Squeezed light from single atoms

Squeezed light from single atoms

  9:11:00 PM    Science Lover

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Max Planck Institute of Quantum Optics scientists generate amplitude-squeezed light fields using single atoms trapped inside optical cavities.

A single rubidium atom in a cavity squeezes the quantum
fluctuations of a weak laser beam, decreasing the fluctuations
of the amplitude at the expense of the phase.
The effect is exaggerated for clarity.

In classical optics light is usually described as a wave, but at the most fundamental quantum level this wave consists of discrete particles called photons. Over the time, physicists developed many tools to manipulate both the wave-like and the particle-like quantum properties of the light. For instance, they created single photon sources with single atoms, using their ability to absorb and emit photons one by one. A team around Professor Gerhard Rempe, Director at the Max Planck Institute of Quantum Optics (Garching near Munich) and head of the Quantum Dynamics Division, has now observed that the light emitted by a single atom may exhibit much richer dynamics (Nature 474, 623, June 30, 2011). Strongly interacting with light inside a cavity, the atom modifies the wave-like properties of the light field, reducing its amplitude or phase fluctuations below the level allowed for classical electromagnetic radiation. This is the very first observation of “squeezed” light produced by a single atom.

The “graininess” of the photons in a light wave causes small fluctuations of the wave’s amplitude and phase. For classical beams, the minimal amount of amplitude and phase fluctuations is equal. However, by creating interactions between the photons, one can “squeeze” the fluctuations of the amplitude below this so-called “shot noise” level at the expense of increasing the fluctuations of the phase, and vice-versa. Unfortunately, the photonic interactions inside standard optical media are very weak, and require bright light beams to be observed. Single atoms are promising candidates to enable such interactions at a few-photon level. Their ability to generate squeezed light has been predicted 30 years ago, but the amount of light they emit is very tiny and so far all attempts to set this idea into realization have failed. In the Quantum Dynamics Division at MPQ sophisticated methods for cooling, isolating and manipulating single atoms have been developed over many years, and made this observation possible.



A single rubidium atom is trapped inside a cavity made of two very reflective mirrors in a distance of about a tenth of a millimetre from each other. When weak laser light is injected into this cavity, the atom can interact with one photon many times, and forms a kind of artificial molecule with the photons of the light field. As a consequence, two photons can enter the system at the same time and become correlated. “According to the model of Bohr, a single atom emits exactly one single energy quantum, i.e., one photon. That means that the number of photons is exactly known, but the phase of the light is not defined”, Professor Gerhard Rempe explains. “But the two photons that are emitted by this strongly coupled atom are indistinguishable and oscillate together. Therefore this time the wave-like properties of the propagating light field are modified.”

When the physicists use a laser beam which is resonant with the excitation frequency of the atom, the measurements show a suppression of the phase fluctuations. If the laser light is resonant with the cavity, they observe a squeezing of the amplitude instead.

The latter situation is illustrated in the figure: The atom in the cavity turns a laser beam into light which has less amplitude and more phase fluctuations than the shot-noise limit. “Our experiment shows that the light emitted by single atoms is much more complex than in the simple view of Albert Einstein concerning photo-emission”, Dr. Karim Murr emphasizes. “The squeezing that we observe is due to the coherent interaction between the two photons emitted from the system. Our measurement is in excellent agreement with the predictions of quantum electrodynamics in the strong-coupling regime.” And Dr. Alexei Ourjoumtsev, who has been working on the experiment as a post doc, adds: “Usually single quantum objects are used to manipulate the particle-like properties of light. It is interesting to see that they can also modify its wave-like properties, and create observable squeezing with excitations beams containing only two photons on average”.

So far squeezed light has only been generated with systems containing many atoms, such as crystals, using very high intensity beams, i.e. many photons. For the first time now physicists have succeeded in generating this kind of non-classical radiation with single atoms and extremely weak light fields. The ability of a single atom to induce strong coherent interactions between propagating photons opens up new perspectives for photonic quantum logic with single emitters.

More information: A. Ourjoumtsev, A. Kubanek, M. Koch, C. Sames, P. W. H. Pinkse, G. Rempe, & K. Murr

Observation of squeezed light from one atom excited with two photons , Nature 474, 623, 30 June 2011. Provided by Max-Planck-Gesellschaft

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Optical circuit enables new approach to quantum technologies

Optical circuit enables new approach to quantum technologies

  1:29:00 AM    Science Lover

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An international research group led by scientists from the University of Bristol, UK, and the Universities of Osaka and Hokkaido, Japan, has demonstrated a fundamental building block for quantum computing that could soon be employed in a range of quantum technologies.

