What is a quantum computer?

A quantum computer is a machine that performs calculations based on the laws of quantum mechanics, which is the behavior of particles at the sub-atomic level.

Representation of Data  - Qubits

A bit of data is represented by a single atom that is in one of two states denoted by |0> and |1>.  A single bit of this form is known as a qubit
A physical implementation of a qubit could use the two energy levels of an atom.  An excited state representing |1> and a ground state representing |0>.

Current technology Limitations

Intel has announced new SRAM chips for high density memory.

Contains 330 million transistors.

Pentium IV has 30 million transistors

Problem…….

Current technology is not having difficulty  adding more transistors…. 

At current rate transistors will be as small as  an atom.

If scale becomes too small, Electrons tunnel  through micro-thin barriers between  wires corrupting signals.

Other uses of Quantum Computing

Modeling large complex systems

the brain

the universe

Can describe an atom with a few bits

Takes 100 bits to describe atoms interacting

2100 or 10100 bits, 1090 particles in whole universe

Few 100 qubits easily solves this problem

Operations on Qubits - Reversible Logic

Due to the nature of quantum physics, the destruction of information in a gate will cause heat to be evolved which can destroy the superposition of qubits.

Quantum Computing problems

Current technology
≈ 40 Qubit operating machine needed to rival current classical equivalents.
Errors
Decoherence -  the tendency of a quantum computer to decay from a given quantum state into an incoherent state as it interacts with the environment.
Interactions are unavoidable and induce breakdown of information stored in the quantum computer resulting in computation errors.
Error rates are typically proportional to the ratio of operating time to decoherence time
operations must be completed much quicker than the decoherence time.