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

NIST Team Breaks Distance Record for Quantum Teleportation

Researchers at the National Institute of Standards and Technology (NIST) have “teleported” or transferred quantum information carried in light particles over 100 kilometers (km) of optical fiber, four times farther than the previous record. 

The experiment confirmed that quantum communication is feasible over long distances in fiber. Other research groups have teleported quantum information over longer distances in free space, but the ability to do so over conventional fiber-optic lines offers more flexibility for network design. 

Not to be confused with Star Trek’s fictional “beaming up” of people, quantum teleportation involves the transfer, or remote reconstruction, of information encoded in quantum states of matter or light. Teleportation is useful in both quantum communications and quantum computing, which offer prospects for novel capabilities such as unbreakable encryption and advanced code-breaking, respectively. The basic method for quantum teleportation was first proposed more than 20 years ago and has been performed by a number of research groups, including one at NIST using atoms in 2004. 

The new record, described in Optica,* involved the transfer of quantum information contained in one photon—its specific time slot in a sequence—to another photon transmitted over 102 km of spooled fiber in a NIST laboratory in Colorado. 

The lead author, Hiroki Takesue, was a NIST guest researcher from NTT Corp. in Japan. The achievement was made possible by advanced single-photon detectors designed and made at NIST. 

“Only about 1 percent of photons make it all the way through 100 km of fiber,” NIST’s Marty Stevens says. “We never could have done this experiment without these new detectors, which can measure this incredibly weak signal.” 

Until now, so much quantum data was lost in fiber that transmission rates and distances were low. The new NTT/NIST teleportation technique could be used to make devices called quantum repeaters that could resend data periodically in order to extend network reach, perhaps enough to eventually build a “quantum internet.” Previously, researchers thought quantum repeaters might need to rely on atoms or other matter, instead of light, a difficult engineering challenge that would also slow down transmission. 

Various quantum states can be used to carry information; the NTT/NIST experiment used quantum states that indicate when in a sequence of time slots a single photon arrives. The teleportation method is novel in that four of NIST’s photon detectors were positioned to filter out specific quantum states. (See graphic for an overview of how the teleportation process works.) The detectors rely on superconducting nanowires made of molybdenum silicide.** They can record more than 80 percent of arriving photons, revealing whether they are in the same or different time slots each just 1 nanosecond long. The experiments were performed at wavelengths commonly used in telecommunications. 

Because the experiment filtered out and focused on a limited combination of quantum states, teleportation could be successful in only 25 percent of the transmissions at best. Thanks to the efficient detectors, researchers successfully teleported the desired quantum state in 83 percent of the maximum possible successful transmissions, on average. All experimental runs with different starting properties exceeded the mathematically significant 66.7 percent threshold for proving the quantum nature of the teleportation process. 

As a non-regulatory agency of the U.S. Department of Commerce, NIST promotes U.S. innovation and industrial competitiveness by advancing measurement science, standards and technology in ways that enhance economic security and improve our quality of life. To learn more about NIST, visit www.nist.gov. 

* H. Takesue, S.D. Dyer, M.J. Stevens, V. Verma, R.P. Mirin, and S.W. Nam. 2015. Quantum teleportation over 100 km of fiber using highly efficient superconducting nanowire single-photon detectors. Optica Vol. 2, Issue 10, page 832. DOI: 10.1364/OPTICA.2.000832 

** The molybdenum silicide is an advance in the detector design described in the 2011 NIST Tech Beat article, “Key Ingredient: Change in Material Boosts Prospects of Ultrafast Single-photon Detector.”

NIST

Nanotechnology World Association

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 

Donuts, math, and superdense teleportation of quantum information

In superdense teleportation of quantum information, Alice (near) selects a particular set of states to send to Bob (far), using the hyperentangled pair of photons they share. The possible states Alice may send are represented as the points on a donut shape, here artistically depicted in sharp relief from the cloudy silhouette of general quantum state that surrounds them. To transmit a state, Alice makes a measurement on her half of the entangled state, which has four possible outcomes shown by red, green, blue, and yellow points. She then communicates the outcome of her measurement (in this case, yellow, represented by the orange streak connecting the two donuts) to Bob using a classical information channel. Bob then can make a corrective rotation on his state to recover the state that Alice sent.


