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Our detector is both small enough to sit on a human hair and can work in normal room temperature conditions. One day soon, I imagine these devices will be routinely part of the micro-processor on your desktop PC and in your mobile phone to keep them secure. Their study has been published in Physical Review Letters.
Quantum computer parts are sensitive and need to be cooled to very low temperatures. Their size makes them particularly susceptible to temperature increases from the thermal noise in the surrounding environment and that caused by other components nearby. Their theoretical approach relies on quantum interference.
Normally, if a hotter object is placed next to a cooler one, the heat can only flow from the hotter object to the cooler one. Therefore, cooling an object that is already cooler than its surroundings requires energy. A new method for cooling down the elements of quantum devices such as qubits, the tiny building blocks of quantum computers, was now theoretically proven to work by a group of physicists. But here, we are using a quantum mechanical principle to realize it," explains Shabir Barzanjeh, the lead author of the study and postdoc in the research group of Professor Johannes Fink.
They used a heat sink connected to both devices, showing that it is possible to control its heat flow such that it cancels the heat coming from the warm object directly to the cool one via special quantum interference. This is quite different, because a signal is coherent, and the noise isn't. The difficulty with this task is that, on the smallest scales, the universe operates under strange rules: Particles can be here and there at the same time; objects separated by immense distances can influence each other instantaneously; the simple act of observing can change the outcome of reality.
Today, nearly 40 years later, such computers are starting to become a reality, and they pose a unique opportunity for particle physicists. You have plenty of problems that we would like to be able to solve accurately without making approximations that we hope we will be able to do on the quantum computer.
They take advantage of a phenomenon known as superposition, in which a particle such as an electron exists in a probabilistic state spread across multiple locations at once. Unlike a classical computer bit, which can be either on or off, a quantum bit—or qubit—can be on, off, or a superposition of both on and off, allowing for computations to be performed simultaneously instead of sequentially.
This not only speeds up computations; it makes currently impossible ones possible. A problem that could effectively trap a normal computer in an infinite loop, testing possibility after possibility, could be solved almost instantaneously by a quantum computer. This processing speed could be key for particle physicists, who wade through enormous amounts of data generated by detectors. They created Higgs bosons by converting the energy of particle collisions temporarily into matter. Those temporary Higgs bosons quickly decayed, converting their energy into other, more common particles, which the detectors were able to measure.
Scientists identified the mass of the Higgs boson by adding up the masses of those less massive particles, the decay products. But to do so, they needed to pick out which of those particles came from the decay of Higgs bosons, and which ones came from something else.
To a detector, a Higgs boson decay can look remarkably similar to other, much more common decays. LHC scientists trained a machine learning algorithm to find the Higgs signal against the decay background—the needle in the haystack. This training process required a huge amount of simulated data. Physicist Maria Spiropulu, who was on the team that discovered the Higgs the first time around, wanted to see if she could improve the process with quantum computing. The group she leads at CalTech used a quantum computer from a company called D-Wave to train a similar machine learning algorithm.
They found that the quantum computer trained the machine learning algorithm on a significantly smaller amount of data than the classical method required. In theory, this would give the algorithm a head start, like giving someone looking for the needle in the haystack expert training in spotting the glint of metal before turning their eyes to the hay. In the quantum annealer, we have a hint that it can learn with small data, and if you learn with small data you can use it as initial conditions later.
Quantum sensors Quantum mechanics is also disrupting another technology used in particle physics: In the quantum world, energy is discrete. Using technology originally developed for quantum computers, Chou and his team are building ultrasensitive detectors for a type of theorized dark matter particle known as an axion.
For Spiropulu, these applications of quantum computers represent an elegant feedback system in the progression of technology and scientific application. Basic research in physics led to the initial transistors that fed the computer science revolution, which is now on the edge of transforming basic research in physics. Phase-change memory PCM devices have in recent years emerged as a game-changing alternative to computer random-access memory.
Using heat to transform the states of material from amorphous to crystalline, PCM chips are fast, use much less power and have the potential to scale down to smaller chips — allowing the trajectory for smaller, more powerful computing to continue. However, manufacturing PCM devices on a large scale with consistent quality and long endurance has been a challenge.
