Quantum computing is expected to increase significantly the computational speed since it operates on completely different principles in comparison to classical computers. In addition, it was predicted that quantum computers can increase the computational speed of at least some problems. For instance, a sub-exponential speed-up is expected if the quantum Shor algorithm is applied to the problem of finding the prime factors of an integer number, the quantum Grover algorithm quadratically increases the speed of searching for an item in an unordered list, and an exponential speed-up is foreseen for simulating the dynamics of quantum systems by quantum computers.
Normal digital computers operate on the basis of a binary code composed of bits with a value of either 0 or 1. In quantum computers, the bits are replaced by qubits, which can be in two states simultaneously, with arbitrary superposition. This significantly boosts their calculation and storage capacity for certain classes of applications. But making qubits is no mean feat: quantum phenomena require highly controlled conditions, including very low temperatures.
On the other hand, quantum computing systems are very sensitive to the environment, and therefore can only be implemented in small physical systems, with only few degrees of freedom, in which the energy difference between the quantized states are larger than the thermal energy.
In the race to produce a quantum computer, a number of projects are seeking a way to create quantum bits – or qubits – that are stable, meaning they are not much affected by changes in their environment. This normally needs highly nonlinear non-dissipative elements capable of functioning at very low temperatures. Quantum computing can be implemented in small, nano-sized systems, such as trapped atom or ions, spin states, superconductors or conductors in the ballistic transport regime. Graphene is a suitable material for implementing quantum logic gates due to its large room-temperature mean free path.
Graphene for implementing Quantum computer
Graphene, having spurred research into numerous novel directions, is naturally also considered as a candidate material host for qubits. For example, back in 2013, a team of researchers from MIT found that graphene can be made into a topological insulator – meaning that electrons with one spin direction move around the graphene edges clockwise, whereas those that have the opposite spin move counterclockwise. They made this happen by applying two magnetic fields: one perpendicular to the graphene sheet, to make the electrons flow at sheet edges only, and another parallel to the sheet, that separates the two spin contributions.
Electron spin has long been considered a candidate qubit, because it is inherently a quantum system that is in a superposition of states. In graphene, the spins move along the sheet edges robustly, without much decoherence. Furthermore, the same research showed switching the spin selection on and off, an important feature of q-bit transistors. Nevertheless, extreme conditions such as strong magnetic fields and temperatures near absolute zero are required for this effect in graphene, raising questions about real-world applicability.
This year, the same group discovered a new kind of quantum state that appears when graphene is sandwiched between two superconductors. In this situation the electrons in graphene, formerly behaving as individual, scattering particles, instead pair up in “Andreev states” — a fundamental electronic configuration that allows a conventional, non-superconducting material to carry a “super-current,” an electric current that flows without dissipating energy.
Andreev states, like the spin qubits, have very little decoherence, due to their paired configuration. These states are predicted to give rise to Majorana fermions, exotic particles that can be used for quantum computing. Although this experiment is also performed at low temperatures, it is an important proof-of-concept that should in the future open doors towards practical realizations of quantum computing.
University of Vienna and Institute of Photonic Sciences propose graphene-based, two-photon quantum logic gates
In May 2019, Researchers from the University of Vienna and Institute of Photonic Sciences in Barcelona have proposed graphene-based, two-photon quantum logic gate for use in universal quantum computing.
Optical approaches to quantum computing have long looked interesting because of the robustness and mobility of single photons but those approaches have been difficult to achieve. “We propose a universal two-qubit quantum logic gate, where qubits are encoded in surface plasmons in graphene nanostructures, that exploits graphene’s strong third-order nonlinearity and long plasmon lifetimes to enable single-photon-level interactions,” report the researchers led by Philip Walther at the University of Vienna.
“Our gate does not require any cryogenic or vacuum technology, has a footprint of a few hundred nanometers, and reaches fidelities and success rates well above the fault-tolerance threshold, suggesting that graphene plasmonics offers a route towards scalable quantum technologies,” write the researchers in their paper.
As the Phys.org article notes, it was only recently realized that nonlinear interactions can be greatly enhanced by using plasmons. In a plasmon, “light is bound to electrons on the surface of the material. These electrons can then help the photons to interact much more strongly.” However, plasmons in standard materials decay before the needed quantum effects can take place.
“In their proposed graphene quantum logic gate, the scientists show that if single plasmons are created in nanoribbons made out of graphene, two plasmons in different nanoribbons can interact through their electric fields. Provided that each plasmon stays in its ribbon multiple gates can be applied to the plasmons which is required for quantum computation. “We have shown that the strong nonlinear interaction in graphene makes it impossible for two plasmons to hop into the same ribbon,” says Irati Alonso Calafell, first author of the study.
