Tag Archives: #photons

Observing Quantum Coherence From Photons Scattered in Free-space (Physics)

Quantum coherence is a key ingredient in many fundamental tests and applications of quantum technology including quantum communication, imaging, computing, sensing and metrology. However, the transfer of quantum coherence in free-space has so far been limited to direct line-of-sight channels as atmospheric turbulence and scattering degrade the quality of coherence severely.

In a new paper published in Light: Science & Applications, researchers from the University of Waterloo have successfully demonstrated the transfer and recovery of quantum coherence using photons scattered in free-space for the first time, enabling new research opportunities and applications in fields ranging from quantum communication to imaging and beyond.

“The ability to transfer quantum coherence via scattered photons means that now you can do many things that previously required direct line-of-sight free-space channels,” said Shihan Sajeed, lead author on the paper and a postdoctoral fellow at the Institute for Quantum Computing (IQC) and in the Department of Physics and Astronomy at the University of Waterloo in Ontario, Canada.

Normally, if you try to send and receive photons through the air (free-space) for quantum communication or any other quantum-encoded protocol, you need a direct line-of-sight between transmitter and receiver. Any objects–from as big as a wall to as small as a molecule–in the optical path will reflect some photons and scatter others, depending on how reflective the surface is. Any quantum information encoded in the photons is typically lost in the scattered photons, interrupting the quantum channel.

Together with Thomas Jennewein, principal investigator of the Quantum Photonics lab at IQC, Sajeed found a way to encode quantum coherence in pairs of photon pulses sent one after the other so that they would maintain their coherence even after scattering from a diffuse surface.

The researchers emitted a train of pulse pairs with a specific phase-coherence that could be measured from the scattered photons using quantum interference. They also used a single-photon-detector-array sensor that, in addition to solving wavefront distortions caused by atmospheric turbulence, acted as an imager thereby allowing to observe single-photon interference and imaging simultaneously. They placed the detector where they would only absorb scattered photons from the laser pulses, and observed a visibility of over 90%, meaning that the scattered photons maintained their quantum coherence even after smashing against an object.

Their novel technique required custom hardware to make use of the coherent light they were generating. The single photon detector array could detect one billion photons every second with a precision of 100 picoseconds. Only cutting-edge time-tagging electronics could handle the demands of this flow of light, and the team had to design their own electronics adapter board to communicate between the detectors and the computer that would process the data.

“Our technique can help image an object with quantum signals or transmit a quantum message in a noisy environment,” said Sajeed. “Scattered photons returning to our sensor will have a certain coherence, whereas noise in the environment will not, and so we can reject everything except the photons we originally sent.”

Sajeed expects their findings will stimulate new research and new applications in quantum sensing, communication, and imaging in free space environments. The duo demonstrated quantum communication and imaging in their paper, but Sajeed said further research is required to find out how their techniques could be used in various practical applications.

“We believe this could be used in quantum enhanced Lidar (Light Detection and Ranging), quantum sensing, non-line-of-sight imaging, and many other areas–the possibilities are endless,” said Sajeed.

Featured image: Each optical pulse from the laser is sent through a phase Converter, which creates two coherent pulses, while the multi-mode Analyzer measures the signals scattered off the target surface, implemented with regular bright paper. A single-photon-detector-array is used as the detection device, with 8 x 8 individual pixels which are each time-tagged separately. © by Shihan Sajeed, Thomas Jennewein

Reference: Sajeed, S., Jennewein, T. Observing quantum coherence from photons scattered in free-space. Light Sci Appl 10, 121 (2021). https://doi.org/10.1038/s41377-021-00565-y

Provided by CIOMP

LHAASO Discovers a Dozen PeVatrons and Photons Exceeding 1 PeV and Launches Ultra-high-energy Gamma Astronomy Era

China’s Large High Altitude Air Shower Observatory (LHAASO)—one of the country’s key national science and technology infrastructure facilities—has found a dozen ultra-high-energy (UHE) cosmic accelerators within the Milky Way. It has also detected photons with energies exceeding 1 peta-electron-volt (quadrillion electron-volts or PeV), including one at 1.4 PeV. The latter is the highest energy photon ever observed.

These findings overturn the traditional understanding of the Milky Way and open up an era of UHE gamma astronomy. These observations will prompt people to rethink the mechanism by which high-energy particles are generated and propagated in the Milky Way, and will encourage people to explore more deeply violent celestial phenomena and their physical processes as well as test basic physical laws under extreme conditions.

These discoveries are published in the journal Nature on May 17. The LHAASO International Collaboration, which is led by the Institute of High Energy Physics (IHEP) of the Chinese Academy of Sciences, completed this study.

The LHAASO Observatory is still under construction. The cosmic accelerators—known as PeVatrons since they accelerate particles to the PeV range—and PeV photons were discovered using the first half of the detection array, which was finished at the end of 2019 and operated for 11 months in 2020.

Photons with energies exceeding 1 PeV were detected in a very active star-forming region in the constellation Cygnus. LHAASO also detected 12 stable gamma ray sources with energies up to about 1 PeV and significances of the photon signals seven standard deviations greater than the surrounding background. These sources are located at positions in our galaxy that can be measured with an accuracy better than 0.3°. They are the brightest Milky Way gamma ray sources in LHAASO’s field of view.

