Quantum Harmonic Oscillator 2006 Quantum Mechanics Prof. Y. F. Chen Quantum Harmonic Oscillator
1D S.H.O.:linear restoring force , k is the force constant
& parabolic potential
.
harmonic potential’s minimum at = a point of stability in a system
2006 Quantum Mechanics Prof. Y. F. Chen
Quantum Harmonic OscillatorQuantum Harmonic Oscillator
xkxF −=)(
2/)( 2xkxV =
A particle oscillating in a harmonic potential
0=x
Ex:the positions of atoms that form a crystal are stabilized by the
presence of a potential that has a local min at the location of each atom
→
∵ the atom position is stabilized by the potential, a local min results in
the first derivative of the series expansion = 0
∴
→ a local min in V(x) is only approximated by the quadratic function of a
H.O.
2006 Quantum Mechanics Prof. Y. F. Chen
Quantum Harmonic OscillatorQuantum Harmonic Oscillator
∑∞
= −
−=0
)()(!
1)(n
no
xxn
n
xxdx
xVdn
xVo
L+−+−+=−−
22
2
)()(21)()()()( o
xxo
xxo xx
dxxVdxx
dxxdVxVxV
oo
L+−+=−
22
2
)()(21)()( o
xxo xx
dxxVdxVxV
o
for the H.O. potential , the time-indep Schrödinger
wave eq.:
use(1) & (2)
→
making the substitution
→
called Hermite functions.
2006 Quantum Mechanics Prof. Y. F. Chen
Schrödinger Wave Eq. for 1D Harmonic OscillatorQuantum Harmonic Oscillator
2/)( 22 xmxV ω=
)()(21
222
2
22
xExxmxd
dm nnn ψψω =⎥
⎦
⎤⎢⎣
⎡+−
h
xmh
ωξ =ω
εh
nn
E2=
( ) 0)(~)(~2
2
2
=−+ ξψξεξξψ
nnn
dd
)()(~ 2/2
ξξψ ξnn He−=
( ) 0)(1)(2)(2
2
=−+− ξεξξξ
ξξ
nnnn H
ddH
dHd
One important class of orthogonal polynomials encountered in QM &
laser physics is the Hermite polynomials, which can be defined by the
formula
the first few Hermite polynomials are:
in general:
.
2006 Quantum Mechanics Prof. Y. F. Chen
Hermite FunctionsQuantum Harmonic Oscillator
L,2,1,0,)1()(2
2
=−=−
ndedeH n
nn
n ξξ
ξξ
ξξξξξξξξ 128)(,24)(,2)(,1)( 33
2210 −=−=== HHHH
knn
n
k
n knknH 2
]2/[
0
)2()!2(!
!)1()( −
=∑ −
−= ξξ
the Hermite polynomials come from the generating function:
.
→ Taylor series:
.
→
substituting into :
→ recurrence relation:
2006 Quantum Mechanics Prof. Y. F. Chen
Hermite FunctionsQuantum Harmonic Oscillator
∞<== ∑∞
=
+− tntHetg
n
nn
tt ,!
)(),(0
22
ξξ ξ
∞<∂∂
== ∑∞
= =
+− ttg
ntetg
n tn
nntt ,
!),(
0 0
22 ξξ
)()1(2
222
0
)(
0
ξξ
ξξξn
un
unn
t
tn
n
tn
n
Hudedee
te
tg
≡−=∂∂
=∂∂
=
−
=
−−
=
2 2
0
( , ) ( )!
nt t
nn
tg t e Hn
ξξ ξ∞
− +
=
= =∑ gttg )22( −=
∂∂ ξ
L,2,1,)(2)(2)( 11 =−= −+ nHnHH nnn ξξξξ
substituting into :
→ recurrence relation:
with &
→ 2nd-order ordinary differential equation for
eigenvalues of the 1D quantum H.O.:
2006 Quantum Mechanics Prof. Y. F. Chen
Hermite FunctionsQuantum Harmonic Oscillator
2 2
0
( , ) ( )!
