chapter 11 quantum physics · 2010-04-25 · wave&aspectof&light • lightpassing&...
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Chapter 11 Quantum Physics • Ma#er and force fields display a universal duality.
• Such stuff appears both wave-‐like and par=cle-‐like.
• Both wave and par=cle aspects are essen=al to understanding microscopic physics.
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Wave aspect of light
• Light passing through two slits displays a wave-‐like interference pa#ern
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Par=cle aspect of light
• Examined closely, it appears that light energy interacts with ma#er in =ny chunks or quanta called photons.
• The wave-‐like interference pa#ern appears in the pa#ern of individual photon interac=ons.
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Wave and par=cle aspects of electrons
• Electrons and other material par=cles show similar behavior.
• Wave interference is observed when passing a beam of material par=cles through two slits.
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The energy of photons
• The energy E of a quantum of light in a wave of frequency f is proportional to frequency.
• E = hf, h=6.626068 × 10-34 Joule-s • h is called Planck’s constant • High frequency quanta (such as X-rays) bear
more energy and are more particle-like than lower frequency (longer wavelength) quanta such as those in a radio wave.
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Ques=on
• Compared to an infrared photon, an ultraviolet photon carries
• 1) less energy • 2) the same energy
• 3) more energy
• 4) some=me less, some=mes more, depends on frequency
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Par=cle proper=es of photons
• Many experiments show that photons carry not just energy E=hf but linear momentum (p = E/c = hf/c) and move at light speed and are massless.
• The values are established by sca#ering photons from material par=cles and observing energy and momentum conserva=on.
• Photons also carry an intrinsic spin angular momentum S=+-‐ h/(2 pi) associated with L and R circular polariza=on.
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Energy conserva=on and photoelectric effect
• Light of a fixed frequency f ejects electrons from atoms. The ejected electrons have (max) energy E=hf-‐W where W is a constant atom specific binding energy.
• Einstein successful described this as the absorp=on of single photons by electrons.
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Compton sca#ering
• In sca#ering from free (not bound) electrons, both energy and momentum are found to be conserved if for photons p=E/c=hf/c.
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Par=cle proper=es of electrons
• Electrons carry not just energy but spin +-‐ h/(4pi) and (rela=vis=c) linear momentum
• Neutrinos are essen=ally electrically neutral nearly massless electrons.
• These are examples of fundamental fermionic ma#er known as leptons and quarks.
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Matter waves • The wavelength associated with a freely moving ma#er par=cle of momentum p is given by the de Broglie rela=on
• No=ce that higher momentum corresponds to short wavelength.
• An electron of kine=c energy 1 eV has a wavelength of 1.23 nm, the atomic scale.
• Such wavelengths are measured by diffrac=on from atoms in a crystal.
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Bound ma#er waves • Like sound, a ma#er wave in a box has a fundamental and harmonic modes of oscilla=on.
• The wavelength takes discrete values and the energy of the ma#er par=cle is quan=zed.
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The wave nature of atomic electrons
• An electron wave is bound by the Coulomb force of a#rac=on to nuclei.
• The binding force is like a box with soa sides and a deep center.
• The ma#er wave can oscillate in its fundamental (ground state) and harmonic modes in 3-‐d
• The energy of the electron is quan=zed • The characteris=c frequencies are the natural “sounds” of ma#er waves in atoms.
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Shape of ma#er wave states • Various representa=ons indicate electron density in atomic ma#er waves.
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Hydrogen atom
• The energy of the natural electron/ma#er wave modes for hydrogen is given by a simple formula
• E(n)= -‐13.6 eV/n2
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Molecules
• In the presence of mul=ple nuclei, the ma#er wave is shared.
• Again there is a fundamental mode (ground state) and discrete/quan=zed excited states.
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Chemistry and beyond
• The ma#er wave aspect of electrons governs the binding of atoms into complex molecules and solids.
• Ma#er waves resonant between atoms in a molecule and are distributed in space.
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Radia=on and atoms • A photon of energy E
(ini=al)-‐E(final) is emi#ed when an electron makes a transi=on between ini=al and final bound energy levels.
• A photon is absorbed only if its energy matches a difference between electron energy levels.
• The emission and absorp=on “frequency spectrum” is different for every atom.
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Quantum dots
• It is possible to fabricate nanoscale structures which confine electrons in =ny boxes.
• Their size determines the electron excita=on spectrum and characteris=c color.
• Various sizes permit various custom spectra.
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Lasers • Laser light sources use one and only one interlevel atomic transi=on to produce single frequency (“monochroma=c”) light.
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Spontaneous and s=mulated emission
• An electron in an excited state spontaneously radiates a photon and jumps to a lower energy state in a =me typically of order 1 ns
• The presence of light of the correct frequency can =ckle an atom to emit light heading in the same direc=on. This is called s=mulated emission.
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Light Amplification by Stimulated Emission
• Excite atoms by electron bombardment or a light source. Use mirrors to contain light of a certain direc=on. Exponen=al growth of s=mulated emission yields a preponderance of light of that direc=on. Let it leak out of a mirror to form the beam.
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Laser guts
• Solid and gases work. • Excita=on by flash lamp or collisions
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The mystery of waves and par=cles
• A wave pulse (or packet) is a localized moving wave, somehwat par=cle like.
• It can be understood as a superposi=on of harmonic waves.
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Construc=ng a packet
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440 Hz + 439 Hz
440 Hz + 439 Hz + 438 Hz
440 Hz + 439 Hz + 438 Hz + 437 Hz + 436 Hz
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The components of a packet • A general wave packet is a superposi=on of harmonic waves with a range of wavelengths or “wave vectors” k=1/lambda = p/h. A par=cle associated with a packet has a corresponding range of momenta.
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Uncertainty principle
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A single wavelength wave is infinitely distributed throughout space. A localized wave pulse has some width dx in space and a range dk of wave vectors with non vanishing amplitude. These are inversely related:
Ma#er waves: A par=cle of sharp momentum p is spread throughout space. A par=cle localized to a region dx has a momentum uncertainty of
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Lesson from free par=cle wave packets
• The proper=es of a par=cle-‐wave are changeable and blurred.
• A quantum par=cle can not simultaneously have a well defined posi=on and momentum.
• If we force it to be localized (dx~0), its wave will have a large range of momenta, and the wave will subsequently rapidly disperse.
• If the force it to have a sharp momentum/wavelength, is must be spread throughout space.
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Heisenberg uncertainty principle
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Consider a prototypical experiment in which light is used to “see” an electron with minimal disturbance.
Suppose for simplicity unit magnifica=on with a single lens as in the eye. Let f= focal length, D= lens diameter.
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Analysis
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Because light is a wave, diffrac=on implies an image size (f=focal length, D= lens diameter)
Individual photons have momentum in a range intercepted by the lens or
In the single quantum sca#ering, the photons transfer this uncertain momentum to the electron and so..
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Interpreta=on
• In so far as all ma#er and light has the dual wave/par=cle character, a measurement that determines the posi=on of a par=cle to within a range dx necessarily implies it imparts an uncertainty in momentum dp=h/dx.
• No par=cle has simultaneously a unique posi=on and momentum.
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Two slit experiment
• At low beam intensity, one observes the quantum nature of ma#er and light -‐ single par=cle events assembling randomly to form the wave interference pa#ern.
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Two slits versus one
• If one slit or the other is blocked, one observes a single slit diffrac=on pa#ern. With both slits open, the pa#ern is NOT the sum. Instead a two slit interference pa#ern with nega=ve inteference at some points.
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The weirdness of interference again
• Star=ng from one slit open, opening the other yields at some angles no par=cles where there were some previously (destruc=ve interference) and more than double the number at other angles (construc=ve interference).
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Outsmar=ng two slits
• It seems a par=cle explore both slits simultaneously. Suppose we place some electrons behind one slit to see which slit each par=cle is going through.
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We are outsmarted
• The target par=cles must have some dy<<D and by the uncertainty principle some momentum dpy~h/dy>>h/D
• This momentum uncertainty will be transferred to the beam par=cles and will wash out the interference pa#ern!
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This experiment is different and gives different results! The uncertainty principle s=ll rules!
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Lesson
• It is not possible to establish that a par=cle follows a trajectory in the classical sense except within the limita=ons prescribed by the uncertainty principle.
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If you can’t observe it it doesn’t exist -‐ a quantum par=cle does not have a trajectory.
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Tunneling
• Waves bound in a box leak out beyond the region classically allowed by energy conserva=on.
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More tunneling
• Upon encountering a poten=al energy barrier, waves are in part reflected and in part transmi#ed. The wave “tunnels” through the barrier.
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Two places at once • An electron associated with a wave packet is already in many places at once, partly delocalized.
• The par=al reflec=on and par=al transmission of a packet make being in two places at once painfully obvious.
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Tunneling between conductors • The space between two conductors forms a poten=al barrier. As they approach one another, tunneling gives an exponen=al increase in current.
• => The onset of conduc=on is exponen=ally sensi=ve to the separa=on.
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Scanning tunnel microscope
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STM images of atoms on surfaces
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Atomic scale manipula=on
• A probe =p may be used to place atoms, to write your name in atoms!
• The Kanji characters for "atom." The literal transla=on is something like "original child.”
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Nanotechnology Center
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Nanoscale Science and Engineering Center
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Nanophysics
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Promises, promises
• If I were asked for an area of science and engineering that will most likely produce the breakthroughs of tomorrow, I would point to nanoscale science and engineering.
• -‐Neal Lane, Assistant to the President for Science and Technology
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Where can this technology go?
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Richard Feynman
“There is plenty of room at the bo#om!”
h#p://www.its.caltech.edu/~feynman/plenty.html
BTW, Feynman is also remembered for saying “I think I can safely say that nobody understands Quantum Mechanics."