Quantum Information Processing, Vol. 5, No. 4, August 2006 (© 2006)
In Praise of Measurement
N. David Mermin
Received October 10, 2005; accepted December 29, 2005; Published online June 6, 2006
The role of measurement in quantum computation is examined in the light of
John Bell’s critique of the how the term “measurement” is used in quantum
mechanics. I argue that within the ﬁeld of quantum computer science the concept
of measurement is precisely deﬁned, unproblematic, and forms the foundation of
the entire subject.
KEY WORDS: Quantum computation; measurement.
PAC S : 03.67Lx; 03.65 Ta; 03.67.-a
Here are some words which ...have no place in a formulation with any pretension to
physical precision: system, apparatus, environment, microscopic, macroscopic, revers-
ible, irreversible, observable, information, measurement. On this list of bad words the
worst of all is “measurement”. ... What exactly qualiﬁes some physical systems to
play the role of “measurer”? ... The word has had such a damaging effect on the
discussion, that I think it should now be banned altogether in quantum mechanics.
J. S. B e l l
In our description of nature the purpose is not to disclose the real essence of the
phenomena but only to track down, so far as it is possible, relations between the
manifold aspects of our experience. Niels Bohr
In his elegant tirade against measurement, John Bell declared that we
lack “an exact formulation of some serious part of quantum mechanics.”
He explained that by “exact” he meant “fully formulated in mathematical
terms, with nothing left to the discretion of the theoretical physicist.” And
by “serious” he meant that “some substantial fragment of physics should
be covered” and that “ ‘apparatus’ should not be separated off from the
rest of the world into black boxes.”
Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, NY 14853-2501,
USA. E-mail: firstname.lastname@example.org
1570-0755/06/0800-0239/0 © 2006 Springer Science+Business Media, Inc.