Quantum Quantum ComputersComputersAs if computers weren’t fast enough As if computers weren’t fast enough already…already…

Brief OutlineBrief Outline Introduction – What is a quantum Introduction – What is a quantum

computer?computer? Differences from classical computersDifferences from classical computers Power(speed,efficiency, etc.)Power(speed,efficiency, etc.) HistoryHistory ProblemsProblems ConclusionConclusion

What is a quantum computer?What is a quantum computer?

Computer using the laws of quantum mechanicsComputer using the laws of quantum mechanics Utilizes physical phenomenaUtilizes physical phenomena

Quantum interferenceQuantum interference SuperpositionSuperposition

May seem to defy logicMay seem to defy logic Mostly theoretical – many of the ideas cannot work due Mostly theoretical – many of the ideas cannot work due

to quantum natureto quantum nature

Classical Idea

Quantum Idea

Photon Split? ->

No!

The difference between classical and The difference between classical and quantum computersquantum computers

ClassicalClassical Bits – storage in 0’s and 1’sBits – storage in 0’s and 1’s Uses Boolean logic gates to manipulate bitsUses Boolean logic gates to manipulate bits Macroscopic physical storageMacroscopic physical storage

Charge, magnetization, etc.Charge, magnetization, etc. Governed by the same laws as everyday phenomenaGoverned by the same laws as everyday phenomena Can only manipulate one piece of data at a timeCan only manipulate one piece of data at a time

QuantumQuantum Qubits – storage in 0’s, 1’s, or a Qubits – storage in 0’s, 1’s, or a superpositionsuperposition of both. of both. Executes quantum gates to act as Executes quantum gates to act as unitary transformationsunitary transformations on on

qubitsqubits Uses quantum laws which differ greatly from everyday Uses quantum laws which differ greatly from everyday

phenomenaphenomena Can manipulate many pieces of data at onceCan manipulate many pieces of data at once

Computing speed and efficiencyComputing speed and efficiency

Qubit can hold as many values at once as Qubit can hold as many values at once as a bit has range to holda bit has range to hold 3 bits: can hold one number up to 83 bits: can hold one number up to 8 3 qubits: can hold 8 numbers up to 83 qubits: can hold 8 numbers up to 8

Can perform many computations at the Can perform many computations at the same time, as opposed to performing same time, as opposed to performing them one at a timethem one at a time Factoring, as an exampleFactoring, as an example

Peter Shor, AT&T Bell Laboratories, New JerseyPeter Shor, AT&T Bell Laboratories, New Jersey

HistoryHistory Idea developed from the idea of elements being small enough to behave Idea developed from the idea of elements being small enough to behave

on the quantum levelon the quantum level 1982 – Feynman comes up with idea for computations. Says quantum 1982 – Feynman comes up with idea for computations. Says quantum

computers can be used for quantum simulations.computers can be used for quantum simulations. 1985 – Duetsch realized Feynman’s assertion could lead to a general 1985 – Duetsch realized Feynman’s assertion could lead to a general

purpose computer and published a paper that showed how.purpose computer and published a paper that showed how. 1994 – Shor circulates a preprint of a paper showing how quantum 1994 – Shor circulates a preprint of a paper showing how quantum

computers could solve many mathematical problems many times faster computers could solve many mathematical problems many times faster than classical computers (namely factorization).than classical computers (namely factorization).

1995 – Theory of quantum error correction1995 – Theory of quantum error correction 1998 – Researchers at Los Alamos National Laboratory and MIT led by 1998 – Researchers at Los Alamos National Laboratory and MIT led by

Raymond Laflamme spread a qubit across three nuclear spins in each Raymond Laflamme spread a qubit across three nuclear spins in each molecule of a liquid solution of alanine or trichloroethylene molecules – molecule of a liquid solution of alanine or trichloroethylene molecules – possible with NMR (nuclear magnetic resonance)possible with NMR (nuclear magnetic resonance) Spread makes it harder to corrupt – can indirectly measure Spread makes it harder to corrupt – can indirectly measure

decoherencedecoherence and compare spin states and compare spin states Present - Present - http://www.cs.berkeley.edu/~vandam/homes.htmlhttp://www.cs.berkeley.edu/~vandam/homes.html

ProblemsProblems Decoherence – tendancy of quantum computer to decay Decoherence – tendancy of quantum computer to decay

as it interacts (entangles) with the outside environmentas it interacts (entangles) with the outside environment Qubits cannot be directly measured – lose superposition Qubits cannot be directly measured – lose superposition

and change to 0 or 1 when attemptedand change to 0 or 1 when attempted May try to set up an algorithm to cycle through qubits after they May try to set up an algorithm to cycle through qubits after they

are manipulated, narrowing down the possible answers, until the are manipulated, narrowing down the possible answers, until the probability of the right answer showing up is close to 100%probability of the right answer showing up is close to 100%

Phase coherence may be used for error correctionPhase coherence may be used for error correction Architecture is too complicated, large, or expensiveArchitecture is too complicated, large, or expensive World is too dependent on current computer systemsWorld is too dependent on current computer systems Ideas are fundamentally more difficult than classical Ideas are fundamentally more difficult than classical

computerscomputers

ConclusionConclusion Quantum computer research is making breakthroughsQuantum computer research is making breakthroughs

We may someday have quantum computers for generic useWe may someday have quantum computers for generic use Quantum computers could provide faster or better Quantum computers could provide faster or better

methods of doing most anything (encryption, as an methods of doing most anything (encryption, as an example)example)

There are too many problems for such breakthroughs There are too many problems for such breakthroughs to occur in the near futureto occur in the near future

For those of you who have taken Quantum A & B:For those of you who have taken Quantum A & B: Quantum physics IS practical for something in our Quantum physics IS practical for something in our everyday lives – your nights and weekends spent on everyday lives – your nights and weekends spent on homework were not spent in vain! homework were not spent in vain!

ReferencesReferences http://library.thinkquest.org/C008537/quahttp://library.thinkquest.org/C008537/qua

ntum/computers/computers.htmlntum/computers/computers.html http://www.cs.caltech.edu/~westside/http://www.cs.caltech.edu/~westside/

quantum-intro.htmlquantum-intro.html