TL;DR

• In a laboratory experiment, researchers have succeeded in realizing an effective spacetime that can be manipulated. In their research on ultracold quantum gases, they were able to simulate an entire family of curved universes to investigate different cosmological scenarios and compare them with the predictions of a quantum field theoretical model.
• Researchers have successfully extended the quantum phase difference estimation algorithm, a general quantum algorithm for the direct calculations of energy gaps, to enable the direct calculation of energy differences between two different molecular geometries. This allows for the computation, based on the finite difference method, of energy derivatives with respect to nuclear coordinates in a single calculation.
• Scientists have developed a quantum experiment that allows them to study the dynamics, or behavior, of a special kind of theoretical wormhole.
• Water that simply will not freeze, no matter how cold it gets — a research group has discovered a quantum state that could be described in this way. Experts have managed to cool a special material to near absolute zero temperature. They found that a central property of atoms — their alignment — did not ‘freeze’, as usual, but remained in a ‘liquid’ state. The new quantum material could serve as a model system to develop novel, highly sensitive quantum sensors.
• Machine learning drives self-discovery of pulses that stabilize quantum systems in the face of environmental noise.
• Recently, researchers have developed an integrated electro-optic modulator that can efficiently change the frequency and bandwidth of single photons. The device could be used for more advanced quantum computing and quantum networks.
• Researchers report the synthesis of semiconductor ‘giant’ core-shell quantum dots with record-breaking emissive lifetimes. In addition, the lifetimes can be tuned by making a simple alteration to the material’s internal structure.
• It was believed that it was impossible to differentiate the enantiomers of a chiral molecule using helical light beams — until now that is. Researchers have now developed a new chiroptical technique to differentiate the two non-superimposable mirror images of a chiral molecule. Its efficiency can even be scaled and controlled by using linear polarized helical light beams.
• Researchers have demonstrated a way to entangle atoms to create a network of atomic clocks and accelerometers. The method has resulted in greater precision in measuring time and acceleration.
• Quantum computers promise significantly shorter computing times for complex problems. But there are still only a few quantum computers worldwide with a limited number of so-called qubits. However, quantum computer algorithms can already run on conventional servers that simulate a quantum computer. A team has succeeded in calculating the electron orbitals and their dynamic development using an example of a small molecule after a laser pulse excitation. In principle, the method is also suitable for investigating larger molecules that cannot be calculated using conventional methods.
• And more!

Quantum Computing Market

According to the recent market research report ‘Quantum Computing Market with COVID-19 impact by Offering (Systems and Services), Deployment (On Premises and Cloud Based), Application, Technology, End-use Industry and Region — Global Forecast to 2026’, published by MarketsandMarkets, the Quantum Computing market is expected to grow from USD 472 million in 2021 to USD 1,765 million by 2026, at a CAGR of 30.2%. The early adoption of quantum computing in the banking and finance sector is expected to fuel the growth of the market globally. Other key factors contributing to the growth of the quantum computing market include rising investments by governments of different countries to carry out research and development activities related to quantum computing technology. Several companies are focusing on the adoption of QCaaS post-COVID-19. This, in turn, is expected to contribute to the growth of the quantum computing market. However, stability and error correction issues are expected to restrain the growth of the market.

According to ‘Quantum Computing Market Research Report: By Offering, Deployment Type, Application, Technology, Industry — Industry Share, Growth, Drivers, Trends and Demand Forecast to 2030’ report, the quantum computing market is projected to reach $64,988 million by 2030. Machine learning (ML) is expected to progress at the highest CAGR, during the forecast period, among all application categories, owing to the fact that quantum computing is being integrated in ML for improving the latter’s use case.

Latest Research

Quantum field simulator for dynamics in curved spacetime

by Celia Viermann, Marius Sparn, Nikolas Liebster, Maurus Hans, Elinor Kath, Álvaro Parra-López, Mireia Tolosa-Simeón, Natalia Sánchez-Kuntz, Tobias Haas, Helmut Strobel, Stefan Floerchinger, Markus K. Oberthaler in Nature

In a laboratory experiment, researchers from Heidelberg University have succeeded in realising an effective spacetime that can be manipulated. In their research on ultracold quantum gases, they were able to simulate an entire family of curved universes to investigate different cosmological scenarios and compare them with the predictions of a quantum field theoretical model.

According to Einstein’s Theory of Relativity, space and time are inextricably connected. In our Universe, whose curvature is barely measurable, the structure of this spacetime is fixed. In a laboratory experiment, researchers from Heidelberg University have succeeded in realising an effective spacetime that can be manipulated. In their research on ultracold quantum gases, they were able to simulate an entire family of curved universes to investigate different cosmological scenarios and compare them with the predictions of a quantum field theoretical model.

