The quantum Fredkin gate has been experimentally realised for the first time

Researchers from Griffith University and the University of Queensland have overcome one of the key challenges to quantum computing by simplifying a complex quantum logic operation. They demonstrated this by experimentally realising a challenging circuit -- the quantum Fredkin gate -- for the first time.

"The allure of quantum computers is the unparalleled processing power that they provide compared to current technology," said Dr Raj Patel from Griffith's Centre for Quantum Dynamics.

"Much like our everyday computer, the brains of a quantum computer consist of chains of logic gates, although quantum logic gates harness quantum phenomena."

The main stumbling block to actually creating a quantum computer has been in minimising the number of resources needed to efficiently implement processing circuits.

"Similar to building a huge wall out lots of small bricks, large quantum circuits require very many logic gates to function. However, if larger bricks are used the same wall could be built with far fewer bricks," said Dr Patel.

"We demonstrate in our experiment how one can build larger quantum circuits in a more direct way without using small logic gates."

At present, even small and medium scale quantum computer circuits cannot be produced because of the requirement to integrate so many of these gates into the circuits. One example is the Fredkin (controlled- SWAP) gate. This is a gate where two qubits are swapped depending on the value of the third.

Usually the Fredkin gate requires implementing a circuit of five logic operations. The research team used the quantum entanglement of photons -- particles of light -- to implement the controlled-SWAP operation directly.

"There are quantum computing algorithms, such as Shor's algorithm for factorising prime numbers, that require the controlled-SWAP operation.

The quantum Fredkin gate can also be used to perform a direct comparison of two sets of qubits (quantum bits) to determine whether they are the same or not. This is not only useful in computing but is an essential feature of some secure quantum communication protocols where the goal is to verify that two strings, or digital signatures, are the same," said Professor Tim Ralph from the University of Queensland.

Professor Geoff Pryde, from Griffith's Centre for Quantum Dynamics, is the project's chief investigator.

"What is exciting about our scheme is that it is not limited to just controlling whether qubits are swapped, but can be applied to a variety of different operations opening up ways to control larger circuits efficiently," said Professor Pryde.

"This could unleash applications that have so far been out of reach."

The team is part of the Australian Research Council's Centre for Quantum Computation and Communication Technology, an effort to exploit Australia's strong expertise in developing quantum information technologies.

The research has been published as A quantum Fredkin gate in Science Advances(DOI:10.1126/sciadv.1501531)

Griffith University

New quantum computer, based on five atoms

Researchers have designed and built a quantum computer from five atoms in an ion trap. The computer uses laser pulses to carry out Shor’s algorithm on each atom, to correctly factor the number 15. Image: Jose-Luis Olivares/MIT

The beginning of the end for encryption schemes?

What are the prime factors, or multipliers, for the number 15? Most grade school students know the answer — 3 and 5 — by memory. A larger number, such as 91, may take some pen and paper. An even larger number, say with 232 digits, can (and has) taken scientists two years to factor, using hundreds of classical computers operating in parallel.

Because factoring large numbers is so devilishly hard, this “factoring problem” is the basis for many encryption schemes for protecting credit cards, state secrets, and other confidential data. It’s thought that a single quantum computer may easily crack this problem, by using hundreds of atoms, essentially in parallel, to quickly factor huge numbers.

In 1994, Peter Shor, the Morss Professor of Applied Mathematics at MIT, came up with a quantum algorithm that calculates the prime factors of a large number, vastly more efficiently than a classical computer. However, the algorithm’s success depends on a computer with a large number of quantum bits. While others have attempted to implement Shor’s algorithm in various quantum systems, none have been able to do so with more than a few quantum bits, in a scalable way.

Now, in a paper published today in the journal Science, researchers from MIT and the University of Innsbruck in Austria report that they have designed and built a quantum computer from five atoms in an ion trap. The computer uses laser pulses to carry out Shor’s algorithm on each atom, to correctly factor the number 15. The system is designed in such a way that more atoms and lasers can be added to build a bigger and faster quantum computer, able to factor much larger numbers. The results, they say, represent the first scalable implementation of Shor’s algorithm.

