High-speed quantum memory for photons

Physicists from the University of Basel have developed a memory that can store photons. These quantum particles travel at the speed of light and are thus suitable for high-speed data transfer. The researchers were able to store them in an atomic vapor and read them out again later without altering their quantum mechanical properties too much. This memory technology is simple and fast and it could find application in a future quantum Internet. The journal Physical Review Letters has published the results.

Even today, fast data transfer in telecommunication networks employs short light pulses. Ultra broadband technology uses optical fiber links through which information can be transferred at the speed of light. At the receiver's end, the transmitted information has to be stored quickly and without errors so that it can be processed further electronically on computers. To avoid transmission errors, each bit of information is encoded in relatively strong light pulses that each contain at least several hundreds of photons.

For several years, researchers all over the world have been working on operating such networks with single photons. Encoding one bit per photon is not only very efficient, but it also allows for a radically new form of information processing based on the laws of quantum physics. These laws allow a single photon to encode not only the states 0 or 1 of a classic bit, but also to encode a superposition of both states at the same time. Such quantum bits are the basis for quantum information processing that could make unconditionally secure communication and super fast quantum computers possible in the future. The ability to store and retrieve single photons from a quantum memory is a key element for these technologies, which is intensively investigated.

Simple and fast

A team of physicists led by the professors Philipp Treutlein and Richard Warburton from the University of Basel has now developed a particularly simple and fast quantum memory that stores photons in a gas of rubidium atoms. A laser controls the storage and retrieval processes. The technology used does not require cooling devices or complicated vacuum equipment and can be implemented in a highly compact setup. The researchers were also able to verify that the memory has a very low noise level and is suitable for single photons.

One step closer to the quantum internet

"The combination of a simple setup, high bandwidth and low noise level is very promising for future application in quantum networks," says Janik Wolters, first author of the study. The development of such quantum networks is one of the goals of the National Center of Competence in Quantum Science and Technology (NCCR QSIT) and of the EU Framework Programme for Research and Innovation that have funded this study. In the future, quantum networks could lead to unconditionally secure communication, the networking of different quantum computers and the simulation of complex physical, chemical and biological systems.

Simple Atomic Quantum Memory Suitable for Semiconductor Quantum Dot Single Photons
Janik Wolters, Gianni Buser, Andrew Horsley, Lucas Béguin, Andreas Jöckel, Jan-Philipp Jahn, Richard J. Warburton, and Philipp Treutlein
Phys. Rev. Lett. 119, 060502 – Published 8 August 2017

DOI: https://doi.org/10.1103/PhysRevLett.119.060502

University of Basel

Nano 'hall of mirrors' causes molecules to mix with light

When a molecule emits a blink of light, it doesn't expect it to ever come back. However researchers have now managed to place single molecules in such a tiny optical cavity that emitted photons, or particles of light, return to the molecule before they have properly left. The energy oscillates back and forth between light and molecule, resulting in a complete mixing of the two.

Previous attempts to mix molecules with light have been complex to produce and only achievable at very low temperatures, but the researchers, led by the University of Cambridge, have developed a method to produce these 'half-light' molecules at room temperature.

These unusual interactions of molecules with light provide new ways to manipulate the physical and chemical properties of matter, and could be used to process quantum information, aid in the understanding of complex processes at work in photosynthesis, or even manipulate the chemical bonds between atoms. The results are reported in the journal Nature.

To use single molecules in this way, the researchers had to reliably construct cavities only a billionth of a metre (one nanometre) across in order to trap light. They used the tiny gap between a gold nanoparticle and a mirror, and placed a coloured dye molecule inside.

"It's like a hall of mirrors for a molecule, only spaced a hundred thousand times thinner than a human hair," said Professor Jeremy Baumberg of the NanoPhotonics Centre at Cambridge's Cavendish Laboratory, who led the research.

In order to achieve the molecule-light mixing, the dye molecules needed to be correctly positioned in the tiny gap. "Our molecules like to lie down flat on the gold, and it was really hard to persuade them to stand up straight," said Rohit Chikkaraddy, lead author of the study.

To solve this, the team joined with a team of chemists at Cambridge led by Professor Oren Scherman to encapsulate the dyes in hollow barrel-shaped molecular cages called cucurbiturils, which are able to hold the dye molecules in the desired upright position.

When assembled together correctly, the molecule scattering spectrum splits into two separated quantum states which is the signature of this 'mixing'. This spacing in colour corresponds to photons taking less than a trillionth of a second to come back to the molecule.

A key advance was to show strong mixing of light and matter was possible for single molecules even with large absorption of light in the metal and at room temperature. "Finding single-molecule signatures took months of data collection," said Chikkaraddy.

The researchers were also able to observe steps in the colour spacing of the states corresponding to whether one, two, or three molecules were in the gap.