The researchers believe their work will indeed be applicable in many quantum information science applications: “By combining ideas from quantum optics with nanoplasmonics, our work opens up an entirely new and promising avenue in the search for single-photon nonlinearities. While we have studied the application of graphene nanoplasmonics to a quantum logic gate, this could also be used for deterministic optical implementations of quantum teleportation,cluster state generation,and single- photon sources,underlining the applicability of this platform.”
EPFL’s Laboratory of Photonics and Quantum Measurements build a quantum capacitor that can create stable qubits
Researchers at EPFL’s Laboratory of Photonics and Quantum Measurements have been working to build a quantum capacitor that can create stable qubits (the units of information storage in quantum computers) that are also resistant to common electromagnetic interference. Such a capacitor is easier to produce using a two dimensional material — such as graphene.
In pursuit of this goal, researchers at EPFL’s Laboratory of Photonics and Quantum Measurements LPQM (STI/SB), have investigated a nonlinear graphene-based quantum capacitor, compatible with cryogenic conditions of superconducting circuits, and based on two-dimensional (2D) materials.
The At EPFL’s capacitor consists of insulating boron nitride sandwiched between two graphene sheets. Thanks to this sandwich structure and graphene’s unusual properties, the incoming charge is not proportional to the voltage that is generated. This nonlinearity is a necessary step in the process of generating quantum bits. In this system small changes in, for example, the intensity of an incident laser beam, give rise to large changes in the measured capacitance of the device.
The researchers calculate that one single incident photon could be enough to change qubit states, which is an ideal case of a qubit. Again, low temperatures are required for operation, however a significant advantage of this design is that there is no need for external magnetic fields, rendering this solution a step closer to practical applications.
When connected to a circuit, this capacitor has the potential to produce stable qubits and also offers other advantages, such as being relatively easier to fabricate than many other known nonlinear cryogenic devices, and being much less sensitive to electromagnetic interference. This research was published in NPJ 2D Materials and Applications.
This device could significantly improve the way quantum information is processed but there are also other potential applications too. It could be used to create very nonlinear high-frequency circuits – all the way up to the terahertz regime – or for mixers, amplifiers, and ultra strong coupling between photons.
Radiation-free quantum technology with graphene reported in July 2021
Rare-earth compounds have fascinated researchers for decades due to the unique quantum properties they display, which have so far remained totally out of reach of everyday compounds. One of the most remarkable and exotic properties of those materials is the emergence of exotic superconducting states, and particularly the superconducting states required to build future topological quantum computers. While these specific rare-earth compounds, known as heavy fermion superconductors, have been known for decades, making usable quantum technologies out of them has remained a critically open challenge. This is because these materials contain critically radioactive compounds, such as uranium and plutonium, rendering them of limited use in real-world quantum technologies.
New research has now revealed an alternative pathway to engineer the fundamental phenomena of these rare-earth compounds solely with graphene, which has none of the safety problems of traditional rare-earth compounds. The exciting result in the new paper shows how a quantum state known as a “heavy fermion” can be produced by combining three twisted graphene layers. A heavy fermion is a particle — in this case an electron — that behaves like it has a lot more mass than it actually does. The reason it behaves this way stems from unique quantum many-body effects that were mostly only observed in rare-earth compounds until now. This heavy fermion behavior is known to be the driving force of the phenomena required to use these materials for topological quantum computing. This new result demonstrates a new, non-radioactive way of achieving this effect using only carbon, opening up a pathway for sustainably exploiting heavy fermion physics in quantum technologies.
In the paper authored by Aline Ramires, (Paul Scherrer Institute, Switzerland) and Jose Lado (Aalto University), the researchers show how it is possible to create heavy fermions with cheap, non-radioactive materials. To do this, they used graphene, which is a one-atom thick layer of carbon. Despite being chemically identical to the material that is used in regular pencils, the sub-nanometre thickness of graphene means that it has unexpectedly unique electrical properties. By layering the thin sheets of carbon on top of one another in a specific pattern, where each sheet is rotated in relation to the other, the researchers can create the quantum properties effect that results in the electrons in the graphene behaving like heavy fermions.
“Until now, practical applications of heavy fermion superconductors for topological quantum computing has not been pursued much, partially because it required compounds containing uranium and plutonium, far from ideal for applications due to their radioactive nature,” says Professor Lado, “In this work we show that one can aim to realize the exactly very same physics just with graphene. While in this work we only show the emergence of heavy fermion behavior, addressing the emergence of topological superconductivity is a natural next step, which could potentially have a groundbreaking impact for topological quantum computing.”
Topological superconductivity is a topic of critical interest for quantum technologies, also tackled by alternative strategies in other papers from Aalto University Department of Applied Physics, including a previous paper by Professor Lado. “These results potentially provide a carbon-based platform for exploitation of heavy fermion phenomena in quantum technologies, without requiring rare-earth elements,” concludes Professor Lado.
References and Resources also include: