coherent manipulation of spins in quantum dotskoenig/dpg_school_10/economou.pdf · coherent...
TRANSCRIPT
Coherent manipulation of spins in quantum dots
Sophia E Economou
NRL
DPG Physics School 2010, Bad Honnef, September 2010
Naval Research Lab
Washington DC
-2.2 -2.0 -1.8 -1.6 -1.4 -1.2 -1.0 -0.81256
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Ene
rgy
(meV
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Bias (V)
growthspectroscopy
controltheory
Outline• Motivation: quantum information processing
• Quantum dots
• Basics of quantum control
• Coherently controlled spins in QDs– Focus on optical control– Spin rotations– Entangling gates between spins
R. Landauer: “Information is physical”
Why quantum information? (e.g. |0>≡|↑>,|1>≡ |↓>)
• Principles of quantum mechanics determine the properties of information encoded in quantum systems– Superposition
– Entanglement of two spins
– Measurement
] in bit classical [cf. ↓↑↓ +↑ = orβαψ
↑↓ −↓ ↑ = Ψsinglet spin :Example
Not in product form
spin second determinesspin first measuring : Ψfor
0 , 1: two states of a physical system
Some consequences of these principles• N-qubit system described by 2N
parameters [while classical N bits by O(N)]because the different contributions to the superposition evolve ‘in parallel’
Potential computational power: A programmable quantum computer may solve exponentially difficult problems
• Unknown quantum states cannot be clonedOtherwise measurement axiom would be violated
Quantum information cannot be intercepted (cryptography)
Motivation-Why optically controlled spins in QDs?
• Quantum information processing– Spin is naturally a qubit
– Long coherence times ms
– Ultrafast preparation, control & measurement (ps timescale)
– Semiconductor: integrability
– Fixed location of qubits
• New & interesting physics– Interplay of solid state environment & confinement provide new playground to explore quantum effects
– Coherent control
QIP with optically controlled spins in QDs• Quantum dot with single excess electron• e spin carries quantum information
• Operations: optically by Raman transitions via trion (bound state of electron and exciton)
• Each QD emits at a different frequency(Large inhomogeneous broadening allows for ‘zip‐code’assigning and optically selective addressing)
• Inter‐dot coupling: – With common cavity mode
– Optically generated extended state
Alternative use of quantum dots: Emitters of single photons and of entangled photon states for use in Quantum Information tasks
Imamoglu et al, PRL 2001Steel+Gammon, Physics Today 1998
Quantum dots• Semiconductor nanostructures with 3D nanometer confinement for electrons/holes
• Atomic‐like energy levels
D. Gammon et al., PRL 76, 305 (1996)
J. P. Reithmaier et al.,Nature 432, 197 (2004)
J. M. Elzerman et al., Nature 430, 431 (2004)
Energy levels: quantization & HH-LH splittingQuantum dotParticle in a box with effective mass from each band(large enough to have underlying structure; see also shallow impurities)
2/3 h, ±
2/1 h, ±
2/1 e, ±Bands of III-V compounds
21
=J
23
=J
Bulk
2/3 h,±
2/1 h, ±
2/1 e, ±21
=J
23
=J
Confinement—induced H-Lhole splitting
trion
Self assembled quantum dots: growth & spectroscopy
Substrate GaAs: Small lattice constant
Deposited Material InAs: Large lattice constant
WLQDs
Stranski-Krastanov ‘self organized’ QDs
excite quantum dotsthrough submicronapertures
1 mm
0.2-25μm
1.25 1.30 1.35 1.40
norm
. Int
ensi
tät
Energie [eV]
100nm
200nm
300nm
ref
single dot spectroscopy
MBE growth
2/3 h, ±
2/1 h, ±
2/1 e, ±21
=J
23
=J
From band picture to total energy representation
|3/2> |-3/2>
2/1 2/1−
Single particle statesParticle interactions cannot be represented
Energy eigenstates
•The 2 spin states form the qubit•Need spin rotations
Spin control (qubit rotation)• Time-dependent magnetic field (pulsed)
– Direct coupling of spin states
• Time-dependent electric field (gate voltage)– Indirect coupling by
• Spin-orbit coupling• Local change of g factor
• Optical (pulsed laser)– Use of optically excited states
outside qubit subspace
B(t)
Ω1(t)Ω2(t)
Spin rotationsSome basics• A spin 1/2 (and any quantum two level system) can be
represented as a 3-vector
• ANY unitary operation U can be mapped to a rotation of this vector in 3-space
• Bloch sphere & Bloch vector
• Difference between design of an operationand population transfer: we want to construct U (independence of initial state)
• To construct an arbitrary rotation, rotations about two orthogonal axes suffice
• Rotation operator is R=exp(-isnφ)=
• A rotation about the quantization axis looks like a phase
See Quantum Mechanics textbooks such as e.g. Shankar or Sakurai
1
0
θ
φ
Ψ
12
sin02
cos θθ φie+=Ψ
Exercise: show that any unitary operation in a 2x2 Hilbert space is a (pseudo)spin rotation
⎥⎦
⎤⎢⎣
⎡ −
2/
2/
00φ
φ
i
i
ee
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
++−
−−−
2sin
2cos
2sin
2sin
2sin
2sin
2sin
2cos
φφφφ
φφφφ
zyx
yxz
innin
ninin
Spin rotations with direct coupling between the two spin states: the Rabi solution
Everything henceforth applies to an arbitrary two‐level system
•Spin states talk to each other through external time dependent magnetic field
•Estimate of field duration: splitting is ωe=g B, with typical QD g-factors |g|=0.2-0.5; we~0.1 meV t~ℏ/ωe ~ 1 ns (useful to remember that ℏ~0.65 meV*ps)
•The Hamiltonian is⎥⎦
⎤⎢⎣
⎡⋅
⋅−2/)()(2/
e
e
tBtB
ωμμω
•We take the field to be product of an envelope function and an oscillation:B(t) = Bo(t)*cos(ωe*t)•cos(ωe*t) = ½ (eiωet + e-iωet): keep only the energy preserving terms: rotating wave approximation (RWA)
After the RWA the Hamiltonian becomes
⎥⎦
⎤⎢⎣
⎡
ΩΩ−
=− 2/)(
)(2/
eti
tie
etet
Hω
ωω
ω
Where we defined ½ μBo(t)=Ω(t)
We can eliminate the oscillating exponents by a transformationU=diageiωt/2, e-iωt/2; this is equivalent to going to the rotating frame of the laser
We then get a new Hamiltonian ⎥⎦
⎤⎢⎣
⎡Δ−Ω
ΩΔ=
2/)( )(2/
tt
Hωω +−=Δ eWhere is the detuning
Now we want to solve the differential equations:
ttittH
∂Ψ∂
=Ψ)()()(
Show how we get this
⎥⎦
⎤⎢⎣
⎡Δ−Ω
ΩΔ=
2/)( )(2/
)(t
ttH t
tittH∂
Ψ∂=Ψ
)()()(
In general, we cannot solve analytically the time-dep Schroedinger eqn, even for the two-level system.
There are some known solutions for specific pulse shapes Ω(t) the simplest being the square pulse (Rabi, Phys Rev 1937)
The Hamiltonian reduces to a time-independent one
Which we know how to solve.
Exercise: use a change of basis to diagonalize this Hamiltonian, find Ud in that basis and transform back to get the evolution operator in the lab frame
t
Ω(t)Ωο
⎥⎦
⎤⎢⎣
⎡Δ−ΩΩΔ
=2/
2/
0
0H
Back in the laser frame the evolution operator is
⎥⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢⎢
⎣
⎡
⎟⎟⎠
⎞⎜⎜⎝
⎛ Ω
ΩΔ
+⎟⎟⎠
⎞⎜⎜⎝
⎛ Ω⎟⎟⎠
⎞⎜⎜⎝
⎛ Ω
ΩΩ
−
⎟⎟⎠
⎞⎜⎜⎝
⎛ Ω
ΩΩ
−⎟⎟⎠
⎞⎜⎜⎝
⎛ Ω
ΩΔ
−⎟⎟⎠
⎞⎜⎜⎝
⎛ Ω
=
2sin
2cos
2sin2
2sin2
2sin
2cos
0
0
τττ
τττ
eff
eff
effeff
eff
eff
eff
eff
eff
eff
ii
iiU
Since any unitary operator in a 2x2 Hilbert space can be mapped on a rotation, we should be able to do this for U.A rotation is parameterized by 3 real numbers
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
++−
−−−== ⋅−
2sin
2cos
2sin
2sin
2sin
2sin
2sin
2cos
2/
φφφφ
φφφφσφ
zyx
yxzni
innin
ninineR
rr
By equating U=R we find eff
zeff
x nnΩΔ
=ΩΩ
=Δ+Ω= 2 τ4 0220φ
where22
04 Δ+Ω=Ωeff
Find the evolution operator and identify the rotation in the lab frame
To summarize so far:Time-dependent magnetic field spin control
– Conceptually straightforward• Time of pulse determines rot. angle
– Rotations in ~ns timescales (compare to ms coherence time)
– No spectral+spatial focusing
