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John Jurgiel
Schrödinger's Cat and Computer Mice
You have a cat. One day, for reasons unfathomable, you decide it is time to put
Whiskers in a box with a curious device. After acquiring some small scratches, you manage to
seal your cat in a box with a canister of poison, with a 50% chance of killing your cat. But you
love your cat, and feel bad about how you treated poor Whiskers. Do not worry, your cat is still
alive. And dead. Whiskers is alive, sure, but he's also dead. What?
This scenario is the famous example of quantum superposition, dubbed "Schrödinger's
cat," and demonstrates how strange quantum mechanics can be. While common sense would
say the cat has to be alive or dead, quantum mechanics says it can be both, and it can seem like
magic. Cute, you might think, but what does this have to do with me? Even if you are not a
physicist, and you love your cats dearly, the mystique of quantum physics will soon be affecting
you too: quantum computers are being developed and used right now, and soon the
technology might even be in your home. Just how weird are quantum computers though? And
what do they do that modern computers cannot? The answers are: very weird, and
revolutionize computing as we know it.
What Makes a Quantum Computer
The only difference between quantum computers and modern computers is that
quantum computers use something called qubits instead of bits to store information.
Computers today operate in bits, which are either a 0 or 1, generally represented with an
electrical charge. Qubits, which can be represented using many different mediums like the spin
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of electrons or polarization of photons, store data in what physicists call a "superposition" of 0
and 1 [1]. Superposition is a fancy term, but what does it mean?
A superposition is partly a measurement of probability; you can think of a coin flip as a
superposition of heads and tails, with a 50% chance of each. This probability does not need to
be even -- a random playing card is a superposition of spades (25%) and not-spades (75%) -- nor
just between two variables -- a random playing card is a superposition of all four suits (25%
each).
But superposition is more than just a set of probabilities: superposition also has the
same dual-reality component described by the Schrödinger's cat concept. If you flipped a coin
using the rules of superposition, but do not check the result, is it heads or tails? Common sense
dictates that it must be one or the other, but you do not know which. Quantum mechanics
disagrees, and according to the rules of superposition, says the result of the flip is both heads
and tails, until the result is observed or interacted with by some outside source. Schrödinger's
cat demonstrates the same concept, but with a cat's life rather than a coin flip. Superposition
means that the system described is both results until observed: the coin flip is heads and tails,
the qubit is a 0 and a 1, and Schrödinger's cat is alive and dead until we check and see the flip is
actually just tails, the qubit is a 1, and the Whiskers is alive and angry about being put in a box.
Now wait a minute, this makes no sense. How can the qubit be a 0 and a 1? And why
does looking at a result change the value? The second question has to do with the nature of
quantum mechanics: objects measured are so small and sensitive, that even the most subtle
detection methods will change their properties (and eliminates the superposition state). This is
called the "uncertainty principle," and while it can be used in some technologies, generally
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makes quantum technology too sensitive to be very useful. Unfortunately, for the first question,
there is no simple answer as to how, and it is strange to think about. The best answer is an
unjustified and unsatisfying "because." And designing a computer off this characteristic has
powerful implication on computing methods.
Faster Computers
Graph illustrating Moore's Law, courtesy of singularity.com
Moore's Law
describes an interesting
phenomenon he noticed
about progress in
integrated circuits: about
every two years, the
number of transistors
that designers can fit on a
silicon chip doubles with
no increase in cost [2].
While Gordon Moore was
only talking about
integrated circuits, this pattern actually describes computers since their conception: about
every two years they get twice as fast. While we still use integrated circuits today, but we are
approaching the limit of how small we can make them. Physical limitations mean we cannot
build integrated circuits smaller than the atomic level, and developers are already very close to
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this limit. In order to build faster computers, and continue the trend defined by of Moore's Law,
a different smaller system must be developed.
Quantum computers can process much faster than modern computers can because of
the superposition of their qubits. Qubits represent both a 0 and a 1 simultaneously, so each
single operation tests both by the nature of quantum computer design [1]. This may not sound
significant, but when you have a two-qubit input, the algorithm tests four inputs (00, 01, 10, 11)
in the time it normally takes to test one; three qubits, eight tests, eight times faster. For n
qubits, the algorithm tests 2n inputs and finishes 2n times faster, which makes random search
algorithms exponentially faster [3]. Processors as large as 128-qubits have been developed [4],
which operates at 2128 or 3.403 x 1038 times the speed of classical computers. Systems this fast
will have a huge impact on computing methods, especially in security.