The emergence of space and time on cosmic time scales from the Big Bang to the present is the subject of current research that can only be based on the observation of our single Universe. The expansion and curvature of space are essential to cosmological models. In a flat space like our current Universe, the shortest distance between two points is always a straight line.

“It is conceivable, however, that our Universe was curved in its early phase. Studying the consequences of a curved spacetime is therefore a pressing question in research,” states Prof. Dr Markus Oberthaler, a researcher at the Kirchhoff Institute for Physics at Heidelberg University. With his “Synthetic Quantum Systems” research group, he developed a quantum field simulator for this purpose.

Theoretical prediction for density-contrast correlation functions in real space.

The quantum field simulator created in the lab consists of a cloud of potassium atoms cooled to just a few nanokelvins above absolute zero. This produces a Bose-Einstein condensate — a special quantum mechanical state of the atomic gas that is reached at very cold temperatures. Prof. Oberthaler explains that the Bose-Einstein condensate is a perfect background against which the smallest excitations, i.e. changes in the energy state of the atoms, become visible. The form of the atomic cloud determines the dimensionality and the properties of spacetime on which these excitations ride like waves. In our Universe, there are three dimensions of space as well as a fourth: time.

In the experiment conducted by the Heidelberg physicists, the atoms are trapped in a thin layer. The excitations can therefore only propagate in two spatial directions — the space is two-dimensional. At the same time, the atomic cloud in the remaining two dimensions can be shaped in almost any way, whereby it is also possible to realise curved spacetimes. The interaction between the atoms can be precisely adjusted by a magnetic field, changing the propagation speed of the wavelike excitations on the Bose-Einstein condensate.

“For the waves on the condensate, the propagation speed depends on the density and the interaction of the atoms. This gives us the opportunity to create conditions like those in an expanding universe,” explains Prof. Dr Stefan Flörchinger.

The researcher, who previously worked at Heidelberg University and joined the University of Jena at the beginning of this year, developed the quantum field theoretical model used to quantitatively compare the experimental results.

Using the quantum field simulator, cosmic phenomena, such as the production of particles based on the expansion of space, and even the spacetime curvature can be made measurable.

“Cosmological problems normally take place on unimaginably large scales. To be able to specifically study them in the lab opens up entirely new possibilities in research by enabling us to experimentally test new theoretical models,” states Celia Viermann, the primary author of the article.

“Studying the interplay of curved spacetime and quantum mechanical states in the lab will occupy us for some time to come,” says Markus Oberthaler, whose research group is also part of the STRUCTURES Cluster of Excellence at Ruperto Carola.

Quantum Algorithm for Numerical Energy Gradient Calculations at the Full Configuration Interaction Level of Theory

by Kenji Sugisaki, Hiroyuki Wakimoto, Kazuo Toyota, Kazunobu Sato, Daisuke Shiomi, Takeji Takui in The Journal of Physical Chemistry Letters

In recent years, research and development on quantum computers has made considerable progress. Quantum chemical calculations for electronic structures of atoms and molecules are attracting great attention as one of the most promising applications of quantum computers. In order to utilize quantum chemical calculations for chemistry and related fields, it is essential to develop geometry optimization methods for finding the most stable structure of molecules. The geometry optimization requires calculations of energy derivatives with respect to nuclear coordinates of molecules.

The finite difference method is one approach for energy derivative calculations. On a classical computer, calculations based on this method for one-dimensional systems require at least two evaluations of the energy. Previous research has shown that a quantum computer, in contrast, requires only a single query to calculate the energy derivatives based on the finite difference method, regardless of the number of degrees of freedom. However, quantum circuits relevant to quantum algorithms capable of performing energy derivative calculations have not been implemented.

A research group including Dr. Kenji Sugisaki, Professor Kazunobu Sato, and Professor Emeritus Takeji Takui from the Graduate School of Science at Osaka Metropolitan University has successfully extended the quantum phase difference estimation algorithm, a general quantum algorithm for the direct calculations of energy gaps, to enable the direct calculation of energy differences between two different molecular geometries. This allows for the computation, based on the finite difference method, of energy derivatives with respect to nuclear coordinates in a single calculation.

Furthermore, the research group has applied the developed energy derivative calculations to execute geometry optimizations of H2, LiH, BeH2, and N2 molecules without calculating the total energies, demonstrating the usefulness of the developed method. The group also discussed how quantum circuits can be assembled according to different degrees of freedom of the molecules.

This research is the latest in a series of the researchers’ articles on quantum chemical calculations on quantum computers.

“Our latest findings bring us one step closer to applying quantum chemical calculations on a quantum computer to real-world problems,” said Dr. Sugisaki. “Since energy derivative calculations are used for not only molecular geometry optimizations but also various calculations for molecular properties, the application of our method is expected to play a very important role in a wide range of related fields, such as in silico drug discovery/design and materials development.”

