From Niels Bohr Institute [Niels Bohr Institutet] (DK): “New invention keeps qubits of light stable at room temperature”

From Niels Bohr Institute [Niels Bohr Institutet] (DK)

at

University of Copenhagen [Københavns Universitet] [UCPH] (DK)

17 June 2021

Eugene Simon Polzik, Professor
The Niels Bohr Institute
University of Copenhagen
+45 23 38 20 45
polzik@nbi.ku.dk

Ida Eriksen, Journalist
Faculty of Science
University of Copenhagen
+4593516002
ier@science.ku.dk

QUANTUM RESEARCH-Researchers from University of Copenhagen have developed a new technique that keeps quantum bits of light stable at room temperature instead of only working at -270 degrees. Their discovery saves power and money and is a breakthrough in quantum research.

Photo: Eugene Simon Polzik.

As almost all our private information is digitalized, it is increasingly important that we find ways to protect our data and ourselves from being hacked. Quantum Cryptography is the researchers’ answer to this problem, and more specifically a certain kind of qubit – consisting of single photons: particles of light.

Single photons or qubits of light as they are also called are extremely difficult to hack. However, in order for these qubits of light to be stable and work properly they need to be stored at temperatures close to absolute zero – that is minus 270 C – something that requires huge amounts of power and resources.

Yet in a recently published study [Nature Communications], researchers from University of Copenhagen, demonstrate a new way to store these qubits at room temperature for a hundred times longer than ever shown before.

“We have developed a special coating for our memory chips that helps the quantum bits of light to be identical and stable while being in room temperature. In addition, our new method enables us to store the qubits for a much longer time, which is milliseconds instead of microseconds – something that has not been possible before. We are really excited about it,” says Eugene Simon Polzik, professor in quantum optics at the Niels Bohr Institute.

The special coating of the memory chips makes it much easier to store the qubits of light without big freezers, which are troublesome to operate and require a lot of power. Therefore, the new invention will be cheaper and more compatible with the demands of the industry in the future.

“The advantage of storing these qubits at room temperature is that it does not require liquid helium or complex laser-systems for cooling. Also it is a much more simple technology that can be implemented more easily in a future quantum internet,” says Karsten Dideriksen, a UCPH-PhD on the project.

Photo of the memory chip, protected in a glasscell. Credit: Eugene Simon Polzik.

A special coating keeps the qubits stable

Normally warm temperatures disturb the energy of each quantum bit of light.

“In our memory chips, thousands of atoms are flying around emitting photons also known as qubits of light. When the atoms are exposed to heat, they start moving faster and collide with one another and with the walls of the chip. This leads them to emit photons that are very different from each other. But we need them to be exactly the same in order to use them for safe communication in the future,” explains Eugene Polzik and adds:

“That is why we have developed a method that protects the atomic memory with the special coating for the inside of the memory chips. The coating consists of paraffin that has a wax like structure and it works by softening the collision of the atoms, making the emitted photons or qubits identical and stable. Also we used special filters to make sure that only identical photons were extracted from the memory chips”.

Even though the new discovery is a breakthrough in quantum research it stills needs more work.

“Right now we produce the qubits of light at a low rate – one photon per second, while cooled systems can produce millions in the same amount of time. But we believe there are important advantages to this new technology and that we can overcome this challenge in time,” Eugene concludes.

See the full article here .

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Niels Bohr Institutet (DK) is a research institute of the Københavns Universitet [UCPH] (DK). The research of the institute spans astronomy, geophysics, nanotechnology, particle physics, quantum mechanics and biophysics.

The Institute was founded in 1921, as the Institute for Theoretical Physics of the Københavns Universitet [UCPH] (DK), by the Danish theoretical physicist Niels Bohr, who had been on the staff of the University of Copenhagen since 1914, and who had been lobbying for its creation since his appointment as professor in 1916. On the 80th anniversary of Niels Bohr’s birth – October 7, 1965 – the Institute officially became The Niels Bohr Institutet (DK). Much of its original funding came from the charitable foundation of the Carlsberg brewery, and later from the Rockefeller Foundation.

During the 1920s, and 1930s, the Institute was the center of the developing disciplines of atomic physics and quantum physics. Physicists from across Europe (and sometimes further abroad) often visited the Institute to confer with Bohr on new theories and discoveries. The Copenhagen interpretation of quantum mechanics is named after work done at the Institute during this time.

On January 1, 1993 the institute was fused with the Astronomic Observatory, the Ørsted Laboratory and the Geophysical Institute. The new resulting institute retained the name Niels Bohr Institutet (DK)).