nt t
nn
tg t e Hn
ξξ ξ∞
− +
=
= =∑ gtxg 2=
∂∂
1
00 !)(2
!)( +
∞
=
∞
=∑∑ =
′ n
n
nn
n
n tn
Htn
H ξξ
L,2,1,)(2)(1 == − nHn
ddH
nn ξξξ
1 1( ) 2 ( ) 2 ( )n n nH H n Hξ ξ ξ ξ+ −= − 1( ) 2 ( )n
ndH n H
dξ ξξ −=
)(ξnH
0)(2)(2)(2
2
=+− ξξξ
ξξξ
nnn Hn
ddH
dHd
ωε h⎟⎠⎞⎜
⎝⎛ +=⇒+=
2112 nEn nn
the eigenfunctions of 1D H.O.:
with the help of , find normalization
constant , →
(i) in CM, the oscillator is forbidden to go beyond the potential, beyond
the turning points where its kinetic energy turns negative.
(ii) the quantum wave functions extend beyond the potential, and thus
there is a finite probability for the oscillator to be found in a classically
forbidden region
2006 Quantum Mechanics Prof. Y. F. Chen
Stationary States of 1D Harmonic OscillatorQuantum Harmonic Oscillator
)()(~ 2/2
ξξψ ξnnn HeC −=
[ ] πξξξ ⋅=∫∞
∞−
− !2)( 22
ndHe nn
nC
[ ] πξξξ ⋅=∫∞
∞−
− !2)( 22
ndHe nn
n=0 n=1
n=2 n=3
n=4 n=5
ξξ
( )ξψ n
n=0 n=1
n=2 n=3
n=4 n=5
ξξ
n=0 n=1
n=2 n=3
n=4 n=5
n=0 n=1
n=2 n=3
n=4 n=5
ξξ
( )ξψ n
2006 Quantum Mechanics Prof. Y. F. Chen
Stationary States of 1D Harmonic OscillatorQuantum Harmonic Oscillator
the classical probability of finding the particle inside a region :
.
the velocity can be expressed as a function of :
→
2006 Quantum Mechanics Prof. Y. F. Chen
Stationary States of 1D Harmonic OscillatorQuantum Harmonic Oscillator
ξΔ
2 / ( )( )2 /cl
t vPT
ξ ξξ ξπ ω
Δ ΔΔ = =
( ) sin ( )v A tξ ω ω= ξ
( )22)( ξωξ −= Av
( )2 2
1 1( )clPA
ξ ξ ξπ ξ
Δ = Δ−
(i) the difference between the two probabilities for n=0 is extremely
striking ∵there is no zero-point energy in CM
(ii) the quantum and classical probability distributions coincide when the
quantum number n becomes large
(iii) this is an evidence of Bohr’s correspondence principle
2006 Quantum Mechanics Prof. Y. F. Chen
Stationary States of 1D Harmonic OscillatorQuantum Harmonic Oscillator
n=0 n=30n=30
(1) classically, the motion of the H.O. is in such a manner that the
position of the particle changes from one moment to another.
(2) however, although there is a probability distribution for any
eigenstate in QM, this distribution is indep of time → stationary states
(3) even so, the Ehrenfest theorem reveals that a coherent
superposition of a number of eigenstates, i.e., so-called “wave packet
state”, will lead to the classical behavior
2006 Quantum Mechanics Prof. Y. F. Chen
Stationary States of 1D Harmonic OscillatorQuantum Harmonic Oscillator
show :
using the generation function , we can have
∵ the orthogonality property, the integration leads to
→
as a consequence, we can obtain
2006 Quantum Mechanics Prof. Y. F. Chen
Stationary States of 1D Harmonic OscillatorQuantum Harmonic Oscillator
[ ] πξξξ ⋅=∫∞
∞−
− !2)( 22
ndHe nn
∑∑∞
=
∞
=
−+−+−− =⋅⋅0 0
22
!!)()(
2222
m
mn
nmn
sstt
mnstHHeeee ξξξξξξ
[ ]∑ ∫∫∞
=
∞
∞−
−∞
∞−
−−− =⋅=⋅0
222)( )(!!