Using in situ transmission electron microscopy TEM at the Yale Institute for Nanoscience and Quantum Engineering YINQE , they observed the device's phase change and how it "self-heals" voids - that is, empty spaces left by the depletion of materials caused by chemical segregation. These kinds of nanoscale voids have caused problems for previous PCM devices. Their results on self-healing of voids are published in Advanced Materials.
By observing the phase-change process through TEM, the researchers saw how the PCM device's self-healing properties come from a combination of the device's structure and the metallic lining, which allow it to control the phase-change of the material. Wanki Kim, an IBM researcher who worked on the project, said the next step is possibly to develop a bipolar operation to switch the direction of voltage, which can control the chemical segregation. In normal operation mode, the direction of voltage bias is always the same.
This next step could prolong the device lifetime even further. Matt Reagor, lead author of the paper, says, "We've developed a technique that enables us to reduce interference between qubits as we add more and more qubits to a chip, thus retaining the ability to perform logical operations that are independent of the state of a large quantum register. Clink a wine glass, and you will hear it ring at its resonant frequency usually around Hz. Likewise, soundwaves at that frequency will cause the same glass to vibrate. Different shapes or amounts of liquid in a glass will produce different clinks, i.
A clinked wine glass will cause identical, nearby glasses to vibrate. Glasses that are different shapes are "off-resonant glasses," meaning they will not vibrate much at all. So, what's the relation between glasses and qubits? Reagor explains that each physical qubit on a superconducting quantum processor stores energy in the form of an oscillating electric current.
In our analogy, this is equivalent to whether or not a wine glass is vibrating. At this tuning point, the "wine glasses" pick up on one another's "vibrations. With qubits, there are tunable circuit elements that fulfill the same purpose. Now we want to tune one glass into resonance with another, without disturbing any of the other glasses. To do that, you could try to equalize the wine levels of the glasses. But that transfer needs to be instantaneous to not shake the rest of the glasses along the way.
Let's say one glass has a resonance at one frequency call it Hz while another, nearby glass has a different one e. Now, we make use of a somewhat subtle musical effect. We are actually going to fill and deplete one of the glasses repeatedly.
By doing so, we create a beat-note for this glass that is exactly resonant with the other. Physicists sometimes call this a parametric process. Our beat-note is "pure"—it does not have frequency content that interferes with the other glasses. That's what we have demonstrated in our recent work, where we navigated a complex eight-qubit processor with parametric two-qubit gates. But finding or designing materials that can host such quantum interactions is a difficult task. But Rondinelli and an international team of theoretical and computational researchers have done just that.
Not only have they demonstrated that multiple quantum interactions can coexist in a single material, the team also discovered how an electric field can be used to control these interactions to tune the material's properties. This breakthrough could enable ultrafast, low-power electronics and quantum computers that operate incredibly faster than current models in the areas of data acquisition, processing, and exchange.
James Rondinelli, the Morris E. Jiangang He, a postdoctoral fellow at Northwestern, and Franchini served as the paper's co-first authors. Quantum mechanical interactions govern the capability of and speed with which electrons can move through a material. This determines whether a material is a conductor or insulator.
It also controls whether or not the material exhibits ferroelectricity, or shows an electrical polarization. Using computational simulations performed at the Vienna Scientific Cluster, the team discovered coexisting quantum-mechanical interactions in the compound silver-bismuth-oxide. Bismuth, a post-transition metal, enables the spin of the electron to interact with its own motion—a feature that has no analogy in classical physics.
It also does not exhibit inversion symmetry, suggesting that ferroelectricity should exist when the material is an electrical insulator. By applying an electric field to the material, researchers were able to control whether the electron spins were coupled in pairs exhibiting Weyl-fermions or separated exhibiting Rashba-splitting as well as whether the system is electrically conductive or not. This rotation could become the building block for a new form of information technology, and for the design of molecular-scale rotors to drive microscopic motors and machines.
The monolayer material, tungsten diselenide WSe2 , is already well-known for its unusual ability to sustain special electronic properties that are far more fleeting in other materials. It is considered a promising candidate for a sought-after form of data storage known as valleytronics, for example, in which the momentum and wavelike motion of electrons in a material can be sorted into opposite "valleys" in a material's electronic structure, with each of these valleys representing the ones and zeroes in conventional binary data. Modern electronics typically rely on manipulations of the charge of electrons to carry and store information, though as electronics are increasingly miniaturized they are more subject to problems associated with heat buildup and electric leaks.
