Quantum entanglement (kinda)

The first universal quantum network
by  / April 12, 2012

German scientists at the Max Planck Institute of Quantum Optics (MPQ) have created the first “universal quantum network” that could be feasibly scaled up to become a quantum internet. So far their quantum network only spans two labs spaced 21 meters apart, but the scientists stress that longer distances and multiple nodes are possible. The network’s construction is ingenious. Each node is represented by a single rubidium atom, trapped inside a reflective optical cavity. These atoms communicate with each other by emitting a single photon over an optical fiber. Each atom is a quantum bit — a qubit — and the polarization of the photon emitted carries the quantum state of the qubit. The receiving qubit absorbs the photon and takes on the quantum state of the transmitter. Voila: A network of qubits that can send, receive, and store quantum information. With this atom/photon setup, the scientists were able to perform a read/write operation between two labs, over a 60-meter run of optic fiber. There aren’t any photos of the equipment used, but I suspect we’re probably talking about very large machines to keep the rubidium atoms near absolute zero.

Historically the difficulty has been getting atoms and photons to interact — they’re both impossibly small, so getting them to collide is tricky. The reflective optical cavity solves this problem — the photon ricochets tens of thousands of times until it eventually collides — but even so, the MPQ scientists still only managed to successfully transfer quantum states 0.2% of the time. In another, probably more exciting test, the emitted photons were actually used to entangle the rubidium atoms. Entangled particles exactly mirror the quantum state of their partner, instantaneously and over any distance. Entangled qubits might be able to form the basis of a quantum network with zero latency over any distance, which would make it rather useful for the intergalactic Galnet that will eventually succeed the internet. The researchers hope that entanglement could be used to mitigate the fickleness of single photons. Back on Earth, though, the goal now is to improve on that 99.8% failure rate and to scale up the number of nodes. I wouldn’t get too excited just yet, but this advance from the Max Planck Institute could mean that quantum networks aren’t actually that far away. Like the internet, the first real quantum network will link up the world’s universities, laboratories, and military installations — and then, eventually, offices and homes.

Rubidium atom/photon quantum network

Physicists Create First Long-Distance Quantum Link
by Jim Heirbaut  /  11 April 2012

For more than a decade, physicists have been developing quantum-mechanical methods to pass secret messages without fear that they could be intercepted. But they still haven’t created a true quantum network—the fully quantum-mechanical analog to an ordinary telecommunications network in which an uncrackable connection can be forged between any two stations or “nodes” in a network. Now, a team of researchers in Germany has built the first true quantum link using two widely separated atoms. A complete network could be constructed by combining many such links, the researchers say. “These results are a remarkable achievement,” says Andrew Shields, an applied physicist and assistant managing director at Toshiba Research Europe Ltd. in Cambridge, U.K., who was not involved in the work. “In the past, we have built networks that can communicate quantum information, but convert it into classical form at the network switching points. [The researchers] report preliminary experiments towards forming a network in which the information remains in quantum form.”

Quantum communication schemes generally take advantage of the fact that, according to quantum theory, it’s impossible to measure the condition or “state” of a quantum particle without disturbing the particle. For example, suppose Alice wants to send Bob a secret message. She can do the encrypting in a traditional way, by writing out the message in the form of a long binary number and zippering it together in a certain mathematical way with a “key,” another long stream of random 0s and 1s. Bob can then use the same key to unscramble the message. But first, Alice must send Bob the key without letting anybody else see it. She can do that if she encodes the key in single particles of light, or photons. Details vary, but schemes generally exploit the fact that an eavesdropper, Eve, cannot measure the individual photons without altering their state in some way that Alice and Bob can detect by comparing notes before Alice encodes and sends her message. Such “quantum key distribution” has already been demonstrated in networks, such as a large six-node network in Vienna in 2008, and various companies offer quantum key distribution devices. Such schemes suffer a significant limitation, however. Although the key is passed from node to node in a quantum fashion, it must be read out and regenerated at each node in the network, leaving the nodes vulnerable to hacking. So physicists would like to make the nodes of the network themselves fully quantum mechanical—say, by forming them out of individual atoms.

