tions, encompassing hundreds of species that
occupy various ecological niches across rep-
licate adaptive radiations.
To keep up with these advances on the
molecular and genomic aspects of cichlid
diversification, it will be important to increase
the efforts at the organismal and life-history
level by surveying ecology, morphology, and
behavior. This integration would make cich-
lids a role model not only for adaptive radia-
tion and explosive speciation but also for the
survey of interactions at all levels of biologi-
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Entangling two photons allows the wave and
particle nature of light to be interchanged even
after the light has already been detected.
Do you have a decision you have to make but you just can’t bring your- self to do it? As the irrevocable
moment approaches, you squirm more and
more, but something inside you says, “Not
now, not yet.” Then when it’s already almost
too late, in a burst of energy and shame, you
come through—or not. Afterward, you are
irrationally resentful, as if someone other
than yourself is responsible for disturbing
your peace of mind. You vow that the next
time a decision arises, you will make it expe-
ditiously. If you are a severe procrastinator
like me (at least when it came to starting
this article), have hope—quantum mechan-
ics is coming to your rescue. On pages 637
and 634 of this issue, experiments by Kai-
) and Peruzzo
) show that
in the presence of quantum entanglement
(in which outcomes of measurements are
tied together), it is possible to hold off mak-
ing a decision, even if events seem to have
already made one. Quantum procrastination
(“proquastination”) allows you to put off for
tomorrow what you should have done today.
The experiments are based on Wheel-
er’s famous delayed-choice experiment (
Although photons are particles of light,
they also possess a wavelike nature and can
exhibit interference effects. Suppose that the
path lengths of a Mach-Zehnder interferom-
) have been tuned to make the pho-
ton come out of one port of the final beam
splitter with probability 1 (see the figure).
After the photon has passed the first beam
splitter, so that it is fully inside the interfer-
ometer, and before it has reached the sec-
ond beam splitter, you decide to whisk away
that second beam splitter, preventing any
interference between the photon’s two paths
from taking place. Without interference, the
photon behaves like a particle and emerges
with equal probability out of either of the
two ports of the apparatus where the second
beam splitter used to be.
Photons Beam splitter
Department of Mechanical Engineering, Massachusetts
Institute of Technology, Cambridge, MA 02139, USA.
the two paths that the single particle takes
in quantum superposition, and the pho-
ton would emerge from only one port with
probability 1. That is, even though you have
delayed the choice of removing the beam
splitter until after the photon—if it really
were a classical particle—should be travel-
ing along one path or the other, by restor-
ing the beam splitter, you can reinstate the
photon’s wavelike nature and have it report
that it was traveling along both paths simul-
Since Wheeler proposed his delayed-
choice gedanken experiment
in 1984, a horde of theories
and experiments exhibit-
ing weird quantum effects
has spread across the sci-
entific landscape, including
tions of Wheeler’s proposal
). Quantum information
theory has supplied a general
language for discussing such
quantum weirdness, and
small but effective quantum
information processors have
provided the wherewithal to
demonstrate virtually any
effect of quantum superpo-
sition and entanglement on
a small number of quantum
). As effects such as
experiment and its relatives,
such as the quantum eraser
), have become common-
place, they have lost some of
their power to amaze.
Welcomed delays. Two studies use quantum entanglement in delayed
choice experiments; the outcome for the first photon detected (whether
it is a particle or a wave or has intermediate character) is determined by
later measurements. Kaiser
. entangle the first photon’s polariza-
tion with that of the second photon, so that its outcome depends on the
second photon’s polarization. Peruzzo
. entangle the photon with
the presence or absence of a beam splitter in the setup and again delay
the outcome of the first photon’s state. If the photon states could be
stored in quantum memories, it might be possible to delay the outcome
of the first photon detection (on a Tuesday) until the observer makes a
choice on Wednesday.
in this issue
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