22
nn
nnststts dHe
nnstedee ξξπξ ξξ
( ) [ ]∑ ∫∑∞
=
∞
∞−
−∞
=
=⋅0
2
0
)(!!!
2 2
nn
nn
n
n
dHennst
nts ξξπ ξ
[ ] πξξξ ⋅=∫∞
∞−
− !2)( 22
ndHe nn
given a mean rate of occurrence r of the events in the relevant interval,
the Poisson distribution gives the probability that exactly n
events will occur
for a small time interval the probability of receiving a call is .
the probability of receiving no call during the same tiny interval is
given by . the probability of receiving exactly n calls in the total
interval is given by
2006 Quantum Mechanics Prof. Y. F. Chen
The Poisson DistributionQuantum Harmonic Oscillator
)( nXP =
tΔ trΔ
tΔ
trΔ−1
tt Δ+
( ) trtPtrtPttP nnn Δ+Δ−=Δ+ − )(1)()( 1
rearranging , dividing through by ,
and letting , the differential recurrence eq. can be found and
written as
for :
which can be integrated to lead to
with the fact that the probability of receiving no calls in a zero time
interval must be equal to unity:
2006 Quantum Mechanics Prof. Y. F. Chen
The Poisson DistributionQuantum Harmonic Oscillator
( ) trtPtrtPttP nnn Δ+Δ−=Δ+ − )(1)()( 1 tΔ
0→Δ t
)()()(
1 tPrtPrtdtdP
nnn −= −
0=n )()(
00 tPrtdtdP
−=
trePtP −= )0()( 00
)0(0P
tretP −=)(0
substituting into for :
, repeating this process, can be found to be
the sum of the probabilities is unity:
the mean of the Poisson distribution:
2006 Quantum Mechanics Prof. Y. F. Chen
The Poisson DistributionQuantum Harmonic Oscillator
tretP −=)(0 )()()(1 tPrtPr
tdtdP
nnn −= − 1=n
tretrtP −= )()(1 )(tPn
( )( )!
nr t
nr tP t en
−=
1!)(
!)()(
000=⋅=== −
∞
=
−∞
=
−∞
=∑∑∑ trtr
n
ntr
n
trn
nn ee
ntree
ntrtP
rtn
rttreentrntnPn
n
ntr
n
trn
nn =
−==>=< ∑∑∑
∞
=
−−
∞
=
−∞
= 1
1
00 !)1()()(
!)()(
in other words, the Poisson distribution with a mean of is given by:
2006 Quantum Mechanics Prof. Y. F. Chen
The Poisson DistributionQuantum Harmonic Oscillator
λλλ −= en
Pn
n !)(
The Schrödinger coherent wave packet state can be generalized as
with
it can be found that the norm square of the coefficient is exactly
the same as the Poisson distribution with the mean of
2006 Quantum Mechanics Prof. Y. F. Chen
Schrödinger Coherent States of the 1D H.O.Quantum Harmonic Oscillator
∑∞
=
−=Ψ
0)(~),(
n
tEi
nn
n
ect hξψξ
2/2
!)( α
φα −= en
ecni
n
2|| nc
2α
substituting & into
using
2006 Quantum Mechanics Prof. Y. F. Chen
Schrödinger Coherent States of the 1D H.O.Quantum Harmonic Oscillator
12nE n ω⎛ ⎞= +⎜ ⎟
⎝ ⎠h ( ) )(!2)(~ 2/2/1 2
ξπξψ ξn
nn Hen −−
⋅=
0( , ) ( ) :
nEi t
n nn
t c eξ ψ ξ∞ −
=
Ψ =∑ h%
2 2
2 2
/ 2 / 2 ( 1/ 2)
0
( )( ) / 2 / 2
1/ 40
( ) 1( , ) ( )! 2 !
