Quantum dots provide complete control of photons

By emitting photons from a quantum dot at the top of a micropyramid, researchers at Link?ping University are creating a polarized light source for such things as energy-saving computer screens and wiretap-proof communications.

Polarized light -- where all the light waves oscillate on the same plane -- forms the foundation for technology such as LCD displays in computers and TV sets, and advanced quantum encryption. Normally, this is created by normal unpolarized light passing through a filter that blocks the unwanted light waves. At least half of the light emitted, and thereby an equal amount of energy, is lost in the process.

A better method is to emit light that is polarized right at the source. This can be achieved with quantum dots -- crystals of semiconductive material so small that they produce quantum mechanical phenomena. But until now, they have only achieved polarization that is either entirely too weak or hard to control.

A semiconductive materials research group led by Professor Per Olof Holtz is now presenting an alternative method where asymmetrical quantum dots of a nitride material with indium is formed at the top of microscopic six-sided pyramids. With these, they have succeeded in creating light with a high degree of linear polarization, on average 84%. The results are being published in the Nature periodical Light: Science & Applications.

"We're demonstrating a new way to generate polarized light directly, with a predetermined polarization vector and with a degree of polarization substantially higher than with the methods previously launched," Professor Holtz says.

In experiments, quantum dots were used that emit violet light with a wavelength of 415 nm, but the photons can in principle take on any colour at all within the visible spectrum through varying the amount of the metal indium.

"Our theoretical calculations point to the fact that an increased amount of indium in the quantum dots further improves the degree of polarization," says reader Fredrik Karlsson, one of the authors of the article.

The micropyramid is constructed through crystalline growth, atom layer by atom layer, of the semiconductive material gallium nitride. A couple of nanothin layers where the metal indium is also included are laid on top of this. From the asymmetrical quantum dot thus formed at the top, light particles are emitted with a well-defined wavelength.

The results of the research are opening up possibilities, for example for more energy-effective polarized light-emitting diodes in the light source for LCD screens. As the quantum dots can also emit one photon at a time, this is very promising technology for quantum encryption, a growing technology for wiretap-proof communications.

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NSA pursues quantum technology

In this month's issue of Physics World, Jon Cartwright explains how the revelation that the US National Security Agency (NSA) is developing quantum computers has renewed interest and sparked debate on just how far ahead they are of the world's major labs looking to develop the same technology.

In 2006 the NSA openly announced a partnership with two US institutions to develop quantum computers. However, according to documents leaked by whistle-blower Edward Snowden, and published last month by the Washington Post, the NSA also wishes to develop the technology so that it is capable of breaking modern Internet security.

The $79.7m project, dubbed "Penetrating Hard Targets," could be made possible by the extraordinary potential of quantum computers to factorize large numbers in a short space of time, quickly deciphering encryption keys that are used to protect sensitive information.

For the NSA, this could mean deciphering banking transactions, private messages and government files; however, many physicists are not surprised and believe this is exactly the type of technology that the NSA is expected to develop.

Raymond Laflamme, a leading quantum information theorist at the University of Waterloo in Canada, said "If you put my level of surprise on a scale from zero to 10, where 10 is very, very surprised, my answer would be zero."

For many other physicists the news has confirmed the need to stay ahead of the game and develop more sophisticated encryption techniques, some of which also take advantage of quantum phenomena.

Quantum key distribution (QKD) is one such technique, which guarantees the security of an encryption key based on fundamental aspects of quantum mechanics, whereby the process of trying to measure or access an encryption key made from various quantum states will automatically destroy it.

The latest leaked documents, however, also reveal that the NSA is attempting to exploit practical loops in QKD under a programme known as "Owning the Net."

Cartwright concludes that quantum computers are still expected to be many years away, with the control of qubits -- the packets of information that quantum computers would process -- a major sticking point for physicists; however, the extent to which the NSA has developed the technology remains largely unknown.

Also in this issue of Physics World, and online today, 31 January, Matin Durrani, editor of the magazine, provides further details of the UK's ?270m investment into quantum technology that was announced by the chancellor George Osborne in last year's Autumn Statement.

The initiative, which will begin in 2015, will focus on areas such as chip-scale atomic clocks for improved GPS communication, quantum-enabled sensors, quantum communication and quantum computing, while some ?4m will go on equipment for the new Advanced Metrology Laboratory being built at the National Physical Laboratory.

The quantum-physics initiative, which has involved careful behind-the-scenes negotiations between the UK physics community, government and industry, was formally put to Osborne last year by a group of physicists led by Professor Sir Peter Knight from Imperial College London.

Jon Cartwright's analysis of NSA developments will be freely available on physicsworld.com from Thursday 6 February 2014.

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The above story is based on materials provided by Physics World. Note: Materials may be edited for content and length.

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Quantum physics could make secure, single-use computer memories possible

Computer security systems may one day get a boost from quantum physics, as a result of recent research from the National Institute of Standards and Technology (NIST). Computer scientist Yi-Kai Liu has devised a way to make a security device that has proved notoriously difficult to build -- a "one-shot" memory unit, whose contents can be read only a single time.

