wavefunctions
Eigen functions
with orthonormal Hermite polynoms
and energies:
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Two – level atomWhy? Is an idealization which is a good approximation to real
physical system in many cases
two level system is connected
with spin ½ algebra using the
Pauli matrices
D. Leibfried, C. Monroe,
R. Blatt, D. Wineland,
Rev. Mod. Phys. 75, 281 (2003)
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Two – level atomWhy? Is an idealization which is a good approximation to real
pyhsical system in many cases
gn ,1
en ,1en,
en ,1
gn ,1gn,
together with the harmonic oscillator leading to the ladder of
eigenstates |g,n>, |e,n>:
levels not coupled
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Laser couplingdipole interaction, Laser radiation with frequency wl, and intensity |E|2
Rabi frequency:
the laser interaction (running laser wave) has a spatial dependence:
Laser
with
momentum kick, recoil:
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Laser couplingin the rotating wave approximation
using
and defining the Lamb
Dicke parameter h:
Raman transition: projection of Dk=k1-k2
x-axis
if the laser direction is at an angle f to the vibration mode direction:
x-axis
single photon transition
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Interaction picture
In the interaction picture defined by
we obtain for the Hamiltonian
with
coupling states with vibration quantum numbers
laser detuning D
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2-level-atom harmonic trap
Laser coupling
dressed system
„molecular
Franck Condon“
picture
dressed system
gn ,1
en ,1en,
en ,1
gn ,1gn,
„energy
ladder“
picture
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Lamb Dicke Regime
carrier:
red sideband:
blue sideband:laser is tuned to
the resonances:
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kicked wave function is non-orthogonal to the other
wave functions
Wavefunctions in momentum space
kick by the laser:
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0,g
0,e1,e
1,g
carrier and sideband
Rabi oscillations
with Rabi frequencies
carrier
sideband
Experimental example
and
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Outside Lamb Dicke Regimecouplin
g s
trength
|n>
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gn ,1
en ,1en,
gn ,1
gn,
1,n e+
strong confinement – well resolved sidebands:
Selective excitation of a single sideband only,
e.g. here the red SB
„Strong confinement“
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weak confinement:
Sidebands are not resolved on that transition.
Simultaneous excitation of several vibrational states
„Weak confinement“
gn ,1
en ,1
en,
gn,
1,n e+
gn ,1
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incoherent: W < g
spont. decay rate g
Rabi frequency W
W/2p = 5MHz
W/2p = 10MHz
W/2p = 100MHz
W/2p = 50MHz
coherent: W > g
g/2p = 15MHzTwo-level system
dynamics
Steady state population of |e>:
Solution of
optical Bloch equations
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Rate equations of absorptionexcitation probabilities in pertubative regime:
incoherent excitation if
photon scatter rate: gn,
detuning D
absorption
spont. decay rate:
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Rate equations of absorption and emissionexcitation probabilities in pertubative regime:
incoherent excitation if
en,
emission
take all physical processes that change n, in lowest order of h
gn ,1
en ,1en,
en ,1
gn ,1gn,
cooling:
gn ,1
en ,1en,
en ,1
gn ,1gn,
heating:
S. Stenholm, Rev. Mod. Phys. 58, 699 (1986)
photon scatter rate:
spont. decay rate:
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gn ,1
en ,1en,
en ,1
gn ,1gn,
Rate equations for cooling and heating
cooling:
gn ,1
en ,1en,
en ,1
gn ,1gn,
heating:
S. Stenholm, Rev. Mod. Phys. 58, 699 (1986)
probability for population in |g,n>: loss and gain from states with |±n>
loss
gain
cooling heating
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Rate equation
different illustration:
n+1
n A-
A- A+
A+
gn ,1
en ,1en,
en ,1
gn ,1gn,
cooling:
gn ,1
en ,1en,
en ,1
gn ,1gn,
heating:
How to reach red detuning
cooling
heating
steady state phonon number cooling rate
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***
n=1
hurra !
n+1
n A-
A- A+
A+
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weak confinement:
Sidebands are not resolved on that transition.
Small differences in
„Weak confinement“
detuning for optimum cooling
gn ,1
en ,1
en,
gn,
1,n e+
gn ,1
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weak confinement:
Sidebands are not resolved on that transition.
Small differences in
„Weak confinement“
detuning for optimum cooling
Lorentzian has the
steepest slope at
Laser
complications:
hlaser < hspontaneous
saturation effects
optical pumping
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„Strong confinement“
Laser
strong confinement – well resolved sidebands:
detuning for optimum cooling
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gn ,1
en ,1
gn ,1
gn,
1,n e+
strong confinement – well resolved sidebands:
detuning for optimum cooling
g
„Strong confinement“
|𝑛, 𝑒 >
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Cooling limit
g,0
e,0e,1
g,1
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Limit of SB cooling
g,0
e,0e,1
g,1
off resonant
carrier excitation:
subsequent blue SB decay:
with an „effective“ g
and the h of spont. emission
leads to heating:
off resonant blue SB excitation
leads to heating:
with:
typical experimental
parameters:x
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Resolved sideband spectroscopy
Select narrow optical transition with:
a) Quadrupole transition
b) Raman transition between Hyperfine ground states
c) Raman transition between Zeeman ground states
d) Octopole transition
e) Intercombination line
f) RF transition
Species and Isotopes:
for (a) 40Ca, 43Ca, 138Ba, 199Hg, 88Sr, ....
for (b) 9Be, 43Ca, 111Cd, 25Mg....
for (c) 40Ca, 24Mg, ....
for (d) 172/172Yb, ....
for (e) 115In, 27Al, ....
for (f) 171Yb, ....
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S1/2
P1/2
D3/2
397 nm
866 nm
729 nm
s2.1D5/2
854 nm
393 nm
P3/2
Level scheme of 40Ca+
narrow S1/2 - D5/2
quadrupole transition excited near 729nm
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P1/2
S1/2
= 7 ns
397 nm
D5/2
= 1 s
729 nm
„qubit“
Ion energy levelsenerg
y
|1>
|0>
Superpositionen of S1/2 and D5/2
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P1/2
S1/2
= 7 ns
397 nm
D5/2
= 1 s
729 nm
Energ
y
|1>
|0>
Specroscopy pulse followed by detection of qubits:
Scatter light near 397nm: S1/2 emits fluorescence D5/2 remains dark
|0> |0>|0>
|1> |1>|1>
|0>|0>
|1>
„qubit
-measurement“
Ion energy levels