Quantum cryptography: Swedish researchers reveal security hole

Hacking the Bell Test using classical light in energy-time entanglement-based quantum key distribution.

Quantum cryptography is considered a fully secure encryption method, but researchers from Linköping University and Stockholm University have discovered that this is not always the case.

They found that energy-time entanglement - the method that today forms the basis for many systems of quantum cryptography - is vulnerable to attack. The results of their research have been published in Science Advances.

"With this security hole, it's possible to eavesdrop on traffic without being detected. We discovered this in our theoretical calculations, and our colleagues in Stockholm were subsequently able to demonstrate it experimentally," says Jan-Åke Larsson, professor at Linköping University's Division of Information Coding.

Quantum cryptography is considered a completely safe method for information transfer, and theoretically it should be impossible to crack. Many research groups around the world are working to make quantum cryptography resistant to various types of disturbance, and so far it has been possible to handle the disturbance that has been detected. Quantum cryptography technology is commercially available, but there is much doubt as to whether it is actually used.

"It's mostly rumours, I haven't seen any system in use. But I know that some universities have test networks for secure data transfer," says Prof Larsson.

The energy-time entanglement technology for quantum encryption studied here is based on testing the connection at the same time as the encryption key is created. Two photons are sent out at exactly the same time in different directions. At both ends of the connection is an interferometer where a small phase shift is added. This provides the interference that is used to compare similarities in the data from the two stations. If the photon stream is being eavesdropped there will be noise, and this can be revealed using a theorem from quantum mechanics - Bell's inequality.

On the other hand if the connection is secure and free from noise, you can use the remaining data, or photons, as an encryption key to protect your message.

What the LiU researchers Jan-Åke Larsson and his doctoral student Jonathan Jogenfors have revealed about energy-time entanglement is that if the photon source is replaced with a traditional light source, an eavesdropper can identify the key, the code string. Consequently they can also read the message without detection. The security test, which is based on Bell's inequality, does not react - even though an attack is underway.

Physicists at Stockholm University have subsequently been able to demonstrate in practical experiments that it is perfectly possible to replace the light source and thus also eavesdrop on the message.

But this problem can also be solved.

"In the article we propose a number of countermeasures, from simple technical solutions to rebuilding the entire machine," said Jonathan Jogenfors.

Linköping University

Quantum computer made of standard semiconductor materials

Magnetic field helps qubit electrons store information longer

 

Physicists at the Technical University of Munich, the Los Alamos National Laboratory and Stanford University (USA) have tracked down semiconductor nanostructure mechanisms that can result in the loss of stored information – and halted the amnesia using an external magnetic field. The new nanostructures comprise common semiconductor materials compatible with standard manufacturing processes.

Quantum bits, qubits for short, are the basic logical elements of quantum information processing (QIP) that may represent the future of computer technology. Since they process problems in a quantum-mechanical manner, such quantum computers might one day solve complex problems much more quickly than currently possible, so the hope of researchers.

In principle, there are various possibilities of implementing qubits: photons are an option equally as viable as confined ions or atoms whose states can be altered in a targeted manner using lasers. The key questions regarding their potential use as memory units are how long information can be stored in the system and which mechanisms might lead to a loss of information.

A team of physicists headed by Alexander Bechtold and Professor Jonathan Finley at the Walter Schottky Institute of the Technical University of Munich and the Excellence Cluster Nanosystems Initiative Munich (NIM) have now presented a system comprising a single electron trapped in a semiconductor nanostructure. Here, the electron’s spin serves as the information carrier.

The researchers were able to precisely demonstrate the existence of different data loss mechanisms and also showed that stored information can nonetheless be retained using an external magnetic field.

Electrons trapped in a quantum dot

The TUM physicists evaporated indium gallium arsenide onto a gallium arsenide substrate to form their nanostructure. As a result of the different lattice spacing of the two semiconductor materials strain is produced at the interface between the crystal grids. The system thus forms nanometer-scale “hills” – so-called quantum dots.

When the quantum dots are cooled down to liquid helium temperatures and optically excited, a singe electron can be trapped in each of the quantum dots. The spin states of the electrons can then be used as information stores. Laser pulses can read and alter the states optically from outside. This makes the system ideal as a building block for future quantum computers.

Spin up or spin down correspond to the standard logical information units 0 and 1. But, on top of this come additional intermediate states of quantum mechanical up and down superpositions.