Københavns Universitet (UCPH) (DK) is the oldest university and research institution in Denmark. Founded in 1479 as a studium generale, it is the second oldest institution for higher education in Scandinavia after Uppsala University [ Uppsala universitet] (SE) (1477). The university has 23,473 undergraduate students, 17,398 postgraduate students, 2,968 doctoral students and over 9,000 employees. The university has four campuses located in and around Copenhagen, with the headquarters located in central Copenhagen. Most courses are taught in Danish; however, many courses are also offered in English and a few in German. The university has several thousands of foreign students, about half of whom come from Nordic countries.

The university is a member of the International Alliance of Research Universities (IARU), along with University of Cambridge (UK), Yale University (US), The Australian National University (AU), and University of California-Berkeley (US), amongst others. The 2016 Academic Ranking of World Universities ranks the University of Copenhagen as the best university in Scandinavia and 30th in the world, the 2016-2017 Times Higher Education World University Rankings as 120th in the world, and the 2016-2017 QS World University Rankings as 68th in the world. The university has had 9 alumni become Nobel laureates and has produced one Turing Award recipient.

From MIT: “Transforming quantum computing’s promise into practice” William Oliver

From MIT News

January 19, 2021
Daniel Ackerman

Electrical engineer William Oliver develops technology to enable reliable quantum computing at scale.

MIT electrical engineer William D. Oliver develops the fundamental technology to enable reliable quantum computers at scale.
Credit: Adam Glanzman.

It was music that sparked William Oliver’s lifelong passion for computers.

Growing up in the Finger Lakes region of New York, he was an avid keyboard player. “But I got into music school on voice,” says Oliver, “because it was a little bit easier.”

But once in school, first at State University of New York at Fredonia then the University of Rochester, he hardly shied away from a challenge. “I was studying sound recording technology, which led me to digital signal processing,” explains Oliver. “And that led me to computers.” Twenty-five years later, he’s still stuck on them.

Oliver, a recently tenured associate professor in MIT’s Department of Electrical Engineering and Computer Science, is building a new class of computer — the quantum computer — with the potential to radically improve how we process information and simulate complex systems. Quantum computing is still in its early days, and Oliver aims to help usher the field out of the laboratory and into the real world. “Our mission is to build the fundamental technologies that are necessary to scale up quantum computing,” he says.

Coast to coast and back again

Oliver’s first stop at MIT was as a master’s student in the Media Lab with adviser Tod Machover. Their interactive Brain Opera project paired Oliver’s love for both music and computing. Oliver orchestrated users’ voices with a computer-generated “angelic arpeggiation of strings and a chorus.” The project was installed at the Haus der Musik museum in Vienna. “It was a fantastic master’s project. I really loved it,” says Oliver. “But the question was ‘okay, what do I do next?’”

Eager for a new challenge, Oliver chose to explore more fundamental research. “I found quantum mechanics to be really puzzling and interesting,” says Oliver. So he traveled to Stanford University to earn a PhD studying quantum optics using free electrons. “I feel very fortunate that I could do those experiments, which have almost no practical application, but that allowed me to think really deeply about quantum mechanics,” he says.

Oliver’s timing was fortunate too. He was delving into quantum mechanics just as the field of quantum computing was emerging. A classical computer, like the one you’re using to read this story, stores information in binary bits, each of which holds a value of 0 or 1. In contrast, a quantum computer stores information in qubits, each of which can hold a 0, 1, or any simultaneous combination of 0 and 1, thanks to a quantum mechanical phenomenon called superposition. That means quantum computers can process information far faster than classical computers, in some cases completing tasks in minutes where a classical computer would take millennia — at least in theory. When Oliver was completing his PhD, quantum computing was a field in its infancy, more idea than reality. But Oliver grasped the potential of quantum computing, so he returned to MIT to help it grow.

The qubit quandary

Quantum computers are frustratingly inconsistent. That’s in part because those qubit superposition states are fragile. In a process called decoherence, qubits can err and lose their quantum information from the slightest disturbance or material defect. In 2003, Oliver took a staff position at MIT’s Lincoln Laboratory to help solve problems like decoherence. His goal, with colleagues Terry Orlando, Leonya Levitov, and Seth Lloyd, was to engineer reliable quantum computing systems that can be scaled up for practical use. “Quantum computing is transitioning from scientific curiosity to technical reality,” says Oliver. “We know that it works at small scale. And we’re now trying to increase the size of the systems so we can do problems that are actually meaningful.”

Even background levels of radiation can trigger decoherence in mere milliseconds. In a recent Nature paper, Oliver and his colleagues, including professor of physics Joe Formaggio, described this problem and proposed ways to shelter qubits from damaging radiation, like shielding them with lead.

