Monday 11 September 2023

Experimental quantum imaging distillation with undetected light


It is possible to image an object with an induced coherence effect by making use of photon pairs to gain information on the item of interest—without detecting the light probing it. While one photon illuminates the object, its partner alone is detected, thereby preventing the measurements of coincidence events to reveal information of the sought after object. This method can be made resilient to noise, as well.

In a new report published in Science Advances, Jorge Fuenzalida and a team in applied optics, precision engineering and theory communications in Germany experimentally showed how the method can be made resilient to noise. They introduced an imaging-distilled approach based on the interferometric modulation of the signal of interest to generate a high-quality image of an object regardless of the extreme noise levels surpassing the actual signal of interest.

Quantum imaging

Quantum imaging is a promising field that is emerging with valid advantages when compared to classical protocols. Researchers have demonstrated this method across different scenarios to work in the low-photon flux regime by making use of undetected probing photons for super-resolution imaging.

Scientists can also develop protocols in quantum imaging without a classical counterpart based on quantum interference and entanglement. Quantum imaging protocols can, however, be made resilient to noise. For instance, distillation or purification can remove decoherence introduced by the environment in a quantum system.

It is also possible to implement quantum imaging distillation with one and several photon pair degrees of freedom. In this work, Fuenzalida and team introduced and experimentally verified a quantum imaging distillation method to detect single photons only.

The method of quantum imaging with undetected light (abbreviated as QIUL) offers a two-photon wide-field interferometric imaging method. During this process, one photon can illuminate an object, while only its partner photon is detected on the camera. Incidentally, the photon illuminating the object remains undetected.

The method offers a unique discovery method to probe samples. The scientists then introduced a source of noise in the quantum imaging scheme to study its resilience to show good performance even for noise intensities above 250 times the quantum signal intensity.

Cleaning a quantum image and generating a photon pair

Quantum imaging distillation is a method in use to clean a quantum image from noise. The team illustrated the distillation method while defining a noise image as an unwanted signal superimposed over a quantum image on the camera. To distill an image, Fuenzalida and team used quantum holography with undetected light (abbreviated QHUL), where the object information was carried into a single-photon interference pattern. If the intensity difference of the method is bigger than the intensity variance of noise, the team could distill the quantum image.

To generate photon pairs mediated by the interaction of an intense pump beam with the atoms of a nonlinear crystal, the team used spontaneous parametric down-conversion. The imaging scheme used an interferometer to generate a pair of signal-idler photons in the forward and backward propagation modes. The noise variance in the setup contributed to the signal intensity variance; where a difference of signal intensity higher than the noise variance can distill the quantum image.
Resilience to different noise intensities. In the top row, the superpositions of quantum (IOF letters) and classical (square shape) images are shown. The ratio between their mean intensities is stated on top of each image. In the middle row, the experimental results for our distillation technique through QHUL are presented. In the last row, a transverse cut of the distilled images is presented. We observe that, while the noise intensity increases, the phase estimation diminishes. Credit: Science Advances, DOI: 10.1126/sciadv.adg9573
Experimental nonlinearity and noise sources

Fuenzalida and colleagues implemented an experimental setup using a nonlinear interferometer in a Michelson configuration and pumped a crystal with a continuous wave laser. Due to the strong nonlinearity of the experiment, the team generated a photon pair via spontaneous parametric down-conversion along the paths, although never simultaneously. They separated the signal, idler, and pump beams in the forward propagation direction by using dichroic mirrors and reflected into the crystal with a series of mirrors.

The camera in the experimental framework showed an interference pattern of signal photons, which the team noted as the transfer of object information obtained by the idler photon to the signal photon interference pattern. The team used a continuous wave diode laser with a variable pump power to introduce noise into the system, and varied the properties of classical illumination, intensity and variance to examine the effects of noise and the distillation performance.
Distillation performance across diverse noise intensities

The scientists superimposed classical and quantum images to perform quantum holography with undetected light to distill or clean the quantum image under diverse intensities of noise. For the quantum image, they used signal photons generated in a single pass through a crystal, where the signal intensity did not change during the experiments. They characterized different noise intensities by superimposing quantum and classical images on the camera, and as the noise intensity increased, they measured the accuracy of the experimental results.