Hitherto unknown memory loss mechanisms

However, there is one problem: “We found out that the strain in the semiconductor material leads to a new and until recently unknown mechanism that results in the loss of quantum information,” says Alexander Bechtold. The strain creates tiny electric fields in the semiconductor that influence the nuclear spin orientation of the atomic nuclei.

“It’s a kind of piezoelectric effect,” says Bechthold. “It results in uncontrolled fluctuations in the nuclear spins.” These can, in turn, modify the spin of the electrons, i.e. the stored information. The information is lost within a few hundred nanoseconds.

In addition, Alexander Bechthold’s team was able to provide concrete evidence for further information loss mechanisms, for example that electron spins are generally influenced by the spins of the surrounding 100,000 atomic nuclei.

Preventing quantum mechanical amnesia

“However, both loss channels can be switched off when a magnetic field of around 1.5 tesla is applied,” says Bechtold. “This corresponds to the magnetic field strength of a strong permanent magnet. It stabilizes the nuclear spins and the encoded information remains intact.”

“Overall, the system is extremely promising,” according to Jonathan Finley, head of the research group. “The semiconductor quantum dots have the advantage that they harmonize perfectly with existing computer technology since they are made of similar semiconductor material.” They could even be equipped with electrical contacts, allowing them to be controlled not only optically using a laser, but also using voltage pulses.

The research was funded by the European Union (S3 Nano and BaCaTeC), the US Department of Energy, the US Army Research Office (ARO), the German Research Foundation DFG (excellence cluster Nanosystems Munich (NIM) and SFB 631), the Alexander von Humboldt Foundation as well as the TUM Institute for Advanced Study (Focus Group Nanophotonics and Quantum Optics).

Publication:

Three-stage decoherence dynamics of an electron spin qubit in an optically active quantum dot; Alexander Bechtold, Dominik Rauch, Fuxiang Li, Tobias Simmet, Per-Lennart Ardelt, Armin Regler, Kai Müller, Nikolai A. Sinitsyn and Jonathan J. Finley; Nature Physics, 11, 1005-1008 (2015) – DOI: 10.1038/nphys3470

Technische Universität München

Researchers discover roots of superfluorescent bursts from quantum wells

Spontaneous bursts of light from a solid block illuminate the unusual way interacting quantum particles behave when they are driven far from equilibrium. The discovery by Rice University scientists of a way to trigger these flashes may lead to new telecommunications equipment and other devices that transmit signals at picosecond speeds.

The Rice University lab of Junichiro Kono found the flashes, which last trillionths of a second, change color as they pulse from within a solid-state block. The researchers said the phenomenon can be understood as a combination of two previously known many-body concepts: superfluorescence, as seen in atomic and molecular systems, and Fermi-edge singularities, a process known to occur in metals.

The team previously reported the first observation of superfluorescence in a solid-state system by strongly exciting semiconductor quantum wells in high magnetic fields. The new process – Fermi-edge superfluorescence – does not require them to use powerful magnets. That opens up the possibility of making compact semiconductor devices to produce picosecond pulses of light.

The results by Rice, Florida State University and Texas A&M University researchers were reported this month in Nature’s online journal, Scientific Reports.

The semiconducting quantum wells at the center of the experiment contain particles – in this case, a dense collection of electrons and holes – and confine them to wiggle only within the two dimensions allowed by the tiny, stacked wells, where they are subject to strong Coulomb interactions.

Previous experiments by Rice and Florida State showed the ability to create superfluorescent bursts from a stack of quantum wells excited by a laser in extreme cold and under the influence of a strong magnetic field, both of which further quenched the electrons’ motions and made an atom-like system. The basic features were essentially the same as those known for superfluorescence in atomic systems.

That was a first, but mysteries remained, especially in results obtained at low or zero magnetic fields. Kono said the team didn’t understand at the time why the wavelength of the burst changed over its 100-picosecond span. Now they do. The team included co-lead authors Timothy Noe, a Rice postdoctoral researcher, and Ji-Hee Kim, a former Rice postdoctoral researcher and now a research professor at Sungkyunkwan University in the Republic of Korea.

In the new results, the researchers not only described the mechanism by which the light’s wavelength evolves during the event (as a Fermi-edge singularity), but also managed to record it without having to travel to the National High Magnetic Field Laboratory at Florida State.

Kono said superfluorescence is a well-known many-body, or cooperative, phenomenon in atomic physics. Many-body theory gives physicists a way to understand how large numbers of interacting particles like molecules, atoms and electrons behave collectively. Superfluorescence is one example of how atoms under tight controls collaborate when triggered by an external source of energy. However, electrons and holes in semiconductors are charged particles, so they interact more strongly than atoms or molecules do.

