Quantum Weirdness Part 6 Quantum Weirdness in Materials Quantum - - PowerPoint PPT Presentation

Quantum Weirdness

Part 6 Quantum Weirdness in Materials Quantum Cryptography Quantum Teleportation Quantum Snake Oil

Quantum Weirdness in Materials

Why Materials Behave as they do

Combining Atoms Into Molecules

• Molecular Orbital Theory

The two 1s levels of two hydrogen atoms combine to form the 1σ orbitals in H2

Still producing discrete energy levels

Gerhard Herzberg, Nobel prize 1971

Carbon Dioxide CO2

• Triatomic molecule

O=C=O

• Shape depends on the

shape of the orbitals, which depends on the wave equations

• Symmetric

stretch does not absorb infra-red radiation

• The three

asymmetric stretches do absorb in the infra-red

• This is what makes CO2 a Greenhouse Gas

Electronic Orbitals in a Solid

• Now we are

combining ~1023 atoms together.

• The discrete energy

levels are so close together, that they form a BAND

• Note this a gap in

energy, not a distance

Band Gap

• The bands in the solid are filled up from the bottom

with the electrons

Part filled Like this multilevel fountain in Garda, Italy

Metal Conduction Band partly filled with electrons

• If a band is full, then the electrons can’t go anywhere and

can’t be used for conducting electricity

• If a band is partly full, then the electrons can slosh about, if

a voltage is applied, and can move

Valence Band Conduction Band Insulator Valence band full Conduction band empty Large energy gap

Δ𝐹𝑕𝑏𝑞

Valence Band Conduction Band

Semiconductor Valence band full Conduction band empty Small energy gap Silicon

𝐹𝑐𝑏𝑜𝑒

• A semiconductor has a

filled band, but the gap to the next level is small.

• At room temperature, a few electrons have sufficient

energy to jump the gap, into the conducting band

Valence Band Conduction Band Conduction Band Valence Band

• Now we have a few electrons

in the conduction band and a partly empty valance band

• Both can now conduct

𝐹𝑐𝑏𝑜𝑒

Semiconductors and Doping

• We can control the conductivity by adding impurities

like Boron or Phosphorus to the semiconductor

12

Valence Band Conduction Band Egap Donor level Valence Band Conduction Band Egap Acceptor level Doping with n-type Doping with p-type

Diodes: Very Useful In Circuits

• Devices which only let current (charge) flow one way
• They were invented in 1905 by Sir John Fleming

(tubes/valves)

Semiconductor Diodes

• A semiconductor device which does the same thing

as the vacuum tube device

• A p-type and an n-type semiconductor placed back-

to-back

p- type Electron deficient n- type Electron rich Current can flow

Valence Band Conduction Band Donor level Valence Band Conduction Band Acceptor level p-type n-type

Inside the diode, electrons are moving between quantum states

Light Emitting Diodes

• If the electrons drop

between levels, they can give off visible light, if the energy difference is correct

Running lights on an Audi

• Very efficient, as they
• nly produce light in a

very narrow set of wavelengths

• Low power
• Good replacements for

incandescent lightbulbs, as they don’t emit in the infra-red

• 2 Watts LED equivalent to a 15

W incandescent

• Some LEDs emit infra-red light, so invisible to the

human eye

• Most TV remote controllers

use an IR-LED to send the signals

• Blue LEDs proved very difficult to make!

Isamu Akasaki, Hiroshi Amano, and Shuji Nakamura: Nobel Prize in Physics 2014

Photodiodes

• The diode can be used to detect photons.

Increased current when light shines on the diode

Valence Band Conduction Band Valence Band Conduction Band

• The TV has an infra-red

photodiode receiver to pick up signals from the remote control

• A solar cell, which

makes energy from sunlight is an example

• f a photodiode

Diode Lasers

• In the ordinary LED, the photons are emitted in

random directions, because it is a spontaneous decay process

• Careful design of the

semiconductor layers can result in photons being produced by stimulated emission

Photodiodes

• Some diodes are designed to be light sensors.
• Light falling on them causes an electron to be

excited into the conduction band, increasing the conductivity

• These photodiodes can be made to detect many

different frequencies of electromagnetic radiation (IR, visible UV being the most common)

Circuit Symbol for Photodiode

• A solar cell, which makes energy from sunlight is an

example of a photodiode

• This type of operation

is known as Photovoltaic Mode

• The photon energy is

used to create an electron-hole pair, which increases conductivity of the device.

