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Survival of the entangled
William Barnes
At the quantum level, common sense is often violated — for example, by
pairs of entangled photons in which each seems to ‘know’ about the state of
the other. Entanglement may be more robust than had been thought.
wo particles are just two particles, aren’t
they? Well, no, actually — not always. In
the weird world of quantum mechanics,
particles can lose their separate identities to
become just one ‘entangled’ object. Entanglement not only represents one of the
biggest differences between the quantum
and the classical, it also provides the key to
exploiting quantum mechanics to build new
technologies such as quantum computers.
But entanglement is fragile: disturb the
system and the entanglement is easily lost.
However, Altewischer et al.1 report, on page
304 of this issue, that entanglement might be
better at surviving than we had suspected.
Their findings could even point to a new way
of manipulating entanglement for quantum
technology.
Altewischer et al. launched photons of
one energy (one wavelength) into a crystal
whose particular properties cause each
photon to be transformed into two new
photons, a process called down-conversion.
By the principle of conservation of energy,
both down-converted photons have half the
energy (twice the wavelength) of the original
photon. Not only is energy conserved, so
too is a more exotic quantity called spin. The
consequence of spin conservation is that the
polarizations of the two down-converted
photons are always orthogonal (at right
angles) to each other. For example, if we
measure one photon to be linearly polarized
in the vertical direction, we always find the
other photon to be linearly polarized in the
horizontal direction.
Now for some quantum weirdness. No
matter which axis is chosen for the measurement of the polarization of one of the
photons, that choice completely determines
what happens when we measure the polarization state of the other photon2. It is as
though the two photons know about each
other instantaneously — in defiance of
special relativity, which states that no signal
could pass between the photons faster than
the speed of light. This strange effect is what
we call entanglement. Even stranger, this
mutual knowledge holds even if the polarization axis for our measurement is not chosen
until after the photons have flown a considerable distance apart.
The key contribution of Altewischer et
al.1 is to show that entanglement can survive
even when one (or both) of the entangled
photons is converted into a ‘surface plasmon’
T
NATURE | VOL 418 | 18 JULY 2002 | www.nature.com/nature
Figure 1 Quantum entanglement. A blue photon undergoes ‘down-conversion’ inside a crystal to
form two lower-energy red photons that are entangled — that is, the measured polarization of one
uniquely determines the polarization of the other. Polarizers placed in the path of these photons are
set so that they pass orthogonal polarizations. The coincident signals in two separate detectors verify
this weird quantum effect. Altewischer et al.1 placed metal films, perforated with arrays of holes
smaller than the photon wavelength, in the path of the photons. The entangled photons are each
transformed into electron vibrations, called surface plasmons, on the metal films. These surface
plasmons are short-lived, and the photons are re-emitted, still in their entangled state. The fact that
quantum entanglement can withstand this conversion raises the possibility of controlling entangled
photons through the manipulation of surface plasmons.
and then back into a photon. Surface plasmons are oscillating electromagnetic fields,
strongly localized at the surface of a metal
and associated with the collective motion of
a large number of electrons. In the Altewischer experiment (Fig. 1), the down-converted photons were fired through metal films
perforated by an array of holes smaller than
the wavelength of the photons. Such metal
films transmit photons surprisingly well3:
incident photons scatter off the periodic
arrangement of holes and are converted into
a surface-plasmon mode on the metal surface; the electromagnetic field associated
with the surface plasmon tunnels through
the holes and is in turn scattered by the
periodicity of the structure, finally being
converted back to a photon. Altewischer
and colleagues found that, although many
photons are lost as a result of absorption in
the metal film, some survive and, amazingly,
are still entangled.
Given the collective nature of the surface
plasmon, which involves a large number of
electrons, it seems remarkable that entanglement should survive. It is perhaps all the
© 2002 Nature Publishing Group
more surprising because surface-plasmon
modes are short-lived; typically they are lost
to absorption in the metal in just a few femtoseconds (1 fs410115 s). But perhaps we
shouldn’t be so surprised. We would expect
entanglement to survive if we simply used
a metallic mirror to reflect an entangled
photon, and we would still be making use of
the collective motion of many electrons to
provide the reflection. In both the surfaceplasmon and simple reflection processes, we
rely on the electron–electron scattering rate
being low enough to allow the electron
motion to remain coherent.
The advantage of using surface plasmons
is that they are well-defined modes, and, by
building suitable nanoscale structures, we
can control them on length scales small
enough to ensure that the electron motion
remains coherent4. In fact, we can control
surface plasmons well enough to achieve
highly concentrated optical fields in volumes
smaller than the wavelength, thus allowing us to control the flow of optical energy
on short length scales. Altewischer et
al.1 have demonstrated that entanglement
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survives when surface plasmons are used.
The challenge now is to see whether we can
exploit the combination of nanotechnology
and surface plasmons to manipulate entanglement for the benefit of emerging quantum technologies.
Despite our increasing ability to control
entanglement, it still seems totally at odds
with common-sense thinking — how can
the photons know about each other when
special relativity precludes signals getting
from one to the other quickly enough to
influence the outcome of our measurements? In finding entanglement difficult
to comprehend we are in good company:
Einstein and others were so disturbed by the
concept that they questioned the validity
of quantum mechanics5. Such conceptual
problems arise because of our natural tendency always to think of photons as separate,
well-defined objects: nature is not subject to
such limitations.