According to quantum mechanics, an atom can have only certain discrete amounts of energy depending on how its innards are gyrating. Bizarrely, an atom can also be in two different energy states—call them 0 and 1—at once, although that uncertain two-states-at-once condition “collapses” into one state or the other as soon as the atom is measured. “Entanglement” takes weirdness to its absurd extreme. Two atoms can be entangled so that both are in an uncertain two-ways-at-once state, but their states are perfectly correlated. For example, if Alice and Bob share a pair of entangled atoms and she measures hers and finds it in the 1 state, then she’ll know that Bob is sure to find his in the 1 state, too, even before he measures it. Obviously, Alice and Bob can generate a shared random key by simply entangling and measuring their atoms again and again. Crucially, if entanglement can be extended to a third atom held by Charlotte, then Alice and Charlotte can share a key. In that case, if Eve then tries to detect the key by surreptitiously measuring Bob’s atom, she’ll mess up the correlations between Alice’s and Charlotte’s atoms in a way that will reveal her presence, making the truly quantum network unhackable, at least in principle. But first, physicists must entangle widely separated atoms. Now, Stephan Ritter of the Max Planck Institute of Quantum Optics in Garching, Germany, and colleagues have done just that, entangling two atoms in separate labs on opposite sides of the street, as they report online today in Nature.

As simple as this may sound, the researchers still needed a complete lab room full of lasers, optical elements, and other equipment for each node. Each atom sat between two highly reflective mirrors 0.5 mm apart, which form an “optical cavity.” By applying an external laser to atom A, Ritter’s team caused a photon emitted by that atom to escape from its cavity and travel through a 60-meter-long optical fiber to the cavity across the street. When the photon was absorbed by atom B, the original quantum information from the first atom was transferred to the second. By starting with just the right state of the first atom, the researchers could entangle the two atoms. According to the researchers, the entanglement could in principle be extended to a third atom, which makes the system scalable to more than two nodes. “Every experimental step had to be just right to make this work,” says Ritter, who works in the group of Gerhard Rempe. “Take, for example, the optical cavity. All physicists agree that atoms and photons are great stuff for building a quantum network, but in free space they hardly interact. We needed to develop the cavity for that.” “This is a very important advance,” says Toshiba’s Shields, because it would enable technologists to share quantum keys on networks where the intermediate nodes can’t be trusted, and it could also lead to more complex multiparty communication protocols based on distributed entanglement. “However,” Shields cautions, “there is still a great deal of work to be done before the technology is practical.” Miniaturization of the components that constitute one node will no doubt be on the researchers’ wish list.

A quantum network built with two atoms and fiber optic cable


An atom in an optical trap. Researchers have created a simple quantum network by linking two such atoms with fiber optic cable. A single photon passing between passes data. In an ordinary computer network, data in the form of binary numbers are transferred from one machine (node) to another via some sort of electronic signal, either electrical or optical. The success of this transfer comes when the recipient has precisely the same set of binary figures that were sent. In a quantum network, the “data” is a quantum state—the particular configuration of an atom’s energy, spin, etc.—and the transfer of information is successful if the state is reproduced in a separate quantum system some distance away.

Extant quantum networks are capable of either receiving or sending signals, but not both simultaneously. A new experiment reported by Stephan Ritter et al. in Nature has achieved a simple two-node quantum network, in which a single photon successfully transferred the spin state of one rubidium atom to a second atom 21 meters away. Since the nodes are identical, both being rubidium atoms, signals are bi-directional. This type of quantum network should be scalable to encompass more than two nodes, leading to the possibility of larger networks with full communication between arbitrary nodes within them.

The network was constructed by Ritter et al. at the Max Planck Institute for Quantum Optics. Each node consisted of a single neutral rubidium atom in an optical trap, which is a standing wave of laser light. Since this kind of trapping is very delicate, the entire system was cooled to 5 mK (5 thousandths of a degree above absolute zero) to minimize the chance that thermal fluctuations would kick the atom out. The researchers used a control laser to send a photon into the atom, which absorbed it and emitted a new photon. When this is done, the polarization (orientation of the electric field) of the second photon will be correlated with the spin state of the atom. To demonstrate that the information was actually carried by the photon, Ritter et al. used a polarized photon, and checked that the same information was present before and after interacting with the atom.

The key portion of the experiment, however, was to copy the quantum state of the first rubidium atom to the second. To accomplish this, Ritter et al. bounced the photon off their atom, then sent the emitted photon down a fiber optic cable 60 meters long to a second optical trap. (The extra cable length ensured that the coherence of the photon was maintained over greater distances than the space between the two optical traps.) The photon was then absorbed by the second atom, which set its spin state based on the photon’s polarization—which in turn was dictated by the spin state of the first atom. In this way, the “data” of the first node was passed to the second node, with an accuracy between 83 and 85 percent.

This is mostly a proof of concept, but it proves a lot of concepts. Since absorption and emission are both repeatable processes, there is no reason the same two atoms couldn’t be used to send and receive additional messages after the first transfer. Similarly, nothing changes if the atoms’ roles are swapped: the second node can just as easily act as the transmitter. Finally, nothing prevents a larger network of atoms from working in exactly the same way—the system is entirely scalable, although we’d face the problem of controlling which path through the network a particular photon should go.

In addition, after the photon exchange, the two atoms’ states may be correlated—entangled with each other. While this only occurs in 2 percent of the cases, it opens up the possibility of distributed quantum computing across the network, since the results of measurements performed on one atom will determine the outcome of measurements on the other, even across wide distances.

Nature, 2012. DOI: 10.1038/nature11023

Quantum network cartoon
Networks based on single atoms, linked by the exchange of single photons, could form the basis of versatile quantum networks. {Image: Andreas Neuzner/M.P.Q.}

Physicists demonstrate a scalable quantum network that ought to be adaptable for all manner of long-distance quantum communication
First Universal Quantum Network Prototype Links 2 Separate Labs
by John Matson  /  April 11, 2012

Physicists have cleared a bit more of the path to a plausible quantum future by constructing an elementary network for exchanging and storing quantum information. The network features two all-purpose nodes that can send, receive and store quantum information, linked by a fiber-optic cable that carries it from one node to another on a single photon. The network is only a prototype, but if it can be refined and scaled up, it could form the basis of communication channels for relaying quantum information. A group from the Max Planck Institute of Quantum Optics (M.P.Q.) in Garching, Germany,described the advance in the April 12 issue of Nature.

Quantum bits, or qubits, are at the heart of quantum information technologies. An ordinary, classical bit in everyday electronics can store one of two values: a 0 or a 1. But thanks to the indeterminacy inherent to quantum mechanics, a qubit can be in a so-called superposition, hovering undecided between 0 and 1, which adds a layer of complexity to the information it carries. Quantum computers would boast capabilities beyond the reach of even the most powerful classical supercomputers, and cryptography protocols based on the exchange of qubits would be more secure than traditional encryption methods. Physicists have used all manner of quantum objects to store qubits—electrons, atomic nuclei, photons and so on. In the new demonstration, the qubit at each node of the network is stored in the internal quantum state of a single rubidium atom trapped in a reflective optical cavity. The atom can then transmit its stored information via an optical fiber by emitting a single photon, whose polarization state carries the mark of its parent atom’s quantum state; conversely, the atom can absorb a photon from the fiber and take on the quantum state imprinted on that photon’s polarization.

Because each node can perform a variety of functions—sending, receiving or storing quantum information—a network based on atoms in optical cavities could be scaled up simply by connecting more all-purpose nodes. “We try to build a system where the network node is universal,” says M.P.Q. physicist Stephan Ritter, one of the study’s authors. “It’s not only capable of sending or receiving—ideally, it would do all of the things you could imagine.” The individual pieces of such a system had been demonstrated—atoms sending quantum information on single emitted photons, say—but now the technologies are sufficiently advanced that they can work as an ensemble. “This has now all come together and enabled us to realize this elementary version of a quantum network,” Ritter says.

Physicists proposed using optical cavities for quantum networks 15 years ago, because they marry the best features of atomic qubits and photonic qubits—namely that atoms stay put, making them an ideal storage medium, whereas photons are speedy, making them an ideal message carrier between stationary nodes. But getting the photons and atoms to communicate with one another has been a challenge. “If you want to use single atoms and single photons, as we do, they hardly interact,” Ritter adds. That is where the optical cavity comes in. The mirrors of the cavity reflect a photon past the rubidium atom tens of thousands of times, boosting the chances of an interaction. “During this time, there’s enough time to really do this information exchange in a reliable way,” Ritter says. “The cavity enhances the coupling between the light field and the atom.”

The M.P.Q. group put their prototype network through a series of tests—transferring a qubit from a single photon to a single atom and reversing the process to transfer information from an atom onto a photon. Combining those read/write operations, the physicists managed to transmit a qubit from one rubidium atom to another located in a separate laboratory 21 meters away, using a messenger photon as the carrier between nodes. (The actual length of optical fiber connecting the two nodes is 60 meters, because it snakes along an indirect route.) A significant number of the photons get lost along the way, limiting the efficiency of the process. But in principle, optical fibers could connect nodes at greater distances. “We’re absolutely not limited to these 21 meters,” Ritter says. “This 21 meters is just the distance that we happened to have between the two labs.”