Although the accumulated data from the first 11 months of operation only allowed people to observe those sources, all of them emit so-called UHE photons, i.e., gamma rays above 0.1 PeV. The results show that the Milky Way is full of PeVatrons, while the largest accelerator on Earth (LHC at CERN) can only accelerate particles to 0.01 PeV. Scientists have already determined that cosmic ray accelerators in the Milky Way have an energy limit. Until now, the predicted limit was around 0.1 PeV, thus leading to a natural cut-off of the gamma-ray spectrum above that.

But LHAASO’s discovery has increased this “limit,” since the spectra of most sources are not truncated. These findings launch an era for UHE gamma astronomic observation. They show that non-thermal radiation celestials, such as young massive star clusters, supernova remnants, pulsar wind nebulas and so on—represented by Cygnus star-forming regions and the Crab nebula—are the best candidates for finding UHE cosmic rays in the Milky Way.

Through UHE gamma astronomy, a century-old mystery-—the origin of cosmic rays—may soon be solved. LHAASO will prompt scientists to rethink the mechanisms of high energy cosmic ray acceleration and propagation in the Milky Way. It will also allow scientists to explore extreme astrophysical phenomena and their corresponding processes, thus enabling examination of the basic laws of physics under extreme conditions.

Extended Materials: 

LHAASO and Its Core Scientific Goals 

LHAASO is a major national scientific and technological infrastructure facility focusing on cosmic ray observation and research. It is located 4,410 meters above sea level on Mt. Haizi in Daocheng County, Sichuan Province. When construction is completed in 2021, LHAASO’s particle detector arrays will comprise 5,195 electromagnetic particle detectors and 1,188 Muon detectors located in the square-kilometer complex array (KM2A), a 78,000 m2 water Cherenkov detector array (WCDA), and 18 wide-field-of-view Cherenkov telescopes (WFCTA). Using these four detection techniques, LHAASO will be able to measure cosmic rays omnidirectionally with multiple variables simultaneously. The arrays will cover an area of about 1.36 km2.

LHAASO’s core scientific goal is to explore the origin of high-energy cosmic rays and study related physics such as the evolution of the universe, the motion and interaction of high-energy astronomical celestials, and the nature of dark matter. LHAASO will extensively survey the universe (especially the Milky Way) for gamma ray sources. It will precisely measure their energy spectra over a broad range—from less than 1 TeV (trillion electron-volts or tera-electron-volts) to more than 1 PeV. It will also measure the components of diffused cosmic rays and their spectra at even higher energies, thus revealing the laws of the generation, acceleration and propagation of cosmic rays, as part of the exploration of new physics frontiers.

PeVatrons and PeV Photons 

The signal of UHE photons around PeVatrons is so weak that just one or two photons at PeV energy can be detected using 1 km2 of detectors per year even when focusing on the Crab Nebula, known as the “standard candle for gamma astronomy.” What’s worse, those one or two photons are submerged in tens of thousands of ordinary cosmic rays. The 1,188 muon detectors in LHAASO’s KM2A are designed to select photon-like signals, making LHAASO the most sensitive UHE gamma ray detector in the world. With its unprecedented sensitivity, in just 11 months, the half-sized KM2A detected one photon around 1 PeV from the Crab Nebula. In addition, KM2A found 12 similar sources in the Milky Way, all of which emit UHE photons and extend their spectra continuously into the vicinity of 1 PeV. Even more important, KM2A has detected a photon with energy of 1.4 PeV—the highest ever recorded. It is clear that LHAASO’s scientific discoveries represent a milestone in identifying the origin of cosmic rays. To be specific, LHAASO’s scientific breakthroughs fall into the following three areas:

1) Revealing the ubiquity of cosmic accelerators capable of accelerating particles to energies exceeding 1 PeV in the Milky Way. All the gamma ray sources that LHAASO has effectively observed radiate photons in the UHE range above 0.1 PeV, indicating that the energy of the parent particles radiating these gamma rays must exceed 1 PeV. As a matter of convention, these sources must have significances of photon signals five standard deviations greater than the surrounding background. The observed energy spectrum of these gamma rays has not truncated above 0.1 peV, demonstrating that there is no acceleration limit below PeV in the galactic accelerators.

This observation violates the prevailing theoretical model. According to current theory, cosmic rays with energies in the PeV range can produce gamma rays of 0.1 PeV by interacting with surrounding gases in the accelerating region. Detecting gamma rays with energies greater than 0.1 PeV is an important way to find and verify PeV cosmic ray sources. Since previous international mainstream detectors work below this energy level, the existence of PeV cosmic ray accelerators had not been solidly confirmed before. But now LHAASO has revealed a large number of PeV cosmic acceleration sources in the Milky Way, all of which are candidates for being UHE cosmic ray generators. This is a crucial step toward determining the origin of cosmic rays.

2) Beginning an era of “UHE gamma astronomy.” In 1989, an experimental group at the Whipple Observatory in Arizona successfully discovered the first object emitting gamma radiation above 0.1 TeV, marking the onset of the era of “very-high-energy” gamma astronomy. Over the next 30 years, more than 200 “very-high-energy” gamma ray sources were discovered. However, the first object emitting UHE gamma radiation was not detected until 2019. Surprisingly, by using a partly complete array for less than a year, LHAASO has already boosted the number of UHE gamma ray sources to 12.

With the completion of LHAASO and the continuous accumulation of data, we can anticipate to find an unexplored “UHE universe” full of surprising phenomena. It is well known that background radiation from the Big Bang is so pervasive it can absorb gamma rays with energies greater than 1 PeV. Even if gamma rays were produced beyond the Milky Way, we wouldn’t be able to detect them. This makes LHAASO’s observational window so special.

3) Photons with energies greater than 1 PeV were first detected from the Cygnus region and the Crab Nebula. The detection of PeV photons is a milestone in gamma astronomy. It fulfills the dream of the gamma astronomy community and has long been a strong driving force in the development of research instruments in the field. In fact, one of the main reasons for the explosion of gamma astronomy in the 1980s was the challenge of the PeV photon limit. The star-forming region in the direction of Cygnus is the brightest area in the northern territory of the Milky Way, with a large number of massive star clusters. Massive stars live only on the order of one million years, so the clusters contain enormous stars in the process of birth and death, with a complex strong shock environment. They are ideal “particle astrophysics laboratories,” i.e., places for accelerating cosmic rays.

The first PeV photons found by LHAASO were from the star-forming region of the constellation Cygnus, making this area the best candidate for exploring the origin of UHE cosmic rays. Therefore, much attention has turned to LHAASO and multi-wavelength observation of this region, which could offer a potential breakthrough in solving the “mystery of the century.”

Extensive observational studies of the Crab Nebula over the years have made the celestial body almost the only standard gamma ray source with a clear emission mechanism. Indeed, precise spectrum measurements across 22 orders of magnitude clearly reveal the signature of an electron accelerator. However, the UHE spectra measured by LHAASO, especially photons at PeV energy, seriously challenge this “standard model” of high-energy astrophysics and even the more fundamental theory of electron acceleration.

Technology Innovations 

LHAASO has developed and/or improved: 1) clock synchronization technology over long distances that ensures timing synchronization accuracy to the sub-nanosecond level for each detector in the array; 2) multiple parallel event trigger algorithms and their realization, with the help of high-speed front-end signal digitization, high-speed data transmission and large on-site computing clusters; and advanced detection technologies include 3) silicon photo multipliers (SiPM) and 4) ultra-large photocathode micro-channel plate photomultiplier tubes (MCP-PMT). They are being employed at LHAASO on a large scale for the first time. They have greatly improved the spatial resolution of photon measurements and lowered the detection energy threshold. These features allow detectors to achieve unprecedented sensitivity in exploring the deep universe at a wide energy range. LHAASO provides an attractive experimental platform for conducting interdisciplinary research in frontier sciences such as atmosphere, high-altitude environment and space weather. It will also serve as a base for international cooperation on high-level scientific research projects.

History of Cosmic Ray Research in China 

Cosmic ray research in China has experienced three stages. LHAASO represents the third generation of high-altitude cosmic ray observatories. High-altitude experiments are a means of making full use of the atmosphere as a detector medium. In this way, scientists can observe cosmic rays on the ground, where the size of the detector can be much larger than in a space-borne detector outside the atmosphere. This is the only way to observe cosmic rays at very high energy.

In 1954, China’s first cosmic ray laboratory was built on Mt. Luoxue in Dongchuan, Yunnan Province, at 3,180 meters above sea level. In 1989, the Sino-Japanese cosmic ray experiment ASg was built at an altitude of 4,300 meters above sea level at Yangbajing, Tibet Autonomous Region.

In 2006, the joint Sino-Italian ARGO-YBJ experiment was built at the same site.

In 2009, at the Xiangshan Science Forum in Beijing, Professor CAO Zhen proposed to build a large-scale composite detection array (i.e., LHAASO) in a high-altitude area. The LHAASO project was approved in 2015 and construction began in 2017. By April 2019, construction was 25% complete and scientific operation had begun. By January 2020, an additional 25% had been completed and put into operation. In December of the same year, 75% of the facility had been completed. The entire facility will be completed in 2021. LHAASO has already become one of the world’s leading UHE gamma detection facilities, and will operate for a long time. With it, scientists will be able to study the origin of cosmic rays from many aspects.

Featured image: Aerial photograph of LHAASO (Image by IHEP)

Reference: Cao, Z., Aharonian, F.A., An, Q. et al. Ultrahigh-energy photons up to 1.4 petaelectronvolts from 12 γ-ray Galactic sources. Nature (2021). https://doi.org/10.1038/s41586-021-03498-z

Provided by Chinese Academy of Sciences

Complex Shapes of Photons To Boost Future Quantum Technologies (Quantum / Physics)

Researchers at Tampere University Photonics Laboratory have demonstrated how two interfering photons can bunch into various shapes. These complex shapes are beneficial for quantum technologies, such as performing fast photonic quantum computations and safe data transfer. The method opens new possibilities also for creating enhanced measurement and sensing techniques.

