Complex hardware systems based on quantum optics

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Complex hardware systems based on quantum optics

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Basics of Quantum Mechanics- Why Quantum Physics? -

• Classical mechanics (Newton's mechanics) and Maxwell's equations (electromagnetics theory) can explain MACROSCOPIC phenomena such as motion of billiard balls or rockets.

• Quantum mechanics is used to explain microscopic phenomena such as photon-atom scattering and flow of the electrons in a semiconductor.

• QUANTUM MECHANICS is a collection of postulates based on a huge number of experimental observations.

• The differences between the classical and quantum mechanics can be understood by examining both– The classical point of view– The quantum point of view

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Basics of Quantum Mechanics- Quantum Point of View -

• Quantum particles can act as both particles and waves WAVE-PARTICLE DUALITY

• Quantum state is a conglomeration of several possible outcomes of measurement of physical properties Quantum mechanics uses the language of PROBABILITY theory (random chance)

• An observer cannot observe a microscopic system without altering some of its properties. Neither one can predict how the state of the system will change.

• QUANTIZATION of energy is yet another property of "microscopic" particles.

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The idea of duality is rooted in a debate over the nature of light and matter dating back to the 1600s, when competing theories of light were proposed by Huygens and Newton. Christiaan Huygens

Dutch 1629-1695light consists of waves

Sir Isaac Newton1643 1727 light consists of particles

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If h is the Planck constant

Then

Louis de BROGLIEFrench (1892-1987)

Max Planck (1901)Göttingen

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- Robert Millikan (1910) showed that it was quantified.

-Rutherford (1911) showed that the negative part was diffuse while the positive part was concentrated.

Soon after theelectron discovery in 1887- J. J. Thomson (1887) Some negative part could be extracted from the atoms

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Quantum numbers

In mathematics, a natural number (also called counting number) has two main purposes: they can be used for counting ("there are 6 apples on the table"), and they can be used for ordering ("this is the 3rd largest city in the country").

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Johannes Rydberg 1888Swedish n1 → n2 name Converge

s to (nm)

1 → ∞ Lyman 91

2 → ∞ Balmer 365

3 → ∞ Pashen 821

4 → ∞ Brackett 1459

5 → ∞ Pfund 2280

6 → ∞ Humphreys

3283

Atomic Spectroscopy

Absorption or Emission

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Wavelengths and energy

• Understand that different wavelengths of electromagnetic radiation have different energies.

• c=vλ– c=velocity of wave – v=(nu) frequency of wave– λ=(lambda) wavelength

Bohr and Quantum Mechanical Model

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• Bohr also postulated that an atom would not emit radiation while it was in one of its stable states but rather only when it made a transition between states.

• The frequency of the radiation emitted would be equal to the difference in energy between those states divided by Planck's constant.

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E2-E1= hvh=6.626 x 10-34 Js = Plank’s constantE= energy of the emitted light (photon)v = frequency of the photon of light

• This results in a unique emission spectra for each element, like a fingerprint.

• electron could "jump" from one allowed energy state to another by absorbing/emitting photons of radiant energy of certain specific frequencies.

• Energy must then be absorbed in order to "jump" to another energy state, and similarly, energy must be emitted to "jump" to a lower state.

• The frequency, v, of this radiant energy corresponds exactly to the energy difference between the two states.

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• In the Bohr model, the electron is in a defined orbit

• Schrödinger model uses probability distributions for a given energy level of the electron.

Orbitals and quantum numbers

• Solving Schrödinger's equation leads to wave functions called orbitals

• They have a characteristic energy and shape (distribution).

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• The lowest energy orbital of the hydrogen atom has an energy of -2.18 x 10 18 J and the shape in the above figure. Note that in the Bohr model we had the same energy for the electron in the ground state, but that it was described as being in a defined orbit.

• The Bohr model used a single quantum number (n) to describe an orbit, the Schrödinger model uses three quantum numbers: n, l and ml to describe an orbital

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The principle quantum number 'n'

• Has integral values of 1, 2, 3, etc.

• As n increases the electron density is further away from the nucleus

• As n increases the electron has a higher energy and is less tightly bound to the nucleus

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The azimuthal or orbital (second) quantum number

'l' • Has integral values from 0 to (n-1) for

each value of n

• Instead of being listed as a numerical value, typically 'l' is referred to by a letter ('s'=0, 'p'=1, 'd'=2, 'f'=3)

• Defines the shape of the orbital

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The magnetic (third) quantum number 'ml'

Has integral values between 'l' and -'l', including 0 Describes the orientation of the orbital in space

For example, the electron orbitals with a principle quantum number of 3

n l Subshell ml Number of orbitals in subshell

3 0 3s 0 1

1 3p -1,0,+1 3

2 3d -2,1,0,+1,+2 5

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• the third electron shell (i.e. 'n'=3) consists of the 3s, 3p and 3d subshells (each with a different shape)

• The 3s subshell contains 1 orbital, the 3p subshell contains 3 orbitals and the 3d subshell contains 5 orbitals. (within each subshell, the different orbitals have different orientations in space)

• Thus, the third electron shell is comprised of nine distinctly different orbitals, although each orbital has the same energy (that associated with the third electron shell) Note: remember, this is for hydrogen only.

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Subshell Number of orbitals

s 1

p 3

d 5

f 7

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Practice:

• What are the possible values of l and ml for an electron with the principle quantum number n=4?

• If l=0, ml=0

• If l=1, ml= -1, 0, +1

• If l=2, ml= -2,-1,0,+1, +2

• If l=3, ml= -3, -2, -1, 0, +1, +2, +3

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Problem #2

• Can an electron have the quantum numbers n=2, l=2 and ml=2?

• No, because l cannot be greater than n-1, so l may only be 0 or 1.

• ml cannot be 2 either because it can never be greater than l

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In order to explain the line spectrum of hydrogen, Bohr made one more addition to his model. He assumed that the electron could "jump" from one allowed energy state to another by absorbing/emitting photons of radiant energy of certain specific frequencies. Energy must then be absorbed in order to "jump" to another energy state, and similarly, energy must be emitted to "jump" to a lower state. The frequency, v, of this radiant energy corresponds exactly to the energy difference between the two states. Therefore, if an electron "jumps" from an initial state with energy Ei to a final state of energy Ef, then the following equality will hold:

(delta) E = Ef - E i = hv

To sum it up, what Bohr's model of the hydrogen atom states is that only the specific frequencies of light that satisfy the above equation can be absorbed or emitted by the atom.

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Johannes Rydberg 1888Swedish

IR

VISIBLE

UV

Atomic Spectroscopy

Absorption or Emission

Emission

-R/12

-R/22

-R/32

-R/42

-R/52

-R/62-R/72

Quantum numbers n, levels are not equally spaced R = 13.6 eV

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Photoelectric Effect (1887-1905)discovered by Hertz in 1887 and explained in 1905 by Einstein.

Vide

I

i

e

ee

Vacuum

Heinrich HERTZ (1857-1894)

Albert EINSTEIN(1879-1955)

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I

00

T (énergie cinétique)Kinetic energy

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Compton effect 1923playing billiards assuming =h/p

p

h/ '

h/

p2/2m

h'

h

Arthur Holly ComptonAmerican 1892-1962

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Davisson and Germer 1925

Clinton DavissonLester GermerIn 1927

d

2d sin = k

Diffraction is similarly observed using a mono-energetic electron beam

Bragg law is verified assuming =h/p

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Wave-particle Equivalence.

•Compton Effect (1923)•Electron Diffraction Davisson and Germer (1925)•Young's Double Slit Experiment

In physics and chemistry, wave–particle duality is the concept that all matter and energy exhibits both wave-like and particle-like properties. A central concept of quantum mechanics, duality, addresses the inadequacy of classical concepts like "particle" and "wave" in fully describing the behavior of small-scale objects. Various interpretations of quantum mechanics attempt to explain this apparent paradox.