The latest study, published online this week in the journal Science, provides a possible path to overcome these issues. It reports that some of the material's phonons, a term describing collective vibrations in atomic crystals, are naturally rotating in a certain direction. This property is known as chirality — similar to a person's handedness where the left and right hand are a mirror image of each other but not identical. Controlling the direction of this rotation would provide a stable mechanism to carry and store information. Researchers prepared a "sandwich" with four sheets of centimeter-sized monolayer WSe2 samples placed between thin sapphire crystals.
They synced ultrafast lasers to record the time-dependent motions. The two laser sources converged on a spot on the samples measuring just 70 millionths of a meter in diameter. One of the lasers was precisely switched between two different tuning modes to sense the difference of left and right chiral phonon activity.
A so-called pump laser produced visible, red-light pulses that excited the samples, and a probe laser produced mid-infrared pulses that followed the first pump pulse within one trillionth of a second. About one mid-infrared photon in every million is absorbed by WSe2 and converted to a chiral phonon. The researchers then captured the high-energy luminescence from the sample, a signature of this rare absorption event. Through this technique, known as transient infrared spectroscopy, researchers not only confirmed the existence of a chiral phonon but also accurately obtained its rotational frequency.
So far, the process only produces a small number of chiral phonons. A next step in the research will be to generate larger numbers of rotating phonons, and to learn whether vigorous agitations in the crystal can be used to flip the spin of electrons or to significantly alter the valley properties of the material. Spin is an inherent property of an electron that can be thought of as its compass needle — if it could be flipped to point either north or south it could be used to convey information in a new form of electronics called spintronics.
In addition, this work allows the possibility of using the rotating atoms as little magnets to guide the spin orientation. Now, a team of researchers at MIT and elsewhere has found novel topological phenomena in a different class of systems—open systems, where energy or material can enter or be emitted, as opposed to closed systems with no such exchange with the outside.
This could open up some new realms of basic physics research, the team says, and might ultimately lead to new kinds of lasers and other technologies. The complexities involved in measuring or analyzing phenomena in which energy or matter can be added or lost through radiation generally make these systems more difficult to study and analyze in a controlled fashion.
But in this work, the team used a method that made these open systems accessible, and "we found interesting topological properties in these non-Hermitian systems," Zhou says. In particular, they found two specific kinds of effects that are distinctive topological signatures of non-Hermitian systems.
One of these is a kind of band feature they refer to as a bulk Fermi arc, and the other is an unusual kind of changing polarization, or orientation of light waves, emitted by the photonic crystal used for the study. Photonic crystals are materials in which billions of very precisely shaped and oriented tiny holes are made, causing light to interact in unusual ways with the material. Such crystals have been actively studied for the exotic interactions they induce between light and matter, which hold the potential for new kinds of light-based computing systems or light-emitting devices.
But while much of this research has been done using closed, Hermitian systems, most of the potential real-world applications involve open systems, so the new observations made by this team could open up whole new areas of research, the researchers say. Fermi arcs, one of the unique phenomena the team found, defy the common intuition that energy contours are necessarily closed curves.
They have been observed before in closed systems, but in those systems they always form on the two-dimensional surfaces of a three-dimensional system. In the new work, for the first time, the researchers found a Fermi arc that resides in the bulk of a system. This bulk Fermi arc connects two points in the emission directions, which are known as exceptional points—another characteristic of open topological systems. The other phenomenon they observed consists of a field of light in which the polarization changes according to the emission direction, gradually forming a half-twist as one follows the direction along a loop and returns back to the starting point.
Zhen adds that "now we have this very interesting technique to probe the properties of non-Hermitian systems. The new findings were made possible by earlier research by many of the same team members, in which they found a way to use light scattered from a photonic crystal to produce direct images that reveal the energy contours of the material, rather than having to calculate those contours indirectly. Photonic crystals are generally made by drilling millions of closely spaced, minuscule holes in a slab of transparent material, using variations of microchip-fabrication methods.
Depending on the exact orientation, size, and spacing of these holes, these materials can exhibit a variety of peculiar optical properties, including "superlensing," which allows for magnification that pushes beyond the normal theoretical limits, and "negative refraction," in which light is bent in a direction opposite to its path through normal transparent materials. But to understand exactly how light of various colors and from various directions moves through photonic crystals requires extremely complex calculations.
