Quantum
Computers
As if computers weren’t fast enough
already…
Brief Outline
 Introduction – What is a quantum
computer?
 Differences from classical computers
 Power(speed,efficiency, etc.)
 History
 Problems
 Conclusion
What is a quantum computer?
 Computer using the laws of quantum mechanics
 Utilizes physical phenomena
 Quantum interference
 Superposition
 May seem to defy logic
 Mostly theoretical – many of the ideas cannot work due
to quantum nature
Photon Split?
->
No!
Classical
Quantum
Idea
Idea
The difference between classical
and quantum computers
 Classical
 Bits – storage in 0’s and 1’s
 Uses Boolean logic gates to manipulate bits
 Macroscopic physical storage
 Charge, magnetization, etc.
 Governed by the same laws as everyday phenomena
 Can only manipulate one piece of data at a time
 Quantum
 Qubits – storage in 0’s, 1’s, or a superposition of both.
 Executes quantum gates to act as unitary transformations on
qubits
 Uses quantum laws which differ greatly from everyday
phenomena
 Can manipulate many pieces of data at once
Computing speed and efficiency
 Qubit can hold as many values at once
as a bit has range to hold
 3 bits: can hold one number up to 8
 3 qubits: can hold 8 numbers up to 8
 Can perform many computations at the
same time, as opposed to performing
them one at a time
 Factoring, as an example
 Peter Shor, AT&T Bell Laboratories, New Jersey
History
 Idea developed from the idea of elements being small enough to
behave on the quantum level
 1982 – Feynman comes up with idea for computations. Says
quantum computers can be used for quantum simulations.
 1985 – Duetsch realized Feynman’s assertion could lead to a
general purpose computer and published a paper that showed
how.
 1994 – Shor circulates a preprint of a paper showing how quantum
computers could solve many mathematical problems many times
faster than classical computers (namely factorization).
 1995 – Theory of quantum error correction
 1998 – Researchers at Los Alamos National Laboratory and MIT
led by Raymond Laflamme spread a qubit across three nuclear
spins in each molecule of a liquid solution of alanine or
trichloroethylene molecules – possible with NMR (nuclear
magnetic resonance)
 Spread makes it harder to corrupt – can indirectly measure
decoherence and compare spin states
 Present - http://www.cs.berkeley.edu/~vandam/homes.html
Problems
 Decoherence – tendancy of quantum computer to
decay as it interacts (entangles) with the outside
environment
 Qubits cannot be directly measured – lose
superposition and change to 0 or 1 when attempted
 May try to set up an algorithm to cycle through qubits after they
are manipulated, narrowing down the possible answers, until
the probability of the right answer showing up is close to 100%
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Phase coherence may be used for error correction
Architecture is too complicated, large, or expensive
World is too dependent on current computer systems
Ideas are fundamentally more difficult than classical
computers
Conclusion
 Quantum computer research is making breakthroughs
 We may someday have quantum computers for generic use
 Quantum computers could provide faster or better
methods of doing most anything (encryption, as an
example)
 There are too many problems for such breakthroughs
to occur in the near future
 For those of you who have taken Quantum A & B:
Quantum physics IS practical for something in our
everyday lives – your nights and weekends spent on
homework were not spent in vain!
References
 http://library.thinkquest.org/C008537/qua
ntum/computers/computers.html
 http://www.cs.caltech.edu/~westside/qua
ntum-intro.html