Science behind Quantum Computing

Introduction
Basics
Breakdown in Classical Physics
Heisenberg’s Uncertainty Principle
Schrodinger’s Cat
Science behind Quantum Computing
TCS Innovation Labs
Cyber Physical Systems Lab
July 26, 2013
Sibi Quantum Mechanics

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Introduction
Basics
Schrodinger’s Cat
Outline
1 Introduction
2 Basics
3 Breakdown in Classical Physics
Anomalous stability of atom and molecule
Rayleigh Jeans Catastrophe
Wave Particle Duality
4 Heisenberg’s Uncertainty Principle
5 Schrodinger’s Cat

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Introduction
Basics
Schrodinger’s Cat
No one fully understands quantum mechanics. - Steven
Weinberg

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Introduction
Basics
Schrodinger’s Cat
Deﬁnition
Branch of physics which deals with physical phenomena at
microscopic scales.
Best understood by unlearning many classical physics
concepts ( Similar to learning functional language from an
imperative background.)

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Introduction
Basics
Schrodinger’s Cat
Quantum Mechanics asserts that a particle’s does not
have a position and a velocity.
Aim of Quantum Mechanics is to calculate the range of
possible particle positions and the relative probability of
these functions.
Explanation of particle has a position of 100 (+ or -) 1:
Classical Mechanics: Particle has a position in the range
99-101.
Quantum Mechanics: Particle is simultaneously all over the
range 99-101. (Thinking about wave which can spread out
helps here.)

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Introduction
Basics
Schrodinger’s Cat
Outline
1 Introduction
2 Basics

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Introduction
Basics
Schrodinger’s Cat
According to classical physics, an electron orbiting a
nucleus should lose energy by emission of radiation.
Because of energy loss, it should gradually spiral in
towards the nucleus.
But this does not happen.

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Introduction
Basics
Schrodinger’s Cat
Outline
1 Introduction
2 Basics

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Introduction
Basics
Schrodinger’s Cat
Also famously called as the Ultraviolet Catastrophe
Prediction of late 19’th century classical physics.
Prediction: An ideal black body at thermal equilibrium will
emit radiation with inﬁnite power.
This too doesn’t happen.

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Introduction
Basics
Schrodinger’s Cat
Outline
1 Introduction
2 Basics

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Introduction
Basics
Schrodinger’s Cat

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Introduction
Basics
Schrodinger’s Cat
Classical Physics can deal with wave or particles.
However various experiments show that waves sometimes
act as if they were stream of particles, and stream of
particles act as if they were waves.
Einstein’s Photoelectric Effect showed light behaved as a
particle.
Young’s double slit experiment showed light behaves like a
wave.

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Introduction
Basics
Schrodinger’s Cat
Both position and velocity of a particle cannot be
measured accurately.
This is because by taking a measurement we inadvertently
disturb the system as well.

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Introduction
Basics
Schrodinger’s Cat
Thought Experiment
Experiment involves an imaginary setup.
A cat sealed inside a box with Geiger counter attached to it.
Inside the Geiger counter is a small amount of radioactive
substance.
The box is set up so that when the radioactive substance
decays, a device smashes a vial of toxic gas, killing the cat
inside.
After an hour, until the box is open, there is no way to
determine whether the cat inside is alive or dead, thus
allowing the cat to be both alive and dead.

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Introduction
Basics
Schrodinger’s Cat
Interpretation
HUP show’s that it’s impossible to directly observe this
decay because it involves particles so small that any
equipment we have to monitor them would interfere with
what they’re doing.
What actually happens is the entire world splits into a
superposition of worlds where cat is alive, and those
where it is dead.
Subatomic particles routinely exist in superposition of
states and correct predictions of the outcomes of certain
experiments can only be made by taking these effects into
account.
In Quantum mechanical world, it is possible for a atom in
the combined state ’decayed-not decayed’ in which it is
neither one nor the other but somewhere in between.

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Introduction
Basics
Schrodinger’s Cat
Superpositon
Atomic particles can exist in two or more states at once.
Example: Room with ﬁve chairs all exactly the same size.
Classical theory: You sit in one of the chair. End of story.
Quantum theory: Electron enters the room and sits down
in all ﬁve of them at once!

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Introduction
Basics
Schrodinger’s Cat
Entanglement
When two particles interact, the can become linked such
that the alteration of one causes a mirrored response in the
other, despite a separation by arbitrarily large distances.
It is a sub case of quantum superposition: Two particles
continue to form a system even after they have been
separated and continue to share a quantum state until a
measurement has been made.
Measuring the state of one, causes the other to take
different state.

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Introduction
Basics
Schrodinger’s Cat
Recent development
23 July, 2013 (2 days ago) it was presented in Moscow that
the rules of quantum mechanics may extend to much
larger objects than we thought.
Earlier people have been able to build superposition states
containing only a few photons, but now it has been
demonstrated successfully with 160 million photons.
Raises interesting question:
Is there a border between micro and macro level where
quantum rules apply ?
Or does quantum mechanics applies on all scales ?

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Science behind Quantum Computing

Science behind Quantum Computing

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