Traversable wormhole dynamics on a quantum processor

by Daniel Jafferis, Alexander Zlokapa, Joseph D. Lykken, David K. Kolchmeyer, Samantha I. Davis, Nikolai Lauk, Hartmut Neven, Maria Spiropulu in Nature

Scientists have, for the first time, developed a quantum experiment that allows them to study the dynamics, or behavior, of a special kind of theoretical wormhole. The experiment has not created an actual wormhole (a rupture in space and time), rather it allows researchers to probe connections between theoretical wormholes and quantum physics, a prediction of so-called quantum gravity. Quantum gravity refers to a set of theories that seek to connect gravity with quantum physics, two fundamental and well-studied descriptions of nature that appear inherently incompatible with each other.

“We found a quantum system that exhibits key properties of a gravitational wormhole yet is sufficiently small to implement on today’s quantum hardware,” says Maria Spiropulu, the principal investigator of the U.S. Department of Energy Office of Science research program Quantum Communication Channels for Fundamental Physics (QCCFP) and the Shang-Yi Ch’en Professor of Physics at Caltech. “This work constitutes a step toward a larger program of testing quantum gravity physics using a quantum computer. It does not substitute for direct probes of quantum gravity in the same way as other planned experiments that might probe quantum gravity effects in the future using quantum sensing, but it does offer a powerful testbed to exercise ideas of quantum gravity.”

The study’s first authors are Daniel Jafferis of Harvard University and Alexander Zlokapa (BS ‘21), a former undergraduate student at Caltech who started on this project for his bachelor’s thesis with Spiropulu and has since moved on to graduate school at MIT.

Wormholes are bridges between two remote regions in spacetime. They have not been observed experimentally, but scientists have theorized about their existence and properties for close to 100 years. In 1935, Albert Einstein and Nathan Rosen described wormholes as tunnels through the fabric of spacetime in accordance with Einstein’s general theory of relativity, which describes gravity as a curvature of spacetime. Researchers call wormholes Einstein-Rosen bridges after the two physicists who invoked them, while the term “wormhole” itself was coined by physicist John Wheeler in the 1950s.

Traversable wormhole in spacetime and in the holographic dual.

The notion that wormholes and quantum physics, specifically entanglement (a phenomenon in which two particles can remain connected across vast distances), may have a connection was first proposed in theoretical research by Juan Maldacena and Leonard Susskind in 2013. The physicists speculated that wormholes (or “ER”) were equivalent to entanglement (also known as “EPR” after Albert Einstein, Boris Podolsky [PhD ‘28], and Nathan Rosen, who first proposed the concept). In essence, this work established a new kind of theoretical link between the worlds of gravity and quantum physics. “It was a very daring and poetic idea,” says Spiropulu of the ER = EPR work.

Later, in 2017, Jafferis, along with his colleagues Ping Gao and Aron Wall, extended the ER = EPR idea to not just wormholes but traversable wormholes. The scientists concocted a scenario in which negative repulsive energy holds a wormhole open long enough for something to pass through from one end to the other. The researchers showed that this gravitational description of a traversable wormhole is equivalent to a process known as quantum teleportation. In quantum teleportation, a protocol that has been experimentally demonstrated over long distances via optical fiber and over the air, information is transported across space using the principles of quantum entanglement.

The present work explores the equivalence of wormholes with quantum teleportation. The Caltech-led team performed the first experiments that probe the idea that information traveling from one point in space to another can be described in either the language of gravity (the wormholes) or the language of quantum physics (quantum entanglement).

A key finding that inspired possible experiments occurred in 2015, when Caltech’s Alexei Kitaev, the Ronald and Maxine Linde Professor of Theoretical Physics and Mathematics, showed that a simple quantum system could exhibit the same duality later described by Gao, Jafferis, and Wall, such that the model’s quantum dynamics are equivalent to quantum gravity effects. This Sachdev-Ye-Kitaev, or SYK model (named after Kitaev, and Subir Sachdev and Jinwu Ye, two other researchers who worked on its development previously) led researchers to suggest that some theoretical wormhole ideas could be studied more deeply by doing experiments on quantum processors.

Furthering these ideas, in 2019, Jafferis and Gao showed that by entangling two SYK models, researchers should be able to perform wormhole teleportation and thus produce and measure the dynamical properties expected of traversable wormholes.

In the new study, the team of physicists performed this type of experiment for the first time. They used a “baby” SYK-like model prepared to preserve gravitational properties, and they observed the wormhole dynamics on a quantum device at Google, namely the Sycamore quantum processor. To accomplish this, the team had to first reduce the SYK model to a simplified form, a feat they achieved using machine learning tools on conventional computers.

“We employed learning techniques to find and prepare a simple SYK-like quantum system that could be encoded in the current quantum architectures and that would preserve the gravitational properties,” says Spiropulu. “In other words, we simplified the microscopic description of the SYK quantum system and studied the resulting effective model that we found on the quantum processor. It is curious and surprising how the optimization on one characteristic of the model preserved the other metrics! We have plans for more tests to get better insights on the model itself.”

In the experiment, the researchers inserted a qubit — the quantum equivalent of a bit in conventional silicon-based computers — into one of their SYK-like systems and observed the information emerge from the other system. The information traveled from one quantum system to the other via quantum teleportation — or, speaking in the complementary language of gravity, the quantum information passed through the traversable wormhole.

“We performed a kind of quantum teleportation equivalent to a traversable wormhole in the gravity picture. To do this, we had to simplify the quantum system to the smallest example that preserves gravitational characteristics so we could implement it on the Sycamore quantum processor at Google,” says Zlokapa.

Co-author Samantha Davis, a graduate student at Caltech, adds, “It took a really long time to arrive at the results, and we surprised ourselves with the outcome.”

In the study, the physicists report wormhole behavior expected both from the perspectives of gravity and from quantum physics. For example, while quantum information can be transmitted across the device, or teleported, in a variety of ways, the experimental process was shown to be equivalent, at least in some ways, to what might happen if information traveled through a wormhole. To do this, the team attempted to “prop open the wormhole” using pulses of either negative repulsive energy pulse or the opposite, positive energy. They observed key signatures of a traversable wormhole only when the equivalent of negative energy was applied, which is consistent with how wormholes are expected to behave.

“The high fidelity of the quantum processor we used was essential,” says Spiropulu. “If the error rates were higher by 50 percent, the signal would have been entirely obscured. If they were half we would have 10 times the signal!”?

In the future, the researchers hope to extend this work to more complex quantum circuits. Though bona fide quantum computers may still be years away, the team plans to continue to perform experiments of this nature on existing quantum computing platforms.

“The relationship between quantum entanglement, spacetime, and quantum gravity is one of the most important questions in fundamental physics and an active area of theoretical research,” says Spiropulu. “We are excited to take this small step toward testing these ideas on quantum hardware and will keep going.”

Spin–orbital liquid state and liquid–gas metamagnetic transition on a pyrochlore lattice

by Nan Tang, Yulia Gritsenko, Kenta Kimura, Subhro Bhattacharjee, et al in Nature Physics

Water that simply will not freeze, no matter how cold it gets — a research group involving the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) has discovered a quantum state that could be described in this way. Experts from the Institute of Solid State Physics at the University of Tokyo in Japan, Johns Hopkins University in the United States, and the Max Planck Institute for the Physics of Complex Systems (MPI-PKS) in Dresden, Germany, managed to cool a special material to near absolute zero temperature. They found that a central property of atoms — their alignment — did not “freeze,” as usual, but remained in a “liquid” state. The new quantum material could serve as a model system to develop novel, highly sensitive quantum sensors.

On first sight, quantum materials do not look different from normal substances — but they sure do their own thing: Inside, the electrons interact with unusual intensity, both with each other and with the atoms of the crystal lattice. This intimate interaction results in powerful quantum effects that not only act on the microscopic, but also on the macroscopic scale. Thanks to these effects, quantum materials exhibit remarkable properties. For example, they can conduct electricity completely loss-free at low temperatures. Often, even slight changes in temperature, pressure, or electrical voltage are enough to drastically change the behavior of the material.

In principle, magnets can also be regarded as quantum materials; after all, magnetism is based on the intrinsic spin of the electrons in the material. “In some ways, these spins can behave like a liquid,” explains Prof. Jochen Wosnitza from the Dresden High Field Magnetic Laboratory (HLD) at HZDR. “As temperatures drop, these disordered spins can then freeze, much like water freezes into ice.” For example, certain kind of magnets, so-called ferromagnets, are non-magnetic above their “freezing,” or more precisely ordering point. Only when they drop below it can they become permanent magnets.

Crystal structure analysis by single crystal synchrotron X-ray diffraction of Pr2Zr2O7.

The international team intended to create a quantum state in which the atomic alignment that is associated with the spins did not order, even at ultracold temperatures — similar to a liquid that will not solidify, even in extreme cold. To achieve this state, the research group used a special material — a compound of the elements, praseodymium, zirconium, and oxygen. They assumed that in this material, the properties of the crystal lattice would enable the electron spins to interact with their orbitals around the atoms in a special way.

“The prerequisite, however, was to have crystals of extreme purity and quality,” Prof. Satoru Nakatsuji of the University of Tokyo explains. It took several attempts, but eventually the team was able to produce crystals pure enough for their experiment: In a cryostat, a kind of super thermos flask, the experts gradually cooled their sample down to 20 millikelvin — just one fiftieth of a degree above absolute zero. To see how the sample responded to this cooling process and inside the magnetic field, they measured how much it changed in length. In another experiment, the group recorded how the crystal reacted to ultrasound waves being directly sent through it.

