A quantum low pass for photons

The probability to find a certain number of photons inside a laser pulse usually corresponds to a classical distribution of independent events, the so-called Poisson-distribution. There are, however, light sources with non-classical photon number distributions that can only be described by the laws of quantum mechanics. A well-known example is the single-photon source that may find application in quantum cryptography for secret key distribution or in quantum networks for connecting quantum memories and processors. However, for many applications in nonlinear quantum optics light pulses with a certain fixed number of photons, e.g. two, three or four, are highly desirable. A team of scientists from the Quantum Dynamics Division of Professor Gerhard Rempe at the Max Planck Institute of Quantum Optics (Garching near Munich) has now succeeded to make the first steps in this direction. Using a strongly coupled atom-cavity system, they were the first to observe the so-called two-photon blockade: the system emits at most two photons at the same time since its storage capacity is limited to that number (PRL, 31 March 2017).

A naive approach for generating a stream of single photons would be to sufficiently attenuate the intensity of a laser beam. But in this case the number of photons still varies from pulse to pulse, and only when averaging over many pulses a mean photon number of one is observed. Applications instead require a fixed number of exactly one photon per pulse. The fluctuations of the photon number per pulse can be strongly reduced by using a single atom as a single-photon source. When the atom is illuminated by a laser beam, it can absorb only one photon at a time, thereby making a transition from the ground state to an excited state. A second photon can only be absorbed after the atom has fallen back to the ground state by emitting a photon. Therefore, no more than one photon is detected in the emitted light field at the same time, an effect that is known as "single-photon blockade."
In order to extend this principle to a "two-photon blockade" one has to go beyond a single atom and look for a system that can store more than one photon, but not more than two. To this end, the MPQ physicists combine the single atom with a cavity that provides additional storage capacities. A cavity can absorb an unlimited number of photons and exhibits a correspondingly large number of energy states that lie -- similar to a "ladder" -- in exactly the same distance from each other. Inserting a single atom into the cavity introduces a nonlinear element. This causes the energy levels to split by a different amount for each of the 'ladder steps'. Hence, laser light can excite the system only up to the level to which it is tuned to. The number of photons that can be stored is thus limited to a certain number, and therefore, not more photons than that can be emitted.
In the experiment, the physicists hold a single rubidium atom in an optical trap inside a cavity made of two high-finesse mirrors. The frequency of the incoming laser beam is tuned to an energy level requiring the absorption of two photons for its excitation. During the five seconds of atom storage time around 5000 measurement cycles are carried out, during which the system is irradiated by a probe laser and emission from the cavity is recorded via single-photon detectors. "Interestingly, the fluctuations in the number of emitted photons does strongly depend on whether we excite the cavity or the atom," points out the project leader Dr. Tatjana Wilk. "The effect that the absorption of two photons suppresses further absorption leading to emission of two or less photons is only achieved in case of atomic excitation. This quantum effect does not appear when we excite the cavity. In this case, we observe an enhanced signal of three and more photons per light pulse."
Christoph Hamsen, doctoral candidate at the experiment, explains the underlying processes: "When the atom is excited we are dealing with the interplay between two conflicting mechanisms. On the one hand, the atom can absorb only one photon at a time. On the other hand, the strongly coupled atom-cavity system is resonant with a two-photon transition. This interplay leads to a sequence of light pluses with a non-classical photon distribution." And Nicolas Tolazzi, another doctoral candidate, adds: "We were able to observe this behaviour in correlations between detected photons where the coincidence of three photons was significantly suppressed compared to the expectation for the classical case."
Prof. Gerhard Rempe gives an outlook on possible extensions of the experiment: "At present, our system emits light pulses with two photons at maximum, but also pulses with fewer, one or even zero, photons. It acts like a kind of 'low pass'. There are, however, a number of applications for quantum communicating and quantum information processing where exactly two, three or four photons are required. Our ultimate goal is the generation of pure states where each light pulse contains exactly the same desired number of photons. The two-photon blockade demonstrated in our experiment is the first step in this direction." Olivia Meyer-Strength

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Internet atlas maps the physical internet to enhance security

Despite the internet-dependent nature of our world, a thorough understanding of the internet's physical makeup has only recently emerged, thanks to painstaking work by University of Wisconsin-Madison researchers and their collaborators.