Wave–particle duality

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Young's Double Slit Experiment

Ecran Plaque photo

F2

F1

Source

ScreenMask with 2 slits

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Young's Double Slit ExperimentThis is a typical experiment showing the wave nature of

light and interferences.What happens when we decrease the light intensity ?If radiation = particles, individual photons reach one

spot and there will be no interferencesIf radiation particles there will be no spots on the

screenThe result is ambiguous

There are spotsThe superposition of all the impacts make interferences

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Young's Double Slit Experiment

Assuming a single electron each timeWhat means interference with itself ?

What is its trajectory?If it goes through F1, it should ignore the presence of F2

Ecran Plaque photo

F2

F1

Source


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Young's Double Slit ExperimentThere is no possibility of knowing through which split the photon went!

If we measure the crossing through F1, we have to place a screen behind.Then it does not go to the final screen.

We know that it goes through F1 but we do not know where it would go after.These two questions are not compatible

Ecran Plaque photo

F2

F1

Source


Two important differences with classical physics:

• measurement is not independent from observer

• trajectories are not defined; h goes through F1 and F2 both! or through them with equal probabilities!

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de Broglie relation from relativityPopular expressions of relativity:m0 is the mass at rest, m in motion

E like to express E(m) as E(p) with p=mv

Ei + T + Erelativistic + ….

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de Broglie relation from relativity

Application to a photon (m0=0)

To remember

To remember

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Max Planck

Useful to remember to relate energy

and wavelength

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A New mathematical tool: Wave functions and Operators

Each particle may be described by a wave function (x,y,z,t), real or complex,having a single value when position (x,y,z) and time (t) are defined.If it is not time-dependent, it is called stationary.The expression =Aei(pr-Et) does not represent one molecule but a flow of particles: a plane wave

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Wave functions describing one particle

To represent a single particle (x,y,z) that does not evolve in time, (x,y,z) must

be finite (0 at ∞). In QM, a particle is not localized but has a probability to be in a given volume:dP= * dV is the probability of finding the particle in the volume dV. Around one point in space, the density of probability is dP/dV= * has the dimension of L-1/3 Integration in the whole space should give oneis said to be normalized.

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Normalization

An eigenfunction remains an eigenfunction when multiplied by a constant

O()= o() thus it is always possible to normalize a finite function

Dirac notations <I>

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Mean value

• If 1 and 2 are associated with the same eigenvalue o: O(a1 +b2)=o(a1 +b2)

• If not O(a1 +b2)=o1(a1 )+o2(b2)

we define ō = (a2o1+b2o2)/(a2+b2)

Dirac notations

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Introducing new variables

Now it is time to give a physical meaning.

p is the momentum, E is the Energy

H=6.62 10-34 J.s

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Plane waves

This represents a (monochromatic) beam, a continuous flow of particles with the same velocity (monokinetic).

k, , , p and E are perfectly definedR (position) and t (time) are not defined.*=A2=constant everywhere; there is no

localization.If E=constant, this is a stationary state,

independent of t which is not defined.

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Niels Henrik David Bohr Danish1885-1962

Correspondence principle 1913/1920

For every physical quantity one can define an operator. The definition uses formulae from classical physics replacing quantities involved by the corresponding operators

QM is then built from classical physics in spite of demonstrating its limits

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Operators p and H

We use the expression of the plane wave which allows defining exactly p and E.