Researchers often use highly simplified approaches; for example they may only calculate the behavior of light along a single direction or for a single color. Instead, the new technique makes the full range of information directly visible. The discovery of this new technique, Zhen explains, came about by looking closely at a phenomenon that the researchers had noticed and even made use of for years, but whose origins they hadn't previously understood.
Patterns of scattered light seemed to fan out from samples of photonic materials when the samples were illuminated by laser light. The scattering was surprising, since the underlying crystalline structure was fabricated to be almost perfect in these materials. Upon careful analysis, they realized the scattering patterns were generated by tiny defects in the crystal—holes that were not perfectly round in shape or that were slightly tapered from one end to the other. By illuminating the sample in turn with a sequence of different colors, it is possible to build up a full display of the relative paths light beams take, all across the visible spectrum.
The scattered light produces a direct view of the iso-frequency contours—a sort of topographic map of the way light beams of different colors bend as they pass through the photonic crystal. The finding could potentially be useful for a number of different applications, the team says. For example, it could lead to a way of making large, transparent display screens, where most light would pass straight through as if through a window, but light of specific frequencies would be scattered to produce a clear image on the screen.
Or, the method could be used to make private displays that would only be visible to the person directly in front of the screen. Because it relies on imperfections in the fabrication of the crystal, this method could also be used as a quality-control measure for manufacturing of such materials; the images provide an indication of not only the total amount of imperfections, but also their specific nature—that is, whether the dominant disorder in the sample comes from noncircular holes or etches that aren't straight—so that the process can be tuned and improved.
Now Ghimire and two colleagues at the Stanford PULSE Institute have invented a new way to probe the valence electrons of atoms deep inside a crystalline solid. In a report today in Nature Physics, they describe using laser light to excite some of the valence electrons, steer them around inside the crystal and bounce them off other atoms.
This produces high-energy bursts of light that are invisible to our eyes, but carry clues to the material's atomic structure and function. It was honored with the Nobel Prize in physics. But STM senses valence electrons from only the top two or three layers of atoms in a material. A flow of those electrons into the instrument's tip creates a current that allows it to measure the distance between the tip and the surface, tracing the bumps where atoms poke up and the valleys between them.
This creates an image of the atoms and yields information about the bonds that hold them together. Now the new technique will give scientists the same level of access to the valence electrons deep inside the solid. The experiments, carried out in a SLAC laser lab by PULSE postdoctoral researcher Yong Sing You, involved crystals of magnesium oxide or magnesia, a common mineral used to make cement, preserve library books and clean up contaminated soil, among a host of other things.
These crystals also have the ability to shift incoming laser light to much shorter wavelengths and higher energies — much as pressing down on a guitar string produces a higher note — through a process called high harmonic generation, or HHG. Steering Electrons to Generate Light In this case, the scientists carefully adjusted the incoming infrared laser beam so it would excite valence electrons in the crystal's oxygen atoms.
Those electrons oscillated, like vibrating guitar strings, and generated light of much shorter wavelengths — in the extreme ultraviolet range — through HHG. But when they adjusted the polarization of the laser beam to steer the excited electrons along different trajectories within the crystal, they discovered that HHG only took place when an electron hit a neighboring atom, and was most efficient when it hit the atom dead center.
Further, the wavelength of the harmonically generated light coming out — which was 13 to 21 times shorter than the light that went in — revealed the density of the neighboring atom's valence electrons, the size of the atom and even whether it was an atom of oxygen or magnesium. Understanding simple systems like this builds a foundation for understanding more complex systems. Materials come in all types. A number of their intriguing properties originate in the way a material's electrons "dance" with its lattice of atomic nuclei, which is also in constant motion due to vibrations known as phonons.
This coupling between electrons and phonons determines how efficiently solar cells convert sunlight into electricity. It also plays key roles in superconductors that transfer electricity without losses, topological insulators that conduct electricity only on their surfaces, materials that drastically change their electrical resistance when exposed to a magnetic field, and more.
This ability is central to the lab's mission of developing new materials for next-generation electronics and energy solutions. Two recent studies made use of these capabilities to study electron-phonon interactions in lead telluride, a material that excels at converting heat into electricity, and chromium, which at low temperatures has peculiar properties similar to those of high-temperature superconductors.
