ISIS Report 16/03/04
Announcing special series
Nature is Quantum, Really
Quantum Phases and Quantum Coherence
How not to Collapse the Wave Function
The Quantum Information Revolution: Freezing Light
Quantum World Coming
A more technical version of this article complete with illustrations
and references is posted on ISIS members’ website. Full details here.
Until fairly recently, the conventional view held by most physicists is
that nature is somewhat sharply divided into the classical domain of every day
objects in which Newton’s laws of mechanics hold, and the weird and
wonderful world of quantum systems at the scale of elementary particles, atoms
and simple molecules, in which ‘things’ are both wave and particle,
and can be in two places or multiple, contradictory states at the same time.
Quantum systems are destroyed by the act of measurement, which brings
them abruptly into the ordinary classical world.
Austrian physicist Erwin Schrödinger, who, like Albert Einstein,
never really believed in quantum theory, invented the story of a cat, now named
after him, to illustrate how absurd the situation is. Schrödinger’s
cat is locked in a box containing a capsule of deadly cyanide gas that would be
released the moment that a radioactive nuclide undergoes radioactive decay. The
way to find out if the cat is dead or alive is to open the lid of the box,
which is equivalent to performing a ‘measurement’ and bringing the
‘quantum system’ of the cat in the box abruptly into the classical
world.
Schrödinger’s cat asleep by Mae-Wan
Ho
But before someone - a conscious being - opens the lid, the cat in the
box is neither dead nor alive, but both. It is said to be in a superposition of
two alternative states: being dead and being alive, or more accurately, all
possible combinations of being both dead and alive at the same time.
Someone opening the lid instantaneously ‘collapses’ the quantum
superposition (or the wave-function describing this state) and only a classical
result can be observed. But can’t the cat surely collapse its own wave
function by experiencing itself either dead or alive?
Over the past 20 years, the scale at which quantum effects can be
observed has become increasingly large, so the problem of
Schrödinger’s cat is all the more relevant to our picture of physical
reality. Could there be some conceptual error involved in the idea of
measurement and the collapse of the wave function itself? Many surprising
discoveries are raising questions over the standard interpretation of quantum
theory, and that is perhaps the most exciting development in
contemporary western science in the 21st century.
The mere promise of quantum computing is enough to send people into a
frenzy of speculation on the coming quantum information revolution that will
make current information technology look Stone Age. Quantum computing not only
provides an exponential increase in computing power, but can also solve
problems that the classical computer can’t handle. However, there appears
to be insurmountable engineering hurdles in actually building a quantum
computer. There may well be deeper problems involved with the whole idea of a
quantum computer that we can actually control and use.
A bit closer to realisation is quantum communication based on entirely
new interactions between light and matter in quantum optics, and quantum
cryptography to keep military and commercial secrets snoop-proof; potentially a
boon for dictators, corporations and terrorists alike, but what’s in it
for ordinary people?
The way I see it, the quantum age entails a shift to a truly organic way
of living and perceiving the world that will reconnect western science to the
deeply ecological and holistic knowledge systems of all indigenous cultures,
most of which are facing extinction. It will make us realise how urgently we
need to protect and revitalize them as the real "common heritage" of the human
species.
A quantum world is a radically interconnected, interdependent world
where every entity evolves like an organism, entangled with all that there is.
ISIS will be circulating a unique series of articles that will change
your life. So look out!
Quantum World Coming
Nature is Quantum, Really
Matter, even big clumps of it, is simultaneously wave and particle.
Dr. Mae-Wan Ho explains
Which slit did the buckyball go through?
One of the first experiments to show up the strangeness of the quantum
world consisted of shining a light through two narrow slits onto a photographic
plate placed some distance behind the slits (Fig. 1).
Figure 1. The two-slit experiment
When only one slit is opened, an image of the slit is recorded on the
photographic plate, which, when viewed under the microscope, would reveal tiny
discrete spots. And this is consistent with the interpretation that individual
particle-like photons, on passing through the slit, have landed on the
photographic plate, where each photon causes a single silver grain to be
deposited.
When both slits are opened, an interference pattern of alternating
bright and dark zones forms on the photographic plate, which is consistent with
a wave-like behaviour of the light: the two wave trains, on passing through the
slits, arrive at different parts of the photographic plate either in phase,
where they reinforce each other to give a bright zone, or out of phase, where
they cancel out to give a dark zone. On examining the plate under the
microscope, however, the same graininess appears, as though the light waves
become individual particles as soon as they strike the plate.