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Momentum and Energy Operators

Remember during this chapter

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Stationary state E=constant

Remember for 3 slides after

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Kinetic energy

Classical quantum operator

In 3D :

Calling the laplacian

Pierre Simon, Marquis de Laplace (1749 -1827)

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Correspondence principleangular momentum

Classical expression Quantum expression

lZ= xpy-ypx

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49

50

51

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Erwin Rudolf Josef Alexander SchrödingerAustrian 1887 –1961

Without potential E = TWith potential E = T + V

Time-dependent Schrödinger Equation

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Schrödinger Equation for stationary states

Kinetic energyTotal energy

Potential energy

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Schrödinger Equation for stationary states

H is the hamiltonian

Sir William Rowan Hamilton Irish 1805-1865

Half penny bridge in Dublin

Remember

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Chemistry is nothing but an application of Schrödinger Equation (Dirac)

Paul Adrien Dirac 1902 – 1984Dirac’s mother was British and his father was Swiss.

<I> < IOI >

Dirac notations

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Uncertainty principle

the Heisenberg uncertainty principle states that locating a particle in a small region of space makes the momentum of the particle uncertain; and conversely, that measuring the momentum of a particle precisely makes the position uncertain

We already have seen incompatible operators

Werner HeisenbergGerman1901-1976

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It is not surprising to find that quantum mechanics does not predict the position of an electron exactly. Rather, it provides only a probability as to where the electron will be found. We shall illustrate the probability aspect in terms of the system of an electron confined to motion along a line of length L. Quantum mechanical probabilities are expressed in terms of a distribution function.For a plane wave, p is defined and the position is not.With a superposition of plane waves, we introduce an uncertainty on p and we localize. Since, the sum of 2 wavefucntions is neither an eigenfunction for p nor x, we have average values.With a Gaussian function, the localization below is 1/2

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p and x do not commute and are incompatibleFor a plane wave, p is known and x is not (*=A2 everywhere)Let’s superpose two waves… this introduces a delocalization for p and may be localize x

At the origin x=0 and at t=0 we want to increase the total amplitude, so the two waves 1 and 2 are taken in phaseAt ± x/2 we want to impose them out of phaseThe position is therefore known for x ± x/2 the waves will have wavelengths

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Superposition of two waves

0 1 2 3 4 5-2

-1

0

1

2

a (radians)

4.95

enveloppe

x/2

x/(2x(√2))

Factor 1/2 a more realistic localization

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Uncertainty principle

Werner HeisenbergGerman1901-1976

A more accurate calculation localizes more(1/2 the width of a gaussian) therefore one gets

x and p or E and t play symmetric roles in the plane wave expression;Therefore, there are two main uncertainty principles

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the total energy of matter related to the frequency ν of the wave is

E=hν

the momentum of matter related to the wavelength λ of the wave is

p=h/λ

Matter wave: de Broglie

Ex: (a) the de Broglie wavelength of a baseball moving at a speed v=10m/s, and m=1kg. (b) for a electron K=100 eV.

)baseball()electron(

2.1106.1100101.92/106.6

2// (b)

106.6106.6)101/(106.6/ (a)

193134

253534

o

o

A

mKhph

Amph

De Broglie’s postulate: wavelike properties of particles

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The experiment of Davisson and Germer

(1) A strong scattered electron beam is detected at θ=50o for V=54 V.

(2) The result can be explained as a constructive interference of waves scattered by the periodic arrangement of the atoms into planes of the crystal.

(3) The phenomenon is analogous to the Bragg-reflections (Laue pattern).

1927, G. P. Thomson showed the diffraction of electron beams passing through thin films confirmed the de Broglie relation λ=h/p. (Debye-Scherrer method)

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Bragg reflection:


constructive interference:

)wavelength (electron

65.1106.154101.92/106.6

2/

eV 54 electronfor

)wavelength ray-(X

65.165sin91.02sin2

1for

652/90 and 50 , 91.0

sin2

193134o

oo

oooo

A

mKh

K

Ad

n

Ad

nd

consistent

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Debye-Scherrer diffraction

X-ray diffraction: electron diffraction : zirconium oxide crystal gold crystal

Laue pattern of X-ray (top) and neutron (bottom) diffraction for sodium choride crystal

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The wave-particle duality


Bohr’s principle of complementarity: The wave and particle models are complementary; if a measurement proves the wave character of matter, then it is impossible to prove the particle character in the same measurement, and conversely

Einstein’s interpretation: for radiation (photon) intensity

is a probability measure of photon density

)/1( 220 NNhcI

2

Max Born: wave function of matter is just as satisfies wave equation

is a measure of the probability of finding a particle in unit volume at a

given place and time. Two superposed matter waves obey a principle of

superposition of radiation.