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The Cambridge team collaborated with theorists Professor Ortwin Hess at the Blackett Laboratory, Imperial College London and Dr Edina Rosta at Kings College London to understand the confinement and interaction of light in such tiny gaps, matching experiments amazingly well.

Reference:

Single-molecule strong coupling at room temperature in plasmonic nanocavities
Nature (2016) doi:10.1038/nature17974

University of Cambridge

New Material for Detecting Photons Captures More Quantum Information

Colorized micrograph of a NIST single-photon detector made of superconducting nanowires patterned on molybdenum silicide.

Photo Credit: Verma/NIST

Detecting individual particles of light just got a bit more precise—by 74 picoseconds to be exact—thanks to advances in materials by National Institute of Standards and Technology (NIST) researchers and their colleagues in fabricating superconducting nanowires.

Although 74 picoseconds may not sound like much—a picosecond is a trillionth of a second—it is a big deal in the quantum world, where light particles, or photons, can carry valuable information. In this case it means that much less “jitter,” or uncertainty in the arrival time of a photon. Less jitter means that photons can be spaced more closely together but still be correctly detected. This enables communications at a higher bit rate, with more information transmitted in the same period.  

Every little bit helps when trying to receive faint signals reliably. It helped, for example, in NIST’s recent quantum teleportation record and difficult tests of physics theories. In such experiments, researchers want to decode as much information as possible from the quantum properties of billions of photons, or determine if “entangled” photons have properties that are linked before—or only after—being measured.  

NIST has made many advances in photon detector designs. In the latest work, described inOptics Express, NIST researchers used an electron beam to pattern nanowires into a thin film made of a heat-tolerant ceramic superconductor, molybdenum silicide. The tiny boost in energy that occurs when a single photon hits is enough to make the nanowires briefly lose their superconducting capability and become normal conductors, signaling the event.  

Nanowire detectors are superfast, counting tens of millions of photons per second, and generating few “dark” (or false) counts. Originally they were inefficient—meaning they missed photons they should have counted—but NIST has been fine-tuning their properties, first by boosting efficiency and now reducing jitter.  

The new design improves on NIST’s 2011 tungsten-silicon alloy material because it can operate at higher (though still cryogenic) temperatures and at a higher electrical current. The higher temperature simplifies refrigeration; the higher current cuts jitter in half, from about 150 picoseconds to 76 picoseconds. NIST researchers enhanced the detector’s light absorption and efficiency by embedding the chip in a cavity made of gold mirrors and layers of other unreactive materials.  

Researchers demonstrated detector efficiencies of 87 percent at wavelengths that are useful in telecommunications. This was almost as efficient as tungsten-silicon devices (93 percent) but with significantly lower jitter.  

The molybdenum-silicide material adds to NIST’s contributions in the competitive international field of quantum information science. Development of next-generation sensors offering high precision is a NIST priority. NIST single-photon detectors are used in a variety of experiments around the world. 

The detectors were made in NIST Boulder’s microfabrication facility. Researchers from the University of Geneva in Switzerland and the Jet Propulsion Laboratory at the California Institute of Technology also contributed to the work. 

NIST

Interacting Ion Qutrits

Enlisting symmetry to protect quantum states from disruptions

Symmetry permeates nature, from the radial symmetry of flowers to the left-right symmetry of the human body. As such, it provides a natural way of classifying objects by grouping those that share the same symmetry. This is particularly useful for describing transitions between phases of matter. For example, liquid and gas phases have translational symmetry, meaning the arrangement of molecules doesn’t change regardless of the direction from which they are observed. On the other hand, the density of atoms in a solid phase is not continuously the same — thus translational symmetry is broken.

In quantum mechanics, symmetry describes more than just the patterns that matter takes — it is used to classify the nature of quantum states. These states can be entangled, exhibiting peculiar connections that cannot be explained without the use of quantum physics. For some entangled states, the symmetry of these connections can offer a kind of protection against disruptions.

Here, the word protection indicates that the system is robust against non-symmetry breaking changes. Like an island in the middle of an ocean, there is not a direct road leading to a symmetry-protected phase or state. This means that the only way to access the state is to change the symmetry itself. Physicists are interested in exploring these classes of protected states because building a useful quantum device requires its building blocks to be robust against outside disturbances that may interfere with device operations.

Recently, JQI researchers under the direction of Christopher Monroe have used trapped atomic ions to construct a system that could potentially support a type of symmetry-protected quantum state. For this research they used a three-state system, called a qutrit, and demonstrated a proof-of-principle experiment for manipulating and controlling multiple qutrits. The result appeared in Physical Review X, an online open-access journal, and is the first demonstration of using multiple interacting qutrits for doing quantum information operations and quantum simulation of the behavior of real materials.