He is quick to emphasize the role of collaboration in solving these complex challenges. “Engineering these quantum systems into useful, larger scale machines is going to require almost every department at the Institute,” says Oliver. In his own research, he builds qubits from electrical circuits in aluminum that are supercooled to just a smidge warmer than absolute zero. At that temperature, the system loses electrical resistance and can be used as an anharmonic oscillator that stores quantum information. Engineering such an intricate system to reliably process information means “we need to bring in a lot of people with their own talents,” says Oliver.

“For example, materials scientists will have a lot to say about the materials and the defects on the surfaces,” he adds. “Electrical engineers will have something to say about how to fabricate and control the qubits. Computer scientists and applied mathematicians will have something to say about the algorithms. Chemists and biologists know the hard problems to solve. And so on.” When he first joined Lincoln Laboratory, Oliver says just two Lincoln staff were focused on quantum technologies. That number now exceeds 100.

In 2015, Oliver founded the Engineering Quantum Systems (EQuS) group to focus specifically on superconducting qubit technology. He is also a Lincoln Laboratory Fellow, director of MIT’s Center for Quantum Engineering, and associate director of the Research Laboratory of Electronics.

A quantum future

Oliver envisions a steadily growing role for quantum computing. Already, Google has demonstrated that for a particular task, a 53-qubit quantum computer can far outpace even the world’s largest supercomputer, which features quadrillions of transistors. “That was like the flight at Kitty Hawk,” says Oliver. “It got off the ground.”

Google quantum computer.

In the near-term, Oliver thinks quantum and classical computers could work as partners. The classical machine would churn through an algorithm, dispatching specific calculations for the quantum computer to run before its qubits decohere. In the longer term, Oliver says that error-correcting codes could enable quantum computers to function indefinitely, even as some individual components remain faulty. “And that’s when quantum computers will basically be universal,” says Oliver. “They’ll be able to run any quantum algorithm at large scale.” That could enable vastly improved simulations of complex systems in fields like molecular biology, quantum chemistry, and climatology.

Oliver will continue to push quantum computing toward that reality. “There are real accomplishments that have been happening,” he says. “At the same time, on the theoretical side, there are real problems we could solve if we just had a quantum computer big enough.” While focused on his mission to scale up quantum computing, Oliver hasn’t lost his passion for music. Although, he says he rarely sings these days: “Only in the shower.”

See the full article here .

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The mission of MIT is to advance knowledge and educate students in science, technology, and other areas of scholarship that will best serve the nation and the world in the twenty-first century. We seek to develop in each member of the MIT community the ability and passion to work wisely, creatively, and effectively for the betterment of humankind.

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From phys.org: “Metasurface enabled quantum edge detection”

From phys.org

December 29, 2020
Thamarasee Jeewandara

The schematics of a metasurface enabled quantum edge detection. (A) The metasurface is designed to perform edge detection for a preferred linear polarization. |V〉, i.e., polarization state is orthogonal to the analyzer. The dashed light red line stands for the electrical path. The question mark means that polarization selection of idler photons of the heralding arm is unknown. If the Schrödinger’s cat is illuminated by unknown linear polarization photons from the polarization entangled source, the image would be a superposition of a regular “solid cat” and an edge-enhanced “outlined cat.” (B) The switch state ON or OFF of the heralding arm. When the idler photons of the heralding arm are projected to |H〉, it indicates the switch OFF state and leads to a solid cat captured. While the heralded photons are projected to |V〉, an edge-enhanced outlined cat is obtained with the switch ON state. (C and D) The calculated and experimental results of a solid cat, respectively. (E and F) The calculated and experimental results of the edge-enhanced outlined cat, respectively. Credit: Science Advances.

Metasurfaces provide unique platforms to realize exotic phenomena including negative refraction, achromatic focusing, and electromagnetic cloaking due to the engineered dielectric or metallic architectures. The intersection of metasurfaces and quantum optics can lead to significant opportunities that remain to be explored. In a new report now published on Science Advances, Junxiao Zhou, Shikai Liu and a research team in quantum information, nano-optoelectronic devices and computer engineering in China and the U.S. proposed and demonstrated a polarization-entangled photon source. They used the source to switch the optical edge mode in an imaging system to ON or OFF states based on a highly dielectric metasurface. The experiment enriched the fields of quantum optics and metamaterials as a promising direction toward quantum edge detection and image processing with a remarkable signal-to-noise ratio.