As the digital revolution has now become mainstream, quantum computing and quantum communication are rising in the consciousness of the field. The enhanced measurement technologies enabled by quantum phenomena, and the possibility of scientific progress using new methods, are of particular interest to researchers around the world.

Recently two researchers at Tampere University, Assistant Professor Robert Fickler and Doctoral Researcher Markus Hiekkamäki, demonstrated that two-photon interference can be controlled in a near-perfect way using the spatial shape of the photon. Their findings were recently published in the prestigious journal Physical Review Letters.

Our report shows how a complex light-shaping method can be used to make two quanta of light interfere with each other in a novel and easily tuneable way”, explains Markus Hiekkamäki.

Single photons (units of light) can have highly complex shapes that are known to be beneficial for quantum technologies such as quantum cryptography, super-sensitive measurements, or quantum-enhanced computational tasks. To make use of these so-called structured photons, it is crucial to make them interfere with other photons.

“One crucial task in essentially all quantum technological applications is improving the ability to manipulate quantum states in a more complex and reliable way.  In photonic quantum technologies, this task involves changing the properties of a single photon as well as interfering multiple photons with each other;” says Robert Fickler, who leads the Experimental Quantum Optics group at the university.

Linear optics bring promising solutions to quantum communications

The demonstrated development is especially interesting from the point of view of high-dimensional quantum information science, where more than a single bit of quantum information is used per carrierThese more complex quantum states not only allow the encoding of more information onto a single photon but are also known to be more noise-resistant in various settings.

The method presented by the research duo holds promise for building new types of linear optical networks. This paves the way for novel schemes of photonic quantum-enhanced computing.

“Our experimental demonstration of bunching two photons into multiple complex spatial shapes is a crucial next step for applying structured photons to various quantum metrological and informational tasks”, continues Markus Hiekkamäki.

The researchers now aim at utilizing the method for developing new quantum-enhanced sensing techniques, while exploring more complex spatial structures of photons and developing new approaches for computational systems using quantum states.

“We hope that these results inspire more research into the fundamental limits of photon shaping. Our findings might also trigger the development of new quantum technologies, e.g. improved noise-tolerant quantum communication or innovative quantum computation schemes, that benefit from such high-dimensional photonic quantum states”, adds Robert Fickler.

Read more about research High-Dimensional Two-Photon Interference Effects in Spatial Modes.

Featured image: Conceptual image of the used method for manipulating the spatial structures of photons using multiple consecutive lossless modulations. Image: Markus Hiekkamäki / Tampere University

Provided by Tampere University

Physicists Observed New State of Light (Physics)

Physicists at the University of Bonn observe new phase in Bose-Einstein condensate of light particles

A single “super photon” made up of many thousands of individual light particles: About ten years ago, researchers at the University of Bonn produced such an extreme aggregate state for the first time and presented a completely new light source. The state is called optical Bose-Einstein condensate and has captivated many physicists ever since, because this exotic world of light particles is home to its very own physical phenomena. Researchers led by Prof. Dr. Martin Weitz, who discovered the super photon, and theoretical physicist Prof. Dr. Johann Kroha have returned from their latest “expedition” into the quantum world with a very special observation. They report of a new, previously unknown phase transition in the optical Bose-Einstein condensate. This is a so-called overdamped phase. The results may in the long term be relevant for encrypted quantum communication. The study has been published in the journal Science.

The Bose-Einstein condensate is an extreme physical state that usually only occurs at very low temperatures. What’s special: The particles in this system are no longer distinguishable and are predominantly in the same quantum mechanical state, in other words they behave like a single giant “superparticle”. The state can therefore be described by a single wave function.

In 2010, researchers led by Martin Weitz succeeded for the first time in creating a Bose-Einstein condensate from light particles (photons). Their special system is still in use today: Physicists trap light particles in a resonator made of two curved mirrors spaced just over a micrometer apart that reflect a rapidly reciprocating beam of light. The space is filled with a liquid dye solution, which serves to cool down the photons. This is done by the dye molecules “swallowing” the photons and then spitting them out again, which brings the light particles to the temperature of the dye solution – equivalent to room temperature. Background: The system makes it possible to cool light particles in the first place, because their natural characteristic is to dissolve when cooled.

Clear separation of two phases

Phase transition is what physicists call the transition between water and ice during freezing. But how does the particular phase transition occur within the system of trapped light particles? The scientists explain it this way: The somewhat translucent mirrors cause photons to be lost and replaced, creating a non-equilibrium that results in the system not assuming a definite temperature and being set into oscillation. This creates a transition between this oscillating phase and a damped phase. Damped means that the amplitude of the vibration decreases.

Prof. Dr. Martin Weitz, Dr. Julian Schmitt, Dr. Frank Vewinger, Prof. Dr. Johann Kroha and Göran Hellmann from the Institute of Applied Physics at the University of Bonn. © Gregor Hübl/Uni Bonn

“The overdamped phase we observed corresponds to a new state of the light field, so to speak,” says lead author Fahri Emre Öztürk, a doctoral student at the Institute for Applied Physics at the University of Bonn. The special characteristic is that the effect of the laser is usually not separated from that of Bose-Einstein condensate by a phase transition, and there is no sharply defined boundary between the two states. This means that physicists can continually move back and forth between effects.