To date, almost all of the work in quantum information science has focused on manipulating "qubits," or so-called spin-1/2 particles that consist of just two energy levels.  In quantum mechanics, multilevel systems are analogous to the concept of "spin," where the number of energy levels corresponds to the number of possible states of spin. This group has used ion spins to explore a variety of topics, such as the physics of quantum magnetism and the transmission speed of quantum information across a spin-crystal. Increasingly, there is interest in moving beyond spin-½ to control and simulations of higher order spin systems, where the laws of symmetry can be radically altered. “One complication of spin-1 materials is that the added complexity of the levels often makes these systems much more difficult to model or understand. Thus, performing experiments in these higher [spin] dimensional systems may yield insight into difficult-to-calculate problems, and also give theorists some guidance on modeling such systems, ” explains Jake Smith, a graduate student in Monroe’s lab and author on the paper.

To engineer a spin-1 system, the researchers electromagnetically trapped a linear crystal of atomic ytterbium (Yb) ions, each atom a few micrometers from the next. Using a magnetic field, internal states of each ion are tailored to represent a qutrit, with a (+) state, (-) state and (0) state denoting the three available energy levels (see figure). With two ions, the team demonstrated the basic techniques necessary for quantum simulation: preparing initial states (placing the ions in certain internal states), observing the state of the system after some evolution, and verifying that the ions are entangled, here with 86% fidelity (fidelity is a measure of how much the experimentally realized state matches the theoretical target state).

To prepare the system in certain initial states, the team first lowers the system into its ground state, the lowest energy state in the presence of a large effective magnetic field. The different available spin chain configurations at a particular magnetic field value correspond to different energies. They observed how the spin chain reacted or evolved as the amplitude of the magnetic field was lowered. Changing the fields that the ions spins are exposed to causes the spins to readjust in order to remain in the lowest energy configuration.  

By adjusting the parameters (here laser amplitudes and frequencies) the team can open up and follow pathways between different energy levels. This is mostly true, but for some target states a simple trajectory that doesn’t break symmetries or pass through a phase transition does not exist. For instance, when the team added a third ion, they could not smoothly guide the system into its ground state, indicating the possible existence of a state with some additional symmetry protections.

“This result is a step towards investigating quantum phases that have special properties based on the symmetries of the system,” says Smith. Employing these sorts of topological phases may be a way to improve coherence times when doing quantum computation, even in the face of environmental disruptions. Coherence time is how long a state retains its quantum nature. Quantum systems are very sensitive to outside disturbances, and doing useful computation requires maintaining this quantum nature for longer than the time it takes to perform a particular calculation.

Monroe explains, "These symmetry-protected states may be the only way to build a large-scale stable quantum computer in many physical systems, especially in the solid-state.  With the exquisite control afforded atomic systems such as trapped ions demonstrated here, we hope to study and control how these very subtle symmetry effects might be used for quantum computing, and help guide their implementation in any platform."

To further investigate this protected phase, the researchers next intend to address the problem of creating antisymmetric ground states. Smith continues, “The next steps are to engineer more complicated interactions between the effective spins and implement a way to break the symmetries of the interactions.”

http://www.nanotechnologyworld.org/#!Interacting-Ion-Qutrits/c89r/55b65d7e0cf2d0bb156af20f 

Quantum Teleportation May be Possible: Electrons Remain Entangled Even After Separation

A team from the RIKEN Center for Emergent Matter Science, along with collaborators from several Japanese institutions, have successfully produced pairs of spin-entangled electrons and demonstrated, for the first time, that these electrons remain entangled even when they are separated from one another on a chip.

This research could contribute to the creation of futuristic quantum networks operating using quantum teleportation, which could allow information contained in quantum bits—qubits—to be shared between many elements on chip, a key requirement to scale up the power of a quantum computer. The ability to create non-local entangled electron pairs—known as Einstein-Podolsky-Rosen pairs—on demand has long been a dream.

Russell Deacon, who carried out the work, says, "We set out to demonstrate that spin-entangled electrons could be reliably produced. So far, researchers have been successful in creating entangled photons, since photons are extremely stable and do not interact. Electrons, by contrast, are profoundly affected by their environment. We chose to try to show that electrons can be entangled through their spin, a property that is relatively stable."

To perform the feat, Deacon and his collaborators began the painstaking work of creating a tiny device, just a few hundred nanometers in size. The idea was to take a Cooper pair—a pair of electrons that allows electricity to flow freely in superconductors—and get them, while tunneling—a quantum phenomenon—across a junction between two superconductor leads, to pass through two separate “quantum dots"—small crystals that have quantum properties. “If we could detect a superconducting current," Deacon continues, "this would mean that the electrons, which can be used as quantum bits—the qubits, or bits used in quantum computing—remain entangled even when they have been separated between the quantum dots. We confirm this separation by measuring a superconducting current that develops when they split and are recombined in the second lead.”