Combining quantum entanglement and edge detection

Photonic metasurfaces are two-dimensional (2-D) ultrathin arrays of engineered metallic or dielectric structures that can facilitate electromagnetic field manipulation of the local phase, amplitude and polarization. Researchers generally develop such capabilities for a variety of applications in classical optics. Quantum entanglement is essential in quantum optics for many applications including quantum cryptography, teleportation, superresolving metrology and quantum imaging. Recent efforts show a trend to combine the metasurface with entangled photons for potential applications in quantum optics. Edge detection is another factor that contributes to image processing to define the boundaries between regions in an image. It is a basic tool in computer vision to pre-process automations in medical imaging and forms a critical component of autonomous vehicles. Metasurface-enabled edge detection can be used in quantum optics to offer possibilities of remote-controlled image processing and cryptography. In this work, Zhou et al. therefore realized a polarization-entangled photon source and high-efficiency metasurface enabled switchable optical edge detection method. The combined strategy showed a high signal-to-noise (SNR) ratio at the same photon flux level (the number of photons per second per unit area).

Experimental setup and sample characterization. (A) Experimental setup of metasurface enabled quantum edge detection. BDM, broadband dielectric mirror; PBS, polarization beam splitter; DM, dichromatic mirror; FC, fiber coupler; BPF, band-pass filter; ICCD, intensified charge coupled device. By pumping a nonlinear crystal (type II phase-matched bulk PPKTP crystal) with a 405-nm laser, pairs of orthogonally polarized photons with 810-nm wavelength are generated through the spontaneously parametric down-conversion process. The blue (red) light path presents the 405-nm (810 nm) light. Edge detection switch is on the heralding arm. An edge detection imaging system is on the imaging arm. (B) Photograph of the partial metasurface sample. Scale bar, 4 mm. (C) Polariscopic analysis characterized by crossed linear polarizers of the sample area marked in 2a. The blue bars indicate the orientation of rotated nanostructures in one period, which represents the Pancharatnam-Berry phase induced by the laser writing dielectric metasurface. Scale bar, 50 μm. (D) The scanning electron microscopy image of the sample area marked in (C). Scale bar, 1 μm. Photo credit: Junxiao Zhou, University of California, San Diego. Credit: Science Advances [above].

Using the “Schrödinger’s cat” concept

Zhou et al. used the Schrödinger’s cat concept to illustrate the expected performance of the switchable quantum edge detection scheme. They reviewed the basic principle of edge detection based on classical continuous wave (CW) light-illumination. In the experimental setup, the edge detection imaging arm was independent of the entangled source and the heralding arm, as well as the coincidence measurement components. When the incident photons achieved a horizontal polarization state, the beam of illuminated light passed through a cat-shaped aperture and an engineered metasurface to separate into a left- and right-handed overlapped polarized image with a horizontal shift. The overlapped components then passed through a horizontally oriented analyzer to form a ‘solid cat’ image. If, however, the incident photons were vertically polarized, the overlapped components recombined to a linear polarized component that is completely blocked by the analyzer to only form an outline of a cat. The researchers therefore used polarization-entangled photons as a source of illumination to develop quantum switchable edge detection in this way.

The experimental setup and polarization-entangled photon pairs

Characterizations of the entangled source. (A) Coincidence counts as a function of the HWP angle θ2 at one output port in 2 s. The red (blue) color of count data and interference corresponds to horizontal (diagonal) projection bases. The solid lines are sinusoidal fits to the data, error bars are estimated by assuming Poisson photon statistics in photon counting. Error bars are obtained from multiple measurements. (B and C) The real and imaginary parts of the reconstructed density matrix ρ of the two-photon states, respectively. Credit: Science Advances [above].

The researchers generated polarization entangled photons using a spontaneous parametric down-conversion process in a 20-mm-long type II phase-matched periodically poled potassium titanyl phosphate (KTiOPO4/PPKTP) crystal embedded in a Sagnac interferometer. They set the temperature of the crystal to 17 degrees Celsius and used two broadband dielectric mirrors and a dual-wavelength polarization beam splitter to form the self-stable Sagnac interferometer. They then used a continuous wave single-frequency diode laser at 405 nm to generate the pump beam focused by a pair of lenses with optimized focal lengths to attain a beam waist approximating 40 microns at the center of the crystal. To balance the power in the clockwise and counter-clockwise-directions, Zhou et al. used a quarter-wave plate (QWP) and a half-wave plate (HWP) in front to the Sagnac loop.

Using a dual-wavelength polarization beam splitter, they separated the down-converted photon pairs pumped by two counter-propagating beams, to send one into the imaging arm and the other to heralding arms, respectively. Zhou et al. also designed the metasurface employed in the setup using the Pancharatnam-Berry phase and fabricated it by scanning a femtosecond pulse laser within a silica slab. Then using scanning electron microscopy, they observed self-assembled nanostructures in the silica slab and showed their origin under intense laser irradiation to generate the metasurface. The team briefly described the quantum state preparation for the polarization entangled degenerate photon pairs generated from the Signac loop. They used the Bell state (the simplest example of nonseparable quantum entanglement) for this work by adjusting the experimental setup. Zhou et al. quantified the entanglement quality of the two-photon state using quantum tomography and reconstructed two-photon density matrix measurements.