Sample quality dependence of physical properties of Pr2Zr2O7.

The result: “Had the spins ordered, it should have caused an abrupt change in the behavior of the crystal, such as a sudden change in length,” Dr. Sergei Zherlitsyn, HLD’s expert in ultrasound investigations, describes. “Yet, as we observed, nothing happened! There were no sudden changes in either length or in its response to ultrasound waves.”

The conclusion: The pronounced interplay of spins and orbitals had prevented ordering, which is why the atoms remained in their liquid quantum state — the first time such a quantum state had been observed. Further investigations in magnetic fields confirmed this assumption.

This basic research result could also have practical implications one day: “At some point we might be able to use the new quantum state to develop highly sensitive quantum sensors,” Jochen Wosnitza speculates. “To do this, however, we still have to figure out how to generate excitations in this state systematically.” Quantum sensing is considered a promising technology of the future. Because their quantum nature makes them extremely sensitive to external stimuli, quantum sensors can register magnetic fields or temperatures with far greater precision than conventional sensors.

Accelerated motional cooling with deep reinforcement learning

by Bijita Sarma, Sangkha Borah, A Kani, Jason Twamley in Physical Review Research

It’s easy to control the trajectory of a basketball: all we have to do is apply mechanical force coupled with human skill. But controlling the movement of quantum systems such as atoms and electrons is much more challenging, as these minuscule scraps of matter often fall prey to perturbations that knock them off their path in unpredictable ways. Movement within the system degrades — a process called damping — and noise from environmental effects such as temperature also disturbs its trajectory.

One way to counteract the damping and the noise is to apply stabilizing pulses of light or voltage of fluctuating intensity to the quantum system. Now researchers from Okinawa Institute of Science and Technology (OIST) in Japan have shown that they can use artificial intelligence to discover these pulses in an optimized way to appropriately cool a micro-mechanical object to its quantum state and control its motion.

Micro-mechanical objects, which are large compared to an atom or electron, behave classically when kept at a high temperature, or even at room temperature. However, if such mechanical modes can be cooled down to their lowest energy state, which physicists call the ground state, quantum behaviour could be realised in such systems. These kinds of mechanical modes then can be used as ultra-sensitive sensors for force, displacement, gravitational acceleration etc. as well as for quantum information processing and computing.

(a) The sche matic workflow of the DRL protocol is shown, where the RLenvironment is either the bipartite magno-mechanical system (b), or the tripartite opto-magno-mechanical system (c). See text for further detail on DRL and the physical models.

“Technologies built from quantum systems offer immense possibilities,” said Dr. Bijita Sarma, the article’s lead author and a Postdoctoral Scholar at OIST Quantum Machines Unit in the lab of Professor Jason Twamley. “But to benefit from their promise for ultraprecise sensor design, high-speed quantum information processing, and quantum computing, we must learn to design ways to achieve fast cooling and control of these systems.”

The machine learning-based method that she and her colleagues designed demonstrates how artificial controllers can be used to discover non-intuitive, intelligent pulse sequences that can cool a mechanical object from high to ultracold temperatures faster than other standard methods. These control pulses are self-discovered by the machine learning agent. The work showcases the utility of artificial machine intelligence in the development of quantum technologies.

Quantum computing has the potential to revolutionise the world by enabling high computing speeds and reformatting cryptographic techniques. That is why, many research institutes and big-tech companies such as Google and IBM are investing a lot of resources in developing such technologies. But to enable this, researchers must achieve complete control over the operation of such quantum systems at very high speed, so that the effects of noise and damping can be eliminated.

“In order to stabilize a quantum system, control pulses must be fast — and our artificial intelligence controllers have shown the promise to achieve such feat,” Dr Sarma said. “Thus, our proposed method of quantum control using an AI controller could provide a breakthrough in the field of high-speed quantum computing, and it might be a first step to achieve quantum machines that are self-driving, similar to self-driving cars. We are hopeful that such methods will attract many quantum researchers for future technological developments.”

Spectral control of nonclassical light pulses using an integrated thin-film lithium niobate modulator

by Di Zhu, Changchen Chen, Mengjie Yu, Linbo Shao, Yaowen Hu, C. J. Xin, Matthew Yeh, Soumya Ghosh, Lingyan He, Christian Reimer, Neil Sinclair, et al in Light: Science & Applications

Optical photons are ideal carriers of quantum information. But to work together in a quantum computer or network, they need to have the same color — or frequency — and bandwidth. Changing a photon’s frequency requires altering its energy, which is particularly challenging on integrated photonic chips.

Recently, researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) developed an integrated electro-optic modulator that can efficiently change the frequency and bandwidth of single photons. The device could be used for more advanced quantum computing and quantum networks.