The quantum dots, each around 100 nanometers in size, were grown at random positions on a semiconductor chip. This chip was painstakingly examined using an atomic force microscope to discover pairs of dots that were close enough that they might function properly. “We observed thousands of dots and identified around a hundred that were suitable. From these we made around twenty devices. Of those just two worked.”

By measuring the superconducting current, the team was able to show clearly that the spin of the electrons remained entangled as they passed through the separate quantum dots. “Since we have demonstrated that the electrons remain entangled even when separated,” says Deacon, “this means that we could now use a similar, albeit more complex, device to prepare entangled electron pairs to teleport qubit states across a chip.”

According to Seigo Tarucha, leader of the laboratory that conducted the work, “This discovery is very exciting, as it could lead eventually to the development of applications such as quantum networks and quantum teleportation. Though it is technically difficult to handle, electron spin is a very promising property for these applications, as it is relatively free from the environment and lasts comparatively long. It could be combined with photons, by using the spin-entangled electrons to create photons that themselves would be entangled. This could allow us to create large networks to share quantum information in a widely distributed way.”

The work, published in Nature Communications, was done by RIKEN in collaboration with the University of Tokyo, University of Osaka, and was funded by JST and DFG.

http://www.nanotechnologyworld.org/#!Quantum-Teleportation-May-be-Possible-Electrons-Remain-Entangled-Even-After-Separation/c89r/55949dc50cf2c7ea473c5edb 

New method of quantum entanglement vastly increases how much information can be carried in a photon

Led by UCLA researchers, research could have applications in finance, health care, government and military communications

A team of researchers led by UCLA electrical engineers has demonstrated a new way to harness light particles, or photons, that are connected to each other and act in unison no matter how far apart they are — a phenomenon known as quantum entanglement.

In previous studies, photons have typically been entangled by one dimension of their quantum properties — usually the direction of their polarization.

In the new study, researchers demonstrated that they could slice up and entangle each photon pair into multiple dimensions using quantum properties such as the photons’ energy and spin. This method, called hyperentanglement, allows each photon pair to carry much more data than was possible with previous methods.

Quantum entanglement could allow users to send data through a network and know immediately whether that data had made it to its destination without being intercepted or altered. With hyperentanglement, users could send much denser packets of information using the same networks.

The research, published today in Nature Photonics, was led by Zhenda Xie, a research scientist in the lab of Chee Wei Wong, a UCLA associate professor of electrical engineering who was the research project’s principal investigator. Researchers from MIT, Columbia University, the University of Maryland and the National Institute of Standards and Technology were also part of the team.

Albert Einstein famously described quantum entanglement as “spooky action at a distance” because it seems so improbable that what happens to one particle in an entangled pair also happens instantly to the other particle, even over great distances. The phenomenon exceeds the speed of light.

In the new study, researchers sent hyperentangled photons in a shape known as a biphoton frequency comb, essentially breaking up entangled photons into smaller parts.

In secure data transfer, photons sent over fiber optic networks can be encrypted through entanglement. With each dimension of entanglement, the amount of information carried on a photon pair is doubled, so a photon pair entangled by five dimensions can carry 32 times as much data as a pair entangled by only one. The result greatly extends from wavelength multiplexing, the method for carrying many videos over a single optical fiber.

“We show that an optical frequency comb can be generated at single photon level,” Xie said. “Essentially, we’re leveraging wavelength division multiplexing concepts at the quantum level.”

Potential applications for the research include secure communication and information processing, in particular for high-capacity data transfer with minimal error. This could be useful for medical servers, government data communications, financial markets and military communication channels, as well as quantum cloud communications and distributed quantum computing.

“We are fortunate to verify a decades-old theoretical prediction by Professor Jeff Shapiro of MIT, that quantum entanglement can be observed in a comb-like state,” Wong said. “With the help of state-of-the-art high-speed single photon detectors at NIST and support from Dr. Franco Wong, Dr. Xie was able to verify the high-dimensional and multi-degrees-of-freedom entanglement of photons. These observations demonstrate a new fundamentally secure approach for dense information processing and communications.”

Co-authors on the paper are Sajan Shrestha, XinAn Xu and Junlin Liang, prior students and postdoctoral scientists at Columbia with Wong; Tian Zhong, professors Jeffrey Shapiro and Franco N.C. Wong of MIT; Yan-Xiao Gong of Southeast University in Nanjing, China; and Joshua Bienfang and Alessandro Restelli, affiliated with both the University of Maryland and the NIST.

The work was funded by the Defense Advanced Research Projects Agency.

http://www.nanotechnologyworld.org/#!New-method-of-quantum-entanglement-vastly-increases-how-much-information-can-be-carried-in-a-photon/c89r/5592af200cf2585ebcda12a4