The researchers conducted a second experiment to quantify the effects of varying noise on the phase accuracy of distilled images by using similar configurations of noise intensities. The experimental behavior was in good agreement to theory, and compared well to existing methods.
Distillation phase variance affected by noise variance. A light diffuser with four different rotation speeds is used to change the properties of the noise illumination; see supplementary text D. The different noise configurations are represented by different colors and symbols; see inset. Data points represent the experimental phase values obtained for different noise variances, and dashed lines represent their fits. A theoretical black solid line representing a Poissonian noise is also included. In all configurations, we observed that a higher noise variance increases the phase inaccuracy in QHUL. We also corroborate that the phase sensitivity is linearly dependent with the noise variance. Credit: Science Advances, DOI: 10.1126/sciadv.adg9573

In this way, Jorge Fuenzalida, and colleagues investigated quantum imaging with undetected light (QIUL) in a two-photon wide-field interferometric imaging method. While one photon illuminated the object of interest and its partner remained on the camera, the illuminating photon remained undetected. The scientists distilled or cleaned the image by using quantum holography with undetected light (QHUL). They proved the imaging method by superimposing partially or completely a classical source of noise on top of the quantum image on the camera. The method worked every time, even with noise intensities higher than the signal intensity.

The team explored the limits of the method by presenting simulations of quantum holography under extreme noise scenarios. The experimental outcomes provide a step forward for quantum imaging in open systems to even examine the limits of innovative versions of quantum-based light detection and ranging (LIDAR), by using undetected light.

#QuantumLight #QuantumOptics   #QuantumFuture  #QuantumInnovation
#QuantumPhotonic #QuantumEntanglement #QuantumTechnology #QuantumPhysics


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Saturday 9 September 2023

Promising quantum state found during error correction research


Window glass, at the microscopic level, shows a strange mix of properties. Like a liquid, its atoms are disordered, but like a solid, its atom are rigid, so a force applied to one atom causes all of them to move.

It's an analogy physicists use to describe a quantum state called a "quantum spin-glass," in which quantum mechanical bits (qubits) in a quantum computer demonstrate both disorder (taking on seemingly random values) and rigidity (when one qubit flips, so do all the others). A team of Cornell researchers unexpectedly discovered the presence of this quantum state while conducting a research project designed to learn more about quantum algorithms and, relatedly, new strategies for error correction in quantum computing.

"Measuring the position of a quantum particle changes its momentum and vice versa. Similarly, for qubits there are quantities which change one another when they are measured. We find that certain random sequences of these incompatible measurements lead to the formation of a quantum spin-glass," said Erich Mueller, professor of physics in the College of Arts and Sciences (A&S). "One implication of our work is that some types of information are automatically protected in quantum algorithms which share the features of our model."

"Subsystem Symmetry, Spin-glass Order, and Criticality From Random Measurements in a Two-dimensional Bacon-Shor Circuit" published on July 31 in Physical Review B. The lead author is Vaibhav Sharma, a doctoral student in physics.

Assistant professor of physics Chao-Ming Jian (A&S) is a co-author along with Mueller. All three conduct their research at Cornell's Laboratory of Atomic and Solid State Physics (LASSP).

"We are trying to understand generic features of quantum algorithms—features which transcend any particular algorithm," Sharma said. "Our strategy for revealing these universal features was to study random algorithms. We discovered that certain classes of algorithms lead to hidden 'spin-glass' order. We are now searching for other forms of hidden order and think that this will lead us to a new taxonomy of quantum states."

Random algorithms are those that incorporate a degree of randomness as part of the algorithm—e.g., random numbers to decide what to do next.

Mueller's proposal for the 2021 New Frontier Grant "Autonomous Quantum Subsystem Error Correction" aimed to simplify quantum computer architectures by developing a new strategy to correct for quantum processor errors caused by environmental noise—that is, any factor, such as cosmic rays or magnetic fields, that would interfere with a quantum computer's qubits, corrupting information.

The bits of classical computer systems are protected by error-correcting codes, Mueller said; information is replicated so that if one bit "flips," you can detect it and fix the error. "For quantum computing to be workable now and in the future, we need to come up with ways to protect qubits in the same way."