Professor Jeremy O’Brien, Director of the University of Bristol’s Centre for Quantum Photonics, and his Japanese colleagues have demonstrated a quantum logic gate acting on four particles of light – photons. The researchers believe their device could provide important routes to new quantum technologies, including secure communication, precision measurement, and ultimately a quantum computer—a powerful type of computer that uses quantum bits (qubits) rather than the conventional bits used in today’s computers.

Unlike conventional bits or transistors, which can be in one of only two states at any one time (1 or 0), a qubit can be in several states at the same time and can therefore be used to hold and process a much larger amount of information at a greater rate.

“We have realised a fundamental element for processing quantum information—a controlled-NOT or CNOT gate—based on a recipe that was theoretically proposed 10 years ago,” said Professor O’Brien. “The reason it has taken so long to achieve this milestone is that even for such a relatively simple circuit we require complete control over four single photons whizzing around at the speed of light!”

The approach taken by Professor O’Brien and his colleagues combined several methods for making optical circuits that must be stable to within a fraction of the wavelength of light, that is, nanometres. In 2001 optical quantum computing became possible when a theoretical recipe for realising this CNOT gate, as well as the other necessary components, was developed. However, the technological challenges associated with making the optical circuits have prevented its realisation until now. The implications for this new approach are far-reaching.

“The ability to implement such a logic gate on photons is critical for building up larger scale circuits and even algorithms,” said Professor O’Brien. “Using an integrated optics on a chip approach that we have pioneered here at Bristol over the last several years will enable this to proceed far more rapidly, paving the way to quantum technologies that will help us understand the most complex scientific problems.”



In the short term, the team expect to apply their new results immediately for developing new approaches to quantum communication and measurement and then for simulation tools in their lab. In the longer term, a small-scale quantum simulator based on a multi-photon optical circuit could be used to simulate processes which themselves are governed by quantum mechanics, such as superconductivity and photosynthesis. “Our technique could improve our understanding of such important processes and help, for example, in the development of more efficient solar cells,” said Professor O’Brien. Other applications include the development of ultra-fast and efficient search engines, designing high-tech materials and new pharmaceuticals.

The leap from using one photon to two photons is not trivial because the two particles need to be identical in every way and because of the way these particles interfere, or interact, with each other. There is no direct analogue of this interaction outside of quantum physics.

“Now that we can implement the fundamental building blocks for quantum circuits, the move to a larger scale devices will become our focus. Because of the increasingly complexity the results will be just as exciting” said Professor O’Brien. “Each time we add a photon, the complexity of the problem we are able to investigate increases exponentially, so if a one-photon quantum circuit has 10 outcomes, a two-photon system can give 100 outcomes and a three-photon system 1000 solutions and so on.”

The Centre for Quantum Photonics now plans to use their chip-based approach to increase the complexity of their experiment not only by adding more photons but also by using larger circuits.

The research is published in Proceedings of the National Academy of Sciences.

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Researchers create light from 'almost nothing'

Researchers create light from 'almost nothing'

  2:27:00 PM    Science Lover

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Several physicists exercising associated with Chalmers College associated with Technologies within Gothenburg, Sweden, possess been successful within showing that which was so far, simply concept; and that's, which noticeable photons might be created from the actual digital contaminants which have been considered to can be found inside a quantum vacuum cleaner. Inside a document released upon arXiv, the actual group explains that they utilized the specifically produced signal known as the superconducting quantum disturbance gadget (SQUID) in order to modulate a little bit of cable duration in a approximately 5 % from the pace associated with gentle, to create noticeable "sparks" in the nothingness of the vacuum.

a) Optical micrograph of the device. Light parts are Al
while dark parts are the Si substrate. The output line is
labeled "CPW" and the drive line enters from the top.
Both lines converge near the SQUID. b) A scanning-
electron micrograph of the SQUID.
Image credit: arXiv:1105.4714v1

The experiment shows that the Casimir effect is not just theory; named after Dutch physicist Hendrik B. G. Casimir who along with Dirk Polderfirst first proposed back in the late 1940’s, the idea of a force that existed in a vacuum; a force that should, if manipulated just right between two plates, or mirrors, result in the creation of photons.