– Time dependent magnetic field is accompanied by time-depelectric field (which is stronger, too): effects on orbital part of state can mess things up
– Experimental demonstration by the Delft groupKoppens et al, Nature 442, 766 (2006)
B(t)
Spin control (qubit rotation)• Time-dependent magnetic field (pulsed)
– Direct coupling of spin states
• Time-dependent electric field (gate voltage)– Indirect coupling by
• Spin-orbit coupling• Local change of g factor
• Optical (pulsed laser)– Use of optically excited states
outside qubit subspace
B(t)
Ω1(t)Ω2(t)
A different way of inducing spin rotations
• Optics more focused
• Faster
• But: design not straightforward
The population exits the spin subspace during the operation
– We have to design the pulses such that it returns to the qubit subspace
The problem• Objective: given the 4 states and selection rules
implement the desired rotations– Find laser fields that will yield desired
evol. operator U(tf) from H(t) U=i dU/dt
• During evolution, trion states (decaying) are populated We need:– Population to return to qubit subspace– Operation to be fastWe’d like to find ways to reduce a 3‐ or 4‐level system to an effective 2‐level system (that we have ways to solve)
σ-σ+
|3/2> |-3/2>
|1/2> |-1/2>
B = 0
No Raman transitions |1/2> |-1/2> possible Cannot implement arbitrary spin rotation
Perpendicular field mixes spinstates & enables Raman transitions
Selection rulesz basis x basis
|1/2>-|-1/2> ≡|-x>
σ-σ+
|3/2> |-3/2>
σ-σ+
Bx ≠ 0
|1/2>+-1/2> ≡|+x>
An alternative way to represent the levels+transitions
•Selection rules in z basis are simple•B field in x mixes ground states
T T
z z
⎟⎟⎟
⎠
⎞
⎜⎜⎜
⎝
⎛
ΩΩ=
T
e
e
ttH
εω
ω
)(0)(0
00
*
Fixing the polarization to σ+ we have in the z basis |-z>,|z>,|T>:
There are 2 timescales in the problem:
•The pulse duration τ•The precession period 1/ωe
When τ >> 1/ωe we can ignore the spin precession during the pulse 3-level systemreduces to 2-level system •All 2 level systems are equivalent
We can use the previous solution but now we need to return the population back to the spin stateIn the |z>, |T> subspace we have:
eeρ
0.2
0.4
0.6
0.8
1
∫Ω= dttA R )(2
2π rotation: returns to -|g> (not |g>)
z
T
Notice what happens for Δ=0 andπτ 2Ω =
The population returns to the ground stateAND the state picks up a minus sign!•In a 2-level system: global phase•In a 3-level system: relativeRotation about z axis by π!
⎥⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢⎢
⎣
⎡
⎟⎟⎠
⎞⎜⎜⎝
⎛⎟⎟⎠
⎞⎜⎜⎝
⎛Ω
ΩΔ
+⎟⎟⎠
⎞⎜⎜⎝
⎛Ω⎟⎟⎠
⎞⎜⎜⎝
⎛Ω
ΩΩ
−
⎟⎟⎠
⎞⎜⎜⎝
⎛Ω
ΩΩ
−⎟⎟⎠
⎞⎜⎜⎝
⎛⎟⎟⎠
⎞⎜⎜⎝
⎛Ω
ΩΔ
−⎟⎟⎠
⎞⎜⎜⎝
⎛Ω
=Δ
−−Δ
−−
ΔΔ
2sin
2cos
2sin2
2sin2
2sin
2cos
202
022
τττ
τττ
ττε
ττε
ττ
eff
eff
effiieff
eff
ii
eff
eff
ieff
eff
effi
ieeeie
ieieU
TT
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
−−=
100010001
U
We still need: •Rotations by other angles•Rotations about (at least) one more axisNow we consider Δ≠0 but still
πτ 2Ω =eff
We’re only intersted in 2x2 subspace
⎥⎦
⎤⎢⎣
⎡≈⎥
⎦
⎤⎢⎣
⎡=
−
2/
2/
2x2 00
001
φ
φ
φ i
i
i ee
eU where
2τπφ Δ
+=
Issues with the square pulse:•unphysical shape•especially for short pulses, not a good approximation to actual short pulse envelope⎥
⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
−
≡⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
−
−=Δ
−−Δ
−−
Δ
22
2
00
00001
00
00
001
ττε
φ
ττε
τ
ii
i
ii
i
ee
e
ee
eUTT
|1/2>-|-1/2> ≡|-x>
σ-σ+
|3/2> |-3/2>
σ-σ+
Bx ≠ 0
|1/2>+-1/2> ≡|+x>
The one ‘special’ direction was z; we already made use of thatThe other one is the B-field axis (x)
Obvious way to generalize to the x axis: focus to one transition only by narrow-width (i.e. long) pulses
Not practical for ωe small (we shall revisit it later for large values of ωe)
There is a way to do this without using (too) long pulses
But first some background...