Transistors

• The transistor normally uses two p-n junctions back

to back. It is a current amplification device

n-p-n transistor

1955 GE transistor radio

Nanotechnology: Quantum Dots

• These are small particles of semiconductors.

Usually clusters of 100-10,000 atoms.

• They do not show full band structures

Cluster of 5 atoms ΔE

• By varying the size of the

particle, the value of ΔE can be altered.

• Cadmium Selenide

• Lead Selenide, stabilised with oleic acid
• Form regular arrays

when deposited on a surface

• Etch them on silicon
• r other

semiconductors

• Photo NIST, USA

• Quantum dots act as potential wells, to trap

electrons

• The electrons have quantized energy levels

• One application is to make the energy gap equivalent

to visible wavelengths.

• If the upper energy levels are occupied, then the

quantum dot will fluoresce in the visible range as the electron drops back into the lower state

Cluster of 5 atoms ΔE

 hc E  

• Changing the size of the particle, changes the energy gap,

and hence changes the colour of the emitted radiation

http://spectrum.mit.edu/articles/features/smarter-quantum-dots/

Crystals and Quantum Weirdness

Yes, there is Crystal Energy

Crystal Lattices and Quantum Weirdness

Sodium Chloride Space Fill

The regular nature

• f the atoms in a

solid material leads to quantum effects and interactions with the electrons

Sodium Chloride

Sapphire is an aluminium

• xide, Al203

(corundum) with titanium and iron impurities

Sapphires

• Absorbs red and yellow light, white light passing

through it emerges as blue

http://scienceworld.wolfram.com/chemistry/IntervalenceChargeTransfer.html

• An electron bound to an iron impurity jumps

to a nearby titanium impurity

Ruby

• Ruby is also corundum
• Different cause of colour – impurities of chromium.
• Chromium is larger than aluminium, so it distorts

the lattice out of shape, so the orbitals change energy

http://www.webexhibits.org/causes

• fcolor/6AA.html

Natural ruby from Tanzania

Thermal Motion in Solids

• Atoms oscillate around a fixed position
• There are lots of possible oscillations!

http://www.crystal.unito.it/vibs/ICEXI/ Vibrations of one of the structures

• f ice (crystalline water)

Waves and Phonons

• The vibrations inside the crystal are waves
• Because of wave particle duality, we can also think of

them as quasi-particles of energy called PHONONS

• They carry energy through the material

http://kino-ap.eng.hokudai.ac.jp/ripples.html

We can watch the ripples of the waves at the surfaces of crystals using laser light

𝐹 = ℎ𝑔 = ℎ𝑑 𝜇

Phonon energy

The idea of the phonon was introduced in 1932 by the Russian physicist Igor Tamm (Nobel Prize 1958)

• Long wavelength phonons carry

sound through materials

• Short wavelength phonons carry

heat energy through materials

Electrical Conductivity

When Electrons Move Through a Conducting Material

+ + + + + + + + + + + + + + + + + + + +

• Electrons moving through the metal collide with

the positive metal ions and lose energy

• To push them through the metal lattice against

resistance needs energy (from a battery)

• The moving electrons bump into lattice ions, and

give energy to the lattice

• The lattice ions have more energy (get hotter).
• Metal heats up as electric current flows through it

Superconductors

When the Phonons Become Important

Superconductivity

• At extremely low temperatures, many metals (and
• ther materials) become superconductors
• No energy is lost as the electrons pass through the

material

• There are no heat losses

• At low temperature there is an interaction between

the electrons and the phonons in the lattice

• This results in the electrons forming pairs (Cooper

pairs)

• These can pass through the metal without

interacting!

• Very subtle quantum

weirdness, involving interactions between quantum particles and quantum quasi-particle!