The research, which Liu is presenting at this week's Innovations in Theoretical Computer Science conference, shows in theory how the laws of quantum physics could allow for the construction of such memory devices. One-shot memories would have a wide range of possible applications such as protecting the transfer of large sums of money electronically. A one-shot memory might contain two authorization codes: one that credits the recipient's bank account and one that credits the sender's bank account, in case the transfer is canceled. Crucially, the memory could only be read once, so only one of the codes can be retrieved, and hence, only one of the two actions can be performed -- not both.

"When an adversary has physical control of a device -- such as a stolen cell phone -- software defenses alone aren't enough; we need to use tamper-resistant hardware to provide security," Liu says. "Moreover, to protect critical systems, we don't want to rely too much on complex defenses that might still get hacked. It's better if we can rely on fundamental laws of nature, which are unassailable."

Unfortunately, there is no fundamental solution to the problem of building tamper-resistant chips, at least not using classical physics alone. So scientists have tried involving quantum mechanics as well, because information that is encoded into a quantum system behaves differently from a classical system.

Liu is exploring one approach, which stores data using quantum bits, or "qubits," which use quantum properties such as magnetic spin to represent digital information. Using a technique called "conjugate coding, "two secret messages -- such as separate authorization codes -- can be encoded into the same string of qubits, so that a user can retrieve either one of the two messages. But as the qubits can only be read once, the user cannot retrieve both.

The risk in this approach stems from a more subtle quantum phenomenon: "entanglement," where two particles can affect each other even when separated by great distances. If an adversary is able to use entanglement, he can retrieve both messages at once, breaking the security of the scheme.

However, Liu has observed that in certain kinds of physical systems, it is very difficult to create and use entanglement, and shows in his paper that this obstacle turns out to be an advantage: Liu presents a mathematical proof that if an adversary is unable to use entanglement in his attack, that adversary will never be able to retrieve both messages from the qubits. Hence, if the right physical systems are used, the conjugate coding method is secure after all.

"It's fascinating how entanglement -- and the lack thereof -- is the key to making this work," Liu says. "From a practical point of view, these quantum devices would be more expensive to fabricate, but they would provide a higher level of security. Right now, this is still basic research. But there's been a lot of progress in this area, so I'm optimistic that this will lead to useful technologies in the real world."

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Record quantum entanglement of multiple dimensions: Two Schrödinger cats which could be alive, dead, or in 101 other states simultaneously

An international team of researchers, directed by researchers from the Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences, and with participation from the Universitat Aut?noma de Barcelona, has managed to create an entanglement of 103 dimensions with only two photons. The record had been established at 11 dimensions. The discovery could represent a great advance toward the construction of quantum computers with much higher processing speeds than current ones, and toward a better encryption of information.

The states in which elementary particles, such as photons, can be found have properties which are beyond common sense. Superpositions are produced, such as the possibility of being in two places at once, which defies intuition. In addition, when two particles are entangled a connection is generated: measuring the state of one (whether they are in one place or another, or spinning one way or another, for example) affects the state of the other particle instantly, no matter how far away from each other they are.

Scientists have spent years combining both properties to construct networks of entangled particles in a state of superposition. This in turn allows constructing quantum computers capable of operating at unimaginable speeds, encrypting information with total security and conducting experiments in quantum mechanics which would be impossible to carry out otherwise.

Until now, in order to increase the "computing" capacity of these particle systems, scientists have mainly turned to increasing the number of entangled particles, each of them in a two-dimensional state of superposition: a qubit (the quantum equivalent to an information bit, but with values which can be 1, 0 or an overlap of both values). Using this method, scientists managed to entangle up to 14 particles, an authentic multitude given its experimental difficulty.

The research team was directed by Anton Zeilinger and Mario Krenn from the Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences. It included the participation of Marcus Huber, researcher from the Group of Quantum Information and Quantum Phenomena from the UAB Department of Physics, as well as visiting researcher at the Institute of Photonic Sciences (ICFO). The team has advanced one more step towards improving entangled quantum systems.

In a new article posted on arXiv, scientists described how they managed to achieve a quantum entanglement with a minimum of 103 dimensions with only two particles. "We have two Schr?dinger cats which could be alive, dead, or in 101 other states simultaneously," Huber jokes, "plus, they are entangled in such a way that what happens to one immediately affects the other." The results implies a record in quantum entanglements of multiple dimensions with two particles, established until now at 11 dimensions.

Instead of entangling many particles with a qubit of information each, scientists generated one single pair of entangled photons in which each could be in more than one hundred states, or in any of the superpositions of theses states; something much easier than entangling many particles. These highly complex states correspond to different modes in which photons may find themselves in, with a distribution of their characteristic phase, angular momentum and intensity for each mode.

"This high dimension quantum entanglement offers great potential for quantum information applications. In cryptography, for example, our method would allow us to maintain the security of the information in realistic situations, with noise and interference. In addition, the discovery could facilitate the experimental development of quantum computers, since this would be an easier way of obtaining high dimensions of entanglement with few particles," explains UAB researcher Marcus Huber.

