Quantum reality more complex than
previously thought
http://www.eurekalert.org/pub_releases/2013-10/fopu-qrm102813.php
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Imagine you order a delivery of several glass vases in different colors. Each vase is sent as a separate parcel. What would you think of the courier if the parcels arrive apparently undamaged, yet when you open them, it turns out that all the red
vases are intact and all the green ones are smashed to pieces? Physicists from the University of Warsaw and the Gdansk University of Technology have demonstrated that when quantum information is transmitted, nature can be as whimsical as this crazy delivery man.
Experiments on individual photons, conducted by physicists from the Faculty of Physics at the University of Warsaw (FUW) and the Faculty of Applied Physics and Mathematics at the Gdansk University of Technology (PG), have revealed yet another
counterintuitive feature of the quantum world. When a quantum object is transmitted, its quantum property – whether it behaves as a wave or as a particle – appears to depend on other properties that at first glance have nothing to do with the transmission. These surprising results were published in
the research journal Nature Communications.
Wave-interference
experiments are some of the simplest and most elegant, and can be conducted by
almost anyone. When a laser beam is directed at a plate with two slits, we
observe a sequence of light and dark fringes. It has long been known that the
fringes are visible even when just individual particles – single electrons or
photons – pass through the slits. Physicists assume that every individual
particle exhibits wave properties, passing through both slits at once and
interfering with itself.
The situation is very
different when it is possible to detect the path taken by a given photon or
electron and determine which slit the particle has passed through, at least in
principle. When information about the particle path leaks from the system to
the observer, the interference disappears and instead of interference fringes
no pattern is observed.
In order for photons to
exhibit interference, their wavelengths must be the same, while electrons must
have the same energy. However, quantum particles have a number of other
properties. For example, they can be polarized (their electrical field vibrates
in a certain plane) or have different spin orientations (a quantum property
describing the dynamics of an object at rest).
"So far, it has
been generally assumed that additional properties such as spin and polarization
do not have a non-trivial impact on interference. We decided to study the topic
in more depth, and we were surprised by the results we obtained," says
Prof. Konrad Banaszek (FUW).
The experiments by
physicists from the University
of Warsaw and the Gdańsk
University of Technology started by generating heralded photons. "The name
sounds complicated, but the idea is simple in itself," according to Prof.
Czeslaw Radzewicz (FUW). "We generate photons using a process in which
they must be created in pairs. When we register one photon, we can be certain
that the second was also born, and we know its properties such as direction or
wavelength without destroying it. In other words, we use one photon to herald
the generation of the second photon."
Each heralded photon was
directed individually towards an interferometer, comprising two calcite
crystals. In the first crystal, the photon was split and then sent through both
arms of the interferometer at the same time. In each arm, researchers altered
the polarization of the photon (the plane of vibration of its electrical field)
by introducing noise. In the second calcite crystal, the paths were recombined
to create a distinctive set of interference fringes, provided that the system
did not leak any information as to which arm the given photon travelled along.
The final stage of the experiment involved measuring the interference fringes
using silicon avalanche photodiodes.
"It turned out that
we were able to use measurements of interference fringes to determine how much
information had leaked during transmission of the photon through the
interferometer. In other words, we could be certain whether any eavesdropping
had taken place during transmission," says Dr Michal Karpinski (University of Warsaw,
currently University
of Oxford), responsible
for building the experimental system and conducting the measurements.
The results have
revealed a new, surprising property of reality: the polarization of photons, or
other internal degrees of freedom, play a highly non-trivial role in
interference between the two paths.
"It is almost as
though the quality of a courier delivery – for example, whether a glass vase delivered
inside a securely packed parcel is still in one piece – depends on whether the
vase is green or red. In our world the color has no bearing on whether the vase
arrives intact or not. However, the condition of the parcels our 'quantum
courier' delivers does indeed depend on internal properties that seem to have
nothing to do with interference," according to Prof. Pawel Horodecki (PG).
The results allow
physicists to examine the fundamental properties of reality in new, more
comprehensive ways, as well as having practical applications in quantum
cryptography. The Warsaw and Gdansk physicists have successfully derived a
general inequality making it possible to precisely estimate the volume of
information leaking from the measurement system.
###
The research was funded
by the European Union 7th Framework Programme and as part of the Foundation for
Polish Science TEAM Programme, co-financed from EU funds – the European
Regional Development Fund.
Physics and Astronomy
first appeared at the University
of Warsaw in 1816, under
the then Faculty of Philosophy. In 1825 the Astronomical Observatory was
established. Currently, the Faculty of Physics' Institutes include Experimental
Physics, Theoretical Physics, Geophysics, Department of Mathematical Methods
and an Astronomical Observatory. Research covers almost all areas of modern
physics, on scales from the quantum to the cosmological. The Faculty's research
and teaching staff includes ca. 200 university teachers, of which near 80 are
employees with the title of professor. The Faculty of Physics, University of Warsaw, is attended by ca. 1000 students
and more than 140 doctoral students.
CONTACTS:
Prof. Konrad Banaszek
Faculty of Physics, University of Warsaw
tel. +48 22 5532306
email: [email protected]
Prof. Pawel Horodecki
Faculty of Applied Physics and Mathematics, Gdansk University of Technology
tel. +48 58 3486069
email: [email protected]
Prof. Czeslaw Radzewicz
Faculty of Physics, University of Warsaw
tel. +48 22 5532243, +48 22 6254738
email: [email protected]
Dr Michal Karpinski
Department of Physics, University of Oxford
tel. +44 1865 273623
email: [email protected]
RELATED LINKS:
http://www.mif.pg.gda.pl/
Faculty of Applied Physics and Mathematics, Gdansk University of Technology
website.
http://www.fuw.edu.pl/
Faculty of Physics at the University
of Warsaw website.
http://www.fuw.edu.pl/informacje-prasowe.html
Press Office for the Faculty of Physics at the University of Warsaw.
IMAGES:
FUW131028b_fot01s.jpg
HR: http://www.fuw.edu.pl/press/images/2013/FUW131028b_fot01.jpg
Even an individual
photon can travel along both arms of the interferometer at the same time. When
it is unknown which path it is travelling along, we observe interference and
the appearance of interference fringes. A strong signal is visible where the
crests of light waves meet, and a weak signal is obtained at the meeting point
of the troughs. If it is possible to determine which arm the photon travelled
along, following leakage of information from the interferometer, the fringes
disappear. (Source: NLTK/Tentaris/Maciej Frolow)
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