),( tx

2

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The uncertainty principle

Heisenberg uncertainty principle: Experiment cannot simultaneously determine the exact value of momentum and its corresponding coordinate.

2/

2/

tE

xpx

Bohr’s thought experiment2/2

)/(/

2/22/ (2)

2/2sin/sin)/2(

)/ ( sin/

sin)/2(sin2 (1)

2

''

'

''

hxptE

ptxEtxvtvx

pvmppEmpE

hhxp

ax

hpp

x

xxx

xxxxx

x

x

Bohr’s thought experiment:

a diffraction apparatus

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Properties of matter wave

wave propagation velocity:

Why? 2

2/)()(

2

vwv

mv

mv

p

E

h

E

p

hw

a de Broglie wave of a particle

)(2sin),(

1/set )/(2sin),(

txtx

txtx

(1) x fixed, at any time t the amplitude is one, frequency is ν.

(2) t fixed, Ψ(x,t) is a sine function of x.

(3) zeros of the function are at

these nodes move along x axis with a velocity

it is the node propagation velocity (the oscillation velocity)

tnxtnx

nntx

nn

n

)/(2/2/

,.......2,1,0 )(2

// dtdxw n

68

modulate the amplitude of the waves


2/

2/ is wave the of velocity group the (2)

/ is wave individual the of velocity the (1)

)(2sin]22

cos[2 ),(2 and 2for

]2

)2(

2

)2([2sin]

22cos[2),(

])()[(2sin),( ),(2sin),(

),(),(),(

21

21

d

dν

dκ

dνg

w

txtd

xd

txdd

td

xd

td

xd

tx

tdxdtxtxtx

txtxtx

69

group velocity of waves equal to moving velocity of particles

vg

vdp

dEmvdvdEmvpmvE

dp

dE

hdp

hdE

d

dg

h

dpd

h

p

h

dEd

h

E

2

1

/

/

1

2


The Fourier integral can prove the following universal properties

of all wave. 4/1 and ,1/for 4/1 tx

:wavematter for hphp //1/

2/

)/1()/(/

2/

4/1)/1()/(

tE

EthhEthEhE

xp

pxhhpxx

uncertainty principle

uncertainty principle

the consequence of duality

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Ex: An atom can radiate at any time after it is excited. It is found that in a typical case the average excited atom has a life-time of about 10-8 sec. That is, during this period it emit a photon and is deexcited. (a) What is the minimum uncertainty in the frequency of the photon? (b) Most photons from sodium atoms are in two spectral lines at about . What is the fractional width of either line, (c) Calculate the uncertainty in the energy of the excited state of the atom. (d) From the previous results determine, to within an accuracy , the energy E of the excited state of a sodium atom, relative to its lowest energy state, that emits a photon whose wavelength is centered at


o

A5890?/ E

eV 1.2)//(//hh/ (d)

state the of width theeV 103.3104

1063.6

4

4/ (c)

line spectral the of width natural 106.1101.5/108/

sec 101.5105890/103/ (b)

sec 1084/14/1 (a)

88

34

8146

-114810

-16

EEEEt

h

t

hE

c

tt

E o

A 5890

71

uncertainty principle in a single-slit diffraction

for a electron beam:


2

sin

sin ,sin

hyy

hyp

y

pppp

p

p

y

y

yy

y

72


Ex: Consider a microscopic particle moving freely along the x axis. Assume that at the instant t=0 the position of the particle is measured and is uncertain by the amount . Calculate the uncertainty in the measured position of the particle at some later time t.