Turning Heat into Electricity and Vice Versa Lead telluride, a compound of the chemical elements lead and tellurium, is of interest because it is a good thermoelectric: It generates an electrical voltage when two opposite sides of the material have different temperatures. An electrical voltage applied across the material creates a temperature difference, which can be exploited in thermoelectric cooling devices.
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It has two important qualities: It's a bad thermal conductor, so it keeps heat from flowing from one side to the other, and it's also a good electrical conductor, so it can turn the temperature difference into an electric current. The coupling between lattice vibrations, caused by heat, and electron motions is therefore very important in this system. With our study at LCLS, we wanted to understand what's naturally going on in this material.
With our method we can study the forces involved and literally watch them change in response to the infrared laser pulse. The excited electrons stabilize the material by weakening certain long-range forces that were previously associated with the material's low thermal conductivity. Controlling Materials by Stimulating Charged Waves The second study looked at charge density waves — alternating areas of high and low electron density across the nuclear lattice — that occur in materials that abruptly change their behavior at a certain threshold.
This includes transitions from insulator to conductor, normal conductor to superconductor, and from one magnetic state to another. These waves don't actually travel through the material; they are stationary, like icy waves near the shoreline of a frozen lake. Singer and his colleagues reported their results on July 25 in Physical Review Letters. The research team used the chemical element chromium as a simple model system to study charge density waves, which form when the crystal is cooled to about minus degrees Fahrenheit.
They stimulated the chilled crystal with pulses of optical laser light and then used LCLS X-ray pulses to observe how this stimulation changed the amplitude, or height, of the charge density waves. LCLS provides unique opportunities to study such process because it allows us to take ultrafast movies of the related structural changes in the lattice. The light pulse interrupts the electron-phonon interactions in the material, causing the lattice to vibrate. Shortly after the pulse, these interactions form again, which boosts the amplitude of the vibrations, like a pendulum that swings farther out when it receives an extra push.
A Bright Future for Studies of the Electron-Phonon Dance Studies like these have a high priority in solid-state physics and materials science because they could pave the way for new materials and provide new ways to control material properties.
With its ultrabright X-ray pulses per second, LCLS reveals the electron-phonon dance with unprecedented detail. That's what happened when scientists cranked up the intensity of the world's first X-ray laser, at the Department of Energy's SLAC National Accelerator Laboratory, to get a better look at a sample they were studying: The X-rays seemed to go right through it as if it were not there. Now his team has published a paper in Physical Review Letters describing the experiment for the first time. What they saw was a so-called nonlinear effect where more than one photon, or particle of X-ray light, enters a sample at the same time, and they team up to cause unexpected things to happen.
So from the outside, it looked like a single beam went straight through and the sample was completely transparent. I think we're just starting to learn. This is a new phenomenon and I don't want to speculate," he said. They were discovered in thes with the invention of the laser — the first source of light so bright that it could send more than one photon into a sample at a time, triggering responses that seemed all out of proportion to the amount of light energy going in.
Scientists use these effects to shift laser light to much higher energies and focus optical microscopes on much smaller objects than anyone had thought possible. The opening of LCLS as a DOE Office of Science User Facility introduced another fundamentally new tool, the X-ray free-electron laser, and scientists have spent a lot of time since then figuring out exactly what it can do. For instance, a SLAC-led team recently published the first report of nonlinear effects produced by its brilliant pulses. The interaction of the X-rays with the sample is very different, and there are effects you could never see at other types of X-ray light sources.
To enhance the contrast of their image, they tuned the LCLS beam to a wavelength that would resonate with cobalt atoms in the sample and amplify the signal in their detector. The initial results looked great. So they turned up the intensity of the laser beam in the hope of making the images even sharper.
That's when the speckled pattern they'd been seeing in their detector went blank, as if the sample had disappeared. We knew this was strange — that there was something here that needed to be understood. He and Scherz dove deeply into the scientific literature. Meanwhile Wu finished his PhD thesis, which described the experiment and its unexpected result, and went on to a job in industry. But the team held off on publishing their experimental results in a scientific journal until they could explain what happened. The study was published on August 30th in Nature Communications.
