Quantum Computing – Part 2Quantum Computing – Part 2

Amanda Denton – Evil DictatorAmanda Denton – Evil Dictator

Jesse Millikan – Mad ScientistJesse Millikan – Mad Scientist

Lee Ballard – Some Guy With A BeardLee Ballard – Some Guy With A Beard

September 30, 2001

A Brief History ofA Brief History ofQuantum ComputingQuantum Computing

How in the heck did this stuff get How in the heck did this stuff get started anyway?started anyway?

Parallel UniversesParallel Universes

1950 – “The Garden of Forking Paths” is 1950 – “The Garden of Forking Paths” is written by Jorge Luis Borgeswritten by Jorge Luis Borges ““In all fictional works, each time a man is In all fictional works, each time a man is

confronted with several alternatives, he confronted with several alternatives, he chooses one and eliminates the others. In the chooses one and eliminates the others. In the fiction of Ts’ui Pen, he chooses – fiction of Ts’ui Pen, he chooses – simultaneously – all of them.”simultaneously – all of them.”

From Humble OriginsFrom Humble Origins

Rolf Landauer – “computation is physics Rolf Landauer – “computation is physics and you cant understand the limits of and you cant understand the limits of computation without saying what the computation without saying what the physical implementation is.”physical implementation is.”David Deutsch proposes the first quantum David Deutsch proposes the first quantum computer in 1977-78computer in 1977-78 Not really a quantum computer, but actually a Not really a quantum computer, but actually a

conventional computer that operated by conventional computer that operated by quantum means and had additional hardware quantum means and had additional hardware that allowed it to do extra things.that allowed it to do extra things.

The Multi-verseThe Multi-verse

In the early 1980’s, Deutsch proposed an In the early 1980’s, Deutsch proposed an experiment to test the idea of parallel universes.experiment to test the idea of parallel universes.

Only problem was that Deutsch’s experiment Only problem was that Deutsch’s experiment would need to be carried out on a machine that would need to be carried out on a machine that no one had and no one knew how to build.no one had and no one knew how to build.

David Deutsch’s ideas were published in 1985 in David Deutsch’s ideas were published in 1985 in the International Journal of Theoretical Physicsthe International Journal of Theoretical Physics

Douglas Adams and Douglas Adams and Quantum ComputingQuantum Computing

Deutsch’s idea seems very similar to the Deutsch’s idea seems very similar to the idea in “The Hitchhiker’s Guide to the idea in “The Hitchhiker’s Guide to the Galaxy” of building a computer, Galaxy” of building a computer, EarthEarth, to , to find the ultimate answer to the ultimate find the ultimate answer to the ultimate question of life the universe and question of life the universe and everything.everything.

Basic Q.C.Basic Q.C.One moose, two mooseOne moose, two mooseRed moose, blue mooseRed moose, blue mooseLive moose, dead mooseLive moose, dead moose

•A ‘qubit’ can be in an infinite number of states•|Ψ> = a|0> + b|1>•Probability of 0: |a|²•Probability of 1: |b|²•|a|² + |b|² = 1

Superposition StatesSuperposition States

•“A full system of m qubits has a basis of 2m states.”*•A classical system of m bits can be set to any of these states.•A quantum system can be set to all of those states at once.

More on superposition statesMore on superposition states

*Introduction to Quantum Computation and Information (Lo, Popescu, Spiller 2000)

EntanglementEntanglement

•The states of qubits in a closed system are ‘entangled’.•Consider a system of two qubits, A and B.•|Ψ>AB = 2-1/2 (|0>A|0>B + |1>A|1>B)•Cannot be written in factored form.•The two qubits don’t have states of their own - they are ‘entangled.’

Reversible Unitary EvolutionReversible Unitary Evolution

•A.K.A. Reversibility•For any truly closed quantum system, you can reverse the system and get back to the original state•Works on paper, but not usually in theory.

Irreversibility, Measurement, Irreversibility, Measurement, DecoherenceDecoherence

•“[Irreversibility] has to be stopped from biting before some desired unitary quantum evolution of the system has been completed.”*•In short, it has to work right or it won’t work right.

*Introduction to Quantum Computation and Information (Lo, Popescu, Spiller 2000)

No CloningNo Cloning

•No matter how hard you try, you can’t copy the state of a superpositioned quantum system.•If you observe it to copy it, it collapses into a base state.•This makes absolutely secure communication possible using a quantum media and the One-Time Pad, or Vernam’s Cipher

Recent Quantum Recent Quantum ComputingComputing

andand

Bulk Spin Resonance Bulk Spin Resonance Quantum ComputingQuantum Computing

Problems with current computing:Problems with current computing:

As computers get faster and smaller, limits As computers get faster and smaller, limits will be reached that cannot be broken.will be reached that cannot be broken.

If computers are to continue improving, a If computers are to continue improving, a new method of computing must be new method of computing must be created.created.

Quantum ComputingQuantum Computing

Problems with Quantum Problems with Quantum Computing:Computing:

QubitQubit

The qubits need to be protected from the The qubits need to be protected from the environment. This way they stay in the environment. This way they stay in the quantum phase.quantum phase.

The qubits need to be coupled to the The qubits need to be coupled to the environment. It won’t be any good to have environment. It won’t be any good to have a qubit and not be able to read it.a qubit and not be able to read it.

These facts contradict each other.These facts contradict each other.

What do we do about it?What do we do about it?

We know it is hard to make a quantum We know it is hard to make a quantum computer stay a quantum computer.computer stay a quantum computer.

Two scientists, Neil Gershenfeld and Isaac Two scientists, Neil Gershenfeld and Isaac Chuang have developed a new method for Chuang have developed a new method for quantum computing that may bypass the quantum computing that may bypass the original constraints.original constraints.

New method:New method:

Don’t try to isolate separate qubits, but use Don’t try to isolate separate qubits, but use a large thermal ensemble.a large thermal ensemble.

Apply RF pulses to the system, Apply RF pulses to the system, resonances occur and it creates a resonances occur and it creates a deviation from equilibrium that simulates deviation from equilibrium that simulates the behavior of qubits.the behavior of qubits.

New method:New method:

The spin of the particles is isolated from The spin of the particles is isolated from the environment, and can therefore last the environment, and can therefore last much longer than previous computers.much longer than previous computers.

This means that more computations can This means that more computations can be done before the coherence is lost.be done before the coherence is lost.

AlsoAlso

Because the qubits are together in a Because the qubits are together in a group, the quantum state can be read.group, the quantum state can be read.

This couldn’t be done by attempting to This couldn’t be done by attempting to read the state of an individual qubit.read the state of an individual qubit.

AlsoAlso

The most important part of the computer The most important part of the computer uses ordinary molecules. It is nothing uses ordinary molecules. It is nothing new.new.

A source of resonance is needed, such as A source of resonance is needed, such as a Nuclear Magnetic Resonance a Nuclear Magnetic Resonance spectrometer.spectrometer.

This will allow for around 10 qubits.This will allow for around 10 qubits.

If…If…

If this new method works,If this new method works,

The new computer could demonstrate The new computer could demonstrate extremely fast algorithms and quantum extremely fast algorithms and quantum error correction.error correction.

New quantum states could be realized that New quantum states could be realized that violate Bell’s Theorem.violate Bell’s Theorem.

The computer could eventually be the size The computer could eventually be the size of a current desktop.of a current desktop.