“However, in our experiment, the overdamped state of the optical Bose-Einstein condensate is separated by a phase transition from both the oscillating state and a standard laser,” says study leader Prof. Dr. Martin Weitz. “This shows that there is a Bose-Einstein condensate, which is really a different state than the standard laser. “In other words, we are dealing with two separate phases of the optical Bose-Einstein condensate,” he emphasizes.

The researchers plan to use their findings as a basis for further studies to search for new states of the light field in multiple coupled light condensates, which can also occur in the system. “If suitable quantum mechanically entangled states occur in coupled light condensates, this may be interesting for transmitting quantum-encrypted messages between multiple participants,” says Fahri Emre Öztürk.


The study received funding from the Collaborative Research Center TR 185 “OSCAR – Control of Atomic and Photonic Quantum Matter by Tailored Coupling to Reservoirs” of the Universities of Kaiserslautern and Bonn and the Cluster of Excellence ML4Q of the Universities of Cologne, Aachen, Bonn and the Research Center Jülich, funded by the German Research Foundation. The Cluster of Excellence is embedded in the Transdisciplinary Research Area (TRA) “Building Blocks of Matter and Fundamental Interactions” of the University of Bonn. In addition, the study was funded by the European Union within the project “PhoQuS – Photons for Quantum Simulation” and the German Aerospace Center with funding from the Federal Ministry for Economic Affairs and Energy.

Video:  https://youtu.be/PHSNJIu2IVo

Featured image: On the right is a microscope objective used to observe and analyze the light emerging from the Resonator. © Gregor Hübl/Uni Bonn

Publication: Fahri Emre Öztürk, Tim Lappe, Göran Hellmann, Julian Schmitt, Jan Klaers, Frank Vewinger, Johann Kroha & Martin Weitz: Observation of a Non-Hermitian Phase Transition in an Optical Quantum Gas. Science, DOI: 10.1126/science.abe9869

Provided by University of Bonn

HAWC: Are Photons of Extreme Energies Coming From the Galaxy’s Largest Accelerator? (Planetary Science)

For years, in the vastness of our galaxy, astrophysicists have been tracking down pevatrons – natural accelerators of particles with monstrous energies. Thanks to the HAWC Observatory for Cosmic Radiation, another probable trace of their existence has just been found: photons with some of the highest energies. However, what is particularly important is that this time the high-energy photons have not only been recorded, but also their probable place of origin has been determined.

We know they exist, we just don’t know where exactly they are or what they look like. Pevatrons – because this is what we are talking about here – are the largest natural particle accelerators in our galaxy, capable of accelerating protons and electrons to energies even many billions of times greater than the energy of photons of visible light. The problem with detecting pevatrons stems from the fact that the particles they accelerate carry an electric charge and are therefore deflected by magnetic fields in the galaxy. The discovery which has just been made thanks to data collected by the High-Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory brings us significantly closer to finding the first cosmic pevatron and understanding its nature.

The HAWC Observatory is located on the slope of the Sierra Negra volcano in Mexico, at an altitude of 4,100 m. It consists of 300 water tanks, each surrounded by sensitive photomultipliers. When a particle of secondary cosmic radiation travelling at a speed faster than the speed of light in water enters a tank, there is an electromagnetic “boom” – a weak flash of radiation (Cherenkov), detected and amplified by the photomultipliers. A careful analysis of flashes observed at the same time in individual tanks makes it possible to extract information about the type, energy and direction of the particle of the primary cosmic radiation which initiated the recorded cascade of secondary particles.

“Based on data collected by the HAWC, we were able to determine the source of photons with energies of about 200 teraelectronvolts. For photons, this is an extreme value, one hundred trillion times greater than the energy typical of photons perceived by our eyes”, says Dr. Sabrina Casanova from the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Cracow. Together with Dr. Francisco Sales Greus (IFJ PAN & IFIC) and PhD student Dezhi Huang from the Michigan Technological University in Houghton (USA), she is one of the main authors of the analysis published in the excellent astronomical journal The Astrophysical Journal Letters.

Compared to protons and electrons, photons have a pleasant feature: they ignore magnetic fields and run to their target along the shortest path that space-time allows. So, once the direction from which the photons are coming is identified within a galaxy, it is usually possible to determine their source. This is not an easy task, but in this case it was successful. The source of the 200 TeV photons turned out to be a region of the recently discovered pulsar eHWC J1825-134, visible in the southern hemisphere in the background of the Vela constellation, and lying at a distance of about 13 thousand light years from Earth.

Observations of such high-energy photons are rare, and it is even rarer to identify the source. The record currently belongs to photons with energies of 450 TeV, detected with the Sino-Japanese ASgamma detector in Tibet. In that case, the photons came from the vicinity of a pulsar in the famous Crab nebula in the background of the Taurus constellation.