When you point a laser pointer at the screen during a presentation, an immense number of light particles races through the air at a billion kilometers per hour. Sometimes as many as four so-called photons pass by, and other times none at all. You won't notice this during your presentation, but for light-based quantum technology, it is crucial that scientists have control over the number of photons per package.
Quantum dots In theory, you can manipulate photons with real individual atoms, but because of their small size, it is extremely hard to work with them. Now, Leiden physicists have discovered that the same principle goes for large artificial atoms—so-called quantum dots—that are much easier to handle. In fact, they managed to filter light beams with one photon per package out of a laser.
PhD student Giacomo Ferranti explained, "The great thing about the detector is that it works at room temperature. The Electroweak Interaction shows that the Weak Interaction is basically electromagnetic in nature. A next step in the research will be to generate larger numbers of rotating phonons, and to learn whether vigorous agitations in the crystal can be used to flip the spin of electrons or to significantly alter the valley properties of the material. They say the metal has a Ph. Would you like to tell us about a lower price? Further, the wavelength of the harmonically generated light coming out — which was 13 to 21 times shorter than the light that went in — revealed the density of the neighboring atom's valence electrons, the size of the atom and even whether it was an atom of oxygen or magnesium. Investing in futures requires a high level of sophistication since factors such as storage costs and interest rates affect pricin g.
This way, we can manipulate photons much faster. Dirk Bouwmeester is to entangle many photons using quantum dots. This is essential, for example, in techniques like quantum cryptography. And the beauty is that in principle, we don't need large experimental setups. We can just integrate our quantum dots in small microchips. Artificial atoms may also feature properties beyond those of conventional ones, with the potential for many applications for example in quantum computing.
Such additional properties have now been shown for artificial atoms in the carbon material graphene. In semiconductor materials such as gallium arsenide, trapping electrons in tiny confinements has already been shown to be possible. These structures are often referred to as "quantum dots". Just like in an atom, where the electrons can only circle the nucleus on certain orbits, electrons in these quantum dots are forced into discrete quantum states. Even more interesting possibilities are opened up by using graphene, a material consisting of a single layer of carbon atoms, which has attracted a lot of attention in the last few years.
The high symmetry of the graphene lattice allows for four different quantum states. This opens up new pathways for quantum information processing and storage" explains Florian Libisch from TU Wien. However, creating well-controlled artificial atoms in graphene turned out to be extremely challenging. Cutting edge is not enough There are different ways of creating artificial atoms: The simplest one is putting electrons into tiny flakes, cut out of a thin layer of the material.
While this works for graphene, the symmetry of the material is broken by the edges of the flake which can never be perfectly smooth. Consequently, the special four-fold multiplicity of states in graphene is reduced to the conventional two-fold one. Therefore, different ways had to be found: It is not necessary to use small graphene flakes to capture electrons.
Using clever combinations of electrical and magnetic fields is a much better option. With the tip of a scanning tunnelling microscope, an electric field can be applied locally. That way, a tiny region is created within the graphene surface, in which low energy electrons can be trapped. At the same time, the electrons are forced into tiny circular orbits by applying a magnetic field. The exceptionally clean graphene sample came from the team around Andre Geim and Kostya Novoselov from Manchester GB - these two researchers were awarded the Nobel Prize in for creating graphene sheets for the first time.
The new artificial atoms now open up new possibilities for many quantum technological experiments: The electrons can preserve arbitrary superpositions for a long time, ideal properties for quantum computers. In addition, the new method has the big advantage of scalability: So says an international team of researchers, which has found that the reverse process — two excited atoms emitting a single photon — is also possible. According to the team, this process could be used to transmit information in a quantum circuit or computer. Physicists have long known that a single atom can absorb or emit two photons simultaneously.
These two-photon, one-atom processes are widely used for spectroscopy and for the production of entangled photons used in quantum devices. Savasta asked his PhD student at the time, Luigi Garziano, to simulate the process. When Garziano's simulation showed that the phenomenon was possible, Savasta was so excited that he "punched the wall," he told physicsworld. Their simulation found that the phenomenon occurs when the resonant frequency of the optical cavity containing the atoms is twice the transition frequency of an individual atom.
For example, in a cavity whose resonant frequency is three times that of the atomic transition, three atoms can simultaneously absorb or emit a single photon. The optical-cavity's dimensions are determined by this resonant frequency, which must be a standing wave.