Frequency and bandwidth control of light through temporal phase modulation.

Converting a photon from one color to another is usually done by sending the photon into a crystal with a strong laser shining through it, a process that tends to be inefficient and noisy. Phase modulation, in which photon wave’s oscillation is accelerated or slowed down to change the photon’s frequency, offers a more efficient method, but the device required for such a process, an electro-optic phase modulator, has proven difficult to integrate on a chip.

One material may be uniquely suited for such an application — thin-film lithium niobate.

“In our work, we adopted a new modulator design on thin-film lithium niobate that significantly improved the device performance,” said Marko Lončar, the Tiantsai Lin Professor of Electrical Engineering at SEAS and senior author of the study. “With this integrated modulator, we achieved record-high terahertz frequency shifts of single photons.”

Single-photon spectral shearing.

The team also used the same modulator as a “time lens” — a magnifying glass that bends light in time instead of space — to change the spectral shape of a photon from fat to skinny.

“Our device is much more compact and energy-efficient than traditional bulk devices,” said Di Zhu, the first author of the paper. “It can be integrated with a wide range of classical and quantum devices on the same chip to realize more sophisticated quantum light control.”

Di is a former postdoctoral fellow at SEAS and is currently a research scientist at the Agency for Science, Research and Technology (A*STAR) in Singapore. Next, the team aims to use the device to control the frequency and bandwidth of quantum emitters for applications in quantum networks.

Parabolic Potential Surfaces Localize Charge Carriers in Nonblinking Long-Lifetime “Giant” Colloidal Quantum Dots

by Marcell Pálmai, Joseph S. Beckwith, Nyssa T. Emerson, Tian Zhao, Eun Byoel Kim, Shuhui Yin, Prakash Parajuli, Kyle Tomczak, Kai Wang, Bibash Sapkota, Ming Tien, Nan Jiang, Robert F. Klie, Haw Yang, Preston T. Snee in Nano Letters

A new study involving researchers at the University of Illinois Chicago achieved a milestone in the synthesis of multifunctional photonic nanomaterials.

In a paper, they report the synthesis of semiconductor “giant” core-shell quantum dots with record-breaking emissive lifetimes. In addition, the lifetimes can be tuned by making a simple alteration to the material’s internal structure. The group, which included collaborators from Princeton University and Pennsylvania State University, demonstrated a new structure-property concept that imparts the ability to spatially localize electrons or holes within a core/shell heterostructure by tuning the charge carrier’s kinetic energy on a parabolic potential energy surface.

According to UIC chemist Preston Snee, this charge carrier separation results in extended radiative lifetimes and in continuous emission at the single-nanoparticle level.

“These properties enable new applications for optics, facilitate novel approaches such as time-gated single-particle imaging and create inroads for the development of other new advanced materials,” said Snee, UIC associate professor of chemistry and the study’s senior co-author.

(A) Optical data and fluorescent images of lifetime tunable “giant” quantum dots. (B) Typical single-particle emission trajectory of type II CdZnSe/CdS gQDs shows no blinking behavior.

Snee and the study’s first author, Marcell Pálmai, UIC postdoctoral research associate in chemistry, teamed with Haw Yang of Princeton and others to excite the quantum dots particle with light to put it in the “exciton” state. The exciton is an electron/hole charge pair, and in the new materials, the electron becomes displaced from the center to the shell, where it becomes trapped for upwards of 500 nanoseconds, which represents the record for such nanomaterials.

“As emissive materials, quantum dots hold the promise of creating more energy-efficient displays and can be used as fluorescent probes for biomedical research due to their highly robust optical properties. They are 10 times to 100 times more absorptive than organic dyes and are nearly impervious to photobleaching, which is why they are used in the new Samsung QLED-TV,” they write.

These new particles have great efficacy for fundamental biological discovery, according to the researchers. The quantum dots presented in their paper emit at red wavelengths, which minimizes scattering, while the long lifetimes allow for biological imaging to be performed with less background noise. At the single particle level, the new quantum dots emit continuously, so a research scientist can tag proteins relevant to cancer and follow the biological dynamics without losing track of the signal which is currently a common problem with such studies.

In future research, the group plans to demonstrate that the materials make good components for optical devices such as micron-sized lasers.

Nonlinear helical dichroism in chiral and achiral molecules

by Jean-Luc Bégin, Ashish Jain, Andrew Parks, Felix Hufnagel, Paul Corkum, Ebrahim Karimi, Thomas Brabec, Ravi Bhardwaj in Nature Photonics

It was believed that it was impossible to differentiate the enantiomers of a chiral molecule using helical light beams — until now that is, thanks to a group of uOttawa researchers.