0x

xtxx

xmtvtx

xmmpv

xp

x

xx

x

or

2/t timeAt

2//

2/0tAt

0

0

0

0

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Dirac’s relativistic quantum mechanics of electron: 420

22 cmpcE

Dirac’s assumption: a vacuum consists of a sea of electrons in

negative energy levels which are normally filled at all points in space.

Some consequences of the uncertainty principle:

(1) Wave and particle is made to display either face at will but not both

simultaneously.

(2) We can observe either the wave or the particle behavior of radiation;

but the uncertainty principle prevents us from observing both together.

(3) Uncertainty principle makes predictions only of probable behavior of

the particles.


The philosophy of quantum theory:

(1) Neil Bohr: Copenhagen interpretation of quantum mechanics.

(2) Heisenberg: Principally, we cannot know the present in all details.

(3) Albert Einstein: “God does not play dice with the universe”

The belief in an external world independent of the perceiving subject is

the basis of all natural science.

How laser works

Spontaneous emission and stimulated emission

An excited electron may gives off a photon and decay to the ground state by two processes:

•stimulated emission : the excited atoms interact with a pre-existing photon that passes by. If the incoming photon has the right energy, it induces the electron to decay and gives off a new photon. Ex. Laser.

•spontaneous emission: neon light, light bulb

Optical pumping

many electrons must be previously excited and held in an excited state without massive spontaneous emission: this is called population inversion. The process is called optical pumping. Example of Ruby laser.

Optical pumping

Only those perpendicular to the mirrors will be reflected back to the active medium, They travel together with incoming photons in the same direction, this is the directionality of the laser.

Characteristics of laser

• The second photon has the same energy, i.e. the same wavelength and color as the first – laser has a pure color

• It travels in the same direction and exactly in the same step with the first photon – laser has temporal coherence

Comparing to the conventional light, a laser is differentiated by three characteristics. They are: Directionality,

pure color, temporal coherence.

Characteristics of laser

Directionality

Temporal coherence

Pure color

where the unit of power is Joules/s or W.

The power and intensity of a laser

The power P is a measure of energy transfer rate;

The energy encountered by a particular spot area in a unit time is measured by the intensity (or power density):

)22

(cmarea spot

(W) Power) (W/cmIntensity

(s)time exposurte

(J) outputenergy TotalPower(W)

laser versus ordinary lights:

The directionality of laser beam offers a great advantage over ordinary lights since it can be concentrate its energy onto a very small spot area. This is because the laser rays can be considered as almost parallel and confined to a well-defined circular spot on a distant object.

Sample problem: we compare the intensity of the light of a bulb of 10 W and that of a laser with output power of 1mW (10-3 W). For calculation, we consider an imagery sphere of radius R of 1m for the light spreading of the bulb, laser beams illuminate a spot of circular area with a radius r = 1mm.

2522

/108)100(4

10

4

10cmW

cm

W

R

W

A

PI bulbbulb

222

3

2

3

/103)1.0(

1010cmW

cm

W

r

W

A

PI laserlaser

400/108

/10325

22

cmW

cmW

I

I

bulb

laser

Fluence, F is defined as the total energy delivered by a laser on an unit area during an expose time TE,F(J/cm2)=I(watts/cm2) x TE(s)

The advantage of directionality of a laser : we can focus or defocus a laser beam using a lens. This can be used to vary the intensity of the laser.

Incoming parallel ray

f

Focused spotDiverged beam

continuous wave (CW) lasers versus pulsed lasers

• CW lasers has a constant power output during whole operation time.

• pulsed lasers emits light in strong bursts periodically with no light between pulses

usually T>>Tw

• The tw may vary from milliseconds (1ms=10-3 s) to femtoseconds (1fs=10-15s), but typically at nanoseconds (1ns=10-9s).