According to the researchers' calculations, the two atoms would oscillate back and forth between their ground and excited states. Indeed, the atoms would first jointly absorb the photon, ending up in their excited states, before jointly emitting a single photon to return to their ground states.
The cycle would then repeat. In addition, they found that the joint absorption and emission can occur with more than just two atoms. Quantum switch A two-atom, one-photon system could be used as a switch to transmit information in a quantum circuit, Savasta says. One atom would act as a qubit, encoding information as a superposition of the ground and excited states. To transmit the information outside of the cavity, the qubit would need to transfer the information to a photon in the cavity.
The second atom would be used to control whether the qubit transmits the information. If the second atom's transition frequency is tuned to half the resonance frequency of the cavity, the two atoms could jointly absorb and emit a single photon, which would contain the encoded information to be transmitted. To ensure that the atoms do not re-adsorb the photon, the atom's resonant frequency can be changed by applying an external magnetic field. Savasta's group has begun to look for experimental collaborators to produce its theoretical prediction in the lab.
While the experiment could be performed using actual atoms, Savasta plans to use artificial atoms: In addition, controlling real atoms involves expensive technology, while artificial atoms can be created cheaply on solid-state chips. Savasta anticipates that their collaborators will be able to successfully perform the experiment in about a year.
According to Tatjana Wilk at the Max Planck Institute for Quantum Optics in Garching, who was not involved in the current research, speaking to the American Physical Society's Physics Focus, she cautions that the excited states of the atoms may not last long enough to be useful in an actual quantum device. The research is published in Physical Review Letters. The difficulty of such an endeavour is that photons usually do not interact at all but pass each other undisturbed. This makes them ideal for the transmission of quantum information, but less suited for its processing. The scientists overcame this steep hurdle by bringing an ancillary third particle into play: This is done using so-called logic gates which assign predefined output values to each input via deterministic protocols.
Likewise, for the information processing in quantum computers, quantum logic gates are the key elements. To realise a universal quantum computer, it is necessary that every input quantum bit can cause a maximal change of the other quantum bits. The practical difficulty lies in the special nature of quantum information: Therefore, classical methods for error correction cannot be applied, and the gate must function for every single photon that carries information. Because of the special importance of photons as information carriers — for example, for communicating quantum information in extended quantum networks — the realisation of a deterministic photon-photon gate has been a long-standing goal.
One of several possibilities to encode photonic quantum bits is the use of polarisation states of single photons. Then the states "0" and "1" of a classical bit correspond to two orthogonal polarisation states. In the two-photon gate, the polarisation of each photon can influence the polarisation of the other photon. As in the classical logic gate it is specified beforehand which input polarisation leads to which output polarisation. In contrast to classical logic gates, which would be fully specified by such a description, a quantum gate can take on an infinite number of possible input states.
The quantum logic gate has to create the correct combination of output states for each one of these. In the experiment presented here two independently polarised photons impinge, in quick succession, onto a resonator which is made of two high-reflectivity mirrors. Inside a single rubidium atom is trapped forming a strongly coupled system with the resonator. The resonator amplifies the light field of the impinging photon at the position of the atom enabling a direct atom-photon interaction.
As a result, the atomic state gets manipulated by the photon just as it is being reflected from the mirror. This change is sensed by the second photon when it arrives at the mirror shortly thereafter. After their reflection, both photons are stored in a 1. Meanwhile, the atomic state is measured. A rotation of the first photon's polarisation conditioned on the outcome of the measurement enables the back action of the second photon on the first one. Nevertheless, we achieve a maximal interaction between them", explains Bastian Hacker, PhD student at the experiment. The scientists could prove experimentally that — depending on the choice of the photons' polarisations — either the first photon affects the second or vice versa.
To this end, they measured the polarisation states of the two outgoing photons for different input states. From these, they generated "truth tables" which correspond to the expected gate operations and thus demonstrate the diverse operational modes of the photon-photon gate. The case when the input polarisation of the two photons is chosen such that they influence each other is of particular interest: Here the two outgoing photons form an entangled pair. One of the applications of entangled photons is in the teleportation of quantum states", explains Stephan Welte, PhD student at the experiment.