Professor of Computer Sciences Paul Barford, Ph.D. candidate Ramakrishnan (Ram) Durairajan and colleagues have developed Internet Atlas, the first detailed map of the internet's structure worldwide.
While average users rarely think of these elements, things like submarine cables -- buried below the ocean floor -- run between continents to enable communication. Data centers in buildings all over the world are packed with servers storing many types of data. Traffic exchange occurs between different service providers at internet exchange points.
Though these and other elements may be out of sight for the average user, they are crucial pieces of the physical infrastructure that billions of people rely on.
Collaborators on the team include Joel Sommers, a UW-Madison alumnus now on the faculty at Colgate University; Walter Willinger, chief scientist at the NIKSUN Innovation Center; and graduate and undergraduate research assistants who help with data collection.
Supported by a Department of Homeland Security grant, Internet Atlas has already attracted considerable attention from publications like MIT Technology Review, New Scientist and others. And in February, Barford and Durairajan presented their work at the RSA Conference in San Francisco, the world's biggest information security conference.
Internet Atlas was one of a select group of DHS-funded projects invited to present at the conference. "It was nice recognition" to be chosen, says Barford, since RSA is attended by tens of thousands.
Mapping the physical internet helps stakeholders boost performance and guard against a number of threats, from terrorism to extreme weather events like hurricanes. Furthermore, "a lot of infrastructure is by major right-of-ways, like railroad lines," says Barford, meaning that an event like a train derailment could end up disrupting internet communications. "The question of 'how does mapping contribute to security?' is one of our fundamental concerns," says Durairajan.
The project has helped direct attention to the problem of shared risk, the subject of an influential 2015 paper by the team. Physical infrastructure is commonly shared by multiple networking entities, so damage to any particular piece of infrastructure can impact more than one entity. "We quantified that for the first time," says Barford.
Much of the data used to create the Internet Atlas comes from publicly available information, such as what internet service providers publish on their websites. Other data has taken more legwork to uncover, such as combing through mundane items like local permits for laying cables. "The core work is grunt work, but by rolling up our sleeves, we assembled a unique data set," says Barford.
Now, the team is looking to enhance the maps even further and share their work so it can be deployed by others to boost network performance and security.
"We'll complement the static maps with the ability to actually examine the status of the network in real time," says Barford. "We've built certain capabilities that allow exactly that to be done, and one of the important focuses going forward is to enhance that capability, basically putting the maps in motion."

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Researchers unlock hardware's hidden talent for rendering 3-D graphics for science -- and video games

When Shuaiwen Leon Song boots up Doom 3 and Half-life 2, he does so in the name of science. Song studies high performance computing at Pacific Northwest National Laboratory, with the goal of making computers smaller, faster and more energy efficient. A more powerful computer, simply put, can solve greater scientific challenges. Like modeling complex chemical reactions or monitoring the electric power grid.

The jump from supercomputers to video games began when Song asked if hardware called 3D stacked memory could do something it was never designed to do: help render 3D graphics. 3D rendering has advanced science with visualizations, models and even virtual reality. It's also the stuff of video games.
"We're pushing the boundaries of what hardware can do," Song said. "And though we tested our idea on video games, this improvement ultimately benefits science."
Song collaborated with researchers from the University of Houston to develop a new architecture for 3D stacked memory that increases 3D rendering speeds up to 65 percent. The researchers exploited the hardware's feature called "processing in memory," the results of which they presented at the 2017 IEEE Symposium on High Performance Computer Architecture, or HPCA.
A normal graphics card uses a graphics processing unit, or GPU, to create images from data stored on memory. 3D stacked memory has an added logic layer that allows for the memory to do some processing too -- hence the name "processing in memory." This essentially reduces the data that has to travel from memory to GPU cores. And like an open highway, less traffic means faster speeds.
The researchers found the last step in rendering -- called anisotropic filtering -- creates the most traffic. So by moving anisotropic filtering to the first step in the pipeline, and performing that process in memory, the researchers found the greatest performance boost.
Song tested the architecture on popular games such as Doom 3 and Half-life 2. Virtual aliens and demons aside, this research is not so different than Song's other work. For example, Song is exploring how high performance computers can model changing networks of information, and how to predict changes in these graphs. With research questions like these, Song means to push the boundaries of what computers can do.

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Android apps can conspire to mine information from your smartphone

Mobile phones have increasingly become the repository for the details that drive our everyday lives. But Virginia Tech researchers have recently discovered that the same apps we regularly use on our phones to organize lunch dates, make convenient online purchases, and communicate the most intimate details of our existence have secretly been colluding to mine our information.