The quantum well, as before, consisted of stacked blocks of an indium gallium arsenide compound separated by barriers of gallium arsenide. “It’s a unique, solid-state environment where many-body effects completely dominate the dynamics of the system,” Kono said.

“When a strong magnetic field is applied, electrons and holes are fully quantized – that is, constrained in their range of motion — just like electrons in atoms,” he said. “So the essential physics in the presence of a high magnetic field is quite similar to that in atomic gases. But as we decrease and eventually eliminate the magnetic field, we’re entering a regime atomic physics cannot access, where continua of electronic states, or bands, exist.”

The Kono team’s goal was to keep the particles as dense as possible at liquid helium temperatures (about -450 degrees Fahrenheit) so that their quantum states were obvious, or “quantum degenerate,” which happens when the so-called Fermi energy is much larger than the thermal energy. When pumped by a strong laser, these quantum degenerate particles gathered energy and released it as light at the Fermi edge: the energy level of the most energetic particles in the system. As the electrons and holes combined to release photons, the edge shifted to lower-energy particles and triggered more reactions until the sequence played out.

The researchers found the emitted light shifted toward the higher red wavelengths as the burst progressed.

“What’s cool about this is that we have a material, we excite it with a 150-femtosecond pulse, wait for 100 picoseconds, and all of a sudden a picosecond pulse comes out. It’s a long delay,” Kono said. “This may lead to a new method for producing picosecond pulses from a solid. We saw something essentially the same previously, but it required high magnetic fields, so there was no practical application. But now the present work demonstrates that we don’t need a magnet.”

Co-authors are Stephen McGill, an associate scholar and scientist with the National High Magnetic Field Laboratory at Florida State University, and researchers Yongrui Wang and Aleksander Wójcik and Professor Alexey Belyanin of Texas A&M University.

The National Science Foundation and the state of Florida supported the research.

Source: http://news.rice.edu/2013/11/25/flashes-of-brilliance/

Headway in Quantum Information Transfer Using Nanomechanical Coupling of Microwave and Optical States

Fiber optics has made communication faster than ever, but the next step involves a quantum leap –– literally. In order to improve the security of the transfer of information, scientists are working on how to translate electrical quantum states to optical quantum states in a way that would enable ultrafast, quantum-encrypted communications.

A UC Santa Barbara research team has demonstrated the first and arguably most challenging step in the process. The paper, published in Nature Physics, describes a nanomechanical transducer that provides strong and coherent coupling between microwave signals and optical photons. In other words, the transducer is an effective conduit for translating electrical signals (microwaves) into light (photons).

Today's high-speed Internet converts electrical signals to light and sends it through optical fibers, but accomplishing this with quantum information is one of the great challenges in quantum physics. If realized, this would enable secure communication and even quantum teleportation, a process by which quantum information can be transmitted from one location to another.

"There's this big effort going on in science now to construct computers and networks that work on the principles of quantum physics," says lead author Jörg Bochmann, a postdoctoral scholar in UCSB's Department of Physics. "And we have found that there actually is a way to translate electrical quantum states to optical quantum states."

The new paper outlines the concept and presents a prototype device, which uses an optomechanical crystal implemented in a piezoelectric material in a way that is compatible with superconducting qubits, quantum analogs of classical bits. Operating the device at the single phonon limit, the scientists were able generate coherent interactions between electrical signals, very high frequency mechanical vibrations, and optical signals.

Although the first prototype of the transducer has not been operated in the quantum realm, that is, in fact, the next step for the research effort. "In this paper, we're characterizing the system using classical electrical and optical signals and find that the essential parameters look very promising," says Bochmann. "In the next step, we would have to actually input quantum signals from the electrical side and then check whether the quantum properties are preserved in the light."

According to the authors, their prototype transducer is fully compatible with superconducting quantum circuits and is well suited for cryogenic operation. "The coupled dynamics of the system should be the same at low temperatures as in our room temperature measurements, albeit with a lower thermal background," said co-author Andrew Cleland, a professor of physics and associate director of the California Nanosystems Institute at UCSB. "Genuine quantum features and non-classical mechanical states will emerge when we couple a superconducting qubit to the transducer.

"We believe that combining optomechanics with superconducting quantum devices will enable a new generation of on-chip quantum devices with unique capabilities, as well as opening an exciting pathway for realizing entangled networks of electronic and photonic quantum systems," Cleland said.

Source: http://www.ia.ucsb.edu/pa/display.aspx?pkey=3116