The researchers also demonstrated that their photonic link can be used to entangle the two distant atoms. Quantum entanglement is a phenomenon by which two particles share correlated properties—in other words, the quantum state of one particle depends on the state of its entangled partner. Manipulating one of the particles, then, affects the other particle’s state, even if it is located in another laboratory. Researchers hope that entanglement can be harnessed to circumvent the photon losses that come from passage through optical fibers. In a proposed application called a quantum repeater, a series of nodes, linked by entanglement, would extend the quantum connection down the line without depending on any one photon as the carrier. Ritter acknowledges that the new work is simply a prototype, and one for which numerous improvements are possible. For instance, the transfer of a quantum state between labs succeeded only 0.2 percent of the time, owing to various inefficiencies and technical limitations. “Everything is at the edge of what can be done,” he says. “All these characteristics are good enough to do what we’ve done, but there are clear strategies to pursue to make them even better.”

Artist’s rendition of ‘blind quantum computing’ achieved by Broadbent and collaborators


Quantum networking is the practical application of experimental quantum cryptography, like the “blind quantum computing” demonstration by another team of researchers at the University of Vienna’s Center for Quantum Science and Technology earlier this year, which involved transmitting an algorithm to acomputer, running it, and receiving it back without the computer’s operator being able to snoop on those operations. Like its cousin, quantum computing, quantum networking takes advantage of the fact that subatomic particles of matter can exist in multiple states–such as “on” and “off” to reference the binary process by which digital computing operates–at the same time.

Again, this is exceedingly difficult stuff to wrap one’s head around, but suffice to say that these properties enable the quantum bits, or qubits, that power quantum computers and the single-photon data packets developed for the MPQ team’s quantum network to perform their duties much more powerfully and securely than the non-quantum parts used in currently available PC chips and network infrastructure devices. Of course, all of this is still very much in the realm of conjecture. Quantum computing is still highly theoretical, with demonstrations like the MPQ team’s limited to laboratory settings. There are no practical quantum computers,just experimental ones.


Hacker Challenge: Crack Blind Quantum Computing, We Dare You
by Damon Poeter / January 20, 2012

Security professionals would balk at the notion of a computer code so perfectly unbreakable that you couldn’t crack it entering, exiting, or even performing its operations on a system, but a team of scientists says they’ve accomplished exactly that with what’s called “blind quantum computing.” The researchers, led by Stefanie Barz of the University of Vienna’s Center for Quantum Science and Technology, reported in Friday’s issue of Science that they are able to transmit an algorithm to a computer, run it, and receive it back even as the computer’s operator is completely unable to snoop on those operations.

Quantum computing is still highly theoretical, with experiments in the science and its cousin, quantum cryptography, limited to laboratory settings—there are no practical quantum computers, just experimental ones. The basic concept is to use the odd nature of the entangled quantum bits, or qubits, that one uses to build a quantum computer to perform computational tasks much faster and much more securely than is possible on digital computers that use silicon transistors. At the exceedingly tiny level where quantum mechanics operates, particles of matter can exist in multiple states—such as “on” and “off” to reference the binary process by which digital computing operates—at the same time. We may not be able to comprehend what this means outside of mathematics, but scientists have theorized for several decades that harnessing these properties for computing would be a natural way past the issues that loom for today’s nanoscale silicon-based transistors, which are running up against atomic-level barriers to functionality the smaller they get.

While other researchers have described a blind quantum computing protocol, Barz’s team appears to be the first to have actually demonstrated one working. The researchers from Austria, Ireland, Singapore, Canada, and the U.K. write that their findings could pave the way for “unconditionally secure quantum cloud computing.” The team reports that it was able to “exploit the conceptual framework of measurement-based quantum computation that enables a client to delegate a computation to a quantum server” and thus create input, computation, and output processes on the target system in such a way that it “all remain[s] unknown to the computer.”