The thinking goes that in any vacuum, virtual particles come into existence and then disappear on a constant ongoing basis; and they do so in waves. The Casimir effect proposes that if two very tiny mirrors were to be placed very close together; close enough that the distance between them would be smaller than the length of some of the virtual waves, a force would be created as the number of particles outside of the space between the mirrors grows higher than the number that exists between them, causing a pull on the mirrors, dragging them closer together. The force that is created, it has been theorized, could then be used to generate photons.

Later researchers proposed that the same effect could be achieved using just one mirror if it were moved back and forth very quickly; and that’s the approach the team took in the experiment. The quick movement of the mirror serves to separate pairs of virtual particles which then provide the energy to convert the virtual particles into real photons, which is what happened in the SQUID, allowing the team to see the photons that were produced.

Such research, while theoretically satisfying, doesn’t really offer much in the way of practical applications, at least not at this time; but that’s not to say that new developments that arise as a result of this research couldn’t conceivably lead to something more profound, such as a means of harnessing energy from the vacuum of space to be used to push a vehicle as it travels throughout the universe.


More information: Observation of the Dynamical Casimir Effect in a Superconducting Circuit, arXiv:1105.4714v1 [quant-ph] 

Abstract

One of the most surprising predictions of modern quantum theory is that the vacuum of space is not empty. In fact, quantum theory predicts that it teems with virtual particles flitting in and out of existence. While initially a curiosity, it was quickly realized that these vacuum fluctuations had measurable consequences, for instance producing the Lamb shift of atomic spectra and modifying the magnetic moment for the electron. This type of renormalization due to vacuum fluctuations is now central to our understanding of nature. However, these effects provide indirect evidence for the existence of vacuum fluctuations. From early on, it was discussed if it might instead be possible to more directly observe the virtual particles that compose the quantum vacuum. 40 years ago, Moore suggested that a mirror undergoing relativistic motion could convert virtual photons into directly observable real photons. This effect was later named the dynamical Casimir effect (DCE). Using a superconducting circuit, we have observed the DCE for the first time. The circuit consists of a coplanar transmission line with an electrical length that can be changed at a few percent of the speed of light. The length is changed by modulating the inductance of a superconducting quantum interference device (SQUID) at high frequencies (~11 GHz). In addition to observing the creation of real photons, we observe two-mode squeezing of the emitted radiation, which is a signature of the quantum character of the generation process.

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New Switching Device Could Help Build an Ultrafast 'Quantum Internet'

New Switching Device Could Help Build an Ultrafast 'Quantum Internet'

  3:58:00 PM    Science Lover

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Northwestern University researchers have developed a new switching device that takes quantum communication to a new level. The device is a practical step toward creating a network that takes advantage of the mysterious and powerful world of quantum mechanics.

A new switching device could be used to develop a 'quantum Internet,' where encrypted information would be completely secure, and networking superfast quantum computers. (Credit: iStockphoto/Andrey Prokhorov)

 

The researchers can route quantum bits, or entangled particles of light, at very high speeds along a shared network of fiber-optic cable without losing the entanglement information embedded in the quantum bits. The switch could be used toward achieving two goals of the information technology world: a quantum Internet, where encrypted information would be completely secure, and networking superfast quantum computers.

The device would enable a common transport mechanism, such as the ubiquitous fiber-optic infrastructure, to be shared among many users of quantum information. Such a system could route a quantum bit, such as a photon, to its final destination just like an e-mail is routed across the Internet today.

The research -- a demonstration of the first all-optical switch suitable for single-photon quantum communications -- is published by the journal Physical Review Letters.

"My goal is to make quantum communication devices very practical," said Prem Kumar, AT&T Professor of Information Technology in the McCormick School of Engineering and Applied Science and senior author of the paper. "We work in fiber optics so that as quantum communication matures it can easily be integrated into the existing telecommunication infrastructure."

The bits we all know through standard, or classical, communications only exist in one of two states, either "1" or "0." All classical information is encoded using these ones and zeros. What makes a quantum bit, or qubit, so attractive is it can be both one and zero simultaneously as well as being one or zero. Additionally, two or more qubits at different locations can be entangled -- a mysterious connection that is not possible with ordinary bits.

Researchers need to build an infrastructure that can transport this "superposition and entanglement" (being one and zero simultaneously) for quantum communications and computing to succeed.

The qubit Kumar works with is the photon, a particle of light. A photonic quantum network will require switches that don't disturb the physical characteristics (superposition and entanglement properties) of the photons being transmitted, Kumar says. He and his team built an all-optical, fiber-based switch that does just that while operating at very high speeds.