/ 21 ( )!
i ni n t
nnn
ni ti t
nn
et e H e en n
ee e H
n
φα ξ ω
ω φα ξ ω
αξ ξπ
αξ
π
∞− − − +
=
− −∞
− + −
=
Ψ =
⎡ ⎤⎣ ⎦=
∑
∑
2 2
0( , ) ( ) :
!
nt t
nn
tg t e Hn
ξξ ξ∞
− +
=
= =∑
{ }{ }
2 2
2 2
2( ) / 2 / 2 ( ) ( )1/ 4
( ) / 2 / 2 2 2( ) ( )1/ 4
1( , ) exp / 2 2
1 exp / 2 2
i t i t i t
i t i t i t
t e e e e
e e e e
α ξ ω ω φ ω φ
α ξ ω ω φ ω φ
ξ α α ξπ
α α ξπ
− + − − − − −
− + − − − − −
⎡ ⎤Ψ = − +⎣ ⎦
= − +
as a result, the probability distribution of the coherent state is given
by:
it can be clearly seen that the center of the wave packet moves in the
path of the classical motion
2006 Quantum Mechanics Prof. Y. F. Chen
Schrödinger Coherent States of the 1D H.O.Quantum Harmonic Oscillator
{ }
{ }
{ }
2 2( ) 2
2 2 2
2
1( , ) ( , ) ( , ) exp cos[2( )] 2 2 cos( )
1 exp 2 cos ( ) 2 2 cos( )
1 exp [ 2 cos( )]
P t t t e t t
t t
t
α ξξ ξ ξ α ω φ αξ ω φπ
ξ α ω φ α ξ ω φπ
ξ α ω φπ
∗ − += Ψ Ψ = − − + −
= − − − + −
= − − −
)cos(2 φωαξ −= t
with , &
the operator acting on the eigenstate
2006 Quantum Mechanics Prof. Y. F. Chen
Creation & Annihilation OperatorsQuantum Harmonic Oscillator
xmh
ωξ = 1 1( ) 2 ( ) 2 ( )n n nH H nHξ ξ ξ ξ+ −= − ( ) 21/ 2/ 2( ) 2 ! ( )n
n nn e Hξψ ξ π ξ−
−=%
x )(~ ξψ n
( )
( )
( )
[ ])(~)(~12
1
)()(21!2
)(!2
)(!2)(~ˆ
11
112/2/1
2/2/1
2/2/1
2
2
2
ξψξψω
ξξπω
ξξπω
ξπξω
ξψ
ξ
ξ
ξ
−+
−+−−
−−
−−
++⎟⎟⎠
⎞⎜⎜⎝
⎛=
⎥⎦⎤
⎢⎣⎡ +⋅⎟⎟
⎠
⎞⎜⎜⎝
⎛=
⋅⎟⎟⎠
⎞⎜⎜⎝
⎛=
⋅⎟⎟⎠
⎞⎜⎜⎝
⎛=
nn
nnn
nn
nn
n
nnm
HnHenm
Henm
Henm
x
h
h
h
h
in a similar way, the operator acting on the eigenstate
2006 Quantum Mechanics Prof. Y. F. Chen
Creation & Annihilation OperatorsQuantum Harmonic Oscillator
)(~ ξψ nxp
( )
( )
( ) ( ) [ ]
( ) ( )
( ) [ ])(~)(~12
1
)()(21!2
)()()(!2
)(!2
)(!2)(~ˆ
11
112/2/1
2/2/2/1
2/2/1
2/2/1
2
22
2
2
ξψξψω
ξξπω
ξξξπω
ξπξ
ω
ξπξψ
ξ
ξξ
ξ
ξ
−+
−+−−
−−−
−−
−−
−+=
⎥⎦⎤
⎢⎣⎡ −⋅=
′+−⋅−=
⋅⎟⎟⎠
⎞⎜⎜⎝
⎛
∂∂
−=
⋅⎟⎟⎠
⎞⎜⎜⎝
⎛
∂∂
−=
nn
nnn
nnn
nn
nn
nx
nnmi
HnHenmi
HeHenmi
Henmi
Henx
ip
h
h
h
h
h
→
&
consequently, it is convenient to define 2 new operators:
