Detection of single photons via quantum entanglement

Detection of single photons via quantum entanglement

As a ‘quantum pendulum’ the ions swing in both directions at the same time. (Illustration: IQOQI/Knabl)

As a ‘quantum pendulum’ the ions swing in both directions at the same time. (Illustration: IQOQI/Knabl)

A team of quantum physicists in Innsbruck led by Christian Roos and Cornelius Hempel have realised an extremely sensitive method for the spectroscopy of atomic and molecular atoms. This technique can be used to closely study a number of particles. The scientists have published their findings in the journal Nature Photonics.

Almost 200 years ago, Bavarian physicist Joseph von Fraunhofer discovered dark lines in the sun’s spectrum. It was later discovered that these spectral lines can be used to infer the chemical composition and temperature of the sun’s atmosphere. Today we are able to gain information about diverse objects through light measurements in a similar way. Because often very little light needs to be detected for this, physicists are looking for ever more sensitive spectroscopy methods. In extreme cases, also single particles of light (photons) need to be measured reliably, which is technically challenging.
Thus, physicists at the Institute for Quantum Optics and Quantum Information (IQOQI) at the Austrian Academy of Sciences and the Institute for Experimental Physics of the University of Innsbruck take a detour via the technique of quantum logic spectroscopy. It was developed some years ago by the group of Nobel laureate David Wineland to build extremely precise atomic clocks. This is one of the first practical applications of quantum information processing and, in the next few years, may lead to a redefinition of the second in the international system of units.
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Measurement via entanglement

Christian Roos’ and Cornelius Hempel’s team of physicists in Innsbruck isolated single ions in an ion trap to study them under controlled conditions. “We do not try to detect the photon that is emitted or absorbed by an ion, but rather the momentum kick the ion receives upon absorption or emission,” explains Cornelius Hempel. “While this effect is extremely small, we can detect it by means of quantum physics.” The physicists use an additional ‘logic’ ion, on which the measurement is performed. “This calcium ion (40Ca+) can be controlled very well in the experiment,” says Hempel. As spectroscopy ion the researchers use another isotope of calcium (44Ca+).
In the experiment a laser pulse excites the particles and entangles the electronic state of the logic ion with the vibration of the particles. “In this configuration, also called Schrödinger cat state, the ions swing like a classical pendulum in a trap. But as a ‘quantum pendulum’ they swing in both directions at the same time,” describes Hempel the central part of the experiment. “We then excite the ion we want to investigate by applying different laser frequencies. At a certain frequency the ion emits a single photon and receives a minimal momentum kick, which causes the vibrational components to be slightly displaced. This can be observed through the electronic state of the logic ion. Combined with this information, the frequency of the laser then allows us to gain information about the internal state of the spectroscopy ion.” In the current experiment the scientists detected single photons with a probability of 12 %. “We, thus, prove that this technique works in principal. With a technically optimized set-up we will be able to considerably increase the sensitivity,” say Roos and Hempel confidently.

Universal application

“By using the exotic concept of quantum mechanical entanglement we are able to gain practical knowledge about single particles,” says Christian Roos excitedly. “Since our method of measurement does not depend that much on the wave length of the detected photon, it may be used for various purposes,” adds Cornelius Hempel. For example, energy levels of different atoms and molecules could be investigated by using this technique. Because it is difficult to control molecules in an experiment, this method is an enormous progress for studying more complex structures.

This research, carried out at the Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences and at the Institute for Experimental Physics at Innsbruck University, was supported by the European Union.

Links:
Entanglement-enhanced detection of single-photon scattering events. C. Hempel, B. P. Lanyon, P. Jurcevic, R. Gerritsma, R. Blatt, C. F. Roos. Advance online publication. Nature Photonics 2013
Quantum Optics and Spectroscopy

Editors note: Original article can be found here.
Credit: University of Innsbruck

The Riddle of Anti-Matter

The Riddle of Anti-Matter

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In high-res 1080p. Explores one of the deepest mysteries about the origin of our universe. According to standard theory, the early moments of the universe were marked by the explosive contact between subatomic particles of opposite charge. Featuring short interviews with Masaki Hori, Tokyo University and Jeffrey Hangst, Aarhus University.