In less rarified terms, what this means is that in the future you might be able to use a cloud service like Google Docs to do some computational business on someone else’s servers, secure in the knowledge that there is literally no way for the servers’ owner or even the server itself to detect what you’re doing. Thank the Uncertainty Principle for that—simply by observing a quantum computational operation, you would change it. In the case of Deutsch’s algorithm and Grover’s algorithm, which the researchers sent to their quantum computer to perform and then send back to them, it would mean that if you somehow could get a peek at those operations, the intelligibility of what was transpiring would be destroyed before you ever got a chance to look at it. And the process is actually blind going both ways. According to the scientists, whoever sent an algorithm to a quantum computer for it to perform wouldn’t be able to see inside that system either.

Entangled Exploit: Doped Crystals Key to Quantum Networks

We have known for quite some time that quantum storage and quantum communication could vastly improve our current communications technology, but it’s not an easy pursuit. Getting photons to do what we want them to do is even harder than you might expect, so until now quantum communication has been more or less an exercise in educated guessing. Lately,researchers at the University of Calgary along with partners at the German University of Paderborn have been pushing quantum networks closer and closer to reality. They figured out that by “doping” a lithium niobate crystal with rare earth ions and chilling it to -454 Fahrenheit, the crystals can store and retrieve information in entangled photons. It’s quantum memory, the first step toward super-fast and super-secure quantum computers.

Quantum computers make use of that “freaky” quantum phenomenon of entanglement, a fundamental connection between two or more photons that means whatever changes happen to one happen to all other entangled photons. In this study, researchers used the precisely-tuned crystals to produce entangled copies of photons. The crystals and the information-containing photons can be stored and retrieved at will, much in the way that bytes of information are stored in a conventional computer. In other words, information is more or less being stored on a crystal, making quantum networks seem nearly within our reach at last. Similar results were found in a separate study at the University of Geneva, suggesting that the teams are onto something provable.

Atomic blockade: Technique efficiently creates single photons for quantum information processing / Apr 20th, 2012

Using lasers to excite just one atom from a cloud of ultra-cold rubidium gas, physicists have developed a new way to rapidly and efficiently create single photons for potential use in optical quantum information processing – and in the study of dynamics and disorder in certain physical systems. The technique takes advantage of the unique properties of atoms that have one or more electrons excited to a condition of near-ionization known as the Rydberg state. Atoms in this highly excited state – with a principal quantum number greater than 70 – have exaggerated electromagnetic properties and interact strongly with one another. That allows one Rydberg atom to block the formation of additional excited atoms within an area of 10 to 20 microns. That single Rydberg atom can then be converted to a photon, ensuring that – on average – only one photon is produced from a rubidium cloud containing hundreds of densely-packed atoms. Reliably producing a single photon with well known properties is important to several research areas, including quantum information systems.

The new technique was reported April 19 in Science Express (“Photons One-at-a-Time”), the rapid online publication of the journal Science. The research was supported by the National Science Foundation (NSF), and by the Air Force Office of Scientific Research (AFOSR). “We are able to convert Rydberg excitations to single photons with very substantial efficiency, which allows us to prepare the state we want every time,” explained Alex Kuzmich, a professor in the School of Physics at the Georgia Institute of Technology. “This new system offers a fertile area for investigating entangled states of atoms, spin waves and photons. We hope this will be a first step toward doing a lot more with this system.” Kuzmich and co-author Yaroslav Dudin, a graduate research assistant, have been studying quantum information systems that rely on mapping information from atoms onto entangled pairs of photons. But the Raman scattering technique they have been using to create the photons was inefficient and unable to provide the number of entangled photons needed for complex systems. “This new photon source is about a thousand times faster than existing systems,” Dudin said. “The numbers are very good for our first experimental implementation.”

To create a Rydberg atom, the researchers used lasers to illuminate a dense ensemble of several hundred rubidium 87 atoms that had been laser-cooled and confined in an optical lattice. The illumination boosted a single atom from the entire cloud into the Rydberg state. Atoms excited to the Rydberg state strongly interact with other Rydberg atoms, and under suitable conditions, modify the atomic level energies and prevent more than one atom from being transferred into this state – a phenomenon known as the Rydberg blockade. Rydberg atoms show this strong interaction within a range of 10 to 20 microns. By limiting their starting ensemble of rubidium atoms to approximately that distance, Kuzmich and Dudin were able to ensure that no more than one such atom could form. “The excited Rydberg atom needs space around it and doesn’t allow any other Rydberg atoms to come nearby,” Dudin explained. “Our ensemble has a limited volume, so we couldn’t fit more than one of these atoms into the space available.” Kuzmich and Dudin have been using Rydberg atoms with a principal quantum number of approximately 100. These excited atoms are much larger – as much as a half-micron in diameter – than ground state rubidium atoms, which have a quantum number of 5 and a diameter of a few Angstroms.