Some more tools in our toolbox•Analytically solvable pulses: the sech pulse•Approximate methods•Coherent population trapping
Vge=Ω(t) eiΔt =Ω0sech(βt) eiΔt|e>
|g>
t−1β
Ω(t)
History of hyperbolic secant pulse
1932: Rosen & Zener find the solution first in the context of a spin ½ in a magnetic field
1969: McCall & Hahn discover Self Induced Transparency
1975: Allen & Eberly find a solution even when there is frequency modulation
1980s: More general (but complicated) solutions of which the sech is a special case
|e>
|g>
sech pulses in two level systemsRosen & Zener Phys. Rev. ’32
( )
( )[ ]
equation tricHypergeome
1tanh21
variableChangedetuning theis and ,sech
0)/( 2
+=
ΔΩ==+−Δ+
t
tfcfcffic eee
βζ
β&&&&
Schrödinger equation
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
−++−+−
++−+−−=
),,,(),1,,(
),1,,(),,,(
***
*
*
* ζζζ
ζζζ
caaFccacaFcia
ccacaFciacaaF
Uc
c
Then the evolution operator is
Phase induced by sech pulse in 2-level system
|e>
|g>
When Ω = β (notice that we get a 2π pulse independent of detuning) population returns to |g> with an acquired phase:
Economou et al. PRB 74 (2006)
State: |g> eiφ |g> ≡ |g> (Global for 2-level system)In the presence of third level |g’>, it alters the quantum state:
eiφ |g>+|g’>≠ |g>+|g’>
⎟⎠⎞
⎜⎝⎛
Δ=
βφ arctan2
Fast pulse (β >> ωe, ωh)Spin precession ~ ‘frozen’ during pulse2-level system + 2 uncoupled levels
⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢
⎣
⎡
≈−
⇓↑↓⇑↑↓↑↓
10000000000001
φ
φ
i
i
ee
U
⎥⎦
⎤⎢⎣
⎡=⎥
⎦
⎤⎢⎣
⎡≈
−
2/
2/2/
00
001
φ
φφ
φ i
ii
ispin ee
ee
U
z rotations with sech pulse
σ+
|3/2> |-3/2>
|1/2>
z basis
B
|-1/2>
Experimental relevance
No need to change power only detuning
Short pulses can be well approximated by a sechenvelope
⎟⎠⎞
⎜⎝⎛
Δ=
βφ arctan2
z basis
T T
z z zzx −=
zzx +=
TTTx +=TT
Tx
−
=T T
zzx −=
zzx +=
x basis mixed basis
Rotations about other axes: useful to think in various bases
Use of linearly polarized light decouples the 4-level system to two 2-level systems:
•Use a single 2π sech pulse: couples to both transitions•We take advantage of property of sech that 2π pulse is detuning independent to avoid long pulses •A different phase is induced to each of the ground states |x>, |-x>:
⎥⎦
⎤⎢⎣
⎡=⎥
⎦
⎤⎢⎣
⎡≈
−
−−+−
−
−
2/)(
2/)(2/)(
21
2121
2
1
00
00
φφ
φφφφ
φ
φ
i
ii
i
i
spin ee
ee
eU
221
2 )(arctan2x
xx β
βφφφ+ΔΔ
Δ−Δ=≡− 1
21
Economou & Reinecke, Phys. Rev. Lett. 99, 217401 (2007)
H
|3/2> - |-3/2>
|+x>
|3/2> + |-3/2>
H
|-x>
x rotations
General rotations: combine rotations about z and x to make arbitrary rotations
Composite rotations
• Rn(φ)=Rz(θ)Rx(φ)Rz(-θ) where θ=angle (n,z)
Example: π rotation about y axis
z ro
tati
on
x r
ota
tion
z ro
tati
on
Experimental setup (theorist’s view)
Pump‐probe experiment
Sampleτ1
τ2
Detector
Differential transmission as fn of delay time τ2 − τ1
σ+
+z
Bloch sphere diff
. tra
nsm
issi
on
Dutt et al., PRL 2005
Experimental results
Greilich, Economou, Spatzek, Yakovlev, Reuter, Wieck, Reinecke, Bayer,
Nature Physics 5, 262 (2009)
⎟⎠⎞
⎜⎝⎛
Δ=
σφ arctan2
z
control pulse Spin rotations also demonstrated optically by Yamamoto’s group (Nature 2008) and the Steel group (PRL 2010)
Brief summary of yesterday’s lecture
• Spin rotations by – time dependent B field– Optical control
• Analytically solvable pulses: square pulse (Rabi), sechpulse (Rozen & Zener)