■
William Barnes is in the School of Physics,
University of Exeter, Stocker Road,
Exeter EX4 4QL, UK.
e-mail: [email protected]
1. Altewischer, E., van Exter, M. P. & Woerdman, J. P. Nature 418,
304–306 (2002).
2. Aspect, A., Grangier, P. & Roger, G. Phys. Rev. Lett. 47, 460–463
(1981).
3. Ebbesen, T. W., Lezec, H. J., Ghaemi, H. F., Thio, T. & Wolff,
P. A. Nature 391, 667–669 (1998).
4. Krenn, J. R. et al. Phys. Rev. Lett. 82, 2590–2593 (1999).
5. Einstein, A., Podolsky, B. & Rosen, N. Phys. Rev. 47, 777–780
(1935).
Developmental biology
Decisions, decisions!
Brigid Hogan
Early embryo cells can develop either into specialized body cells or into
precursors of eggs or sperm. It is not understood how this crucial decision
is made in mammals, but new work brings us closer to the answer.
ne of the most contentious issues in
biology today is whether a stem cell
from an adult tissue, committed to
generate a few specialized cell types, can be
‘reprogrammed’ to produce many more.
Pushing the envelope, some have even suggested that an adult cell might be persuaded
to give rise to eggs and sperm1 — our precious germ cells, which together can generate
a complete organism. But there are still huge
lacunae in our knowledge of the processes we
are trying to reverse. How do cells of the early
embryo normally become limited in their
developmental potential? And how are a
privileged few embryonic cells chosen to
become germ cells? On page 293 of this
issue, Saitou and colleagues2 describe a bold
approach to these problems. The authors
monitored gene activity in individual mouse
embryonic cells at precisely the time when
a group decision was being reached as to
which would become body cells and which
the germ cells.
In rapidly developing organisms such as
worms and flies, germ-cell fate is decided
very autocratically. Material known as germ
plasm, which dictates germ-cell development, is deposited in the egg before fertilization. It is then parcelled out to specific cells
early in development, sealing their fate3,4. By
contrast, in mammalian embryos, germ-cell
status is acquired in a democratic way, as a
result of interactions between neighbouring
cells. In mice, these interactions are initiated
when there are only three cell layers in
the embryo (Fig. 1). The inner layer (the
epiblast) eventually gives rise to all the
cells of the fetus, including the germ cells,
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while the outer layers (one of which is the
‘extra-embryonic ectoderm’) are supportive
tissues that nevertheless send important
early patterning signals to the epiblast.
Initially, the epiblast is a cup-shaped
sheet, but during the process known as gastrulation cells move towards one side, drop
out of the layer and give rise to a new cell
population, the mesoderm. Most mesoderm
cells move into the developing embryo,
while the remainder contribute to extraembryonic mesodermal support tissues —
the amnion and allantois. Early germ cells
(or ‘primordial germ cells’, PGCs) are first
identified around this time as a cluster of
about 40 to 50 cells expressing high levels of
the enzyme alkaline phosphatase (encoded
by the Tnap gene)3,5. This cluster lies in a
special niche in the no-man’s land between
the embryonic and extra-embryonic mesoderm (Fig. 1).
Where do these PGCs come from? Celllineage studies show that they originate in
the rim of the epiblast cup, right next to
the extra-embryonic ectoderm5. Importantly, the fate of cells in this region is not yet
sealed. Rather, it appears that the selection
of future PGCs is made in two steps. First, in
response to signals from the extra-embryonic ectoderm, epiblast cells in the rim are programmed to become common precursors
of extra-embryonic mesoderm and PGCs. A
second signal then imposes germ-cell status
on a few of these cells; the others differentiate
into extra-embryonic mesoderm.
Experiments have identified growth factors called bone morphogenetic proteins
(Bmp4 and Bmp8b) as components of the
first signal6,7. To learn more about the second
decision-making step, Saitou et al.2 decided
to investigate which genes are active in
individual cells in the region where the PGCs
expressing high levels of Tnap will appear.
Figure 1 Model for the development of primordial germ cells (PGCs) in mouse embryos, according to
Saitou et al.2. a, Six days after fertilization (E6.0) the embryo consists of three layers. The epiblast will
produce the fetus; the extra-embryonic ectoderm and the visceral endoderm are supportive tissues.
Signals from the extra-embryonic ectoderm, including certain Bmp proteins (blue), induce
neighbouring epiblast cells (open circles) to become common precursors of extra-embryonic
mesoderm and PGCs. During gastrulation these cells move (black arrow) towards the posterior and
drop out of the epiblast. Saitou et al. predict that, as the cells migrate, they adhere to one another
through the membrane protein fragilis. b, This clustering becomes more pronounced by E7.5. Cells in
the centre, which express the highest levels of the Tnap and fragilis proteins (dark green), are induced
to express the nuclear protein Pgc7/stella (red) and will become PGCs. Cells at the periphery and
outside the cluster express Hoxb1 (brown). The Bmp present in this region (lighter blue) promotes
the survival of PGCs14. Precisely what drives decision-making in the cluster is not known. c, Once
PGCs are specified, they migrate to the future gonads (dashed arrow).
© 2002 Nature Publishing Group
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