&
2006 Quantum Mechanics Prof. Y. F. Chen
Creation & Annihilation OperatorsQuantum Harmonic Oscillator
)(~1)(~ˆ1ˆ2
11 ξψξψ
ωω
++=⎟⎟⎠
⎞⎜⎜⎝
⎛− nnx np
mixm
hh
)(~)(~ˆ1ˆ2
11 ξψξψ
ωω
−=⎟⎟⎠
⎞⎜⎜⎝
⎛+ nnx np
mixm
hh
⎟⎟⎠
⎞⎜⎜⎝
⎛−= xp
mixma ˆ1ˆ
21ˆ†
hh ωω
⎟⎟⎠
⎞⎜⎜⎝
⎛+= xp
mixma ˆ1ˆ
21ˆ
hh ωω
the operator is the increasing (creation) operator:
this means that operating with on the n-th stationary states yields a
state, which is proportional to the higher (n +1)-th state
the operator is the lowering (annihilation) operator:
this means that operating with on the n-th stationary states yields a
state, which is proportional to the higher (n -1)-th state
2006 Quantum Mechanics Prof. Y. F. Chen
Creation & Annihilation OperatorsQuantum Harmonic Oscillator
†a
)(~1)(~ˆ 1† ξψξψ ++= nn na
†a
a
)(~)(~ˆ 1 ξψξψ −= nn na
a
in terms of & , the operators & can be expressed as:
&
we can find the commutator of these 2 ladder operators:
which is the so-called canonical commutation relation
2006 Quantum Mechanics Prof. Y. F. Chen
Creation & Annihilation OperatorsQuantum Harmonic Oscillator
a †a x xp
( )†ˆˆ2
ˆ aam
x +=ωh ( )†ˆˆ
2ˆ aamipx −−=
ωh
[ ] [ ] 1ˆ,ˆˆ,ˆ21
ˆ1ˆ,ˆ1ˆ21]ˆ,ˆ[ †
=⎟⎠⎞
⎜⎝⎛ +−
=
⎥⎦
⎤⎢⎣
⎡−+=
xpipxi
pm
ixmpm
ixmaa
xx
xx
hh
hhhh ωω
ωω
is the hermitian conjugate :
proof:
2006 Quantum Mechanics Prof. Y. F. Chen
Creation & Annihilation OperatorsQuantum Harmonic Oscillator
†a a∗
= 1†
221 |ˆ||ˆ| ψψψψ aa
1 2 1 2
1 2 1 2
2 1 2 1
2 1
1 1? �| |2
1 1? 2
1 1? 2
1 1? 2
x
x
x
x
ma x i pm
m x i pm
m x i pm
m x i pm
ωψ ψ ψ ψω
ω ψ ψ ψ ψω
ω ψ ψ ψ ψω
ωψ ψω
∗ ∗
∗
= +
⎡ ⎤= +⎢ ⎥
⎢ ⎥⎣ ⎦⎡ ⎤
= +⎢ ⎥⎢ ⎥⎣ ⎦⎡ ⎤⎢ ⎥= −⎢ ⎥⎣ ⎦
h h
h h
h h
h h
†2 1ˆ | |aψ ψ
∗=
with , &
the operator acting on the eigenstate
2006 Quantum Mechanics Prof. Y. F. Chen
Creation & Annihilation Operators
xmh
ωξ = 1 1( ) 2 ( ) 2 ( )n n nH H nHξ ξ ξ ξ+ −= − ( ) 21/ 2/ 2( ) 2 ! ( )n
n nn e Hξψ ξ π ξ−
−=%
x )(~ ξψ n
( )
( )
( )
[ ])(~)(~12
1
)()(21!2
)(!2
)(!2)(~ˆ
11
112/2/1
2/2/1
2/2/1
2
2
2
ξψξψω
ξξπω
ξξπω