The switchable edge detection demonstration. (A to D) The metasurface sample orientation, which is aligned with the xy plane. The inset yellow arrows indicate the phase gradient direction of the metasurface. (E to H) The images of the whole object comprising the separated LCP and RCP components, which is the OFF state of the edge detection mode. (I to L) The images reveal edges along different directions, which is the ON state of the edge detection mode. Photo credit: Junxiao Zhou, University of California, San Diego. Credit: Science Advances [above].

Quantum-entanglement enabled quantum edge detection

After confirming the quality of generated polarization-entangled photon pairs, they demonstrated switchable quantum edge detection. To accomplish this, they prepared the photons in horizontal or vertical linear polarizations states using the setup and coupled the photons into the fiber and sent them to the edge detection image system to capture the final alternative image via an intensified charge-coupled device camera (ICCD). For instance, Zhou et al. obtained two overlapped images with a tiny shift, where the shift direction aligned with the phase gradient direction of the metasurface. When they increased the period of the metasurface structure, they decreased the shift between the two overlapped images to achieve high-resolution edge detection. The quantum edge detection scheme had another advantage due to its high signal-to-noise (SNR) ratio, where the team could significantly reduce the ambient noise in the setup, where noise only accumulated in a very short timeframe. By contrast, in classical optics, the noise would continue to accumulate. As proof of concept, they acquired an edge image with remarkable SNR for improved entanglement-enabled experimental quantum edge detection.

Outlook

In this way, Junxiao Zhou, Shikai Liu and colleagues combined quantum entanglement-enabled quantum edge detection using a metasurface filter combined with a polarization-entangled source. The metasurfaces provided ultrathin and lightweight optical elements with precisely engineered phase profiles to obtain a variety of functions to form a more compact and integrated system. The setup will assist the conception of security applications including image encryption and steganography. The method also offers an appealing signal-to-noise (SNR) ratio suited for a variety of photon-hungry imaging and sensing applications in biomedicine, including tracking enzymatic reactions and observing living organisms or photosensitive cells.

More information:

Flat optics with designer metasurfaces
Nature Materials

Experimental quantum teleportation
Nature

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From The Optical Society via phys.org: “Researchers demonstrate nondestructive mid-infrared imaging using entangled photons”

From The Optical Society

via

phys.org

December 10, 2020

Researchers used entangled photons to increase the penetration depth of OCT for scattering materials. They demonstrated the technique by analyzing two alumina ceramic stacks containing laser-milled microchannels. The mid-infrared illumination allowed the researchers to capture depth information and to create a full 3D reconstruction of the channel structures (pictured). Credit: Aron Vanselow and Sven Ramelow, Humboldt-Universität zu Berlin.

Researchers have shown that entangled photons can be used to improve the penetration depth of optical coherence tomography (OCT) in highly scattering materials. The method represents a way to perform OCT with mid-infrared wavelengths and could be useful for non-destructive testing and analysis of materials such as ceramics and paint samples.

OCT is a nondestructive imaging method that provides detailed 3-D images of subsurface structures. OCT is typically performed using visible or near-infrared wavelengths because light sources and detectors for these wavelengths are readily available. However, these wavelengths don’t penetrate very deeply into highly scattering or very porous materials.

In Optica, The Optical Society’s (OSA) journal for high-impact research, Aron Vanselow and colleagues from Humboldt-Universität zu Berlin (DE), together with collaborators at the Research Center for Non-Destructive Testing GmbH in Austria, demonstrate a proof-of-concept experiment for mid-infrared OCT based on ultra-broadband entangled photon pairs. They show that this approach can produce high quality 2-D and 3-D images of highly scattering samples using a relatively compact, straightforward optical setup.

“Our method eliminates the need for broadband mid-infrared sources or detectors, which have made it challenging to develop practical OCT systems that work at these wavelengths,” said Vanselow. “It represents one of the first real-world applications in which entangled photons are competitive with conventional technology.”

The technique could be useful for many applications including analyzing the complex paint layers used on airplanes and cars or monitoring the coatings used on pharmaceuticals. It can also provide detailed 3-D images that would be useful for art conservation.

Tapping into quantum mechanics

When photons are entangled, they behave as if they can instantly affect each other. This quantum mechanical phenomenon is essential to many quantum technology applications under development, such as quantum sensing, quantum communications or quantum computing.

