Transcript Topic #2

CHEMISTRY 2000
Topics of Interest #2:
Quantum Computers
Quantum Computers Calculate Energy of H2
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This month, scientists at Harvard University and the University of
Queensland (Australia) reported that they had used a quantum
computer to calculate the energy of a hydrogen molecule. This is a
significant advance (compared to other recent reports of using
quantum computers to factor the number 15 or to solve a Sudoku
puzzle).
Conventional computers can perform this calculation, but it is
anticipated that quantum computers will be able to calculate the
exact energies of molecules far larger than those which
conventional computers can handle. (The current limit for exact
calculations on conventional computers is 4-5 atoms. For larger
molecules/systems, we use methods that make approximations.)
http://esciencenews.com/articles/2010/01/10/quantum.computer.calculates.
exact.energy.molecular.hydrogen
Quantum Computers vs. Conventional
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How do quantum computers differ from conventional computers?
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Conventional computers store information in bits. A bit has a value of
either 0 or 1. Several bits are required to store a single number.
e.g. “4” would be stored as 1 0 0 (requiring at least three bits)
Quantum computers store information in qubits. A qubit can store both
values 0 and 1 at the same time. Instead of storing just one number
(e.g. “4”), three qubits can store eight numbers simultaneously:
0 0 0 = “0”
1 0 0 = “4”
0 0 1 = “1”
1 0 1 = “5”
0 1 0 = “2”
1 1 0 = “6”
0 1 1 = “3”
1 1 1 = “7”
This is referred to as “superposition”. The most famous example of
superposition is the thought experiment known as Schrödinger’s Cat.
The superposition will only be broken when the qubit is read.
http://www.toqc.com/Default.aspx?tabid=1 Version 1.1 (an online intro. text)
Quantum Computers vs. Conventional
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Since a three qubit ‘quantum computer’ can store eight times as many
values as a three bit ‘conventional computer’, it should be able to do
calculations eight times faster and therefore potentially eight times
more complex (8 = 23). (Some leeway has been taken here in an order
to avoid maths/physics far beyond the level of this course.)
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A twenty qubit computer would be able to store 220 = 1,048,576 times
as many numbers as a twenty bit computer.
A hundred qubit computer would be able to store 2100 =
1,267,650,600,228,229,401,496,703,205,376 times as many numbers
as a hundred bit computer. (For reference, todays computers typically
contain Gigabytes where 1 byte = 8 bits so 1 GB = 8,000,000,000 bits.
Quantum computers are still operating in the double digit qubit range
but are getting bigger.)
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While this scaling is impressive, the real value of quantum computers is
that their quantum nature allows them to run algorithms that
conventional computers cannot. There is certainly logic in the idea of
applying a quantum computer to solve quantum mechanical problems.
http://www.toqc.com/Default.aspx?tabid=1 Version 1.1 (an online intro. text)
Quantum Computers Still Pretty Small
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16 qubit quantum computer (solved a Sudoku):
© 2007 D-Wave Systems Inc.