A possible application is the development of a super-fast computer and
highly precise clocks that could be the future basis for a new standard
of time
 |
| Serge Haroche & David J.Wineland shared the 2012 Nobel Prize in physics |
Serge Haroche and David Wineland have opened the door to
a new era of experimentation with quantum physics by demonstrating the
direct observation of individual quantum systems without destroying
them.
Through their ingenious laboratory methods they have managed to
measure and control very fragile quantum states, enabling their field of
research to take the very first steps towards building a new type of
super fast computer, based on quantum physics. These methods have also
led to the construction of extremely precise clocks that could become
the future basis for a new standard of time, with more than hundred-fold
greater precision than present-day caesium clocks.
For
single particles of light or matter, the laws of classical physics
cease to apply and quantum physics takes over. But single particles are
not easily isolated from their surrounding environment and they lose
their mysterious quantum properties as soon as they interact with the
outside world.
Both Laureates work in the field of
quantum optics studying the fundamental interaction between light and
matter.
In David Wineland’s laboratory in Boulder,
Colorado, electrically charged atoms or ions are kept inside a trap by
surrounding them with electric fields.
One of the
secrets behind Wineland’s breakthrough is the mastery of the art of
using laser beams and creating laser pulses. A laser is used to put the
ion in its lowest energy state and thus enabling the study of quantum
phenomena with the trapped ion. A carefully tuned laser pulse can be
used to put the ion in a superposition state, which is a
simultaneous existence of two distinctly different states.
For instance, the quantum superposition of the ion’s energy
states can be studied by using the laser pulse to nudge the ion halfway
between the high- and low-energy levels.
Controlling single photons
Serge Haroche and his research group employ a
different method to reveal the mysteries of the quantum world. In their
laboratory in Paris microwave photons bounce back and forth inside a
small cavity between two mirrors, about three centimetres apart. The
mirrors are made of superconducting material and are cooled to a
temperature just above absolute zero. These superconducting mirrors are
so reflective that a single photon can bounce back and forth inside the
cavity for almost a tenth of a second before it is lost or absorbed.
During its long life time, many quantum manipulations can
be performed with the trapped photon. Haroche uses specially prepared
atoms, so-called Rydberg atoms to both control and measure the microwave
photon in the cavity. A Rydberg atom has a radius of about 125
nanometres which is roughly 1,000 times larger than typical atoms. The
Rydberg atoms are sent into the cavity one by one at a carefully chosen
speed, so that the interaction with the microwave photon occurs in a
well-controlled manner.
The Rydberg atom traverses
and exits the cavity, leaving the microwave photon behind. But the
interaction between the photon and the atom creates a change in the
phase of quantum state of the atom: if you think of the atom’s quantum
state as a wave, the peaks and the dips of the wave become shifted. This
phase shift can be measured when the atom exits the cavity, thereby
revealing the presence or absence of a photon inside the cavity. With no
photon there is no phase shift. Haroche can thus measure a single
photon without destroying it.
Physics in the quantum
world has some inherent uncertainty or randomness to it. One example of
this contrary behaviour is superposition, where a quantum particle can
be in several different states simultaneously.
Why
do we never become aware of these strange facets of our world? Why can
we not observe a superposition of quantum marble in our every-day life?
The Austrian physicist and Nobel Laureate (Physics 1933) Erwin
Schrödinger battled with this question. Like many other pioneers of
quantum theory, he struggled to understand and interpret its
implications. As late as 1952, he wrote: “We never experiment with just
one electron or atom or (small) molecule. In thought-experiments we
sometimes assume that we do; this invariably entails ridiculous
consequences...”
In order to illustrate the absurd
consequences of moving between the micro-world of quantum physics and
our every-day macro-world, Erwin Schrödinger described a thought
experiment with a cat: Schrödinger’s cat is completely isolated from the
outside world inside a box. The cat must be in a superposition state of
being both dead and alive.