“We show that Shor’s algorithm, the most complex quantum algorithm known to date, is realizable in a way where, yes, all you have to do is go in the lab, apply more technology, and you should be able to make a bigger quantum computer,” says Isaac Chuang, professor of physics and professor of electrical engineering and computer science at MIT. “It might still cost an enormous amount of money to build — you won’t be building a quantum computer and putting it on your desktop anytime soon — but now it’s much more an engineering effort, and not a basic physics question.”

Seeing through the quantum forest

In classical computing, numbers are represented by either 0s or 1s, and calculations are carried out according to an algorithm’s “instructions,” which manipulate these 0s and 1s to transform an input to an output. In contrast, quantum computing relies on atomic-scale units, or “qubits,” that can be simultaneously 0 and 1 — a state known as a superposition. In this state, a single qubit can essentially carry out two separate streams of calculations in parallel, making computations far more efficient than a classical computer.

In 2001, Chuang, a pioneer in the field of quantum computing, designed a quantum computer based on one molecule that could be held in superposition and manipulated with nuclear magnetic resonance to factor the number 15. The results, which were published in Nature, represented the first experimental realization of Shor’s algorithm. But the system wasn’t scalable; it became more difficult to control the system as more atoms were added.

“Once you had too many atoms, it was like a big forest — it was very hard to control one atom from the next one,” Chuang says. “The difficulty is to implement [the algorithm] in a system that’s sufficiently isolated that it can stay quantum mechanical for long enough that you can actually have a chance to do the whole algorithm.”

“Straightforwardly scalable”

Chuang and his colleagues have now come up with a new, scalable quantum system for factoring numbers efficiently. While it typically takes about 12 qubits to factor the number 15, they found a way to shave the system down to five qubits, each represented by a single atom. Each atom can be held in a superposition of two different energy states simultaneously. The researchers use laser pulses to perform “logic gates,” or components of Shor’s algorithm, on four of the five atoms. The results are then stored, forwarded, extracted, and recycled via the fifth atom, thereby carrying out Shor’s algorithm in parallel, with fewer qubits than is typically required.

The team was able to keep the quantum system stable by holding the atoms in an ion trap, where they removed an electron from each atom, thereby charging it. They then held each atom in place with an electric field.

“That way, we know exactly where that atom is in space,” Chuang explains. “Then we do that with another atom, a few microns away — [a distance] about 100th the width of a human hair. By having a number of these atoms together, they can still interact with each other, because they’re charged. That interaction lets us perform logic gates, which allow us to realize the primitives of the Shor factoring algorithm. The gates we perform can work on any of these kinds of atoms, no matter how large we make the system.”

Chuang’s team first worked out the quantum design in principle. His colleagues at the University of Innsbruck then built an experimental apparatus based on his methodology. They directed the quantum system to factor the number 15 — the smallest number that can meaningfully demonstrate Shor’s algorithm. Without any prior knowledge of the answers, the system returned the correct factors, with a confidence exceeding 99 percent.

“In future generations, we foresee it being straightforwardly scalable, once the apparatus can trap more atoms and more laser beams can control the pulses,” Chuang says. “We see no physical reason why that is not going to be in the cards.”

Mark Ritter, senior manager of physical sciences at IBM, says the group’s method of recycling qubits reduces the resources required in the system by a factor of 3 — a significant though small step towards scaling up quantum computing.

“Improving the state-of-the-art by a factor of 3 is good,” says Ritter. But truly scaling the system “requires orders of magnitude more qubits, and these qubits must be shuttled around advanced traps with many thousands of simultaneous laser control pulses.”

If the team can successfully add more quantum components to the system, Ritter says it will have accomplished a long-unrealized feat.

“Shor's algorithm was the first non-trivial quantum algorithm showing a potential of ‘exponential’ speed-up over classical algorithms,” Ritter says. “It captured the imagination of many researchers who took notice of quantum computing because of its promise of truly remarkable algorithmic acceleration. Therefore, to implement Shor's algorithm is comparable to the ‘Hello, World’ of classical computing.”

What will all this eventually mean for encryption schemes of the future?

“Well, one thing is that if you are a nation state, you probably don’t want to publicly store your secrets using encryption that relies on factoring as a hard-to-invert problem,” Chuang says. “Because when these quantum computers start coming out, you’ll be able to go back and unencrypt all those old secrets.”