"The key to error correction is redundancy," Mueller said. "If I send three copies of a bit, you can tell if there is an error by comparing the bits with one another. We borrow language from cryptography for talking about such strategies and refer to the repeated set of bits as a 'codeword.'"

When they made their discovery about spin-glass order, Mueller and his team were looking into a generalization, where multiple codewords are used to represent the same information. For example, in a subsystem code, the bit "1" might be stored in 4 different ways: 111; 100; 101; and 001.

"The extra freedom that one has in quantum subsystem codes simplifies the process of detecting and correcting errors," Mueller said.

The researchers emphasized that they weren't simply trying to generate a better error protection scheme when they began this research. Rather, they were studying random algorithms to learn general properties of all such algorithms.

"Interestingly, we found nontrivial structure," Mueller said. "The most dramatic was the existence of this spin-glass order, which points toward there being some extra hidden information floating around, which should be useable in some way for computing, though we don't know how yet."

Thursday 31 August 2023

IBM makes major leap in quantum computing error-detection


Quantum computing is on the verge of catapulting the digital revolution to new heights.

Turbocharged processing holds the promise of instantaneously diagnosing health ailments and providing rapid development of new medicines; greatly speeding up response time in AI systems for such time-sensitive operations as autonomous driving and space travel; optimizing traffic control in congested cities; helping aircraft better navigate extreme turbulence; speeding up weather forecasting that better prepares localities facing potential disaster, and optimizing supply chain systems for more efficient delivery times and cost savings.

But we're not there yet. One of the greatest obstacles facing quantum operations is error-correction.

The price for speedier operations in quantum systems is a higher error rate. Quantum computers are highly susceptible to noise such as electromagnetic signals, temperature change and disturbances in the Earth's magnetic field. Such noise triggers errors.

Qubits, the components particular to quantum computing, themselves are prone to error. Faults in frequencies, energy levels and coupling strength can cause miscalculations.

Unlike standard computer bits that are copied reliably most of the time, qubits, by their very nature, cannot be cloned without errors being introduced. Bits store easily replicated binary digit states while qubits store data in a complex mathematical quantum state that can be disrupted during copying. Additionally, qubits age quickly and deterioration can introduce errors.

Researchers at IBM Quantum say they have developed a system that dramatically improves error-detection in quantum computing. In an online post Aug. 28, they explained the challenge: "Standard classical error-correction only needs to correct bit flip errors," said IBM researcher Sergey Bravyi.

"Quantum computers must correct more kinds of errors, like phase errors which can corrupt the extra quantum information that qubits carry … Techniques must [also] correct errors without the ability to copy unknown quantum states, and without destroying the underlying quantum state."

In their research paper, IBM researchers described a process they say greatly trims the required arsenal currently used in quantum computing to catch errors.

Standard computer surface codes have long been successfully used for error-corrections. These are two-dimensional grids resembling a checkerboard. Efficient error-correction for qubits is more challenging.

Bravyi says many experts estimate fault-tolerant quantum computing would require millions of qubits, "a number we believe is too large to be practical at this stage of development."

IBM's solution, improved code and a redesign of qubit placement, achieves results requiring one-tenth the number of physical qubits currently used in error-correction.

"Practical error correction is far from a solved problem," the researchers acknowledged in a paper titled "High-threshold and low-overhead fault-tolerant quantum memory" published Aug. 15 in the preprint server arXiv.

"However, these new codes and other advances across the field are increasing our confidence that fault tolerant quantum computing isn't just possible, but is possible without having to build an unreasonably large quantum computer."

Their approach currently only works on quantum memory and not computational power.

"These techniques are a stepping stone towards a world of fault-tolerant computing," Bravyi says, "and this new … code is bringing that world closer. It's a promising result pointing us where we should look next for even better error correcting codes."

#QuantumComputingRevolution #QubitsAndBeyond #QuantumLeap #ComputingInParallel #EntanglementEra #QuantumBits #SuperpositionComputing #DecoherenceChallenges #QuantumAlgorithm

Sunday 27 August 2023

Visualizing the microscopic phases of magic-angle twisted bilayer graphene


For the first time, the researchers were able to specifically capture unprecedentedly precise visualizations of the microscopic behavior of interacting electrons that give rise to the insulating quantum phase of MATBG. Additionally, through the use of novel and innovative theoretical techniques, they were able to interpret and understand these behaviors. Their study is published in the journal Nature.