Putting a hole in the center of the donut—a mid-nineteenth-century invention—allows the deep-fried pastry to cook evenly, inside and out. As it turns out, the hole in the center of the donut also holds answers for a type of more efficient and reliable quantum information teleportation, a critical goal for quantum information science.

Quantum teleportation is a method of communicating information from one location to another without moving the physical matter to which the information is attached. Instead, the sender (Alice) and the receiver (Bob) share a pair of entangled elementary particles—in this experiment, photons, the smallest units of light—that transmit information through their shared quantum state. In simplified terms, Alice encodes information in the form of the quantum state of her photon. She then sends a key to Bob over traditional communication channels, indicating what operation he must perform on his photon to prepare the same quantum state, thus teleporting the information.

Quantum teleportation has been achieved by a number of research teams around the globe since it was first theorized in 1993, but current experimental methods require extensive resources and/or only work successfully a fraction of the time.

Now, by taking advantage of the mathematical properties intrinsic to the shape of a donut—or torus, in mathematical terminology—a research team led by physicist Paul Kwiat of the University of Illinois at Urbana-Champaign has made great strides by realizing “superdense teleportation”. This new protocol, developed by coauthor physicist Herbert Bernstein of Hampshire College in Amherst, MA, effectively reduces the resources and effort required to teleport quantum information, while at the same time improving the reliability of the information transfer.

With this new protocol, the researchers have experimentally achieved 88 percent transmission fidelity, twice the classical upper limit of 44 percent. The protocol uses pairs of photons that are “hyperentangled”—simultaneously entangled in more than one state variable, in this case in polarization and in orbital angular momentum—with a restricted number of possible states in each variable. In this way, each photon can carry more information than in earlier quantum teleportation experiments.

At the same time, this method makes Alice’s measurements and Bob’s transformations far more efficient than their corresponding operations in quantum teleportation: the number of possible operations being sent to Bob as the key has been reduced, hence the term “superdense”.

Kwiat explains, “In classical computing, a unit of information, called a bit, can have only one of two possible values—it’s either a zero or a one. A quantum bit, or qubit, can simultaneously hold many values, arbitrary superpositions of 0 and 1 at the same time, which makes faster, more powerful computing systems possible.

“So a qubit could be represented as a point on a sphere, and to specify what state it is, one would need longitude and latitude. That’s a lot of information compared to just a 0 or a 1.”

“What makes our new scheme work is a restrictive set of states. The analog would be, instead of using a sphere, we are going to use a torus, or donut shape. A sphere can only rotate on an axis, and there is no way to get an opposite point for every point on a sphere by rotating it—because the axis points, the north and the south, don’t move. With a donut, if you rotate it 180 degrees, every point becomes its opposite. Instead of axis points you have a donut hole. Another advantage, the donut shape actually has more surface area than the sphere, mathematically speaking—this means it has more distinct points that can be used as encoded information.”

Lead author, Illinois physics doctoral candidate Trent Graham, comments, “We are constrained to sending a certain class of quantum states called ‘equimodular’ states. We can deterministically perform operations on this constrained set of states, which are impossible to perfectly perform with completely general quantum states. Deterministic describes a definite outcome, as opposed to one that is probabilistic. With existing technologies, previous photonic quantum teleportation schemes either cannot work every time or require extensive experimental resources. Our new scheme could work every time with simple measurements.”

This research team is part of a broader collaboration that is working toward realizing quantum communication from a space platform, such as the International Space Station, to an optical telescope on Earth. The collaboration—Kwiat, Graham, Bernstein, physicist Jungsang Kim of Duke University in Durham, NC, and scientist Hamid Javadi of NASA’s Jet Propulsion Laboratory in Pasadena, CA—recently received funding from NASA Headquarter's Space Communication and Navigation program (with project directors Badri Younes and Barry Geldzahler) to explore the possibility.