Quantum Computing

Promising, but early days

Digital Computing: The Bit

• The basic piece of digital data
• Either 0 or 1
• Stored in a logic gate (a collection of a few

transistors)

1 1 1 1 1

(1 × 28) + (0 × 27) + (1 × 26) + (1 × 25) + (1 × 25) + (0 × 23) + (1 × 22) + (0 × 21)

= 186

Qubit

• A two level quantum state
• The information is stored as a quantum

superposition

Spin states

• It contains more information

than a conventional bit

• Conventional bit must be

pointing either up or down

• Qubit can point in any direction
• n what is known as the Bloch

sphere

1

• Each qubit contains information in a superposition
• f states
• If qubits interact with each other, they can process

multiple possible answers simultaneously

• You then measure the final state of the qubits, to

get a final answer

• Do it many times to get a range of possible answers
• To make a qubit, use a superconducting junction

Josephson Junctions

• Two superconductors, with a very thin layer or

insulating material between them

Cooper Pairs of electrons can tunnel from one superconductor to the other

https://en.wikipedia.org/wiki/Superconducting _quantum_computing

The Cooper pairs exist as different quantum levels whilst tunnelling They can act as a Qubit

IBM Quantum Experience

• IBM Q Experience: two 5-qubit processors and a

16-qubit processor. Superconductors – cooled to 0.100 mK (very cold)

3 nm thick layer (0.00000001 cm)

Quantum Supremacy

• A task which can be carried out much more quickly
• n a quantum computer, than can be feasibly

carried out on a regular computer

• Paper published yesterday!
• https://www.nature.com/articles/s41586-019-

1666-5

Quantum Cryptography

Making Unbreakable Codes

Codes and Cyphers

• Transposition of letter for letter
• r letter to number
• Can be done mechanically

(Enigma machine)

• Must have some repeating

patterns to the encoding

• Can be solved, if you have fast

enough computing power

Cyphers – same length as the encoded text Codes – different length

• Mechanical computer – the Bletchley Park Bombe
• Based on the Polish bomba kryptologiczna

(cryptological bomb)

• To crack the German Naval codes (which had an

extra wheel on the Enigma, and were more complicated), they built the first computer Colossus

One Time Pad Cyphers

• One time pad
• Used by both the sender

(Alice) to code

• And the receiver (Bob) to

decode

• If both destroy the pad after

use, then it is unbreakable

• Requires both to share the pad

• Alice and Bob create an entangled quantum

system as the key (a quantum one time pad)

If anyone else tries to break into the system and measure the key, they will change the state of the system, so it has security built in

A B

Quantum Teleportation

Not What You Think It Is!

Moving Matter From Place To Place

• Using a device like the “Transporter” – Star Trek
• https://www.youtube.com/watch?v=jDFI87zn9t0&f

eature=youtu.be

• What’s the reality?

Quantum Teleportation is Different

• It does not transport for matter
• It is a communication stream
• It moves quantum information (a Qubit)
• To reconstruct the quantum object requires

instructions, which must be transmitted conventionally (at the speed of light)

• The process destroys the original qubit

• Alice and Bob share a pair of entangled photons
• Alice now uses her entangled photon to measure

another photon (the one we want to teleport)

• The net result is that Alice’s two photons are now in

undetermined states, and Bob’s photon is now in the same state as Alice’s was originally

• However, you have to transmit the information

about how many possible states there are to Bob by another (conventional) channel – the process is not instantaneous

• We can entangle photons, then send one

elsewhere

• The two photons are still entangled – action

performed one, affects the other instantaneously

143 km

• Satellite to Ground communication using the

Chinese Quantum Space Satellite Micius / Mozi

• 1400 km ranges

Three State Systems

• Recent experiments (15th August 2019!) have

demonstrated that a three state system can be teleported

• Qutrit (states 0, 1 & 2) instead of a qubit (0 & 1)

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.123.070505

Quantum Snake Oil

Fake News

Things to be Cautious About

• Quantum Medicine
• Quantum University
• Quantum Healing
• Quantum Resonance Spectroscopy

Not to be confused with Magnetic Resonance Spectroscopy and Magnetic Resonance Imaging (MRI), which are genuine!

• Look for information coming from a well-known,

publicly funded University. Avoid information from private research organizations, Think-Tanks, etc

Summary of Quantum Weirdness