“We now know of two mechanisms that can explain the existence of photons with energies of 200 TeV and above”, explains Dr Salesa Greus, before elaborating: “According to the first, the source of such energetic photons could be electrons with slightly higher energies, emitted by supernova remnants or pulsars and then interacting with the microwave background radiation filling the Universe. This case seems to fit the Crab nebula. The second variant of the course of events assumes that photons are born due to the interaction of protons emitted by a pulsar with matter in interstellar space. What is particularly interesting in this scenario is that the energies of the protons must be at least an order of magnitude greater than the energies of the observed photons!”

The region of the eHWC J1825-134 pulsar is a complex astronomical structure with several sources of high-energy gamma rays within it. HAWC researchers have determined that the origin of the 200 TeV photons was not the pulsar itself, but a previously unknown source: a nearby cloud of interstellar material. It surrounds a young cluster of stars, about one million years old, designated as [BDS2003] 8. The observed photons could thus have been emitted by protons from the eHWC J1825-134 pulsar which, within the [BDS2003] 8 star cluster, had enough time to accelerate in the local magnetic field to energies of a few petaelectronvolts and produce energetic photons in the interaction with the cloud. If this variant of the course of events is confirmed in subsequent observations, we would be dealing with the first pevatron identified in our galaxy.

“For the time being, we have too little data to unambiguously decide on the nature of the cosmic accelerator responsible for the generation of 200 TeV photons in the region of eHWC J1825-134. If, however, there is a galactic pevatron hiding somewhere, we have managed to find a really excellent candidate”, notes Dr. Casanova.

The research of scientists from the IFJ PAN was funded by a grant from the Polish National Sci – ence Centre.

Featured image: Photons with an energy of 200 teraelectronvolts are most likely emitted by protons colliding with interstellar gas. The primary source of protons is pulsar HAWC J1825-134 (in the orange circle), the role of the actual accelerator is played by the star cluster [BDS2003] 8 (dark blue). (Source: HAWC)

Scientific papers:

„Evidence of 200 TeV Photons from HAWC J1825-134”
A. Albert et al.;
The Astrophysical Journal Letters, 907, 2, 2021;
DOI: https://doi.org/10.3847/2041-8213/abd77b

Provided by IFJ

Photon-photon Polaritons: The Intriguing Particles That Emerge When Two Photons Couple (Particle Physics)

Researchers exploring the interactions between light particles, photons and matter find that optical microresonators host quasiparticles made by two photons.

Scientists at the University of Bath have found a way to bind together two photons of different colours, paving the way for important advancements in quantum-electrodynamics – the field of science that describes how light and matter interact. In time, the team’s findings are likely to impact developments in optical and quantum communication, and precision measurements of frequency, time and distances.

Apple and wave: they both have a mass

An apple falling from a tree has velocity and mass, which together give it momentum. ‘Apple energy’ derived from motion depends on the fruit’s momentum and mass.

Most people find the concept of momentum and energy (and therefore mass) an easy one to grasp when it’s associated with solid objects. But the idea that non-material objects, such as light waves (everything from sunlight to laser radiation), also have a mass is surprising to many. Among physicists, however, it’s a well-known fact. This apparently paradoxical idea that waves have a mass marks the place where quantum physics and the physical world come together.

The wave-particle duality, proposed by French physicist Louis de Broglie in 1924, is a powerful concept that describes how every particle or quantum entity can be described as either a particle or a wave. Many so-called quasiparticles have been discovered that combine either two different types of matter particles, or light waves bound to a particle of matter. A list of exotic quasiparticles includes phonons, plasmons, magnons and polaritons.

The team of physicists at Bath has now reported a way to create quasiparticles that bind together two differently coloured particles of light. They have named these formations photon-photon polaritons.

Detecting photon-photon polaritons

The opportunity to discover, and manipulate, photon-photons is possible thanks to the relatively new development of high-quality microresonators. For light, microresonators act as miniature racetracks, with photons zipping around the internal structure in loops. The signature left by photon-photons in the light exiting the microresonator can be linked to the Autler–Townes effect, a peculiar phenomenon in quantum theory that describes strong photon-atom interactions. To achieve this effect in microresonators, a laser is tuned to the specific resonance frequency where a photon is expected to be absorbed, yet no resonance absorption happens. Instead, the photon-photon interaction makes up two new resonance frequencies away from the old one.

A significant feature that has emerged from the Bath research is that the microresonator provided a whole set of split resonances, where each photon-photon pair displayed its own momentum and energy, allowing the researchers to apply the quasiparticle concept and calculate mass. According to the researchers’ predictions, photon-photons are 1,000+ times lighter than electrons.

Professor Dmitry Skryabin, the physicist who led the research, said: “We now have a situation where microresonators – which are millimeter-scale objects – behave like giant atoms. The artificial atoms concept is rapidly gaining ground in the quantum-electrodynamics of microwaves in superconducting circuits, while here we are looking at the similar opportunity in the optical range of frequencies.

“The small mass of photon-photons could lead to further developments of many important analogies between light and fluids, where other families of quasiparticles have already being used.”