ξπξω
ξψ
ξ
ξ
ξ
−+
−+−−
−−
−−
++⎟⎟⎠
⎞⎜⎜⎝
⎛=
⎥⎦⎤
⎢⎣⎡ +⋅⎟⎟
⎠
⎞⎜⎜⎝
⎛=
⋅⎟⎟⎠
⎞⎜⎜⎝
⎛=
⋅⎟⎟⎠
⎞⎜⎜⎝
⎛=
nn
nnn
nn
nn
n
nnm
HnHenm
Henm
Henm
x
h
h
h
h
Quantum Harmonic Oscillator
in a similar way, the operator acting on the eigenstate
2006 Quantum Mechanics Prof. Y. F. Chen
Creation & Annihilation Operators
)(~ ξψ nxp
( )
( )
( ) ( ) [ ]
( ) ( )
( ) [ ])(~)(~12
1
)()(21!2
)()()(!2
)(!2
)(!2)(~ˆ
11
112/2/1
2/2/2/1
2/2/1
2/2/1
2
22
2
2
ξψξψω
ξξπω
ξξξπω
ξπξ
ω
ξπξψ
ξ
ξξ
ξ
ξ
−+
−+−−
−−−
−−
−−
−+=
⎥⎦⎤
⎢⎣⎡ −⋅=
′+−⋅−=
⋅⎟⎟⎠
⎞⎜⎜⎝
⎛
∂∂
−=
⋅⎟⎟⎠
⎞⎜⎜⎝
⎛
∂∂
−=
nn
nnn
nnn
nn
nn
nx
nnmi
HnHenmi
HeHenmi
Henmi
Henx
ip
h
h
h
h
h
Quantum Harmonic Oscillator
→
&
consequently, it is convenient to define 2 new operators:
&
2006 Quantum Mechanics Prof. Y. F. Chen
Creation & Annihilation Operators
)(~1)(~ˆ1ˆ2
11 ξψξψ
ωω
++=⎟⎟⎠
⎞⎜⎜⎝
⎛− nnx np
mixm
hh
)(~)(~ˆ1ˆ2
11 ξψξψ
ωω
−=⎟⎟⎠
⎞⎜⎜⎝
⎛+ nnx np
mixm
hh
⎟⎟⎠
⎞⎜⎜⎝
⎛−= xp
mixma ˆ1ˆ
21ˆ†
hh ωω
⎟⎟⎠
⎞⎜⎜⎝
⎛+= xp
mixma ˆ1ˆ
21ˆ
hh ωω
Quantum Harmonic Oscillator
the operator is the increasing (creation) operator:
this means that operating with on the n-th stationary states yields a
state, which is proportional to the higher (n +1)-th state
the operator is the lowering (annihilation) operator:
this means that operating with on the n-th stationary states yields a
state, which is proportional to the higher (n -1)-th state
2006 Quantum Mechanics Prof. Y. F. Chen
Creation & Annihilation Operators†a
)(~1)(~ˆ 1† ξψξψ ++= nn na
†a
a
)(~)(~ˆ 1 ξψξψ −= nn na
a
Quantum Harmonic Oscillator
in terms of & , the operators & can be expressed as:
&
we can find the commutator of these 2 ladder operators:
which is the so-called canonical commutation relation
2006 Quantum Mechanics Prof. Y. F. Chen
Creation & Annihilation Operators
a †a x xp
( )†ˆˆ2
ˆ aam
x +=ωh ( )†ˆˆ
2ˆ aamipx −−=
ωh
[ ] [ ] 1ˆ,ˆˆ,ˆ21
ˆ1ˆ,ˆ1ˆ21]ˆ,ˆ[ †
=⎟⎠⎞
⎜⎝⎛ +−
=
⎥⎦
⎤⎢⎣
⎡−+=
xpipxi
pm
ixmpm
ixmaa
xx
xx
hh
hhhh ωω
ωω
Quantum Harmonic Oscillator
is the hermitian conjugate :