MIT

Researchers have created a model biological supercomputer that is both sustainable and highly energy efficient

The model bio-supercomputer is powered by adenosine triphosphate (ATP), the substance that provides energy to all of the cells in a human body. The model is able to process information extremely quickly and accurately using parallel networks, in the same fashion that electronic supercomputers are able to process information.

However, the bio-supercomputer developed by the project team is much smaller and more energy efficient than the current generation of electronic supercomputers, being only the size of a standard-sized book.

The model bio-supercomputer was created with a combination of geometrical modelling and engineering expertise on the nano-scale. Importantly, it is the first step in showing that a biological supercomputer could realistically work in practice.

Small, portable and energy efficient

The circuit created by the researchers is around 1.5 cm square and instead of electrons being propelled by an electrical charge, as is the case with a traditional microchip, short strings of proteins (called ‘biological agents’ by the project team) travel around the circuit in a controlled way. These movements are powered by ATP, a biochemical that enables internal energy transfer among cells.

Traditional supercomputers use a large amount of electricity and thus heat up to such high temperatures that they need to be physically cooled in order to function effectively. To do this, many supercomputers often require their own dedicated power plant.

In contrast, due to being run by biological agents, the bio-supercomputer hardly heats up at all and is consequently much more sustainable and cost-effective. As the technology is developed further over the coming years and possible routes to larger-scale commercialisation are considered, this could become a major selling-point.

Calculating answers to major societal issues

Although the model bio-supercomputer has successfully and efficiently tackled a complex mathematical problem by using parallel computing in the same fashion as traditional supercomputers, the project team recognises that there is still a long way to go between the model and the development of a full-scale functional bio-supercomputer.

It is hoped that an eventual shift to bio-supercomputers will provide solutions to the growing problem of traditional supercomputers being increasingly unable to quickly calculate answers to some of society’s most pressing issues, such as the development of new drugs and ensuring that engineering systems work as they are supposed to. For these problems, computers have to simply go through all of the possible guesses before reaching the correct answer. This means that if the problem size increases even modestly, the computer can no longer solve it quickly enough to be useful.

Next steps: From science fiction to science

The project team has already begun to explore other avenues on how to push their research even further, and hope that other scientists will be encouraged to also construct new models using alternative biological materials.

The eventual goal would be to perfect the design for a new generation of smaller, more portable and more energy efficient bio-supercomputers that can fully replace traditional supercomputers.

Although the research team believes that it will still take some time for this to become a reality, a potential mid-term solution would be to produce a hybrid design, mixing traditional and biological technologies.
 

ABACUS Project

Using Atoms to Turn Optical Nanofiber Guided Light On and Off

Researchers in the Light-Matter Interactions Unit led by Professor Síle Nic Chormaic at the Okinawa Institute of Science and Technology Graduate University (OIST) have developed an on-off switch with ultrathin optical fibers, which could be used for data transfer in the future. This research was published in the New Journal of Physics.

0101000001101000011110010111001101101001011000110111001100100000011010010111001100100000011001100111010101101110 means “Physics is fun” in binary code. Computers translate every letter, number, sign, space, image and sound to a set of 8 ones and zeros. For example, 01010000 corresponds to the letter P. While you type, your computer transfers your words to another distant computer by sending a series of ones and zeros encoded in light through standard optical fibers. Switching the light beam on and off very quickly generates the ones and zeros. These bits of information are converted to electronic signals at a node, usually a router or server, and finally appear as text on the screen of your recipient. While this is the classical way of transferring information online, OIST researchers are exploring more efficient ways of transferring data, using the quantum properties of light and matter. They have managed to create an on/off switch based on the quantum characteristics of rubidium atoms in the presence of light of different wavelengths. This proof-of-concept system could be used as a building block in a quantum network, the future of our internet.

The OIST team’s experimental setup consists of two lasers that produce light at different wavelengths, an optical nanofiber used to guide light, and rubidium atoms trapped around it. The peculiarity of optical nanofibers is their super-thin diameter. For this study the diameter was 350 nanometers, about 300 times thinner than the thickness of a sheet of paper. The diameter is even smaller than the wavelength of the light guided by the fiber. Some of the light, therefore, leaks outside the nanofiber and interacts with the rubidium atoms that are trapped around it. These atoms can function as a quantum node, a redistribution point of a network, the equivalent of today’s servers.