Numerous other more sophisticated experimental configurations have been
devised to investigate this phenomenon, and always the conundrum remains.
Photons are split into superposed reflected and transmitted states, or into
opposite polarized states, that are capable of interfering when brought
together again; but as soon as information is gained as to which path the
photon has taken, or which polarised state it has adopted, then it behaves as
an ordinary particle.
More remarkably, the two-slit experiment has been repeated with
increasingly massive particles and essentially the same results have been
obtained: electrons, neutrons 1800 times as massive as the electron, and more
recently ‘ buckyballs’, a newly identified form of carbon molecule
consisting of 60 atoms of carbon arranged in the shape of a football, and
possibly, even a small protein.
Professor Anton Zeilinger, who leads a group in the University of Vienna
engaged in these experiments, said when giving the 16th
Schrödinger Lecture in London last November that they are planning to try
a small virus next, and is quite confident that it too, will behave as both
wave and particle.
There is quite a gap between virus and a mouse, or a human being, but
who is to say we are not both a wave spread out in space and a seemingly solid
body that can bump into furniture?
Macroscopic quantum objects?
Schrödinger would have been astonished by all these findings if he
were alive today. After all, he invented the parable of the cat named after him
to show what absurd things quantum theory would have us think about: that an
entity could be simultaneously in mutually contradictory states until the
instant it is ‘measured’.
But what constitutes a measurement? Quantum physicists John Bell, who
died a few years ago, had apparently called for the word
‘measurement’ to be banished from quantum theory.
At a workshop in 1990 concerned with how quantum effects can manifest on
a macroscopic scale, the concept of measurement became very ambiguous. Philip
Ball, reporting in Nature, said, "the most profound message from that
meeting was that interpretations of quantum theory are no longer a matter of
philosophical taste." Why? It was because of the development of electronic
systems of remarkable sensitivity, and many ‘thought experiments’
could be directly tested.
It had become possible by then to create individual macroscopic quantum
objects, perhaps a few centimetres in size. Among the first most promising
candidates for displaying macroscopic quantum behaviour were various kinds of
electronic circuits, particularly semiconductor structures, in which electrons
behave like a two-dimensional gas, and super-conducting rings (which conduct
electricity with zero resistance) containing weak links in the SQUID (Super
Quantum Interference Device) magnetometer. SQUID magnetometers are increasingly
used to measure the ultraweak magnetic fields coming from the body as electric
currents flow through it.
At the 1990 workshop, Terry Clark of University of Sussex in Britain
discussed the then state of the art in SQUID ring experiments. The weak link in
these rings – typically made from a low-temperature superconductor such as
niobium - is a point contact, and transport of correlated electron pairs
(called Cooper pairs) across the contact relies on quantum tunnelling through
the energy barrier created by the weak link. This is a probabilistic process
resulting in a build-up of charge on either side of the junction, so the device
develops a capacitance (charge storage).
At the microscopic level, charge Q and magnetic flux f are related, like
position and momentum by the uncertainty principle that’s fundamental in
quantum physics, DfDQ > h/2.
That means if you measure one quantity precisely, the other is totally
uncertain: if you know the exact position of a particle, its momentum (mass x
velocity) could be anything from zero to infinity, and conversely, if you
pinpoint the momentum, then the particle could be anywhere in the universe.
The weak-link ring can adopt two quantum modes – a flux mode, in
which charge flows and could be anywhere in the system, but the magnetic flux
lines through the ring tend to be localized inside the ring; and a charge
(capacitive) mode, in which charge tends to be localized, but not the magnetic
flux. Different quantized (discrete) energy states (eigenstates) of the charge
and flux modes are coupled by some characteristic tunnelling frequency so that
in principle, the ring may lie in a quantum superposition of the two states. Is
it possible to catch the ring in such a superposition?
This is where measurement comes in. According to the standard
‘Copenhagen’ interpretation, the act of measurement
‘collapses’ the quantum superposition. But the hope is that if the
coupling (connection) to a measurement device is very weak, this collapse would
not happen. Terry Clark’s team managed to set up just such a weak
measurement system and obtained results suggesting that the SQUID ring could
exist in a quantum superposition of both the flux mode and the charge mode (see
Box).
So, where does the quantum world stop and the classical start? One
might say I am a quantum being between the acts of living and dying, like
Schrödinger’s cat. Read on.
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