To demonstrate their switch, the researchers first produced pairs of entangled photons using another device developed by Kumar, called an Entangled Photon Source. "Entangled" means that some physical characteristic (such as polarization as used in 3-D TV) of each pair of photons emitted by this device are inextricably linked. If one photon assumes one state, its mate assumes a corresponding state; this holds even if the two photons are hundreds of kilometers apart.

The researchers used pairs of polarization-entangled photons emitted into standard telecom-grade fiber. One photon of the pair was transmitted through the all-optical switch. Using single-photon detectors, the researchers found that the quantum state of the pair of photons was not disturbed; the encoded entanglement information was intact.

"Quantum communication can achieve things that are not possible with classical communication," said Kumar, director of Northwestern's Center for Photonic Communication and Computing. "This switch opens new doors for many applications, including distributed quantum processing where nodes of small-scale quantum processors are connected via quantum communication links."

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Three Solid-State Qubits Entangled: Big Step Toward Quantum Error Correction

Three Solid-State Qubits Entangled: Big Step Toward Quantum Error Correction

  11:19:00 AM    Science Lover

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The rules that govern the world of the very small, quantum mechanics, are known for being bizarre. One of the strangest tenets is something called quantum entanglement, in which two or more objects (such as particles of light, called photons) become inextricably linked, so that measuring certain properties of one object reveals information about the other(s), even if they are separated by thousands of miles. Einstein found the consequences of entanglement so unpalatable he famously dubbed it "spooky action at a distance."

The quantum entanglement of three solid-state qubits, or quantum bits, represents the first step towards quantum error correction, a crucial aspect of future quantum computing. (Credit: iStockphoto/Yenwen Lu)

Now a team led by Yale researchers has harnessed this counterintuitive aspect of quantum mechanics and achieved the entanglement of three solid-state qubits, or quantum bits, for the first time. Their accomplishment, described in the Sept. 30 issue of the journal Nature, is a first step towards quantum error correction, a crucial aspect of future quantum computing.

"Entanglement between three objects has been demonstrated before with photons and charged particles," said Steven Girvin, the Eugene Higgins Professor of Physics & Applied Physics at Yale and an author of the paper. "But this is the first three-qubit, solid-state device that looks and feels like a conventional microprocessor."

The new result builds on the team's development last year of the world's first rudimentary solid-state quantum processor, which they demonstrated was capable of executing simple algorithms using two qubits.

The team, led by Robert Schoelkopf, the William A. Norton Professor of Applied Physics & Physics at Yale, used artificial "atoms" -- actually made up of a billion aluminum atoms that behave as a single entity -- as their qubits. These "atoms" can occupy two different energy states, akin to the "1" and "0" or "on" and "off" states of regular bits used in conventional computers. The strange laws of quantum mechanics, however, allow for qubits to be placed in a "superposition" of these two states at the same time, resulting in far greater information storage and processing power.

In this new study, the team was able to achieve an entangled state by placing the three qubits in a superposition of two possibilities -- all three were either in the 0 state or the 1 state. They were able to attain this entangled state 88 percent of the time.

With the particular entangled state the team achieved, they also demonstrated for the first time the encoding of quantum information from a single qubit into three qubits using a so-called repetition code. "This is the first step towards quantum error correction, which, as in a classical computer, uses the extra qubits to allow the computer to operate correctly even in the presence of occasional errors," Girvin said.

Such errors might include a cosmic ray hitting one of the qubits and switching it from a 0 to a 1 state, or vice versa. By replicating the qubits, the computer can confirm whether all three are in the same state (as expected) by checking each one against the others.

"Error correction is one of the holy grails in quantum computing today," Schoelkopf said. "It takes at least three qubits to be able to start doing it, so this is an exciting step."

Other authors of the paper include Leonardo DiCarlo, Matthew Reed, Luyan Sun, Blake Johnson, Jerry Chow and Michel Devoret (all of Yale University); and Jay Gambetta (University of Waterloo).

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Funneling Solar Energy: Antenna Made of Carbon Nanotubes Could Make Photovoltaic Cells More Efficient

Funneling Solar Energy: Antenna Made of Carbon Nanotubes Could Make Photovoltaic Cells More Efficient

  1:23:00 AM    Science Lover

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Using carbon nanotubes (hollow tubes of carbon atoms), MIT chemical engineers have found a way to concentrate solar energy 100 times more than a regular photovoltaic cell. Such nanotubes could form antennas that capture and focus light energy, potentially allowing much smaller and more powerful solar arrays.