Now that the results demonstrate that obtaining high dimension entanglements is accessible, scientists conclude in the article that the next step will be to search how they can experimentally control these hundreds of spatial modes of the photons in order to conduct quantum computer operations.

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More secure communications thanks to quantum physics

One of the recent revelations by Edward Snowden is that the U.S. National Security Agency is currently developing a quantum computer. Physicists aren't surprised by this news; such a computer could crack the encryption that is commonly used today in no time and would therefore be highly attractive for the NSA.

Professor Thomas Walther of the Institute of Applied Physics at the Technical University of Darmstadt is convinced that "Sooner or later, the quantum computer will arrive." Yet the quantum physicist is not worried. After all, he knows of an antidote: so-called quantum cryptography. This also uses the bizarre rules of quantum physics, but not to decrypt messages at a record pace. Quite the opposite -- to encrypt it in a way that can not be cracked by a quantum computer. To do this, a "key" that depends on the laws of quantum mechanics has to be exchanged between the communication partners; this then serves to encrypt the message. Physicists throughout the world are perfecting quantum cryptography to make it suitable for particularly security-sensitive applications, such as for banking transactions or tap-proof communications. Walther's Ph.D. student Sabine Euler is one of them.

As early as the 1980s, physicists Charles Bennett and Gilles Brassard thought about how quantum physics could help transfer keys while avoiding eavesdropping. Something similar to Morse code is used, consisting of a sequence of light signals from individual light particles (photons). The information is in the different polarizations of successive photons. Eavesdropping is impossible due to the quantum nature of photons. Any eavesdropper will inevitably be discovered because the eavesdropper needs to do measurements on the photons, and these measurements will always be noticed.

"That's the theory" says Walther. However, there are ways to listen without being noticed in practice. This has been demonstrated by hackers who specialize in quantum cryptography based on systems already available on the market. "Commercial systems have always relinquished a little bit of security in the past," says Walther. In order to make the protocol of Bennett and Brassard reality, you need, for example, light sources that are can be controlled so finely that they emit single photons in succession. Usually, a laser that is weakened so much that it emits single photons serves as the light source. "But sometimes two photons can come out simultaneously, which might help a potential eavesdropper to remain unnoticed" says Walther. The eavesdropper could intercept the second photon and transmit the first one.

Therefore, the team led by Sabine Euler uses a light source that transmits a signal when it sends a single photon; this signal can be used to select only the individually transmitted photons for communication. Nevertheless, there are still vulnerabilities. If the system changes the polarization of the light particles during coding, for example, the power consumption varies or the time interval of the pulses changes slightly. "An eavesdropper could tap this information and read the message without the sender and receiver noticing" explains Walther. Sabine Euler and her colleagues at the Institute of Applied Physics are trying to eliminate these vulnerabilities. "They are demonstrating a lot of creativity here" says Walther approvingly. Thanks to such research, it will be harder and harder for hackers to take advantage of vulnerabilities in quantum cryptography systems.

The TU Darmstadt quantum physicists want to make quantum cryptography not only more secure, but more manageable at the same time. "In a network in which many users wish to communicate securely with each other, the technology must be affordable," he says. Therefore, his team develops its systems in such a manner that they are as simple as possible and can be miniaturized.

The research team is part of the Center for Advanced Security Research Darmstadt (CASED), in which the TU Darmstadt, the Fraunhofer Institute for Secure Information Technology and the University of Darmstadt combine their expertise in current and future IT security issues. Over 200 scientists conduct research in CASED, funded by the State Initiative for Economic and Academic Excellence (LOEWE) of the Hessian Ministry for Science and the Arts. "We also exchange information with computer scientists, which is very exciting," says Walther.

After all, the computer science experts deal with many of the same issues as Walther's quantum physicists. For example, Johannes Buchmann of the department of Computer Science at the TU Darmstadt is also working on encryption methods that theoretically can not be cracked by a quantum computer. However, these are not based on quantum physics phenomena, but rather on an unsolvable math problem.

Therefore, it may well be that the answer to the first code-cracking quantum computer comes from Darmstadt.

Bizarre quantum physics and encryption

A quantum computer could quickly crack current encryptions because it can test very many possibilities simultaneously, in the same way as if you could try all possible variations for a password at once. After all, according to the quantum physics principle of superposition, atoms, electrons or photons can have several states simultaneously; for example, they can rotate clockwise and counterclockwise at the same time.

However, if you were to measure a property of a particle, such as the direction of rotation, the superposition is lost. This phenomenon is useful for quantum cryptography. Eavesdroppers inevitably betray themselves because their measurements of the photon change the photon's characteristics. Moreover, quantum physics forbids them to copy the photon with all its properties. Therefore, they can not siphon off any information to retransmit the uninfluenced photons on to the sender of the message.

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The above story is based on materials provided by Technische Universit?t Darmstadt. The original article was written by Christian J. Meier. Note: Materials may be edited for content and length.

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