The off switch condition is obtained when only the laser producing 780 nm is on. In this case, at the point where light leaks outside of the optical nanofiber, the rubidium atoms absorb the maximum amount of light and almost no light can continue to pass along the fiber. In contrast, the switch is turned on when both 776 nm and 780 nm lights are present. In this situation, most of the light is transmitted through the optical nanofiber and the rubidium atoms absorb it only minimally.

Since the optical nanofiber is directly connected to a standard optical fiber, the light can, in principle, be transferred to another quantum system or node some distance away, in the same way you can send a message from your computer to that of your friend’s in another location. 

“Using optical nanofibers would allow us to fully integrate our system with existing fiber-based communication networks. While the current work is far from being a practical solution to quantum information, it brings the notion of using atoms and light to develop real devices based on quantum mechanics ever closer to fulfilment”, explains Professor Síle Nic Chormaic.

While the experiment at OIST currently only generates zeros/off and ones/on consecutively, further exploitation of the quantum behavior of atoms should allow the research team to send light as a combination of “on” and “off” at the same time. In this way, in the future, quantum networks will be able to process more data simultaneously, increase efficiency of information transfer and also provide better cyber security.

“It has been very exciting to work with optical nanofibers which can guide light extremely efficiently even if their diameter is much smaller than the wavelength of light itself. These systems are sure to give us significant progress in quantum networks in the years to come,” enthuses Ravi Kumar, one of the authors of this study and a PhD student at University College Cork in Ireland, doing his research work at OIST.

Okinawa Institute of Science and Technology Graduate University

Tapping Particles of Light

At the Weizmann Institute of Science, researchers have managed to “pluck” a single photon – one particle of light – out of a pulse of light. The findings of this research,which appeared this week in Nature Photonics, bear both fundamental and practical significance: Light is the workhorse of today’s communication systems, and single photons are likely to be the backbone of future quantum communication systems. In addition, say the scientists, the apparatus they have devised will spur further research into the fundamental particle nature of light.  

“Once we move over to quantum communication, information will have to be encoded in single photons,” says Dr. Barak Dayan, head of the Weizmann Institute Quantum Optics group. “Each photon will then represent a single ‘qubit’ – a quantum bit that can exist in more than one state at the same time (for example, an equal combination of both 1 and 0).”

Dayan and his research team, led by Dr. Serge Rosenblum and Orel Bechler, set out to demonstrate a scheme for pulling just one photon out of a stream, on demand. Their mechanism relies on a physical effect that they call single-photon Raman interaction, or SPRINT, which is based on a single atom, or atom-like system. “The advantage of SPRINT,” says Dayan, “is that it is completely passive - it does not require any control fields, just the interaction between the atom and the optical pulse.” In previous research, he and his team had employed SPRINT as a switch for single photons that sent them down different pathways, effectively turning the apparatus into a photonic router. In this work, the atom becomes a tap rather than a switch, snatching one photon from the flow and then turning itself off. “It is not trivial,” says Dayan, “to have a mechanism that continues to function even in high fluxes of photons and to remove just one photon.” 

The experimental setup of Weizmann’s quantum optics group relies on state-of-the-art technologies: laser cooling and trapping of atoms (in this case rubidium), the fabrication of chip-based, ultrahigh-quality glass microspheres, and optical nanofibers. 

“The ability to divert a single photon from a flow could be harnessed for various tasks,” says Dayan, “from creating nonclassical states of light that are useful for basic scientific research, through eavesdropping on imperfect quantum-cryptography systems that rely on single photons, to increasing the security of your own quantum-communication systems. 

The existence of photons was first suggested by Einstein in 1905, yet many of their properties are just now coming to light. Dayan believes their new method will expand our capabilities to study and control them as individual particles.

Weizmann Institute of Science 

What are these electronic nanostars in 2D-superconductors supposed to mean?