For nearly 20 years researchers believed that it was not possible to differentiate the enantiomers of a chiral molecule using helical light beams. Enantiomers are mirror images of a molecule that cannot be superimposed, like our left and right hands that cannot appear identical simply by reorientation. In addition, molecules with symmetrical properties such as achiral molecules are not expected to show any dependence on the helicity of light. Yet this is what a group of researchers at the University of Ottawa have done.

Helical dichroism (Type II) in fenchone with linearly polarized light (s = 0) as a function of peak laser fluence.

The team, led by Professor Ravi Bhardwaj and his PhD students Ashish Jain and Jean-Luc Bégin, with the collaboration of professors Thomas Brabec, Ebrahim Karimi from the uOttawa Nexus for Quantum technology Institute, and Paul Corkum, Canada Research Chair in Attosecond Photonics, developed a new chiroptical technique to differentiate the two non-superimposable mirror images of a chiral molecule. Its efficiency can even be scaled and controlled by using linear polarized helical light beams.

“Our understanding of light-matter interactions is mainly based on the propagation of homogeneously polarized light and the dominance of the dipole-active transitions between different quantum states of matter,” explained Jean-Luc Bégin. “The higher-order multipole effects are often ignored. Our findings demonstrate their importance.”

Their main findings include:

• Enhanced chiral sensitivity can be observed directly using linearly polarized helical light beams without any intermediary.

• Differential absorption of left- and right-asymmetrical helical light can be observed even in achiral molecules that can be scaled and precisely controlled.

• Helicity dependent absorption of light arises due to coupling of electric dipole and electric quadrupole moments and can be tuned by changing laser polarization.

“Detecting enantiomers with enhanced sensitivity is essential in the pharmaceutical industry to eliminate the unwanted side effects of a drug,” added Ashish Jain. “ Moreover, the control of light-matter interactions demonstrated by us with helical light potentially opens new opportunities in spectroscopy, light-driven molecular machines, optical switching and ultrafast probing of magnetic materials.”

Distributed quantum sensing with mode-entangled spin-squeezed atomic states

by Benjamin K. Malia, Yunfan Wu, Julián Martínez-Rincón, Mark A. Kasevich in Nature

Researchers affiliated with the Q-NEXT quantum research center show how to create quantum-entangled networks of atomic clocks and accelerometers — and they demonstrate the setup’s superior, high-precision performance.

For the first time, scientists have entangled atoms for use as networked quantum sensors, specifically, atomic clocks and accelerometers. The research team’s experimental setup yielded ultraprecise measurements of time and acceleration. Compared to a similar setup that does not draw on quantum entanglement, their time measurements were 3.5 times more precise, and acceleration measurements exhibited 1.2 times greater precision. The result is supported by Q-NEXT, a U.S. Department of Energy (DOE) National Quantum Information Science Research Center led by DOE’s Argonne National Laboratory. The research was conducted by scientists at Stanford University, Cornell University and DOE’s Brookhaven National Laboratory.

“The impact of using entanglement in this configuration was that it produced better sensor network performance than would have been available if quantum entanglement were not used as a resource,” said Mark Kasevich, lead author of the paper, a member of Q-NEXT, the William R. Kenan, Jr. professor in the Stanford School of Humanities and Sciences and professor of physics and of applied physics. “For atomic clocks and accelerometers, ours is a pioneering demonstration.”

Apparatus.

What is quantum entanglement? How does it apply to sensors?

• Entanglement, a special property of nature at the quantum level, is a correlation between two or more objects. When two atoms are entangled, one can measure the properties of both atoms by observing only one. This is true no matter how much distance — even if it’s light-years — separates the entangled atoms.
• A helpful everyday analogy: A red marble and a blue marble are placed in a box. If you draw a red marble from the box, you know, without having to look at the other one, that it’s blue. The color of the marbles is correlated, or entangled.
• In the quantum realm, entanglement is subtler. An atom can take on multiple states (colors) at once. If our marbles were like atoms, each marble would be both red and blue at the same time. Neither is fully red or blue while it sits the box. The quantum marble “decides” its color only at the moment of revelation. And once you draw one marble of “decided” color, you know the color of its entangled partner.
• To take a measurement of one member of an entangled pair is effectively to take a simultaneous reading of both.
• Taking this further: Two entangled clocks are practically equivalent to a single clock with two displays. Time measurements taken using entangled clocks can be more precise than measurements from two separate, synchronized clocks.

Greater sensitivity in atomic clocks and accelerometers would lead to more precise timekeeping and navigation systems, such as those used in global positioning systems, in defense and in broadcast communications. Ultraprecise clocks are also used in finance and trading.

“GPS tells me where I am to about a meter right now,” Kasevich said. “But what if I wanted to know where I was to within 10 centimeters? That’s what the impact of better clocks would be.”

One can mark the passage of time by counting the number of pulses in an electromagnetic wave, just as you would count the ticks of a clock. If you know that a particular wave pulses 6 billion times per second, you know that, once you count 6 billion crests of the wave, one second has passed. So knowing the exact frequency of a microwave gives one a precise way to track time.