The amazing properties of twisted bilayer graphene were first discovered in 2018 by Pablo Jarillo-Herrero and his team at the Massachusetts Institute of Technology (MIT). They showed that this material can be superconducting, a state in which electrons flow freely without any resistance. This state is vital to many of our everyday electronics, including magnets for MRIs and particle accelerators as well as in the making of quantum bits (called qubits) that are being used to build quantum computers.

Since that discovery, twisted bilayer graphene has demonstrated many novel quantum physical states, such as insulating, magnetic, and superconducting states, all of which are created by complex interactions of electrons. How and why electrons form insulating states in MATBG has been one of the key unsolved puzzles in the field.

The solution to this puzzle would not only unlock our understanding of both the insulator and the proximate superconductor, but also such behavior shared by many unusual superconductors that scientists seek to understand, including the high-temperature cuprate superconductors.

"MATBG shows a lot of interesting physics in a single material platform-much of which remains to be understood," said Kevin Nuckolls, the co-lead author of the paper, who earned his Ph.D. in 2023 in Princeton's physics department and is now a postdoctoral fellow at MIT. "This insulating phase, in which electrons are completely blocked from flowing, has been a real mystery."

To create the desired quantum effects, researchers place two sheets of graphene on top of each other with the top layer angled slightly. This off-kilter position creates a moiré pattern, which resembles and is named after a common French textile design. Importantly, however, the angle at which the top layer of graphene must be positioned is precisely 1.1 degrees. This is the "magic" angle that produces the quantum effect; that is, this angle induces strange, strongly correlated interactions between the electrons in the graphene sheets.

While physicists have been able to demonstrate different quantum phases in this material, such as the zero-resistance superconducting phase and the insulating phase, there has been very little understanding of why these phases occur in MATBG. Indeed, all previous experiments involving MATBG give good demonstrations of what the system is capable of producing, but not why the system is producing these states.

And that "why" became the basis for the current experiment.

"The general idea of this experiment is that we wanted to ask questions about the origins of these quantum phases—to really understand what exactly are the electrons doing on the graphene atomic scale," said Nuckolls. "Being able to probe the material microscopically, and to take images of its correlated states—to fingerprint them, effectively—gives us the ability to discern very distinctly and precisely the microscopic origins of some of these phases. Our experiment also helps guide theorists in the search for phases that were not predicted."

The study is the culmination of two years of work and was achieved by a team from Princeton University and the University of California, Berkeley. The scientists harnessed the power of the scanning tunneling microscope (STM) to probe this very minute realm. This tool relies on a technique called "quantum tunneling," where electrons are funneled between the sharp metallic tip of the microscope and the sample. The microscope uses this tunneling current rather than light to view the world of electrons on the atomic scale. Measurements of these quantum tunneling events are then translated into high resolution, highly sensitive images of materials.

However, the first step—and perhaps the most crucial step in the experiment's success—was the creation of what the researchers refer to as a "pristine" sample. The surface of carbon atoms that constituted the twisted bilayer graphene sample had to have no flaws or imperfections.
Visualizing the microscopic phases of magic-angle twisted bilayer graphene

"The technical breakthrough that made this paper happen was our group's ability to make the samples so pristine in terms of their cleanliness such that these high-resolution images that you see in the paper were possible," said Ali Yazdani, the Class of 1909 Professor of Physics and Director of the Center for Complex Materials at Princeton University. "In other words, you have to make one hundred thousand atoms without a single flaw or disorder."

The actual experiment involved placing the graphene sheets in the correct "magic angle," at 1.1 degrees. The researchers then positioned the sharp, metallic tip of the STM over the graphene sample and measured the quantum mechanical tunneling current as they moved the tip across the sample.

"Electrons at this quantum scale are not only particles, but they are also waves," said Ryan Lee, a graduate student in the Department of Physics at Princeton and one of the paper's co-lead authors. "And essentially, we're imaging wave-like patterns of electrons, where the exact way that they interfere (with each other) is telling us some very specific information about what is giving rise to the underlying electronic states."