• Obtaining effective 2‐level system by fast pulse approximation
• Resonant 2π pulse induces a minus sign to state
Other ways to get an effective 2‐level system
• Adiabatic elimination or Optical Stark Effect
• Coherent population trapping (CPT)‐based methods
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
ΔΩΩΩΩ−
=**
2/002/
βα
β
α
δδ
H
•This method is appropriate for large detunings Δ>>|δ|,|Ωa||,|Ωb|•Since the detuning is very large the trion population is assumed negligible
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡=
γβα
ψ )(t
The Hamiltonian in the rot. frame is
The state vector is
γβαγ
γβδβ
γαδα
βα
β
α
Δ+Ω+Ω=
Ω+−=
Ω+−=
∗∗)(2
)(
2)(
ti
ti
ti
&
&
&
Eqns of motion areSetting dγ/dt=0 we get
ΔΩ+Ω−= ∗∗ /)( βαγ βα
⎥⎥⎦
⎤
⎢⎢⎣
⎡
ΔΩ−ΔΩΩ−ΔΩΩ−ΔΩ−−
= ∗
∗
/||2////||2/2
2
βαβ
βαα
δδ
effH
for Δ 0 this blows up
Other ways to get an effective 2‐level system
• Adiabatic elimination or Optical Stark Effect
• Coherent population trapping (CPT)‐based methods
Define new basis |D> = Ω1 |g2> - Ω2 | g1 > and |B> = Ω1 | g1 >+ Ω2 | g2 > Take matrix element of laser between |D> and |E> :
<E|V (Ω1 |g2> - Ω2 |g1>) = Ω1<E|V|g2> - Ω2<E|V|g1> =Ω1Ω2-Ω2Ω1=0
State Ω1 |g2> − Ω2 |g1> is decoupled, even though it contains states that couple to the excited state (destructive quantum interference)
The orthogonal state |B> = Ω1 | g1 >+ Ω2 | g2 > can be thought of as the only state coupling to |E>
1g 2g
E
Ω1Ω2
B D
E
Ω
• Above scheme requires large bandwidths for z rotations
• For QDs with large Zeeman splittings such lasers may not be available and/or other levels may couple
Use narrowband pulses to select a Λ systemTotal laser field
Choosing equal detuning and same f(t) creates a coherently trapped state
Alternative spin rotation method based onCoherent population trappingfor large Zeeman splittings
|1/2>-|-1/2>≡ |-x>
σ+
|3/2> |-3/2>
σ+
|1/2>+-1/2> ≡ |+x>
where
Bright state coupling to trion is
where
Coherent population trapping in electron/trion system
For bright/dark states to be time-independent, they are related to the energy eigenstates by the transformation:
xT
B
xT
D
, xB TV
xT|xT|
B| D|
xTBV ,|
Want the total pulse acting on bright state to have area 2π :
Coherent population trapping + 2π sechpulses: analytic sln to Λ system
xT
B
xT
D
, xB TV
Then we have a rotation
Δ is the (common) detuning
Axis of rot. determined by phase and relative strength of two lasers
σ+σ+
Parameters for CdSe QDs used
Example: π/2 rotation about z axis
Other theoretical works using CPT for rotation:Kis and Renzoni, PRA 2002Chen et al, PRB 2004Experimental CPT with CW pulses: Xu et al, Nature 2008
Spin‐spin interactions for 2‐qubit gates & entanglement
• Two‐qubit logic gates are necessary for QIP
• Need for (switchable) interaction between spins
– exchange interaction based (Coulomb nature)
– cavity mediated interactions
• Growth– Coupled quantum dots (quantum dot molecules)
– QD photonic crystal samples
• Design– Switchable interactions
– Given the interactions design gates
Quantum dot ‘molecules’
SAQD
Growth:
TruncatePartial cap Repeat
Strain-inducednucleation
Spectroscopy:
⇓↑
⇑↓
⇓↑
⇑↓
Applied Bias
Photoluminescence
Example: exciton in a quantum dot molecule with hole state tunneling