This filament containing about 30 million carbon nanotubes absorbs energy from the sun as photons and then re-emits photons of lower energy, creating the fluorescence seen here. The red regions indicate highest energy intensity, and green and blue are lower intensity. (Credit: Geraldine Paulus)

"Instead of having your whole roof be a photovoltaic cell, you could have little spots that were tiny photovoltaic cells, with antennas that would drive photons into them," says Michael Strano, the Charles and Hilda Roddey Associate Professor of Chemical Engineering and leader of the research team.

Strano and his students describe their new carbon nanotube antenna, or "solar funnel," in the Sept. 12 online edition of the journal Nature Materials. Lead authors of the paper are postdoctoral associate Jae-Hee Han and graduate student Geraldine Paulus.

Their new antennas might also be useful for any other application that requires light to be concentrated, such as night-vision goggles or telescopes.

Solar panels generate electricity by converting photons (packets of light energy) into an electric current. Strano's nanotube antenna boosts the number of photons that can be captured and transforms the light into energy that can be funneled into a solar cell.

The antenna consists of a fibrous rope about 10 micrometers (millionths of a meter) long and four micrometers thick, containing about 30 million carbon nanotubes. Strano's team built, for the first time, a fiber made of two layers of nanotubes with different electrical properties -- specifically, different bandgaps.

In any material, electrons can exist at different energy levels. When a photon strikes the surface, it excites an electron to a higher energy level, which is specific to the material. The interaction between the energized electron and the hole it leaves behind is called an exciton, and the difference in energy levels between the hole and the electron is known as the bandgap.

The inner layer of the antenna contains nanotubes with a small bandgap, and nanotubes in the outer layer have a higher bandgap. That's important because excitons like to flow from high to low energy. In this case, that means the excitons in the outer layer flow to the inner layer, where they can exist in a lower (but still excited) energy state.

Therefore, when light energy strikes the material, all of the excitons flow to the center of the fiber, where they are concentrated. Strano and his team have not yet built a photovoltaic device using the antenna, but they plan to. In such a device, the antenna would concentrate photons before the photovoltaic cell converts them to an electrical current. This could be done by constructing the antenna around a core of semiconducting material.

The interface between the semiconductor and the nanotubes would separate the electron from the hole, with electrons being collected at one electrode touching the inner semiconductor, and holes collected at an electrode touching the nanotubes. This system would then generate electric current. The efficiency of such a solar cell would depend on the materials used for the electrode, according to the researchers.

Strano's team is the first to construct nanotube fibers in which they can control the properties of different layers, an achievement made possible by recent advances in separating nanotubes with different properties.

While the cost of carbon nanotubes was once prohibitive, it has been coming down in recent years as chemical companies build up their manufacturing capacity. "At some point in the near future, carbon nanotubes will likely be sold for pennies per pound, as polymers are sold," says Strano. "With this cost, the addition to a solar cell might be negligible compared to the fabrication and raw material cost of the cell itself, just as coatings and polymer components are small parts of the cost of a photovoltaic cell."

Strano's team is now working on ways to minimize the energy lost as excitons flow through the fiber, and on ways to generate more than one exciton per photon. The nanotube bundles described in the Nature Materials paper lose about 13 percent of the energy they absorb, but the team is working on new antennas that would lose only 1 percent.

Funding: National Science Foundation Career Award, MIT Sloan Fellowship, the MIT-Dupont Alliance and the Korea Research Foundation.

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Researchers Create 'Quantum Cats' Made of Light

Researchers Create 'Quantum Cats' Made of Light

  9:18:00 PM    Science Lover

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Researchers at the National Institute of Standards and Technology (NIST) have created "quantum cats" made of photons (particles of light), boosting prospects for manipulating light in new ways to enhance precision measurements as well as computing and communications based on quantum physics.

These colorized plots of electric field values indicate how closely the NIST "quantum cats" (left) compare with theoretical predictions for a cat state (right). The purple spots and alternating blue contrast regions in the center of the images indicate the light is in the appropriate quantum state. (Credit: Gerrits/NIST)

The NIST experiments, described in a forthcoming paper, repeatedly produced light pulses that each possessed two exactly opposite properties -- specifically, opposite phases, as if the peaks of the light waves were superimposed on the troughs. Physicists call this an optical Schrödinger's cat. NIST's quantum cat is the first to be made by detecting three photons at once and is one of the largest and most well-defined cat states ever made from light. (Larger cat states have been created in different systems by other research groups, including one at NIST.)