PhD student Vlad Pankratov, who participated in the project, said: “After a year of running models and collecting data, these are incredibly exciting findings for us. The potential applications of our results are in the terabit and quantum optical communication schemes, and in the area of precision measurements.”

This work was supported by the European Union Marie Skłodowska-Curie Actions (812818, MICROCOMB)

The paper Photon-photon polaritons in χ(2) microresonators is published in Physical Review Research.

Featured image: Photon-photon polaritons in microresonators © University of Bath

Provided by University of Bath

Scientists Observe Very Slow Hot Electron Cooling in Electron-doped Quantum Dots (Chemistry)

In conventional solar cells, the excessive energy of carriers above the band gap of a semiconductor is lost in the form of heat due to rapid cooling of hot carriers via emission of phonons. This is a major reason for Shockley-Queisser limit of solar cell efficiency.

Extracting the hot carrier to selective contacts has the potential to boost power conversion efficiency of solar cells to as high as 66%. The challenge in efficient hot carrier extraction is that intraband cooling of hot carriers usually occurs in a sub-picosecond timescale.

A research group led by Prof. WU Kaifeng from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS) recently observed hot electrons as long-lived as more than 300 picoseconds in electron-doped (n-doped) colloidal CdSe quantum dots.

This study was published in Nature Communications on Jan. 22.

For lightly-doped dots they observed a slow 1Pe hot electron relaxation (~10 picosecond) resulting from a Pauli spin blockade of the preoccupying 1Se electron.

For heavily-doped dots, a large number of electrons residing in the surface states introduced picosecond Auger recombination which annihilated the valance band hole. Thus, the researchers could observe 300-picosecond-long hot electrons as a manifestation of a phonon bottleneck effect. This decreased the hot electron energy loss rate to a level of sub-meV per picosecond from a usual level of 1 eV per picosecond.

These results offered exciting opportunities of hot electron harvesting by exploiting carrier-carrier, carrier-phonon and spin-spin interactions in doped quantum dots.

“The observation of long-lived hot electrons in QDs sheds light on the utilization of long-lived hot electrons for high-efficiency photovoltaics and photocatalysis,” said Prof. WU.

This work was supported by the Ministry of Science and Technology of China, the Chinese Academy of Sciences, the National Natural Science Foundation of China and the Liaoning Revitalization Talents Program.

Featured image: Carrier-carrier, carrier-phonon and spin-spin interactions in doped quantum dots result in long-lived hot electrons (Image by WANG Junhui)

Reference: Wang, J., Wang, L., Yu, S. et al. Spin blockade and phonon bottleneck for hot electron relaxation observed in n-doped colloidal quantum dots. Nat Commun 12, 550 (2021). https://doi.org/10.1038/s41467-020-20835-4

Provided by Chinese Academy of Sciences

Physicists Develop Record-breaking Source for Single Photons (Physics)

Researchers at the University of Basel and Ruhr University Bochum have developed a source of single photons that can produce billions of these quantum particles per second. With its record-breaking efficiency, the photon source represents a new and powerful building-block for quantum technologies.

Quantum cryptography promises absolutely secure communications. A key component here are strings of single photons. Information can be stored in the quantum states of these light particles and transmitted over long distances. In the future, remote quantum processors will communicate with each other via single photons. And perhaps the processor itself will use photons as quantum bits for computing.

A basic prerequisite for such applications, however, is an efficient source of single photons. A research team led by Professor Richard Warburton, Natasha Tomm and Dr. Alisa Javadi from the University of Basel, together with colleagues from Bochum, now reports in the journal Nature Nanotechnology on the development of a single-photon source that significantly surpasses previously known systems in terms of efficiency.

“Funnel” guides light particles

Each photon is created by exciting a single “artificial atom” (a quantum dot) inside a semiconductor. Usually, these photons leave the quantum dot in all possible directions and thus a large fraction is lost. In the photon source now presented, the researchers have solved this problem by positioning the quantum dot inside a “funnel” to send all photons in a specific direction.

The “funnel” is a novel micro-cavity that represents the real innovation of the research team: The micro-cavity captures almost all of the photons and then directs them into an optical fiber. The photons, each about two centimeters long, emerge at the end of an optical fiber.

The efficiency of the entire system – that is, the probability that excitation of the quantum dot actually results in a usable photon – is 57 percent, more than double that of previous single-photon sources. “This is a really special moment,” explains lead author Richard Warburton. “We’ve known for a year or two what’s possible in principle. Now we’ve succeeded in putting our ideas into practice.”

Enormous increase in computing power

The increase in efficiency has significant consequences, Warburton adds: “increasing the efficiency of single photon creation by a factor of two adds up to an overall improvement of a factor of one million for a string of, say, 20 photons. In the future, we’d like to make our single-photon source even better: We’d like to simplify it and pursue some of its myriad applications in quantum cryptography, quantum computing and other technologies.”

The project was funded by the Swiss National Science Foundation, the National Center of Competence in Research “Quantum Science and Technology” (NCCR QSIT), and the European Union under the Horizon2020 programme.