This information allowed the researchers to make some very incisive interpretations about the quantum phases that were produced by the twisted bilayer graphene. Importantly, the researchers used this information to focus on and solve the long-standing puzzle that for many years has challenged researchers working in this field, namely, the quantum insulating phase that occurs when graphene is tuned to its magic angle.

To help understand this from a theoretical viewpoint, the Princeton researchers collaborated with a team from the University of California-Berkeley, led by physicists B. Andrei Bernevig at Princeton and Michael Zaletel at Berkeley. This team developed a novel and innovative theoretical framework called "local order parameter" analysis to interpret the STM images and understand what the electrons were doing—in other words, how they were interacting—in the insulating phase. What they discovered was that the insulating state occurs because of the strong repulsion between the electrons, on the microscopic level.

"In magic-angle twisted bilayer graphene, the challenge was to model the system," said Tomohiro Soejima, a graduate student and theorist at U.C. Berkeley and one of the paper's co-lead authors. "There were many competing theories, and no one knew which one was correct. Our experiment of 'finger-printing' was really crucial because that way we could pinpoint the actual electronic interactions that give rise to the insulating phase."

By using this theoretical framework, the researchers were able, for the first time, to make a measurement of the observed wave functions of the electrons. "The experiment introduces a new way of analyzing quantum microscopy," said Yazdani.

The researchers suggest the technology—both the imagery and the theoretical framework—can be used in the future to analyze and understand many other quantum phases in MATBG, and ultimately, to help comprehend new and unusual material properties that may be useful for next-generation quantum technological applications.

"Our experiment was a wonderful example of how Mother Nature can be so complicated—can be really confusing—until you have the right framework to look at it, and then you say, 'oh, that's what's happening,'" said Yazdani.

#HomogeneousPhase #MoirePatternPhase #MottInsulatorPhase #SuperconductingPhase
#FerromagneticPhase #AntiferromagneticPhase #TopologicalPhase)

Friday 25 August 2023

Long-lived quantum state points the way to solving a mystery in radioactive nuclei


Timothy Gray of the Department of Energy's Oak Ridge National Laboratory led a study that may have revealed an unexpected change in the shape of an atomic nucleus. The surprise finding could affect our understanding of what holds nuclei together, how protons and neutrons interact and how elements form.

"We used radioactive beams of excited sodium-32 nuclei to test our understanding of nuclear shapes far from stability and found an unexpected result that raises questions about how nuclear shapes evolve," said Gray, a nuclear physicist. The results are published in Physical Review Letters.

The shapes and energies of atomic nuclei can shift over time between different configurations. Typically, nuclei live as quantum entities that have either spherical or deformed shapes. The former look like basketballs, and the latter resemble American footballs.

How shapes and energy levels relate is a major open question for the scientific community. Nuclear structure models have trouble extrapolating to regions with little experimental data.

For some exotic radioactive nuclei, the shapes predicted by traditional models are the opposite of those observed. Radioactive nuclei that were expected to be spherical in their ground states, or lowest-energy configurations, turned out to be deformed.
What can turn a quantum state on its head?

In principle, the energy of an excited deformed state can drop below that of a spherical ground state, making the spherical shape the high-energy one. Unexpectedly, this role reversal appears to be happening for some exotic nuclei when the natural ratio of neutrons to protons becomes unbalanced. Yet, the post-reversal excited spherical states have never been found. It is as though once the ground state becomes deformed, all the excited states do, too.

Many examples exist of nuclei with spherical ground states and deformed excited states. Similarly, plenty of nuclei have deformed ground states and subsequent excited states that are also deformed—sometimes with different amounts or kinds of deformation. However, nuclei with both deformed ground states and spherical excited states are much more elusive.

Using data collected in 2022 from the first experiment at the Facility for Rare Isotope Beams, or FRIB, a DOE Office of Science user facility at Michigan State University, Gray's team discovered a long-lived excited state of radioactive sodium-32. The newly observed excited state has an unusually long lifetime of 24 microseconds—about a million times longer than a typical nuclear excited state.

Long-lived excited states are called isomers. A long lifetime indicates that something unanticipated is going on. For example, if the excited state is spherical, a difficulty in returning to a deformed ground state could account for its long life.

The study involved 66 participants from 20 universities and national laboratories. Co-principal investigators came from Lawrence Berkeley National Laboratory, Florida State University, Mississippi State University, the University of Tennessee, Knoxville, and ORNL.