Interferometer sequence timing.

The entanglement: Rubidium atoms, trapped inside a cavity, are separated into two groups of about 100,000 atoms each. The groups sit between two mirrors. Light is made to bounce back and forth between the mirrors, tracing its way through the groups of atoms with every shot. The ricocheting light entangles them.

The sensing: A microwave ripples through the two groups of atoms. The atoms that happen to resonate with the microwave’s particular frequency respond by changing to a different state, like the wine glass that vibrates when a soprano hits just the right note. Similarly, when a particular acceleration is applied to the atomic groups, some fraction of the atoms in each group responds by changing state.

The measurement: The two entangled atomic groups behave like two faces of a single clock, or two readings of one accelerometer. The research team measured the number of atoms that changed state — the ones that vibrated like a wine glass — in each group. Then they used the numbers to calculate the difference in the microwave frequencies applied to the two groups, and therefore the difference in the groups’ readings of time or acceleration.

Increased precision: The Kasevich team found that entanglement improves the precision in the frequency or acceleration difference read by the displays.

In their setup, the measurement of time in two locations was 3.5 times more precise when the clocks were entangled than if they were operating independently. For acceleration, the measurement was 1.2 times more precise with entanglement.

“If you want to know how long something takes, you might look at one clock as a starting point and then run to another room to look at another clock, the end point,” Kasevich said. “Our method exploits the entanglement principle to make that comparison as precise as possible.”

The researchers also successfully networked four groups of atoms in four separate locations using this configuration. In the team’s experiment, the two groups of atoms were separated by about 20 micrometers, close to the average width of a human hair. Their work means that time or acceleration can be compared, with unprecedented sensitivity, between four separate, albeit close-together, locations.

“In the future, we want to push them out to longer distances. The world wants clocks whose time can be compared. It’s the same with accelerometers. There are sensing configurations where you want to be able to read out the difference in the acceleration of one group with respect to another. We were able to show how to do that,” Kasevich said.

“This is a tour de force result from Mark and his team,” said Q-NEXT Deputy Director JoAnne Hewett, who is also the SLAC National Accelerator Laboratory associate director of fundamental physics and chief research officer as well as a Stanford professor of particle physics and astrophysics. “This means we can harness entanglement to develop sensors that are far more powerful than those we use today. We are another step closer to wielding quantum phenomena to improve our everyday lives.”

Quantum-Compute Algorithm for Exact Laser-Driven Electron Dynamics in Molecules

by Fabian Langkabel, Annika Bande in Journal of Chemical Theory and Computation

Quantum computers promise significantly shorter computing times for complex problems. But there are still only a few quantum computers worldwide with a limited number of so-called qubits. However, quantum computer algorithms can already run on conventional servers that simulate a quantum computer. A team at HZB has succeeded to calculate the electron orbitals and their dynamic development on the example of a small molecule after a laser pulse excitation. In principle, the method is also suitable for investigating larger molecules that cannot be calculated using conventional methods.

“These quantum computer algorithms were originally developed in a completely different context. We used them here for the first time to calculate electron densities of molecules, in particular also their dynamic evolution after excitation by a light pulse,” says Annika Bande, who heads a group on theoretical chemistry at HZB. Together with Fabian Langkabel, who is doing his doctorate with Bande, she has now shown in a study how well this works.

The calculations allow the electron densities and the changes after excitation to be determined with high spatial and temporal resolution. Here, the example of the lithium hydride molecule shows the shift of electron density from cyanide (red) to lithium (green) during a laser pulse.

“We developed an algorithm for a fictitious, completely error-free quantum computer and ran it on a classical server simulating a quantum computer of ten Qbits,” says Fabian Langkabel. The scientists limited their study to smaller molecules in order to be able to perform the calculations without a real quantum computer and to compare them with conventional calculations.

Indeed, the quantum algorithms produced the expected results. In contrast to conventional calculations, however, the quantum algorithms are also suitable for calculating significantly larger molecules with future quantum computers: “This has to do with the calculation times. They increase with the number of atoms that make up the molecule,” says Langkabel. While the computing time multiplies with each additional atom for conventional methods, this is not the case for quantum algorithms, which makes them much faster.

The study thus shows a new way to calculate electron densities and their “response” to excitations with light in advance with very high spatial and temporal resolution. This makes it possible, for example, to simulate and understand ultrafast decay processes, which are also crucial in quantum computers made of so-called quantum dots. Also predictions about the physical or chemical behaviour of molecules are possible, for example during the absorption of light and the subsequent transfer of electrical charges. This could facilitate the development of photocatalysts for the production of green hydrogen with sunlight or help to understand processes in the light-sensitive receptor molecules in the eye.

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