Quantum dot ‘molecules’For a switchable interaction we want:•Ground state not coupled•Excited state coupled
Need the 2‐qubit gate to be compatible with the spin rotations:•Additional states to realize two‐qubit gate (i.e., we do not want the trion states used before to tunnel‐couple)
•Subband states can be used
2/3 h, ±
2/1 h, ±
2/1 e, ±21
=J
23
=J
Quantum dot ‘molecules’ for 2‐qubit gates
Find Coulomb interactions: direct interactions strongest
Exchange interactions: e‐e strongest, diagonalize first
Electron‐hole interactions are small but nonzero and they’ll prove to be very important!
3 electrons and 1 hole
3 distinct orbital electron states available: all spin configurations (no Pauli excl.)
3 spin ½ particles: total e‐spin can be 3/2, ½, ½
See also earlier proposal by Lu Sham group using RKKY type interaction, Piermarocchi et al, PRL 2002
Exchange interactions—general• Exchange interactions are between spins
• Arise from Coulomb interaction + Pauli principle
• Electric in nature, so stronger than magnetic dipole
• Example: 2 spins
• Possible spin states are singlet and 3 triplets (multiplied by the symm. + asymm. orbital states respectively for antisymmetrization)
First find energy eigenstates of 3e and 1h without EHEI
• External B field along x (in-plane direction)
• Diagonalize using 3 electronic orbital states & tensor product with hole
2Jee
Jee
Se=1/2
Se=1/2
Se=3/2
L RE
electron‐hole exchange interactions
• In our picture, electrons and holes are different quasiparticles (in conduction and valence band respectively)
• But, there is a mixing term (k dot p) that couples different bands (see effective mass approximation)
• 3rd order process:
v
c
pk⋅ pk⋅
VCoulomb
Pikus & Bir,
Electron-hole exchange• EHEI of electron i can be written using hole
pseudospin j as
• Total spin Hamiltonian of 3 e’s and hole can be written as
• Parity good quantum number Hspin is block diagonal
λλλ
λα jsrH iizyx
iexch )(
,,∑=
=
.. )(3
)()(
3
)()()(
2121
,,
321
pcjssrr
jSrrrjSHzyx
xhxespin
+−−
+
++++= ∑
=
λλλλλ
λλλ
λλλ
αα
αααωω
jSJ
He spinJi x
+=
= ,0],[ π
Diagonalizing Hspin gives the excited (trion) states
• Color corresponds to odd and even parity
• States mostly mix within Se
2Jee
Jee
Se=1/2
Se=1/2
Se=3/2
EHEI
We focus on the 4x4 subspace in the dashed box
Describe entangling gates, CNOT, CZ• For quantum information processing we need single qubit gates & 1 entangling two‐qubit gate
• The most well‐known is probably the CNOT:– If spin 1 is up, flip the other spin; if it is down, do nothing
• In the basis
• An alternative is the CZ gate
• Entangling gates between spins also useful for entangled photon generation!
↓↓↓↑↑↓↑↑ ,,,
⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢
⎣
⎡
=
1000010000010010
cnot
⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢
⎣
⎡−
=
1000010000100001
czExercise: show that the CZ gate is equivalent to the CNOT up to single qubit operations
ideal
ikikki
kkiii
ii
UUI
IIIIIf
+
∗
≠
∗
=
++= ∑∑ )(201||
101 2
Single transition isolated by EHEI• Left: without EHEI the transitions
are doubly degenerate• Cannot address only one
transition• EHEI enables a new transition
with non degenerate frequency ω
• Now CZ gate can be implementedUideal= diag-1,1,1,1
• Fidelity calculation
ω0
ω0
ω+Δω
EHEI
↑↑
↓↑↑↓ ,
↓↓
ω
~90%
What about experimental demonstration?