A "cat state" is a curiosity of the quantum world, where particles can exist in "superpositions" of two opposite properties simultaneously. Cat state is a reference to German physicist Erwin Schrödinger's famed 1935 theoretical notion of a cat that is both alive and dead simultaneously.

"This is a new state of light, predicted in quantum optics for a long time," says NIST research associate Thomas Gerrits, lead author of the paper. "The technologies that enable us to get these really good results are ultrafast lasers, knowledge of the type of light needed to create the cat state, and photon detectors that can actually count individual photons."

The NIST team created their optical cat state by using an ultrafast laser pulse to excite special crystals to create a form of light known as a squeezed vacuum, which contains only even numbers of photons. A specific number of photons were subtracted from the squeezed vacuum using a device called a beam splitter. The photons were identified with a NIST sensor that efficiently detects and counts individual photons. Depending on the number of subtracted photons, the remaining light is in a state that is a good approximation of a quantum cat says Gerrits -- the best that can be achieved because nobody has been able to create a "real" one, by, for instance, the quantum equivalent to superimposing two weak laser beams with opposite phases.

NIST conducts research on novel states of light because they may enhance measurement techniques such as interferometry, used to measure distance based on the interference of two light beams. The research also may contribute to quantum computing -- which may someday solve some problems that are intractable today -- and quantum communications, the most secure method known for protecting the privacy of a communications channel. Larger quantum cats of light are needed for accurate information processing.

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Quantum mechanics not in jeopardy: Born's rule

Quantum mechanics not in jeopardy: Born's rule

  10:56:00 PM    Science Lover

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When waves -- regardless of whether light or sound -- collide, they overlap creating interferences. Austrian and Canadian quantum physicists have now been able to rule out the existence of higher-order interferences experimentally and thereby confirmed an axiom in quantum physics: Born's rule.

When waves -- regardless of whether light or sound -- collide, they overlap creating interferences. Austrian and Canadian quantum physicists have now been able to rule out the existence of higher-order interferences experimentally and thereby confirmed an axiom in quantum physics: Born's rule. (Credit: Graphic by IQC)

They have published their findings in the scientific journal Science.

In quantum mechanics many propositions are made in probabilities. In 1926 German physicist Max Born postulated that the probability to find a quantum object at a certain place at a certain time equals the square of its wave function. A direct consequence of this rule is the interference pattern as shown in the double slit diffraction experiment. Born's rule is one of the key laws in quantum mechanics and it proposes that interference occurs in pairs of possibilities. Interferences of higher order are ruled out. There was no experimental verification of this proposition until now, when the research group led by Prof. Gregor Weihs from the University of Innsbruck and the University of Waterloo has confirmed the accuracy of Born's law in a triple-slit experiment. "The existence of third-order interference terms would have tremendous theoretical repercussions -- it would shake quantum mechanics to the core," says Weihs. The impetus for this experiment was the suggestion made by physicists to generalize either quantum mechanics or gravitation -- the two pillars of modern physics -- to achieve unification, thereby arriving at a one all-encompassing theory. "Our experiment thwarts these efforts once again," explains Gregor Weihs.

Triple-slit experiment

Gregor Weihs -- Professor of Photonics at the University of Innsbruck -- and his team are investigating new light sources to be used for transmitting quantum information. He developed a single-photon source, which served as the basis for testing Born's rule. Photons were sent through a steel membrane mask which has three micrometer sized slits cut into it. Measurements were performed with the slits closed individually resulting in eight independent slit combinations. The data taken was then used to calculate whether Born's rule applies. "In principle, this experiment is very simple," says Gregor Weihs "and we were quite surprised to find that nobody hadn't performed this experiment before." However, the physicists were struggling with measurement errors, which they were eventually able to overcome during their two year long Sisyphean task. "Our measurements show that we can rule out the existence of third-order interference up to a certain bound," says a happy experimental physicist Weihs. His next step will be to considerably lower the bound with an improved experiment.

Master of light particles

The experiment was performed at the Institute for Quantum Computing at the University of Waterloo in Canada, where Prof. Gregor Weihs worked before his appointment at the University of Innsbruck. Since 2008 he has been setting up his own research group at the Institute for Experimental Physics in Innsbruck, which now comprises twelve group members. The group, whose members come from all over the world, investigates the development of novel single-photon sources and entangled photon pairs from semiconductor nanostructures. The researcher's ultimate goal is to integrate quantum optical experiments with functions on semiconductor chips.

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