• energy is stored up and emitted during a brief time tw,

• this results in a very high instantaneous power Pi • the average power Pave delivered by a pulsed laser is

low. Instantaneous power Pi

w

pulsei t

EP

Average power Pave

RET

EP pulse

pulseave

Where R is the repetition rate

Example A pulsed laser emits 1 milliJoule (mJ) energy that lasts for 1 nanoseconds (ns), if the repetition rate R is 5 Hz, comparing their instantaneous power and average power. (The repetition rate is the number of pulses per second, so the repetition rate is related to the time interval by R=1/T).

Ws

J

ns

mlliJoule

t

EP

w

pulsei

69

3

1010

10

1

1

WHzJHzRJET

EP pulse

pulseave

33 105510)()(

Mechanisms of laser interaction with human tissues

When a laser beam projected to tissue

•reflection,•transmission, •scattering, •re-emission, •absorption.

Five phenomena:

Laser light interacts with tissue and transfers energy of photons to tissue because absorption occurs.

Photocoagulation

What is a coagulation? • A slow heating of muscle and other tissues is like a

cooking of meat in everyday life. • The heating induced the destabilization of the

proteins, enzymes.• This is also called coagulation. • Like egg whites coagulate when cooked, red meat

turns gray because coagulation during cooking.

Consequence of photocoagulation

When lasers are used to photocoagulate tissues during surgery, tissues essentially becomes cooked:

• they shrink in mass because water is expelled, • the heated region change color and loses its

mechanical integrity • cells in the photocoagulated region die and a

region of dead tissue called photocoagulation burn develops

• can be removed or pull out,

Applications of photocoagulation

• destroy tumors• treating various eye conditions like retinal

disorders caused by diabetes• hemostatic laser surgery - bloodless incision,

excision:due to its ability to stop bleeding during surgery. A blood vessel subjected to photocoagulation develops a pinched point due to shrinkage of proteins in the vessel’s wall. The coagulation restriction helps seal off the flow, while damaged cells initiate clotting.

Photo-vaporization

With very high power densities, instead of cooking, lasers will quickly heat the tissues to above 100o C , water within the tissues boils and evaporates. Since 70% of the body tissue is water, the boiling change the tissue into a gas. This phenomenon is called photo-vaporization. Photo- vaporization results in complete removal of the tissue, making possible for :

• hemostatic incision,or excision.

• complete removal of thin layer of tissue. Skin rejuvenation, resurfacing

Conditions for photo-vaporization

1.the tissue must be heated quickly to above the boiling point of the water, this require very high intensity lasers,

2.a very short exposure time TE, so no time for heat to flow away while delivering enough energy,

highly spatial coherence (directionality) of lasers over other light sources is responsible for providing higher intensities

Intensity requirement

Intensity (W/cm2) Resulting processes

Low (<10) General heatingModerate (10 – 100) PhotocoagulationHigh (>100) Photo-vaporization

Photochemical ablation

When using high power lasers of ultraviolet wavelength, some chemical bonds can be broken without causing local heating; this process is called photo-chemical ablation.

The photo-chemical ablation results in clean-cut incision. The thermal component is relatively small and the zone of thermal interaction is limited in the incision wall.

Selective absorption of laser light by human tissues

Selective absorption

Selective absorption occurs when a given color of light is strongly absorbed by one type of tissue, while transmitted by another. Lasers’ pure color is responsible for selective absorption. The main absorbing components of tissues are:

• Oxyhemoglobin (in blood): the blood’s oxygen carrying protein, absorption of UV and blue and green light,

• Melanin (a pigment in skin, hair, moles, etc): absorption in visible and near IR light (400nm – 1000nm),

• Water (in tissues): transparent to visible light but strong absorption of UV light below 300nm and IR over 1300nm

Selective absorption

Applications of lasers

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Complex hardware systems based on quantum optics