>Once a highly excited atom was created, the researchers used an additional laser field to convert the excitation into a quantum light field that has the same statistical properties as the excitation. Because the field was produced by a single Rydberg atom, it contained just one photon, which can be used in a variety of protocols. For the Georgia Tech group, the next goal may be development of a quantum gate between light fields. The quantum gating of photons has been proposed and pursued by many research groups, so far unsuccessfully. “If this can be realized, such quantum gates would allow us to deterministically create complex entangled states of atoms and light, which would add valuable capabilities to the fields of quantum networks and computing,” Kuzmich said. “Our work points in this direction.” Beyond quantum information systems, the new single-photon system could also help scientists investigating other areas of physics. “Our results also hold promise for studies of dynamics and disorder in many-body systems with tunable interactions,” Kuzmich explained. “In particular, translational symmetry breaking, phase transitions and non-equilibrium many-body physics could be investigated in the future using strongly-coupled Rydberg excitations of an atomic gas.”

The single-photon work complements research being done in the Kuzmich lab on long-lived quantum memories. A new Air Force Office of Scientific Research Multidisciplinary University Research Initiative (MURI) was recently awarded to a consortium of seven U.S. universities that will work together to determine the best approach for generating quantum memories based on interaction between light and matter. Georgia Tech leads the MURI. “With this new work, we have demonstrated a new, deterministic source of single photons,” Kuzmich said. “In its first experimental realization, it already out-performs other types of single photons that have been pursued during the past decade around the world, including in our group. With further increases in efficiency and generation rate – and integration with long-lived quantum memories being developed in related work – such a single-photon source may make possible optical quantum information processing.”

On the Border Between Matter and Anti-Matter: Nanoscientists Find Long-Sought Majorana Particle / Apr. 13, 2012

Scientists at TU Delft’s Kavli Institute and the Foundation for Fundamental Research on Matter (FOM Foundation) have succeeded for the first time in detecting a Majorana particle. In the 1930s, the brilliant Italian physicist Ettore Majorana deduced from quantum theory the possibility of the existence of a very special particle, a particle that is its own anti-particle: the Majorana fermion. That ‘Majorana’ would be right on the border between matter and anti-matter.
Nanoscientist Leo Kouwenhoven already caused great excitement among scientists in February by presenting the preliminary results at a scientific congress. Today, the scientists have published their research in Science. The research was financed by the FOM Foundation and Microsoft.

Quantum computer and dark matter
Majorana fermions are very interesting — not only because their discovery opens up a new and uncharted chapter of fundamental physics; they may also play a role in cosmology. A proposed theory assumes that the mysterious ‘dark matter’, which forms the greatest part of the universe, is composed of Majorana fermions. Furthermore, scientists view the particles as fundamental building blocks for the quantum computer. Such a computer is far more powerful than the best supercomputer, but only exists in theory so far. Contrary to an ‘ordinary’ quantum computer, a quantum computer based on Majorana fermions is exceptionally stable and barely sensitive to external influences.

For the first time, scientists in Leo Kouwenhoven’s research group managed to create a nanoscale electronic device in which a pair of Majorana fermions ‘appear’ at either end of a nanowire. They did this by combining an extremely small nanowire, made by colleagues from Eindhoven University of Technology, with a superconducting material and a strong magnetic field. “The measurements of the particle at the ends of the nanowire cannot otherwise be explained than through the presence of a pair of Majorana fermions,” says Leo Kouwenhoven.

Particle accelerators
It is theoretically possible to detect a Majorana fermion with a particle accelerator such as the one at CERN. The current Large Hadron Collider appears to be insufficiently sensitive for that purpose but, according to physicists, there is another possibility: Majorana fermions can also appear in properly designed nanostructures. “What’s magical about quantum mechanics is that a Majorana particle created in this way is similar to the ones that may be observed in a particle accelerator, although that is very difficult to comprehend,” explains Kouwenhoven. “In 2010, two different groups of theorists came up with a solution using nanowires, superconductors and a strong magnetic field. We happened to be very familiar with those ingredients here at TU Delft through earlier research.” Microsoft approached Leo Kouwenhoven to help them lead a special FOM programme in search of Majorana fermions, resulting in a successful outcome..