Physicists from France and Russia have discovered magnetic disturbances in two-dimensional (2D) layered superconductors, resembling small oscillating stars. These star-like electronic excitations are located around individual magnetic atoms put inside the superconducting material. This experimental observation, made by means of scanning tunneling spectroscopy at only 0.3 degrees above the absolute zero-temperature, is the direct confirmation of the famous theory by Yu-Shiba-Rusinov who predicted these quantum bound magnetic sates to exist. Moreover, it was found out that in the two-dimensional systems the magnetic excitations spread over longer distances as compared to ordinary three dimensional superconducting materials. This discovery opens a route for generating more complex quantum states, topologically protected from decoherence, out of chains or clusters of magnetic atoms in superconductors. Building and manipulating such protected states is a crucial step towards the long-awaited quantum computers. The results were published in Nature Physics this week.

Prof. Dmitri Roditchev from the Superior School of Industrial Physics and Chemistry (ESPCI ParisTech, Paris), Gerbold Ménard,Dr. Christophe Brun, Dr. Tristan Cren from the Institute of Nanosciences of Paris at Sorbonne University, Dr. Vasily Stolyarov from the Laboratory of Topological Quantum Phenomena in Superconducting Systems at MIPT, and their colleagues from Paris-Saclay University studied the emergence of Yu-Shiba-Rusinov (YSR) states bound around single magnetic atoms embedded in a two-dimensional superconductor. YSR states were theoretically predicted in the 1960s, but very few evidences for them have been experimentally revealed till now. In the present work it was found that in two-dimensional systems, magnetic excitations extend over a greater distance as compared to ordinary three-dimensional superconductors, and the emergent YSR quantum states are more stable, which makes them more suitable for developing a new generation of quantum electronics.

A crystal lattice of a layered superconducting material  -niobium diselenide - was used in the tests. With an ultra-low-temperature scanning tunnelling microscope built by Roditchev, the researchers were able to observe , for the first time, YSR states around single magnetic atoms of iron.  “We have demonstrated that the use of two-dimensional superconductors instead of the three dimensional ones results in an increase in the spatial extension of YSR states for several dozen nanometres, i.e. ten times further than in “normal” three-dimensional superconductors. And the area of excitation was shaped like a six-fold electronic “star” with its rays extending along the axis of the crystal lattice of niobium diselenide. The observed “stars” are more stable and more suitable to creating new topologically protected states. Non-Abelian anyons can be collected from the YSR state chains, and can then be used as elements in quantum computers of the future,” says Vasily Stolyarov, a co-author of the study and the head of the Laboratory of Topological Quantum Phenomena in Superconducting Systems at MIPT.

The experiments described in the article were conducted in Paris. Work is underway at MIPT’s Laboratory of Topological Quantum Phenomena in Superconducting Systems to create the experimental conditions necessary to obtain such high quality results. The Laboratory was set up in 2014 using funds from a mega grant awarded to Alexander Golubov, a professor  at the University of Twente (Netherlands). The main purpose of the Laboratory is to study the quantum properties of new superconductors and topologically protected materials, and also hybrid artificial systems based on these materials.

 The laboratory is being equipped in close cooperation with Roditchev’s and Cren’s groups and it is expected that the facilities at both laboratories will complement one another.

 Yu-Shiba-Rusinov states were predicted in the late 1960s by three physicists from China, the USSR, and Japan independently of one another. They suggested that magnetic atoms introduced into a superconductor must create special states of excitation around themselves – electron-hole standing waves named after their discoverers. Calculations show that areas of topological conductivity may form around these states, where the current is only able to flow in one direction. Until recently, however, it had not been possible to confirm this prediction experimentally.

For the last 20 years, scientists have been attempting to create quantum systems that will outperform traditional semiconductor-based computers, the development potential of which is now almost exhausted. A number of potential “candidate” systems to be used as a base to build the components of a quantum computer are currently being investigated. The main problem preventing the development of these computers is the high sensitivity of the nanoworld to external influences that destroy quantum states. One promising option is to use topologically protected electron states that are resistant to decoherence. Non-Abelian anyons may be perfect for this; they are not negative ions, but rather special excitations in two-dimensional quantum systems in a magnetic field. 

The theory predicts that such non-Abelian anyons may occur in a two-dimensional “liquid” of electrons in a superconductor under the influence of a local magnetic field. The electron liquid thus becomes degenerate, i.e. the electrons can have different states at the same energy level. The superposition of several anyons cannot be affected without moving them, therefore they are completely protected from disturbances.

Moscow Institute of Physics and Technology