Featured image: The new single-photon source is based on excitation of a quantum dot (shown as a bulge on the bottom left), which then emits photons. A micro-cavity ensures that the photons are guided into an optical fiber and emerge at its end. (Figure: University of Basel, Department of Physics)

Reference: Natasha Tomm, Alisa Javadi, Nadia O. Antoniadis, Daniel Najer, Matthias C. Löbl, Alexander R. Korsch, Rüdiger Schott, Sascha R. Valentin, Andreas D. Wieck, Arne Ludwig, and Richard J. Warburton
A bright and fast source of coherent single photons
Nature Nanotechnology (2021), doi: 10.1038/s41565-020-00831-x

Provided by University of Basel

Navigation by Atom – Coming to a Vehicle Near You (Physics)

Instruments normally found in physics labs are making their way into everyday applications. Institute scientists have greatly expanded these instruments’ capabilities.

Satellite-based GPS is a wondrous invention, but even GPS has its limitations – such as in navigating underwater or when one of the orbiting satellites malfunctions. GPS, like atlases and paper maps, works on parameters of distance and height, but there are other ways to measure locations and routes – for example, by charting the Earth’s gravitational field or, in the near future, by basing very small guidance systems on the trajectories of atoms. Such atomic systems could be used for navigation without GPS.

These atomic systems, known as cold-atom interferometers, work on a well-known quantum principle. Quantum particles like photons or electrons can act as waves; as waves they split into two in a device like an interferometer and then recombine, the patterns of interference between the two waves revealing information about that particle’s trajectory. Creating interferometers that work with atomic matter waves is significantly more challenging than with light, because to perform properly as quantum particles, atoms must be cooled to temperatures of just a few millionths of a degree above absolute zero. Nonetheless, researchers in several leading labs around the globe have been developing the technology, in part as a highly accurate means of navigation. The method can also measure slight changes in gravitational forces acting on the atom.

A cloud of cold atoms filmed at 0.1-second intervals in free-fall. The atoms’ motion is parabolic, due to gravity, while the cloud expands radially due to its finite (non-zero) temperature. As they fall, they are interrogated by three laser pulses, at the beginning, middle and end of their trajectory © Weizmann Institute of Science

Weizmann Institute of Science researchers recently demonstrated a way to make this technology even more useful, by greatly extending the range of the measurements it can perform. Chen Avinadav and Dr. Dimitry Yankelev in the groups of Dr. Ofer Firstenberg and Prof. Nir Davidson, all in the Physics of Complex Systems Department, led this research.

Interferometers are generally highly sensitive, but they are but limited in range. Measuring with an instrument like an interferometer has the same limitations as weighing something on a mechanical scale: There is always a trade-off between the range of measurement and the sensitivity. Your bathroom scale, for example, has a range of around 100 kilograms, but it cannot tell if your weight has changed by a few grams. Your kitchen scale, on the other hand, has a range of around a kilogram but is sensitive down to a gram or less.

(l-r) Prof. Nir Davidson, Chen Avinadav, Dr. Ofer Firstenberg and Dr. Dimitry Yankelev © WIS

The Institute researchers found a way around this limitation for atomic interferometers by performing several measurements at slightly different scales. When these are combined, the interfering waves generate a so-called “moire” pattern, which extends the range of the measurement many times over. In the present study, the research team succeeded in building on their previous improvements in obtaining moire measurements, as well as reducing the noise inherent in the process. They developed a system based on simultaneous measurements of two scales in interferometer setups that were identical in all other ways.

The range of measurement could be extended up to a thousand times over, with almost no sacrifice in the sensitivity.

When two or three such measurements were taken in quick succession, the range of measurement could be extended up to a thousand times over, with almost no sacrifice in the sensitivity of those measurements. And this might be only the beginning, says Firstenberg. The two labs are planning future experiments with even more scales, to see if it is possible to extend the range even further.

Such atomic interferometers are touted for two main applications: inertial navigation, which works by dead reckoning from a starting point, and gravitational mapping which, among other things, can assist in the search for natural resources. The new technique could represent a significant advance in the efforts to create everyday applications in the field.

The moire effect: two nearly-identical fine scales provide for a high measurement sensitivity, while the coarse scale generated in their overlap increases the measurement’s range © WIS

Prof. Nir Davidson is Head of the André Deloro Institute for Advanced Research in Space and Optics; and Head of the Center for Experimental Physics. His research is also supported by the Veronika A. Rabl Physics Discretionary Fund; Dana and Yossie Hollander; the Norman E. Alexander Family M Foundation; and Paul and Tina Gardner. Prof. Davidson is the incumbent of the Peter and Carola Kleeman Professorial Chair of Optical Sciences.  Dr. Ofer Firstenberg’s research is supported by the Laboratory in Memory of Leon and Blacky Broder.

Reference: Dimitry Yankelev, Chen Avinadav, Nir Davidson, Ofer Firstenberg, “Atom interferometry with thousand-fold increase in dynamic range”, Science Advances  04 Nov 2020: Vol. 6, no. 45, eabd0650 DOI: 10.1126/sciadv.abd0650 https://advances.sciencemag.org/content/6/45/eabd0650

Provided by Weizmann Institute of Science