The scientists envision that the new photon-photon gate could pave the way towards all-optical quantum information processing. Quantum dots in electrically-controlled cavities yield bright, nearly identical photons Optical quantum technologies are based on the interactions of atoms and photons at the singleparticle level, and so require sources of single photons that are highly indistinguishable — that is, as identical as possible.
Conversely, parametric down conversion sources yield photons that while being highly indistinguishable have very low brightness. The researchers state that by demonstrating efficient generation of a pure single photon with near-unity indistinguishability, their novel approach promises significant advances in optical quantum technology complexity and scalability. Pascale Senellart and Phys. Moreover, all the photons should be identical in spatial shape, wavelength, polarization, and a spectrum that is the Fourier transform of its temporal profile," Senellart tells Phys.
With this technique, we can position a single quantum dot with 50 nm accuracy at the center of a micronsized pillar. However, our study showed that collisions of the carriers with phonons and fluctuation of charges around the quantum dot were the main limitations. This in turn removed the noise. Moreover, she adds, this electrical control allows tuning the quantum dot wavelength — a process that was previously done by increasing temperature at the expense of increasing vibration. That is, we control emission wavelength, emission lifetime and coupling to the environment, all in a fully deterministic and scalable way.
Even in the highest-quality semiconductors, charges bound to defects fluctuate and create a fluctuating electric field3. In the meantime, several colleagues were observing very low charge noise in structures where an electric field was applied to the quantum dot — but this was not combined with a cavity structure. Senellart says that the connected pillars geometry was the key to both controlling the quantum wavelength of dot and efficiently collecting its emission4. In demonstrating the efficient generation of a pure single photon with near-unity indistinguishability, Senellart continues, the researchers had one last step — combining high photon extraction efficiency and perfect indistinguishability — which they did by implementing a resonant excitation scheme of the quantum dot.
Their result was beautiful, but again, not combined with an efficient extraction of the photons. The experimental challenge here is to suppress the scattered light from the laser and collect only the single photons radiated by the quantum dot. It turns out that we send only a few photons — that is, less than 10 — on the device to have the quantum dot emitting one photon. This beautiful efficiency, also demonstrated in the excitation process, which we report in another paper6, made this step quite easy. However, this was no longer the case in etched structures, where a strong charge noise is always measured on very short time scale — less than 1 ns — that prevents the photon from being indistinguishable.
Therefore, this number will determine the complexity of any quantum computation or simulation scheme one can implement. In addressing how these de novo devices may lead to new levels of complexity and scalability in optical quantum technologies, Senellart first discusses the historical sources used develop optical quantum technologies. She makes the point that all previous implementations of optical quantum simulation or computing have been implemented using Spontaneous Parametric Down Conversion SPDC sources, in which pairs of photons are generated by the nonlinear interaction of a laser on a nonlinear crystal, wherein one photon of the pair is detected to announce the presence of the other photon.
This so-called heralded source can present strongly indistinguishable photons, but only at the cost of extremely low brightness. Nevertheless, with these sources, the quantum optics community has demonstrated many beautiful proofs of concept of optical quantum technologies, including long-distance teleportation, quantum computing of simple chemical or physical systems, and quantum simulations like BosonSampling. It takes typically hundreds of hours to manipulate three photons, and the measurement time increases exponentially with the number of photons.
Obviously, with the possibility to generate more many indistinguishable photons with an efficiency more than one order of magnitude greater than SPDC sources, our devices have the potential to bring optical quantum technologies to a whole new level. Actually, very recently — in the first demonstration of the superiority of our new single photon sources — our colleagues in Brisbane made use of such bright indistinguishable quantum dot-based single photon sources to demonstrate a three photon BosonSampling experiment8, where the solid-state multiphoton source was one to two orders-of-magnitude more efficient than downconversion sources, allowing to complete the experiment faster than those performed with SPDC sources.
Moreover, this is a first step; we'll progressively increase the number of manipulated photons, in both quantum simulation and quantum computing tasks. Read more Read less. Kindle Cloud Reader Read instantly in your browser. Customers who bought this item also bought. Page 1 of 1 Start over Page 1 of 1. History, Manifests, Analysis, Photos. Searching for the Treasure of the Spanish Plate Fleet. Product details File Size: Spyglass Publications November 28, Publication Date: November 28, Sold by: Share your thoughts with other customers. Write a customer review. Showing of 1 reviews.
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