Science Magazine - 2 November 2012

PERSPECTIVES
For quantum weirdness with more kick
to it, we need look no further than the two
delayed-choice experiments of Kaiser et
al. and Peruzzo et al. Both experiments use
quantum entanglement to delay the choice
of what quantum effects are demonstrated
not merely until after the photon has entered
the interferometer, but until after the pho-
ton has emerged from the interferometer
and the measurement that detects it has
already taken place. In the first proquasti-
nation experiment, polarizing beam split-
ters ensure that vertically polarized photons
entering the Mach-Zehnder interferometer
undergo quantum interference, while hori-
zontally polarized photons do not. Photons
whose polarization is in between vertical
and horizontal—diagonally polarized pho-
tons—exhibit partial interference.
There is nothing here that the two Lud-
wigs, Mach and Zehnder, couldn’t already
have observed in the early 1890s, but now
the tricky part comes in. Kaiser et al. do not
send a photon with a definite polarization
into the interferometer. Rather, they send a
photon whose polarization is entangled with
the polarization of a second photon. After
the first photon has already emerged from
the interferometer and the port by which it
has emerged has been detected, Kaiser et
al. measured the polarization of the second
photon. If they measure the polarization of
the second photon along the vertical/hori-
zontal axis and obtain the result “horizon-
tal,” then the first photon has behaved like a
particle: No interference has taken place. If
they obtain the result “vertical,” then the first
particle has behaved like a wave, and inter-
ference has taken place.

Although the two quantum procrastina-
tion experiments reported here delay the
choice of whether to exhibit wave- or par-
ticle-like nature of entangled particles for
just a few nanoseconds, if one has access
to quantum memory in which to store the
entanglement, the decision could be put off
until tomorrow (or for as long as the memory
works reliably). So why decide now? Just let
those quanta slide! Sadly, the applications of
quantum procrastination are for the moment
limited to making only a few highly quan-
tum types of decision ex post facto. I wish
I had decided to start writing this article a
week before it was due, but no amount of
entanglement can hide that I decided to the
day before.
References
1. F. Kaiser et al., Science 338, 637 (2012).
2. A. Peruzzo, P. Shadbolt, N. Brunner, S. Popescu,
J. L. O’Brien, Science 338, 634 (2012).
3. J. A. Wheeler, in Quantum Theory and Measurement, J. A.
Wheeler, W. H. Zurek, Eds. (Princeton Univ. Press, Princ-
eton, NJ, 1984), pp. 182–213.
4. L. Zehnder, Z. Instrumentenkunde 11, 275 (1891).
5. L. Mach, Z. Instrumentenkunde 12, 89 (1891).
6. V. Jacques et al., Science 315, 966 (2007).
7. M. A. Nielsen, I. L. Chuang, Quantum Computation and
Quantum Information (Cambridge Univ. Press, Cam-
bridge, UK, 10th anniversary ed., 2011).
8. Y.-H. Kim, R. Yu, S. P. Kulik, Y. Shih, M. O. Scully, Phys.
Rev. Lett. 84, 1 (2000).
9. J. S. Bell, Physics 1, 3 (1964
10.1126/science.1229825
PLANT SCIENCE
Chloroplast Delivery by UPS
Felix Kessler

Identification of a membrane-anchored E3
ligase in plants reveals a role for the ubiquitin
proteasome system in chloroplast development.
Chloroplasts are the organelles of photosynthesis in plants and are responsibleformuchofthefoodand
biomass production on our planet. But chlo-
roplasts are only the best-known members
of an extended family of organelles termed
plastids. Their name suggests plasticity
and, indeed, plastids exist in various incar-
nations depending on developmental cues
(e.g., nonphotosynthetic etioplasts in dark-
grown leaves, colored chromoplasts in pet-
als and fruit, and starch-storing amyloplasts
in roots). Yet, the mechanisms underlying
the transformation from one plastid type to
another are largely unknown. On page 655
in this issue, Ling et al. (1) show that the
Institute of Biology, University of Neuchâtel, CH-2000
Neuchâtel, Switzerland. E-mail: felix.kessler@unine.ch
ubiquitin-26S proteasome system (UPS)
directly targets plastids and promotes chlo-
roplast biogenesis, controlling yet another
important facet of cell biology.
Plastids originate from an endosymbiotic
process that started ~1.5 billion years ago
when a eukaryotic host cell engulfed a pho-
tosynthetic prokaryote. Over time, the two
organisms became almost completely inte-
grated. A permanent and ongoing flow of
genetic material from the prokaryotic endo-
symbiont resulted in the transfer of most
plastid protein-encoding genes to the host
nucleus (2). The Arabidopsis chloroplast
today has ~2000 proteins (3, 4), only 87 of
which are encoded in the organelle. Con-
currently with their transfer to the nucleus,
the former endosymbiont genes acquired
genetic information encoding amino-termi-
nal targeting sequences resulting in synthe-
sis of preproteins in the cytosol. The amino-
terminal sequences enable the recognition
and the translocation of preproteins across
the dual-membrane chloroplast envelope
and are later removed.

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