For this technique, the researchers developed and patented a nonlinear crystal that creates broadband photon pairs with very different wavelengths. One of the photons has a wavelength that can be easily detected with standard equipment while the other photon is in the mid-infrared range, making it difficult to detect. When the hard-to-detect photons illuminate a sample, they change the signal in a way that can be measured using only the easy-to-detect photons.

“Our technique makes it easy to acquire useful measurements at what is a traditionally hard-to-handle wavelength range due to technology challenges,” said Sven Ramelow, who conceived and guided the research. “Moreover, the lasers and optics we used are not complex and are also more compact, robust and cost-effective than those used in current mid-infrared OCT systems.”

Imaging with less light

To demonstrate the technique, the researchers first confirmed that the performance of their optical setup matched theoretical predictions. They found that they could use six orders of magnitude less light to achieve the same signal-to-noise ratio as the few conventional mid-infrared OCT systems that have been recently developed.

“We were positively surprised that we did not see any noise in the measurements beyond the intrinsic quantum noise of the light itself,” said Ramelow. “This also explained why we can achieve a good signal-to-noise ratio with so little light.”

The researchers tested their setup on a range of real-world samples, including highly scattering paint samples. They also analyzed two 900-micron thick alumina ceramic stacks containing laser-milled microchannels. The mid-infrared illumination allowed the researchers to capture depth information and to create a full 3-D reconstruction of the channel structures. The pores in alumina ceramics make this material useful for drug testing and DNA detection but also highly scattering at the wavelengths traditionally used for OCT.

The researchers have already begun to engage with partners from industry and other research institutes to develop a compact OCT sensor head and full system for a pilot commercial application.

See the full article here .

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The Optical Society (OSA) is a professional association of individuals and companies with an interest in optics and photonics. It publishes journals, and organizes conferences and exhibitions. In 2019 it had about 22,000 members in more than 100 different countries, including some 300 companies

From University of Rochester: “Building a quantum network one node at a time”

From University of Rochester

November 3, 2020
Bob Marcotte
bmarcotte@ur.rochester.edu

This illustration of a nanoscale node created by the lab of Nick Vamivakas, professor of quantum optics and quantum physics, shows a closeup of one of an array pillars, each a mere 120 nanometers high. Each pillar serves as a location marker for a quantum state that can interact with photons. A novel alignment of tungsten diselenide (WSe2) is draped over the pillars with an underlying, highly reactive layer of chromium triiodide (CrI3). Where the atomically thin, 12-micron area layers touch, the CrI3 imparts an electric charge to the WSe2, creating a “hole” alongside each of the pillars. Credit: University of Rochester Michael Osadciw.

New research demonstrates a way to use quantum properties of light to transmit information, a key step on the path to the next generation of computing and communications systems.

Researchers at the University of Rochester and Cornell University have taken an important step toward developing a communications network that exchanges information across long distances by using photons, mass-less measures of light that are key elements of quantum computing and quantum communications systems.

The research team has designed a nanoscale node made out of magnetic and semiconducting materials that could interact with other nodes, using laser light to emit and accept photons.

The development of such a quantum network—designed to take advantage of the physical properties of light and matter characterized by quantum mechanics—promises faster, more efficient ways to communicate, compute, and detect objects and materials as compared to networks currently used for computing and communications.

Described in the journal Nature Communications, the node consists of an array of pillars a mere 120 nanometers high. The pillars are part of a platform containing atomically thin layers of semiconductor and magnetic materials.

The array is engineered so that each pillar serves as a location marker for a quantum state that can interact with photons and the associated photons can potentially interact with other locations across the device—and with similar arrays at other locations. This potential to connect quantum nodes across a remote network capitalizes on the concept of entanglement, a phenomenon of quantum mechanics that, at its very basic level, describes how the properties of particles are connected at the subatomic level.

“This is the beginnings of having a kind of register, if you like, where different spatial locations can store information and interact with photons,” says Nick Vamivakas, professor of quantum optics and quantum physics at Rochester.

Toward ‘miniaturizing a quantum computer’

The project builds on work the Vamivakas Lab has conducted in recent years using tungsten diselenide (WSe2) in so-called Van der Waals heterostructures. That work uses layers of atomically thin materials on top of each other to create or capture single photons.

The new device uses a novel alignment of WSe2 draped over the pillars with an underlying, highly reactive layer of chromium triiodide (CrI3). Where the atomically thin, 12-micron area layers touch, the CrI3 imparts an electric charge to the WSe2, creating a “hole” alongside each of the pillars.