Associate Professor Daphne Yao and Assistant Professor Gang Wang, both in the Department of Computer Science in Virginia Tech¹s College of Engineering, are part of a research team to conduct the first ever large-scale and systematic study of exactly how the trusty apps on Android phones are able to talk to one another and trade information.
Yao will present the team¹s findings in Dubai at the Association for Computing Machinery Asia Computer and Communications Security Conference on April 3.
"Researchers were aware that apps may talk to one another in some way, shape, or form," said Wang. "What this study shows undeniably with real-world evidence over and over again is that app behavior, whether it is intentional or not, can pose a security breach depending on the kinds of apps you have on your phone."
The types of threats fall into two major categories, either a malware app that is specifically designed to launch a cyberattack or apps that simply allow for collusion and privilege escalation. In the latter category, it is not possible to quantify the intention of the developer, so collusion, while still a security breach, can in many cases be unintentional.
In order to run the programs to test pairs of apps, the team developed a tool called DIALDroid to perform their massive inter-app security analysis. The study, funded by the Defense Advanced Research Projects Agency as part of its Automated Program Analysis for Cybersecurity initiative, took 6,340 hours using the newly developed DIALDroid software, a task that would have been considerably longer without it.
First author of the paper Amiangshu Bosu, an assistant professor at Southern Illinois University, spearheaded the software development effort and the push to release the code to the wider research community. Fang Liu, a fifth year Ph.D. candidate studying under Yao, also contributed to the malware detection research.
"Our team was able to exploit the strengths of relational databases to complete the analysis, in combination with efficient static program analysis, workflow engineering and optimization, and the utilization of high performance computing. Of the apps we studied, we found thousands of pairs of apps that could potentially leak sensitive phone or personal information and allow unauthorized apps to gain access to privileged data," said Yao, who is both an Elizabeth and James E. Turner Jr. '56 and L-3 Faculty Fellow.
The team studied a whopping 110,150 apps over three years including 100,206 of Google Play¹s most popular apps and 9,994 malware apps from Virus Share, a private collection of malware app samples. The set up for cybersecurity leaks works when a seemingly innocuous sender app like that handy and ubiquitous flashlight app works in tandem with a receiver app to divulge a user¹s information such as contacts, geolocation, or provide access to the web.
The team found that the biggest security risks were some of the least utilitarian. Apps that pertained to personalization of ringtones, widgets, and emojis.
"App security is a little like the Wild West right now with few regulations," said Wang. "We hope this paper will be a source for the industry to consider re-examining their software development practices and incorporate safeguards on the front end. While we can¹t quantify what the intention is for app developers in the non-malware cases we can at least raise awareness of this security problem with mobile apps for consumers who previosuly may not have thought much about what they were downloading onto their phones."

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Quantum mechanics is complex enough

Quantum mechanics is based on a set of mathematical rules, describing how the quantum world works. These rules predict, for example, how electrons orbit a nucleus in an atom, and how an atom can absorb photons, particles of light. The standard rules of quantum mechanics work extremely well, but, given that there are still open questions regarding the interpretation of quantum mechanics, scientists are not sure whether the current rules are the final story. This has motivated some scientists to develop alternative versions of the mathematical rules, which are able to properly explain the results of past experiments, but provide new insight into the underlying structure of quantum mechanics. Some of these alternative mathematical rules even predict new effects, which require new experimental tests.

Everyday experience of mathematical rules
In everyday life, if we walk all the way around a park we end up back at the same place regardless of whether we choose to walk clockwise or counter-clockwise. Physicists would say that these two actions commute. Not every action needs to commute, though. If, on our walk around the park, we walk clockwise, and first find money lying on the ground and then encounter an ice cream man, we will exit the park feeling refreshed. However, if we instead travel counter-clockwise, we will see the ice cream man before finding the money needed to buy the ice cream. In that case, we may exit park feeling disappointed. In order to determine which actions commute or do not commute physicists provide a mathematical description of the physical world.
In standard quantum mechanics, these mathematical rules use complex numbers. However, recently an alternative version of quantum mechanics was proposed which uses more complex, so-called "hyper-complex" numbers. These are a generalization of complex numbers. With the new rules, physicists can replicate most of the predictions of standard quantum mechanics. However, hyper-complex rules predict that some operations that commute in standard quantum mechanics do not actually commute in the real world.
Searching for hyper-complex numbers
A research team led by Philip Walther has now tested for deviations from standard quantum mechanics predicted by the alternative hyper-complex quantum theory. In their experiment the scientists replaced the park with an interferometer, a device which allows a single photon to travel two paths at the same time. They replaced the money and ice cream with a normal optical material and a specially designed metamaterial. The normal optical material slightly slowed down light as it passed through, whereas the metamaterial slightly sped the light up.
The rules of standard quantum mechanics dictate that light behaves the same no matter whether it first passes through a normal material and then through a metamaterial or vice versa. In other words, the action of the two materials on the light commutes. In hyper-complex quantum mechanics, however, that might not be the case. From the behavior of the measured photons the physicists verified that hyper-complex rules were not needed to describe the experiment. "We were able to place very precise bounds on the need for hyper-complex numbers to describe our experiment," says Lorenzo Procopio, a lead author of the study. However, the authors say that it is always very difficult to unambiguously rule something out. Lee Rozema, another author of the paper, says "we still are very interested in performing experiments under different conditions and with even higher precision, to gather more evidence supporting standard quantum mechanics." This work has placed tight limits on the need for a hyper-complex quantum theory, but there are many other alternatives which need to be tested, and the newly-developed tools provide the perfect avenue for this.