The box also contains a
bottle of deadly cyanide which is released only after the decay of some
radioactive atom, also inside the box.
The
radioactive decay is governed by the laws of quantum mechanics,
according to which the radioactive material is in a superposition state
of both having decayed and not yet decayed. Therefore the cat must also
be in a superposition state of being both dead and alive. Now, if you
peek inside the box, you risk killing the cat because the quantum
superposition is so sensitive to interaction with the environment that
the slightest attempt to observe the cat would immediately ‘collapse’
the ‘cat-state’ to one of the two possible outcomes — dead or alive.
Instead of Schrödinger’s cat, Haroche and Wineland trap quantum
particles and put them in cat-like superposition states. These quantum
objects are not really macroscopic as a cat, but they are still quite
large by quantum standards.
Inside Haroche’s cavity
microwave photons are put in cat-like states with opposite phases at the
same time, like a stopwatch with a needle that spins both clockwise and
counterclockwise simultaneously. The microwave field inside the cavity
is then probed with Rydberg atoms. The result is another unintelligible
quantum effect called entanglement.
Entanglement has
also been described by Erwin Schrödinger and can occur between two or
more quantum particles that have no direct contact but still can read
and affect the properties of each other. Entanglement of the microwave
field and Rydberg atoms allowed Haroche to map the life and death of the
cat-like state inside his cavity, following it step by step, atom by
atom, as it underwent a transition from the quantum superposition of
states to a well defined state of classical physics.
Computer revolution
A possible application of ion traps that
many scientists dream of is the quantum computer. In present-day
classical computers the smallest unit of information is a bit that takes
the value of either 1 or 0. In a quantum computer, however, the basic
unit of information — a quantum bit or qubit — can be 1 and 0 at the
same time.
Two quantum bits can simultaneously take
on four values — 00, 01, 10 and 11 — and each additional qubit doubles
the amount of possible states. For n quantum bits there are 2 possible
states, and a quantum computer of only 300 qubits could hold 2 values
simultaneously.
Wineland’s group was the first in the
world to demonstrate a quantum operation with two quantum bits. Since
control operations have already been achieved with a few qubits, there
is no reason to believe that it should not be possible to achieve such
operations with many more qubits.
However, to build
such a quantum computer one has to satisfy two opposing requirements:
the qubits need to be adequately isolated from their environment in
order not to destroy their quantum properties, yet they must also be
able to communicate with the outside world in order to pass on the
results of their calculations. David Wineland and his team of
researchers have also used ions in a trap to build a clock that is a
hundred times more precise than the caesium-based atomic clocks which
are currently the standard for our measurement of time. Time is kept by
setting, or synchronizing all clocks against one standard. Caesium
clocks operate in the microwave range whereas Wineland’s ion clocks use
visible light — hence their name: optical clocks.
Optical clocks
An optical clock can consist of just one ion or two ions
in a trap. With two ions, one is used as the clock and the other is used
to read the clock without destroying its state, or causing it to miss a
tick. The precision of an optical clock is better than one part in 10 —
if one had started to measure time at the beginning of the universe in
the Big Bang about 14 billion years ago, the optical clock would only
have been off by about five seconds today.
With such
precision, some extremely subtle and beautiful phenomena of nature have
been observed, such as changes in the flow of time, or minute
variations of gravity, the fabric of space-time. According to Einstein’s
theory of relativity, time is affected by motion and gravity.
The higher the speed and the stronger the gravity, the
slower the passage of time. We may not be aware of these effects, but
they have in fact become part of our everyday life. When we navigate
with the GPS we rely on time signals from satellites with clocks that
are routinely calibrated, because gravity is somewhat weaker several
hundred kilometres altitude.
With an optical clock
it is possible to measure a difference in the passage of time when the
clock’s speed is changed by less than 10 metres per second, or when
gravity is altered as a consequence of a difference in height of only 30
centimetres.