The 2022 experiment that generated the data used for the 2023 result employed the FRIB Decay Station initiator, or FDSi, a modular multidetector system that is extremely sensitive to rare isotope decay signatures.

"FDSi's versatile combination of detectors shows that the long-lived excited state of sodium-32 is delivered within the FRIB beam and that it then decays internally by emitting gamma rays to the ground state of the same nucleus," said ORNL's Mitch Allmond, a co-author of the paper who manages the FDSi project.

To stop FRIB's highly energetic radioactive beam, which travels at about 50% of the speed of light, an implantation detector built by UT Knoxville was positioned at FDSi's center. North of the beam line was a gamma-ray detector array called DEGAi, comprising 11 germanium clover-style detectors and 15 fast-timing lanthanum bromide detectors. South of the beam line were 88 modules of a detector called NEXTi to measure time of flight of neutrons emitted in radioactive decay.

A beam of excited sodium-32 nuclei stopped in the detector and decayed to the deformed ground state by emitting gamma rays. Analysis of gamma-ray spectra to discern the time difference between beam implantation and gamma-ray emission revealed how long the excited state existed. The new isomer's 24-microsecond existence was the longest lifetime seen among isomers with 20 to 28 neutrons that decay by gamma-ray emission. Approximately 1.8% of the sodium-32 nuclei were observed to be the new isomer.

"We can come up with two different models that equally well explain the energies and lifetime that we've observed in the experiment," Gray said.

An experiment with higher beam power is needed to determine whether the excited state in sodium-32 is spherical. If it is, then the state would have six quantized units of angular momentum, which is a quality of a nucleus related to its whole-body rotation or the orbital motion of its individual protons and/or neutrons about the center of mass. However, if the excited state in sodium-32 is deformed, then the state would have zero quantized units of angular momentum.

A planned upgrade to FRIB will provide more power, increasing the number of nuclei in the beam. Data from the more intense beam will enable an experiment that distinguishes between the two possibilities.

"We'd characterize correlations between the angles of two gamma rays that are emitted in a cascade," Gray said. "The two possibilities have very different angular correlations between the gamma rays. If we have enough statistics, we could disentangle the pattern that reveals a clear answer."

Thursday 24 August 2023

Research group detects a quantum entanglement wave for the first time using real-space measurements


Triplons are tricky little things. Experimentally, they're exceedingly difficult to observe. And even then, researchers usually conduct the tests on macroscopic materials, in which measurements are expressed as an average across the whole sample.

That's where designer quantum materials offer a unique advantage, says Academy Research Fellow Robert Drost, the first author of a paper published in Physical Review Letters. These designer quantum materials let researchers create phenomena not found in natural compounds, ultimately enabling the realization of exotic quantum excitations.

"These materials are very complex. They give you very exciting physics, but the most exotic ones are also challenging to find and study. So, we are trying a different approach here by building an artificial material using individual components," says Professor Peter Liljeroth, head of the Atomic Scale physics research group at Aalto University.

Quantum materials are governed by the interactions between electrons at the microscopic level. These electronic correlations lead to unusual phenomena like high-temperature superconductivity or complex magnetic states, and quantum correlations give rise to new electronic states.

In the case of two electrons, there are two entangled states known as singlet and triplet states. Supplying energy to the electron system can excite it from the singlet to the triplet state. In some cases, this excitation can propagate through a material in an entanglement wave known as a triplon. These excitations are not present in conventional magnetic materials, and measuring them has remained an open challenge in quantum materials.
The team's triplon experiments

In the new study, the team used small organic molecules to create an artificial quantum material with unusual magnetic properties. Each of the cobalt-phthalocyanine molecules used in the experiment contains two frontier electrons.

"Using very simple molecular building blocks, we are able to engineer and probe this complex quantum magnet in a way that has never been done before, revealing phenomena not found in its independent parts," Drost says. "While magnetic excitations in isolated atoms have long been observed using scanning tunneling spectroscopy, it has never been accomplished with propagating triplons."

"We use these molecules to bundle electrons together, we pack them into a tight space and force them to interact," continues Drost. "Looking into such a molecule from the outside, we will see the joint physics of both electrons. Because our fundamental building block now contains two electrons, rather than one, we see a very different kind of physics."