In quantum physics, a hole is characterized by the absence of an electron. Each positively charged hole also has a binary north/south magnetic property associated with it, so that each is also a nanomagnet

When the device is bathed in laser light, further reactions occur, turning the nanomagnets into individual optically active spin arrays that emit and interact with photons. Whereas classical information processing deals in bits that have values of either 0 or 1, spin states can encode both 0 and 1 at the same time, expanding the possibilities for information processing.

“Being able to control hole spin orientation using ultrathin and 12-micron large CrI3, replaces the need for using external magnetic fields from gigantic magnetic coils akin to those used in MRI systems,“ says lead author and graduate student Arunabh Mukherjee. “This will go a long way in miniaturizing a quantum computer based on single hole spins. “

Still to come: Entanglement at a distance?

Two major challenges confronted the researchers in creating the device.

One was creating an inert environment in which to work with the highly reactive CrI3. This was where the collaboration with Cornell University came into play. “They have a lot of expertise with the chromium triiodide and since we were working with that for the first time, we coordinated with them on that aspect of it,” Vamivakas says. For example, fabrication of the CrI3 was done in nitrogen-filled glove boxes to avoid oxygen and moisture degradation.

The other challenge was determining just the right configuration of pillars to ensure that the holes and spin valleys associated with each pillar could be properly registered to eventually link to other nodes.

And therein lies the next major challenge: finding a way to send photons long distances through an optical fiber to other nodes, while preserving their properties of entanglement.

“We haven’t yet engineered the device to promote that kind of behavior,” Vamivakas says. “That’s down the road.”

In addition to Vamivakas and Mukherjee, other coauthors of the paper include lead authors Kamran Shayan of Vamivakas’ lab and Lizhong Li, Jie Shan, and Kin Fai Mak at Cornell University.

The National Science Foundation, the Air Force Office of Scientific Research, and the Department of Energy supported the project with funding.

See the full article here .

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U Rochester

The University of Rochester is one of the country’s top-tier research universities. Our 158 buildings house more than 200 academic majors, more than 2,000 faculty and instructional staff, and some 10,500 students—approximately half of whom are women.

Learning at the University of Rochester is also on a very personal scale. Rochester remains one of the smallest and most collegiate among top research universities, with smaller classes, a low 10:1 student to teacher ratio, and increased interactions with faculty.

From Niels Bohr Institute DK: “Photon turnstile brings order to light”

University of Copenhagen DK

From Niels Bohr Institute DK

1 October 2020
Anders Søndberg Sørensen, Professor
anders.sorensen@nbi.ku.dk

Light particles pass through a glass fiber and meet a cloud of atoms. Like a turnstile, the atoms ensure that light particles only pass through one by one. Photo: Humboldt University.

QUANTUM OPTICS: With the creation of a turnstile for light in glass fibers, quantum optics researchers from Germany, Denmark and Austria have succeeded in directly converting laser light in optical fibers into a single file of isolated photons. According to Anders Søndberg Sørensen from the Niels Bohr Institute at the University of Copenhagen, who was involved in the theoretical phase of the experiment, this creation of isolated photons can prove essential in the exploration of quantum communication. The results of the experiment are published in Nature Photonics this week.

Physicists have long studied the interaction of light and matter and the way in which light particles, so called photons, are affected when they meet clouds of atoms. Quantum optics researchers are particularly interested in this because it can help them find more secure ways to process information, for example by sending information in the form of single photons.

Until now, the challenge has been how to ‘feed’ emitted photons in a glass fiber in such a way that they come out in an sorted manner, one after the other. “This is crucial in making quantum technologies where we encode information in individual photons and atoms”, explains Professor Anders Søndberg Sørensen, leader of the Theoretical Quantum Optics groups at the Niels Bohr Institute at Copenhagen University. To send encoded information in an undistorted way, you need to be able to send the photons in an isolated way. “If you can do that, you work towards dramatic new ways of processing information. Single photons can be for instance be used to send encrypted messages which cannot be eavesdropped” Søndberg Sørensen adds.

150 is the magic number

In their experiment, the researchers explored how many atoms a photon should meet for it to come out isolated at the other end by precisely controlling the number of atoms along the laser beam in the glass fiber. The proposal for the experiment came from Søndberg Sørensen and theoretical physicists at the Leibniz University Hannover. The research group of Prof. Dr. Arno Rauschenbeutel at Humboldt University of Berlin then carried out the experiment using a powerful atom-light interface in which atoms are trapped near a so-called optical nanofiber, which is one hundred times thinner than a human hair.

With the use of tweezers of laser lights, the atoms were held in place at precisely 0.2 micrometers from the glass fiber surface, while laser lights were cooling the atoms down to a temperature of a few millionths of a degree above absolute zero. The researchers found that when there were about 150 atoms trapped near the nanofiber, the photons would come out one by one. If they would use less atoms, the photons would be unaffected by the atoms; if they would use more, the photons would come out in pairs.

