Quantum Computing – Part 2 Amanda Denton – Evil Dictator Jesse Millikan – Mad Scientist Lee Ballard – Some Guy With A Beard September 30, 2001

A Brief History of Quantum Computing How in the heck did this stuff get started anyway?

Parallel Universes 1950 – “The Garden of Forking Paths” is written by Jorge Luis Borges “In all fictional works, each time a man is confronted with several alternatives, he chooses one and eliminates the others. In the fiction of Ts’ui Pen, he chooses – simultaneously – all of them.” “In all fictional works, each time a man is confronted with several alternatives, he chooses one and eliminates the others. In the fiction of Ts’ui Pen, he chooses – simultaneously – all of them.”

From Humble Origins Rolf Landauer – “computation is physics and you cant understand the limits of computation without saying what the physical implementation is.” David Deutsch proposes the first quantum computer in Not really a quantum computer, but actually a conventional computer that operated by quantum means and had additional hardware that allowed it to do extra things. Not really a quantum computer, but actually a conventional computer that operated by quantum means and had additional hardware that allowed it to do extra things.

The Multi-verse In the early 1980’s, Deutsch proposed an experiment to test the idea of parallel universes. Only problem was that Deutsch’s experiment would need to be carried out on a machine that no one had and no one knew how to build. David Deutsch’s ideas were published in 1985 in the International Journal of Theoretical Physics

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

Basic Q.C. One moose, two moose Red moose, blue moose Live 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 States

“A full system of m qubits has a basis of 2 m 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 states *Introduction to Quantum Computation and Information (Lo, Popescu, Spiller 2000)

Entanglement 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 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, Decoherence “[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 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 Computing and Bulk Spin Resonance Quantum Computing

Problems with current computing: As computers get faster and smaller, limits will be reached that cannot be broken. If computers are to continue improving, a new method of computing must be created. Quantum Computing

Problems with Quantum Computing: Qubit The qubits need to be protected from the environment. This way they stay in the quantum phase. The qubits need to be coupled to the environment. It won’t be any good to have a qubit and not be able to read it. These facts contradict each other.

What do we do about it? We know it is hard to make a quantum computer stay a quantum computer. Two scientists, Neil Gershenfeld and Isaac Chuang have developed a new method for quantum computing that may bypass the original constraints.

New method: Don’t try to isolate separate qubits, but use a large thermal ensemble. Apply RF pulses to the system, resonances occur and it creates a deviation from equilibrium that simulates the behavior of qubits.

New method: The spin of the particles is isolated from the environment, and can therefore last much longer than previous computers. This means that more computations can be done before the coherence is lost.

Also Because the qubits are together in a group, the quantum state can be read. This couldn’t be done by attempting to read the state of an individual qubit.

Also The most important part of the computer uses ordinary molecules. It is nothing new. A source of resonance is needed, such as a Nuclear Magnetic Resonance spectrometer. This will allow for around 10 qubits.

If… If this new method works, The new computer could demonstrate extremely fast algorithms and quantum error correction. New quantum states could be realized that violate Bell’s Theorem. The computer could eventually be the size of a current desktop.