proof:
2006 Quantum Mechanics Prof. Y. F. Chen
Creation & Annihilation Operators†a a
∗= 1
†221 |ˆ||ˆ| ψψψψ aa
1 2 1 2
1 2 1 2
2 1 2 1
2 1
1 1? �| |2
1 1? 2
1 1? 2
1 1? 2
x
x
x
x
ma x i pm
m x i pm
m x i pm
m x i pm
ωψ ψ ψ ψω
ω ψ ψ ψ ψω
ω ψ ψ ψ ψω
ωψ ψω
∗ ∗
∗
= +
⎡ ⎤= +⎢ ⎥
⎢ ⎥⎣ ⎦⎡ ⎤
= +⎢ ⎥⎢ ⎥⎣ ⎦⎡ ⎤⎢ ⎥= −⎢ ⎥⎣ ⎦
h h
h h
h h
h h
†2 1ˆ | |aψ ψ
∗=
Quantum Harmonic Oscillator
with
&
→
using the commutation relation
→
define the so-called number operator:
→ the H.O. Hamiltonian takes the form:
2006 Quantum Mechanics Prof. Y. F. Chen
Creation & Annihilation Operators
( ) ( ) ( )††††††2
ˆˆˆˆˆˆˆˆ4
ˆˆˆˆ42
ˆaaaaaaaaaaaa
mpx −−+=−−−=
ωω hh
( )( ) ( )††††††22 ˆˆˆˆˆˆˆˆ4
ˆˆˆˆ4
ˆ21 aaaaaaaaaaaaxm +++=++=
ωωω hh
( )aaaaxmm
pH x ˆˆˆˆ
2ˆ
21
2ˆˆ ††22
2
+=+=ωω h
1ˆˆˆˆ]ˆ,ˆ[ ††† =−= aaaaaa
⎟⎠⎞
⎜⎝⎛ +=
21ˆˆˆ †aaH ωh
aaN ˆˆˆ †=
⎟⎠⎞
⎜⎝⎛ +=
21ˆˆ NH ωh
Quantum Harmonic Oscillator
the eigenstates of can be found to be coherent states :
coherent states have the minimum uncertainty
2006 Quantum Mechanics Prof. Y. F. Chen
Creation & Annihilation OperatorsQuantum Harmonic Oscillator
a );0,( αξΨ
∑∞
=
−− ==Ψ0
2/||0
ˆ2/|| )(~!
)(~);0,(2†2
nn
na
neee ξψαξψαξ ααα
†? �( ) ( ,0; ) ( ,0; ) ( ,0; ) ( ,0; )2
2 ( ) cos2
i x a am
m m
ξ α ξ α ξ α ξ αω
α α φω ω
∗
Ψ Ψ = Ψ + Ψ
= + =
h
h h
( )22 �
2 2
? �( ,0; ) ( ,0; ) ( ,0; ) ( ,0; )2
( 1)2
x a am
m
ξ α ξ α ξ α ξ αω
α α α α α αω
∗ ∗ ∗
Ψ Ψ = Ψ + Ψ
⎡ ⎤= + + + +⎣ ⎦
h
h
22 2?( ,0; ) ( ,0; ) ( ,0; ) ( ,0; )2
x x xm
ξ α ξ α ξ α ξ αω
→ Δ = Ψ Ψ − Ψ Ψ =h
as a consequence, we obtain the minimum uncertainty state:
2006 Quantum Mechanics Prof. Y. F. Chen
Creation & Annihilation OperatorsQuantum Harmonic Oscillator
†?�( ) ( ,0; ) ( ,0; ) ( ,0; ) ( ,0; )2
( ) 2 sin2
xmii p i a a
mi m
ωξ α ξ α ξ α ξ α
ω α α ω φ∗
Ψ Ψ = − Ψ − Ψ
= − − =
h
hh
( )22 �
2 2
? �( ,0; ) ( ,0; ) ( ,0; ) ( ,0; )2
( 1)2
xmp a a
m
ωξ α ξ α ξ α ξ α
ω α α α α α α∗ ∗ ∗
Ψ Ψ = − Ψ − Ψ
⎡ ⎤= − + − + −⎣ ⎦
h
h
2);0,(ˆ);0,();0,(ˆ);0,( 222 hωαξαξαξαξ mppp xxx =ΨΨ−ΨΨ=Δ
2h
=Δ⋅Δ xpx