An unexpected result

Søndberg Sørensen is excited about the results of the experiment. Not only was it unexpected that the researchers found the exact interval that leads to the transmission of single photons, but also that it was possible to make it work on weakly coupled atoms. “The beauty of this interface is that it’s fairly simple and that it works with weakly coupled atoms, which means it could also be applied to for example x-rays in the future”, Søndberg Sørensen explains.

What this could mean for the future is an open question. “Such sources have never been available before, so we do not yet understand the full range of applications for them,” says Søndberg Sørensen. “But potentially, they could be used for ultra-precise sensing and allow for much broader exploration of quantum technologies.”

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Niels Bohr Institute DK (Danish: Niels Bohr Institutet) is a research institute of the University of Copenhagen DK. The research of the institute spans astronomy, geophysics, nanotechnology, particle physics, quantum mechanics and biophysics.

The Institute was founded in 1921, as the Institute for Theoretical Physics of the University of Copenhagen DK, by the Danish theoretical physicist Niels Bohr, who had been on the staff of the University of Copenhagen since 1914, and who had been lobbying for its creation since his appointment as professor in 1916. On the 80th anniversary of Niels Bohr’s birth – October 7, 1965 – the Institute officially became The Niels Bohr Institute. Much of its original funding came from the charitable foundation of the Carlsberg brewery, and later from the Rockefeller Foundation.

During the 1920s, and 1930s, the Institute was the center of the developing disciplines of atomic physics and quantum physics. Physicists from across Europe (and sometimes further abroad) often visited the Institute to confer with Bohr on new theories and discoveries. The Copenhagen interpretation of quantum mechanics is named after work done at the Institute during this time.

On January 1, 1993 the institute was fused with the Astronomic Observatory, the Ørsted Laboratory and the Geophysical Institute. The new resulting institute retained the name Niels Bohr Institute DK.

The University of Copenhagen (UCPH) DK (Danish: Københavns Universitet) is the oldest university and research institution in Denmark. Founded in 1479 as a studium generale, it is the second oldest institution for higher education in Scandinavia after Uppsala University (1477). The university has 23,473 undergraduate students, 17,398 postgraduate students, 2,968 doctoral students and over 9,000 employees. The university has four campuses located in and around Copenhagen, with the headquarters located in central Copenhagen. Most courses are taught in Danish; however, many courses are also offered in English and a few in German. The university has several thousands of foreign students, about half of whom come from Nordic countries.

The university is a member of the International Alliance of Research Universities (IARU), along with University of Cambridge, Yale University, The Australian National University, and UC Berkeley, amongst others. The 2016 Academic Ranking of World Universities ranks the University of Copenhagen as the best university in Scandinavia and 30th in the world, the 2016-2017 Times Higher Education World University Rankings as 120th in the world, and the 2016-2017 QS World University Rankings as 68th in the world. The university has had 9 alumni become Nobel laureates and has produced one Turing Award recipient.

From University of Waterloo: “Quantum researchers able to split one photon into three”

From University of Waterloo

February 27, 2020

Researchers from the Institute for Quantum Computing (IQC) at the University of Waterloo report the first occurrence of directly splitting one photon into three.

The occurrence, the first of its kind, used the spontaneous parametric down-conversion method (SPDC) in quantum optics and created what quantum optics researchers call a non-Gaussian state of light. A non-Gaussian state of light is considered a critical ingredient to gain a quantum advantage.

“It was understood that there were limits to the type of entanglement generated with the two-photon version, but these results form the basis of an exciting new paradigm of three-photon quantum optics,” said Chris Wilson, a principal investigator at IQC faculty member and a professor of Electrical and Computer Engineering at Waterloo.

“Given that this research brings us past the known ability to split one photon into two entangled daughter photons, we’re optimistic that we’ve opened up a new area of exploration.”

Lab of Chris Wilson

“The two-photon version has been a workhorse for quantum research for over 30 years,” said Wilson. “We think three photons will overcome the limits and will encourage further theoretical research and experimental applications and hopefully the development of optical quantum computing using superconducting units.”

Wilson used microwave photons to stretch the known limits of SPDC. The experimental implementation used a superconducting parametric resonator. The result clearly showed the strong correlation among three photons generated at different frequencies. Ongoing work aims to show that the photons are entangled.

“Non-Gaussian states and operations are a critical ingredient for obtaining the quantum advantage,” said Wilson. “They are very difficult to simulate and model classically, which has resulted in a dearth of theoretical work for this application.”

Science paper:
“Observation of Three-Photon Spontaneous Parametric Down-Conversion in a Superconducting Parametric Cavity”
Physical Review X

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