Ettore Majorana
The Italian physicist Ettore Majorana was a brilliant theorist who showed great insight into physics at a young age. He discovered a hitherto unknown solution to the equations from which quantum scientists deduce elementary particles: the Majorana fermion. Practically all theoretic particles that are predicted by quantum theory have been found in the last decades, with just a few exceptions, including the enigmatic Majorana particle and the well-known Higgs boson. But Ettore Majorana the person is every bit as mysterious as the particle. In 1938 he withdrew all his money and disappeared during a boat trip from Palermo to Naples. Whether he killed himself, was murdered or lived on under a different identity is still not known. No trace of Majorana was ever found.

Elusive Majorana fermions may be lurking in a cold nanowire
A nanowire (silver color) is attached to a gold electrode and rests against a superconductor (blue). The combination produces quasiparticles that may be Majorana fermions.

Elusive Majorana fermions may be lurking in a cold nanowire

Inside materials, the interactions between groups of electrons and atoms in the crystal lattice can give rise to a variety of interesting phenomena. Their collective behavior, especially at low temperatures, can give rise to quasiparticles: particle-like excitations that have strikingly different properties than the electrons that form them. Quasiparticles have been discovered that have behaviors predicted by particle physics, but have not been observed in particle collidors.

Researchers in the Netherlands have now produced quasiparticles that act like Majorana fermions: electrically-neutral particles that are their own antiparticles, such that if two collide, they annihilate. The existence of Majorana fermions was first predicted in the 1930s, but no individual particles are known to behave that way. V. Mourik et al. found a quasiparticle version by constructing a very thin wire—a nanowire—of semiconductor material and connected it to a superconductor. The specific electronic properties of the hybrid system gave rise to a pair of zero-velocity quasiparticles at two positions in the nanowire, and these showed behavior consistent with Majorana fermions. Some researchers suggest that quasiparticles of this type would be very useful in quantum computing applications.

Fermions vs. Bosons
Particles and quasiparticles come in two basic types, fermions and bosons, depending on the type of spin they have. The elementary particles of matter (electrons, quarks, and neutrinos) are fermions, while photons and other force carriers are bosons. Particles are paired with antiparticles—antimatter electrons are positrons, etc.—but photons are their own antiparticles. To annihilate, particles and antiparticles must have opposite charge, so Majorana fermions, which are their own antiparticles, need to be electrically-neutral. At present, no fermion is known to be its own antiparticle, although neutrinos may have this property (we don’t yet know).

Theorists predicted the existence of Majorana fermion quasiparticles in a materials known as topological superconductors, in which the interior of the material has zero electrical resistance, but the outside behaves like an ordinary conductor. To create a topological superconductor, Mourik et al. connected a semiconducting indium-antimony nanowire (InSb) between a gold electrode and the edge of a superconductor (NbTiN). They deposited the whole system onto a silicon substrate, which itself was printed with set of logic circuits that read the electronic properties of the wire.

By measuring the relationship between current and voltage at various positions along the nanowire, the researchers found a strong response at two points where the Majorana fermions are expected to appear. These quasiparticles didn’t move under the influence of either a magnetic field or an additional current, indicating that they are electrically neutral and trapped in place. This effect was strongest at 60 millikelvins (60 mK, which is 0.06 degrees above absolute zero) and vanished entirely at temperatures higher than 300 mK. Additionally, Mourik et al. confirmed that these Majorana quasiparticles failed to appear when the superconductor was replaced with another gold electrode, showing that the combination of the nanowire with the superconductor was necessary to create the fermions.

As the researchers themselves note, these results are consistent with Majorana fermions, but they have not been able to test for the presence of some of the predicted properties. Specifically, while the quasiparticles in the nanowire are electrically neutral and trapped at the expected positions, they should also behave in a certain way if their positions are swapped. While that can’t be directly tested in this device, this fundamental property of Majorana fermions can be tested using a superconducting device known as a Josephson junction, a standard technique.

Since the quantum states of Majorana quasiparticles in topological superconductors are not independent of each other, the total system represents a qubit (quantum bit), which has been proposed as another way to achieve working quantum computers (although that may be overselling them). Apart from that, from a pure physics point of view, this result is very important: if these quasiparticles indeed turn out to be Majorana fermions, that will be the first confirmed detection in any physical system.

Science, 2012. DOI: 10.1126/science.1222360  (About DOIs).

Leave a Reply