The team monitored magnetic excitations first in individual cobalt-phthalocyanine molecules and later in larger structures like molecular chains and islands. By starting with the very simple and working towards increasing complexity, the researchers hope to understand emergent behavior in quantum materials. In the present study, the team could demonstrate that the singlet-triplet excitations of their building blocks can traverse molecular networks as exotic magnetic quasiparticles known as triplons.

"We show that we can create an exotic quantum magnetic excitation in an artificial material. This strategy shows that we can rationally design material platforms that open up new possibilities in quantum technologies," says Assistant Professor Jose Lado, one of the study's co-authors, who heads the Correlated Quantum Materials research group at Aalto University.

The team plans to extend their approach towards more complex building blocks to design other exotic magnetic excitations and ordering in quantum materials. Rational design from simple ingredients will not only help understand the complex physics of correlated electron systems but also establish new platforms for designer quantum materials.

Physicists use a 350-year-old theorem to reveal new properties of light waves


Since the 17th century, when Isaac Newton and Christiaan Huygens first debated the nature of light, scientists have been puzzling over whether light is best viewed as a wave or a particle—or perhaps, at the quantum level, even both at once. Now, researchers at Stevens Institute of Technology have revealed a new connection between the two perspectives, using a 350-year-old mechanical theorem—ordinarily used to describe the movement of large, physical objects like pendulums and planets—to explain some of the most complex behaviors of light waves.

The work, led by Xiaofeng Qian, assistant professor of physics at Stevens and reported in the August 17 online issue of Physical Review Research, also proves for the first time that a light wave's degree of non-quantum entanglement exists in a direct and complementary relationship with its degree of polarization. As one rises, the other falls, enabling the level of entanglement to be inferred directly from the level of polarization, and vice versa. This means that hard-to-measure optical properties such as amplitudes, phases and correlations—perhaps even these of quantum wave systems—can be deduced from something a lot easier to measure: light intensity.

"We've known for over a century that light sometimes behaves like a wave, and sometimes like a particle, but reconciling those two frameworks has proven extremely difficult," said Qian "Our work doesn't solve that problem—but it does show that there are profound connections between wave and particle concepts not just at the quantum level, but at the level of classical light-waves and point-mass systems."

Qian's team used a mechanical theorem, originally developed by Huygens in a 1673 book on pendulums, that explains how the energy required to rotate an object varies depending on the object's mass and the axis around which it turns. "This is a well-established mechanical theorem that explains the workings of physical systems like clocks or prosthetic limbs," Qian explained. "But we were able to show that it can offer new insights into how light works, too."

This 350-year-old theorem describes relationships between masses and their rotational momentum, so how could it be applied to light where there is no mass to measure? Qian's team interpreted the intensity of a light as the equivalent of a physical object's mass, then mapped those measurements onto a coordinate system that could be interpreted using Huygens' mechanical theorem. "Essentially, we found a way to translate an optical system so we could visualize it as a mechanical system, then describe it using well-established physical equations," explained Qian.

Once the team visualized a light wave as part of a mechanical system, new connections between the wave's properties immediately became apparent—including the fact that entanglement and polarization stood in a clear relationship with one another.

"This was something that hadn't been shown before, but that becomes very clear once you map light's properties onto a mechanical system," said Qian. "What was once abstract becomes concrete: using mechanical equations, you can literally measure the distance between 'center of mass' and other mechanical points to show how different properties of light relate to one another."

Clarifying these relationships could have important practical implications, allowing subtle and hard-to-measure properties of optical systems—or even quantum systems—to be deduced from simpler and more robust measurements of light intensity, Qian explained. More speculatively, the team's findings suggest the possibility of using mechanical systems to simulate and better-understand the strange and complex behaviors of quantum wave systems.

"That still lies ahead of us, but with this first study we've shown clearly that by applying mechanical concepts, it's possible to understand optical systems in an entirely new way," Qian said. "Ultimately, this research is helping to simplify the way we understand the world, by allowing us to recognize the intrinsic underlying connections between apparently unrelated physical laws."

Experimental quantum imaging distillation with undetected light

  It is possible to image an object with an induced coherence effect by making use of photon pairs to gain information on the item of intere...