• Recent progress by NRL experimental group– Kim, Carter, Greilich, Bracker and Gammon, arxiv 2010
• Control of entanglement demonstrated
• Coupling is in the ground state, not excited state
Energy Level Diagram
Applied Bias (V)
2e
Applied Bias (V)
2e
X2-
ST
ST
T
X
S
kineticexchange
Δee
S S
S S
T
Short vs. Long optical pulses
Short pulses Long pulses
Time domain: • Acts on single spin state because faster than exchange interaction
Time domain: • Acts on joint spin state because slower than exchange interaction
Frequency domain: Frequency domain:
T
X
S
T
X
S
Acts on S + T Acts just on S
– Cavity‐waveguide systems with a quantum dot coupled with the cavity at each node
– Need switchable interactions
– Need coherent control design such
that only the target qubit(s) is (are) affected
– Measurement schemes
Current/future work: Multiqubit and long range control: Cavity/waveguide systems
initialize measure
cavity 2cavity 1Entanglement
initialize measure
Photonic coupling
Other related uses of optically active quantum dots
• In many quantum information processing tasks, single and entangle photon sources are needed
• QDs are good single photon emitters (large dipole moments, simple level structure, polarization memory even at B=0)
• Entangled photon emitters– Biexciton
– Periodically pulsed spins
Coupled quantum dots as emitters of entangled photons
Main idea:Entangled emitters emit entangled photons
Measurement‐based or one‐way quantum computing: an alternative way of doing quantum computationRaussendorf & Briegel, Phys. Rev. Lett. 86, 5188 (2001)
• Creation of a highly entangled multi‐qubit state upfront
• Only single qubit measurements needed
• State collapses
• Measurement determines the ‘answer’
• Envisioned as ‘the way to go’ for photonic QC
• Problem: creation of the ‘cluster state’
Single QD based linear cluster state• Broken symmetry of quantum dot along growth axis gives rise to unusual selection rules
• Spontaneous emission rate γvery fast
• Can consider 2 indep. 2‐level systems when ωZeeman<γ
σ+
Bσ-
23,
23
23,
23
−
21,
21
21,
21
−e spin states
Trion states
Spontaneous emission:
23,
23
21,
21
23,
23
−21,
21
−−⎟⎟
⎠
⎞⎜⎜⎝
⎛−−++−++⎟⎟
⎠
⎞⎜⎜⎝
⎛−−++
⎯⎯ →⎯−⎟⎟⎠
⎞⎜⎜⎝
⎛−+−++⎟⎟
⎠
⎞⎜⎜⎝
⎛−+
⎯⎯ →⎯−⎟⎟⎠
⎞⎜⎜⎝
⎛−+−++⎟⎟
⎠
⎞⎜⎜⎝
⎛−+
⎯⎯⎯ →⎯−−++→−+
σσσσσσ
σσ
σσ
σσ
21
21
21
21
23
23
23
23
21
21
21
21
21
21
23
23
decay
excite
precession
Lindner+Rudolph PRL 2009
entangled photon states:
cluster state generation
lines denote entanglement
1st set of photons
time
QD 1
QD 2...
2nd set of
photons
Protocol:• Initialize spins
• Precession under B field
• Apply CZ gate
• Excite each spin to trion with linearly polarized light
• Fast decay+photon emission
• Precession
• …Economou, Lindner, RudolphPRL 2010
Outlook• Multiqubit systems
– Design cavity‐waveguide configurations– Pulse design in many‐qubit context
• Nuclei– Understand physics of simultaneous hyperfine interaction and pulses at the quantum level & compare results from different experiments
– Possible use for quantum memory?
• QDs as emitters of entangled photon states– Design of many particle state production
Summary• Single qubit rotations can be done directly by time‐dep B fields, or by lasers through auxiliary optically excited states
• Square pulse and sech pulse 2 of the few analytically solvable pulses of Schroedingereqn. for 2 level syst.
• Resonant 2π rotation induces a minus sign to a two‐level system
• Exchange interactions (Pauli+Coulomb) and cavity mediated interactions can be used to couple spins in different dots