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New quantum liquid crystals may play role in future of computers

Physicists at the Institute for Quantum Information and Matter at Caltech have discovered the first three-dimensional quantum liquid crystal -- a new state of matter that may have applications in ultrafast quantum computers of the future.

"We have detected the existence of a fundamentally new state of matter that can be regarded as a quantum analog of a liquid crystal," says Caltech assistant professor of physics David Hsieh, principal investigator on a new study describing the findings in the April 21 issue of Science. "There are numerous classes of such quantum liquid crystals that can, in principle, exist; therefore, our finding is likely the tip of an iceberg."
Liquid crystals fall somewhere in between a liquid and a solid: they are made up of molecules that flow around freely as if they were a liquid but are all oriented in the same direction, as in a solid. Liquid crystals can be found in nature, such as in biological cell membranes. Alternatively, they can be made artificially -- such as those found in the liquid crystal displays commonly used in watches, smartphones, televisions, and other items that have display screens.
In a "quantum" liquid crystal, electrons behave like the molecules in classical liquid crystals. That is, the electrons move around freely yet have a preferred direction of flow. The first-ever quantum liquid crystal was discovered in 1999 by Caltech's Jim Eisenstein, the Frank J. Roshek Professor of Physics and Applied Physics. Eisenstein's quantum liquid crystal was two-dimensional, meaning that it was confined to a single plane inside the host material -- an artificially grown gallium-arsenide-based metal. Such 2-D quantum liquid crystals have since been found in several more materials including high-temperature superconductors -- materials that conduct electricity with zero resistance at around -150 degrees Celsius, which is warmer than operating temperatures for traditional superconductors.
John Harter, a postdoctoral scholar in the Hsieh lab and lead author of the new study, explains that 2-D quantum liquid crystals behave in strange ways. "Electrons living in this flatland collectively decide to flow preferentially along the x-axis rather than the y-axis even though there's nothing to distinguish one direction from the other," he says.
Now Harter, Hsieh, and their colleagues at Oak Ridge National Laboratory and the University of Tennessee have discovered the first 3-D quantum liquid crystal. Compared to a 2-D quantum liquid crystal, the 3-D version is even more bizarre. Here, the electrons not only make a distinction between the x, y, and z axes, but they also have different magnetic properties depending on whether they flow forward or backward on a given axis.
"Running an electrical current through these materials transforms them from nonmagnets into magnets, which is highly unusual," says Hsieh. "What's more, in every direction that you can flow current, the magnetic strength and magnetic orientation changes. Physicists say that the electrons 'break the symmetry' of the lattice."
Harter actually hit upon the discovery serendipitously. He was originally interested in studying the atomic structure of a metal compound based on the element rhenium. In particular, he was trying to characterize the structure of the crystal's atomic lattice using a technique called optical second-harmonic rotational anisotropy. In these experiments, laser light is fired at a material, and light with twice the frequency is reflected back out. The pattern of emitted light contains information about the symmetry of the crystal. The patterns measured from the rhenium-based metal were very strange -- and could not be explained by the known atomic structure of the compound.
"At first, we didn't know what was going on," Harter says. The researchers then learned about the concept of 3-D quantum liquid crystals, developed by Liang Fu, a physics professor at MIT. "It explained the patterns perfectly. Everything suddenly made sense," Harter says.
The researchers say that 3-D quantum liquid crystals could play a role in a field called spintronics, in which the direction that electrons spin may be exploited to create more efficient computer chips. The discovery could also help with some of the challenges of building a quantum computer, which seeks to take advantage of the quantum nature of particles to make even faster calculations, such as those needed to decrypt codes. One of the difficulties in building such a computer is that quantum properties are extremely fragile and can easily be destroyed through interactions with their surrounding environment. A technique called topological quantum computing -- developed by Caltech's Alexei Kitaev, the Ronald and Maxine Linde Professor of Theoretical Physics and Mathematics -- can solve this problem with the help of a special kind of superconductor dubbed a topological superconductor.
"In the same way that 2-D quantum liquid crystals have been proposed to be a precursor to high-temperature superconductors, 3-D quantum liquid crystals could be the precursors to the topological superconductors we've been looking for," says Hsieh.
"Rather than rely on serendipity to find topological superconductors, we may now have a route to rationally creating them using 3-D quantum liquid crystals" says Harter. "That is next on our agenda."

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