Scientists are now focusing their most powerful technologies on an effort to figure out exactly what happened. Our understanding of cosmic history hangs on the question: how did matter as we know it survive? And what happened to its birth twin, its opposite, a mysterious substance known as antimatter?

A crew of astronauts is making its way to a launch pad at the Kennedy Space Center in Florida. Little noticed in the publicity surrounding the close of this storied program is the cargo bolted into Endeavor’s hold. It’s a science instrument that some hope will become one of the most important scientific contributions of human space flight.

It’s a kind of telescope, though it will not return dazzling images of cosmic realms long hidden from view, the distant corners of the universe, or the hidden structure of black holes and exploding stars.

Unlike the great observatories that were launched aboard the shuttle, it was not named for a famous astronomer, like Hubble, or the Chandra X-ray observatory.

The instrument, called the Alpha Magnetic Spectrometer, or AMS. The promise surrounding this device is that it will enable scientists to look at the universe in a completely new way.

Most telescopes are designed to capture photons, so-called neutral particles reflected or emitted by objects such as stars or galaxies. AMS will capture something different: exotic particles and atoms that are endowed with an electrical charge. The instrument is tuned to capture “cosmic rays” at high energy hurled out by supernova explosions or the turbulent regions surrounding black holes. And there are high hopes that it will capture particles of antimatter from a very early time that remains shrouded in mystery.

The chain of events that gave rise to the universe is described by what’s known as the Standard model. It’s a theory in the scientific sense, in that it combines a body of observations, experimental evidence, and mathematical models into a consistent overall picture. But this picture is not necessarily complete.

The universe began hot. After about a billionth of a second, it had cooled down enough for fundamental particles to emerge in pairs of opposite charge, known as quarks and antiquarks. After that came leptons and antileptons, such as electrons and positrons. These pairs began annihilating each other.

Most quark pairs were gone by the time the universe was a second old, with most leptons gone a few seconds later. When the dust settled, so to speak, a tiny amount of matter, about one particle in a billion, managed to survive the mass annihilation.

That tiny amount went on to form the universe we can know – all the light emitting gas, dust, stars, galaxies, and planets. To be sure, antimatter does exist in our universe today. The Fermi Gamma Ray Space Telescope spotted a giant plume of antimatter extending out from the center of our galaxy, most likely created by the acceleration of particles around a supermassive black hole.

The same telescope picked up signs of antimatter created by lightning strikes in giant thunderstorms in Earth’s atmosphere. Scientists have long known how to create antimatter artificially in physics labs – in the superhot environments created by crashing atoms together at nearly the speed of light.

Here is one of the biggest and most enduring mysteries in science: why do we live in a matter-dominated universe? What process caused matter to survive and antimatter to all but disappear? One possibility: that large amounts of antimatter have survived down the eons alongside matter.

In 1928, a young physicist, Paul Dirac, wrote equations that predicted the existence of antimatter. Dirac showed that every type of particle has a twin, exactly identical but of opposite charge. As Dirac saw it, the electron and the positron are mirror images of each other. With all the same properties, they would behave in exactly the same way whether in realms of matter or antimatter. It became clear, though, that ours is a matter universe. The Apollo astronauts went to the moon and back, never once getting annihilated. Solar cosmic rays proved to be matter, not antimatter.

It stands to reason that when the universe was more tightly packed, that it would have experienced an “annihilation catastrophe” that cleared the universe of large chunks of the stuff. Unless antimatter somehow became separated from its twin at birth and exists beyond our field of view, scientists are left to wonder: why do we live in a matter-dominated universe?