Quantum Computers

by

Ran Li

Contents

Background

Quantum Characteristics

Applications

Quantum Computer Design

Current Challenges

Predictions For The Future

Background

Moore’s Law. Predicted that the

number of transistors on a chip will double every 18-24 months.

The Wall! Transistors can only be

so small. It is believed this will

happen in the next 20 years.

Background

The idea of the quantum computer first immerged in 1982 by Richard Feynman.

In 1994, Peter W. Shor of AT&T Shor’s algorithm to factorize big number

Basic Idea

A quantum computer uses the physical characteristics of atoms to represent 0 and 1.

For example, spin of electrons.

Electron spin gate

Electron spin gate

Electron spin gate

WRONG !

Electron spin gate

Superposition

Electron is in two states at the same time.

or

|2

1|

2

1

0|2

11|

2

1

Qubit

Quantum Interference

Qubits

Quantum Bits

A normal bit can be either 0 or 1.

Because of superposition, a qubit can be both 0 and 1 at the same time.

There is no equivalency of superposition in classical world.

Qubits

Qubits

An n-qubit register can store 2n

numbers at the same time, or do 2n

operations at one shot.

It’s exponentially faster!

Application(1)

Cryptography :perfectly secure communication Factorizing large numbers very rapidly

(Shor’s algorithm) Today’s computers would take about a billion years to

factor a 400-digit number. Using this new algorithm, a 400 digit number is

estimated to take about a year to factor.

Application(2)

Searching : especially algorithmic searching (Grover’s algorithm) To search 10,000 items

Classical algorithm uses 5,000 queries average

Grover’s algorithm takes 100 queries

Application(3)

Simulating quantum-mechanical system efficiently

Quantum computers will be able to model certain systems better. David Cory at MIT was the first to simulated a

quantum mechanical process. He simulated a “truncated harmonic oscillator” in July of 1999.

Quantum Computer Designs

NMR (Nuclear Magnetic Resonance)

Ion Trap

Quantum Dots

NMR (Nuclear Magnetic Resonance)

Developed at IBM by Issac Chaung in 1996.Protons and Neutrons have spin.Use magnetic field to control the spins.

NMR(example)

Technology already exists.

Too large in size. 10-feet tall.

Current NMR Machine

Ion Traps

Ions in a radio frequency trap interact by exchanging vibrational excitations. Each ion can be controlled by a polarized, properly focused laser beam.

Picture shows the electrode structure.

The electrode is 1mm thick.

Quantum Dots

Current Challenges

Number of bits in a word. 7-qubit machine is the most advanced to date. All the atoms interact with each other and cause

undesirable results.

Physical size of the machines. Current machines are too large to be of practical use to

everyday society.

Error correction. Many advance have been made but this is still an area

of intense research.

Predictions For The Future

In the next 20 years more research will be devoted to quantum computers.

A working version for specific purposes should be ready to be released on the market by in 20 years.

Very little of our population knows of quantum computers. This will change a great deal also in the next 20

years.