“It would be a stepping stone toward building a quantum communications network, a system of nodes on Earth and in space that would enable communication from any node to any other node,” Kwiat explains. “For this, we’re experimenting with different quantum state properties that would be less susceptible to air turbulence disruptions.”

The team’s recent experimental findings are published in the May 28, 2015 issue of Nature Communications, and represent the collaborative effort Kwiat, Graham, and Bernstein, as well as physicist Tzu-Chieh Wei of State University of New York at Stony Brook, and mathematician Marius Junge of the University of Illinois.

This research is funded by NSF Grant No. PHY-0903865, NASA NIAC Program, and NASA Grant No. NNX13AP35A. It is partially supported by National Science Foundation Grants DMS-1201886, No. PHY 1314748, and No. PHY 1333903.
______________________

Contact: Siv Schwink, communications coordinator, Department of Physics, 217/300-2201.

Paul Kwiat, Department of Physics, University of Illinois at Urbana-Champaign.

Image by Precision Graphics, copyright Paul Kwiat, University of Illinois at Urbana-Champaign.

Source: http://engineering.illinois.edu/news/article/11151?

Energy can be teleported over long distances, say physicists

Squeezed vacuum states could allow long-distance energy teleportation. (Courtesy: iStockphoto/agsandrew)



The ability to teleport energy from one location to another could revolutionise the way quantum devices operate, but only if it can be made to work over practical distances. Now physicists think they know how.






Teleportation is the transfer of an object from one point in the universe to another without travelling through the space in between. It is common practice in many labs around the world. Since the early 90s, physicists have used it to teleport increasingly complex objects starting with photons and more recently with atoms and ions.

But that’s just the beginning. Back in 2010, we looked at the extraordinary work of Masahiro Hotta at Tohoku University in Japan who has worked out that it ought to be possible to teleport energy too. That’s something that could have profound implications for the way quantum devices and machines might be made to work in future.

But energy teleportation has an important limitation–the distance over which it can be sent. The limitations are so severe that it’s hard to see how energy teleportation could help even at the nanoscale. This “strong distance limitation has hampered experimental verification,” says Hotta.

But now he and a couple of mates say they’ve discovered a way round this limitation that allows energy to be teleported over almost any distance. And this new protocol for energy teleportation should allow experimental verification for the first time.

First some background. Energy teleportation relies on the natural quantum variations that occur in a vacuum on the smallest scale. On this scale, a vacuum is far from empty.

Instead, physicists think of it as a maelstrom of virtual quantum particles and antiparticles constantly leaping in and out of existence. That’s OK and does not violate any physical laws as long as the average energy of this vacuum is zero.

It also ensures that regions of space are entangled over these short distances. So what happens in one region immediately influences the region it is entangled with.

Hotta’s idea is to create a pair of entangled photons and allow one of them to interact with one region of space, thus injecting energy into the vacuum.

It then becomes possible to extract this energy from a nearby, entangled region of space using the other photon. That ensures that any increase in energy in one region is balanced by a decrease in another nearby region.

What limits this process is the distance over which regions of space are entangled, which is not very far, on the order of the Planck scale which is 10^-35 metres. And therein lies the problem.

Now Hotta and pals say they’ve found a way round this using an exotic quantum effect known as a squeezed state, which minimises the quantum noise in a system. They say that preparing the original photons in a squeezed state overcomes the distance limitation.

Instead of relying on entangled regions of space to balance the energy between one point and another, the squeezed state itself does the balancing. And that makes it possible to teleport energy over almost any distance.

Hotta and co say this should make the experimental verification of energy teleportation much easier. If they’re right, we could see the first energy teleportation experiments in the coming months and years.

Ref: arxiv.org/abs/1305.3955 : Quantum Energy Teleportation without Limit of Distance

Source: http://www.technologyreview.com/view/523716/energy-teleportation-overcomes-distance-limit/