Security Systems
Quantum computing is so fast, it would render most modern computer security systems
obsolete with its vivacious processing abilities. Many security processes rely on solutions to
crack them taking too long to be practical, but with quantum computers guessing so quickly,
these systems would not be secured [5]. One of the most well known and inefficient methods
of hacking into a computer system is called the "brute-force" method, where a computer cycles
through all possible password combinations, trying them one by one to try and gain access to a
system. On a modern computer, these blind guesses take years to complete, and the practice is
not very often used. However, quantum computers try multiple combinations at the same time,
and make the process almost practical to apply. Many other security systems rely on processes
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that are not impossible to crack, but just take too long to do so, and they would also become
invalidated [5].
Quantum technology does offer more secure methods of communication, however.
According to quantum mechanics, as previously mentioned, it is impossible to know everything
about a quantum object because all methods we could use to measure it end up changing the
object. Just like opening up Whisker's box to see how he feels about the situation means he is
either alive or dead, but not both anymore, measuring a quantum object destroys its
superposition. This is called the uncertainty principle, and it means that trying to record
quantum data will change it. If we use quantum techniques to generate a signal, then observing
the signal will actually change it. Using this principle, engineers have established systems of
quantum communications where anyone trying to eavesdrop on the message only hears
gibberish, because by listening he corrupts the data. Furthermore, because the signal has been
tampered with, the receiving party also hears gibberish, and knows someone is trying to listen
to his communications. Banks have been using this technology since 2004, and not only does it
greatly increase security, but it also remains secure even when quantum computers are used
[6].
Technical Difficulties
While dreams and development for the quantum computer began in 1982 with Nobel
physicist Richard Feynman, one huge technical problem continues to make quantum computers
expensive and difficult to operate: decoherence [7]. Decoherence occurs when outside forces
interfere with qubits and change their values. Decoherence for qubits is like rubbing a magnet
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over your modern computer: the magnet will change the electrical charge of the bits in your
computer and destroy all information you have. Likewise, decoherence breaks the
superposition of qubits, and erases data. Unlike modern computers though, decoherence
occurs much more easily, and exactly how depends on what medium is used. Furthermore,
designers cannot simply isolate all qubits from the outside world, because they still need to
interact with the computer. Trying to appropriately isolate the qubit from decoherence is the
biggest obstacle in making quantum computers commonplace today: it can be done, but
methods are impractical [5].
While not presently functional, parts of quantum computer are being built. USC has
recently paired with a significant industry leader to house and operate a 128-qubit quantum
processor [4], and many other organizations possess parts of varying complexity. However, the
hardware requirements are very stringent: facilities must keep hardware very close to absolute
zero (USC's operates at 20 microKelvin), and elaborate electromagnetic shielding must be
implemented to prevent decoherence [4]. Furthermore, not all parts of the quantum computer
have been created, and connecting the pieces that do is still a big obstacle to creating the
quantum computer. Due to the fragility of qubits, quantum computers will probably not
become commonplace within the decade, but the technology is far from undeveloped, and
hopefully soon will be making a large impact on the world around it [8].
Image suggestions for Illumen: possibly a more detailed chart demonstrating Moore's law,
possibly a picture or chart to illustrate the magnitude of how much faster quantum computers can
operate, or possibly just a picture of a computer or a cat for the relevant sections.
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Sources
1
: Joseph B Altepeter. (2010). A tale of Two Qubits: How Quantum Computers Work. [Online].
Available: http://arstechnica.com/science/guides/2010/01/a-tale-of-two-qubits-howquantum-computers-work.ars/1
2
: PBS. (1999). Gordon Moore.[Online]. Available:
http://www.pbs.org/transistor/album1/moore/index.html
3
: A. Barenco, A. Ekert, A. Sanpera, and C. Machiavello. (2011). A Short Introduction to
Quantum Computation. [Online]. Available: http://www.qubit.org/tutorials/25-quantumcomputing.html
4
: Viterbi School of Engineering. (Oct 29, 2011). Operational Quantum Computing Center
Established at USC. [Online]. Available:
http://viterbi.usc.edu/news/news/2011/operational-quantum-computing334119.htm
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: Alan Boyle. (May 18, 2000). A Quantum Leap in Computing. [Online]. Available:
http://www.msnbc.msn.com/id/3077363/ns/technology_and_science-science/t/quantumleap-computing/#.TrjMX0P7g8V
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: Bank of Austria Creditanstalt. (April 21, 2004). World Premiere: Bank Transfer via Quantum
Cryptography Based on Entangled Photons. [Press Conference, summary online].
Available: http://colossalstorage.net/quantum_entanglement_austria.pdf
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: Simone Bone and Matias Castro. A Brief History of Quantum Computing. [Online]. Available:
http://www.doc.ic.ac.uk/~nd/surprise_97/journal/vol4/spb3/#6. Current progress & future
prospects
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: Matt Buchanan. (Aug 19, 2009). Giz Explains: Why Quantum Computing is the Future (But a
Distant One). [Online]. Available: http://gizmodo.com/5335901/giz-explains-whyquantum-computing-is-the-future-but-a-distant-one
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