13.6 Conclusion 293desired energy level.(3) Ion traps confined the ions for long periods with buffer gas cooling.
(4) The advent of laser cooling of atoms meant that spectroscopists were
no longer passive observers, but could magnetically trap neutral
atoms and control the atom’s motion. Laser cooling of ions can put
them into the ground state of the trap to give a completely defined
external state.
(5) Quantum computing techniques allow the manipulation of both the
external and internal state of the trapped ion qubits with laser light,
to put a system of many ions intoa particular quantum state.
(6) Quantum error correction can be regarded as a refined form of laser
cooling. QEC puts the ions back into a coherent superposition of
the desired states of the system (actually a particular sub-space of
Hilbert space), rather than the lowest energy state as in conven-
tional cooling. It was a real surprise that decoherence in quantum
mechanics can be overcome in this way and actively stabilise quan-
tum information.
13.6 Conclusion
At heart, quantum mechanics remains mysterious. Certain aspects of
quantum behaviour seem strange to our intuition based on the classi-
cal world that we experience directly, e.g. the well-known examples of
‘which way the photon goes’ in a Young’s double-slit experiment and
Schr ̈odinger’s cat paradox.^12 These examples have provoked much dis-^12 See quotation from Heisenberg in the
background reading section of the Pref-
ace.
cussion and thought over the years about the so-called quantum mea-
surement problem. Nowadays, physicists do not regard the peculiarities
of quantum systems as a problem at all, but rather as an opportunity.
A proper appreciation of the profoundly different properties of multiple-
particle systems and the nature of entanglement has shown how to use
their unique behaviour in quantum computing. Just as a car mechanic
uses her practical knowledge to get an engine working, without worry-
ing about the details of internal combustion, so a quantum mechanic
(or quantum state engineer) can design a quantum computer using the
known rules of quantum mechanicswithout worrying too much about
philosophical implications. Physicists, however, strive towards a bet-
ter understanding of the quantum world and consideration of the pro-
found and subtle ideas in quantum information theory sheds new light
on aspects of quantum mechanics. In physics research there often exists
a symbiotic relationship between theory and experimental work, with
each stimulating the other; a prime example is quantum error correc-
tion which was not thought about until people started to face up to
the fact that none of the systems used in existing experiments has any
possibility of working reliably with a useful number of qubits (without
error correction)—of course, the problem of decoherence has always been
recognised but experimental results focused attention on this issue.