Quantum technologies have the potential to spur revolutions in computing, sensing, cryptography and beyond. Quantum technologies make use of the often peculiar rules that govern the behavior of individual particles. In the quantum world, particles can behave as if they are in more than one state at a given time, and influence each other’s behavior even if they are far away in space. By taking advantage of those properties, quantum computers can process information in new ways, potentially performing calculations far beyond the reach of even the fastest of today’s supercomputers. Quantum sensors far more powerful than those used today could be useful in applications ranging from medicine to seismology. And quantum cryptography could lead to intrinsically secure communication.
For all the promise, however, there are obstacles to overcome before these quantum technologies deliver their full potential. One problem is that quantum states are extremely fragile — the slightest disturbance can destroy them. And scientists still don’t fully understand how to model the complex correlations between particles in quantum systems. Those microscopic correlations are critically important, because they ultimately determine the properties of a material at the macroscopic scale.
There are many types of quantum bits, or qubits, ranging from those using trapped ions, superconducting loops or photons. Quantum dots have also been suggested as implementations of qubits for quantum information processing. Quantum dots (QD) are very small semiconductor particles, that have a radius of a few nanometres (1 nm = a billionth of a metre). They are a central theme in nanotechnology. Many types of quantum dot will emit light of specific frequencies if electricity or light is applied to them, and these frequencies can be precisely tuned by changing the dots’ size, shape and material, giving rise to many applications.
The quantum computing concepts based on quantum dots can be subdivided in two main branches: optical concepts and electrical concepts. In most of the optical concepts, the two level system representing the quantum bit (qubit) consists of exciton states. These are manipulated using polarized light. When a QD is provided with energy,” explains Prof. Felici, “via luminous radition or electric impluses, for example, the system first shifts to an excited state and then loses energy by emitting light or photons. By correctly exciting a QD, it can be primed to emit a single photon for each excitation impulse. This ability is extremely interesting, especially in view of the use of single photons as “quantum bits” or qubits in computing and quantum information protocols.
In electrical concepts, the spin states of electrons are used as qubit and manipulation can be done all-electrically. Quantum dots enable stable spin qubits with low error rates for building Quantum computers. A ‘spin qubit’ is a quantum bit that encodes information based on the quantised magnetic direction of a quantum object, such as an electron. “
Electrical engineering professor Supriyo Bandyopadhyay is attempting to make quantum computers with quantum dots.”The process is difficult” said Bandyopadhyay, “but the payoff is tremendous.”
Artificial atoms create stable qubits for quantum computing, reported in Feb 2020
In a breakthrough for quantum computing, researchers at UNSW Sydney have made improved qubits using concepts from high school chemistry. In a paper published in Nature Communications, UNSW quantum computing researchers describe how they created artificial atoms in a silicon ‘quantum dot’, a tiny space in a quantum circuit where electrons are used as qubits (or quantum bits), the basic units of quantum information.
Scientia Professor Andrew Dzurak explains that unlike a real atom, an artificial atom has no nucleus, but it still has shells of electrons whizzing around the centre of the device, rather than around the atom’s nucleus “The idea of creating artificial atoms using electrons is not new, in fact it was first proposed theoretically in the 1930s and then experimentally demonstrated in the 1990s – although not in silicon. We first made a rudimentary version of it in silicon back in 2013,” says Professor Dzurak, who is an ARC Laureate Fellow and is also director of the Australian National Fabrication Facility at UNSW, where the quantum dot device was manufactured.
“But what really excites us about our latest research is that artificial atoms with a higher number of electrons turn out to be much more robust qubits than previously thought possible, meaning they can be reliably used for calculations in quantum computers. This is significant because qubits based on just one electron can be very unreliable.”
Professor Dzurak likens the different types of artificial atoms his team has created to a kind of periodic table for quantum bits, which he says is apt given that 2019 – when this ground-breaking work was carried out – was the International Year of the Periodic Table. “If you think back to your high school science class, you may remember a dusty chart hanging on the wall that listed all the known elements in the order of how many electrons they had, starting with Hydrogen with one electron, Helium with two, Lithium with three and so on. “You may even remember that as each atom gets heavier, with more and more electrons, they organise into different levels of orbit, known as ‘shells’. “It turns out that when we create artificial atoms in our quantum circuits, they also have well organised and predictable shells of electrons, just like natural atoms in the periodic table do.”
Professor Dzurak and his team from UNSW’s School of Electrical Engineering – including PhD student Ross Leon who is also lead author in the research, and Dr Andre Saraiva – configured a quantum device in silicon to test the stability of electrons in artificial atoms. They applied a voltage to the silicon via a metal surface ‘gate’ electrode to attract spare electrons from the silicon to form the quantum dot, an infinitesimally small space of only around 10 nanometres in diameter. “As we slowly increased the voltage, we would draw in new electrons, one after another, to form an artificial atom in our quantum dot,” says Dr Saraiva, who led the theoretical analysis of the results.
“In a real atom, you have a positive charge in the middle, being the nucleus, and then the negatively charged electrons are held around it in three dimensional orbits. In our case, rather than the positive nucleus, the positive charge comes from the gate electrode which is separated from the silicon by an insulating barrier of silicon oxide, and then the electrons are suspended underneath it, each orbiting around the centre of the quantum dot. But rather than forming a sphere, they are arranged flat, in a disc.”Mr Leon, who ran the experiments, says the researchers were interested in what happened when an extra electron began to populate a new outer shell. In the periodic table, the elements with just one electron in their outer shells include Hydrogen and the metals Lithium, Sodium and Potassium.
“When we create the equivalent of Hydrogen, Lithium and Sodium in the quantum dot, we are basically able to use that lone electron on the outer shell as a qubit,” Ross says. “Up until now, imperfections in silicon devices at the atomic level have disrupted the way qubits behave, leading to unreliable operation and errors. But it seems that the extra electrons in the inner shells act like a ‘primer’ on the imperfect surface of the quantum dot, smoothing things out and giving stability to the electron in the outer shell.”
Achieving stability and control of electrons is a crucial step towards silicon-based quantum computers becoming a reality. Where a classical computer uses ‘bits’ of information represented by either a 0 or a 1, the qubits in a quantum computer can store values of 0 and 1 simultaneously. This enables a quantum computer to carry out calculations in parallel, rather than one after another as a conventional computer would. The data processing power of a quantum computer then increases exponentially with the number of qubits it has available.It is the spin of an electron that we use to encode the value of the qubit, explains Professor Dzurak.
“Spin is a quantum mechanical property. An electron acts like a tiny magnet and depending on which way it spins its north pole can either point up or down, corresponding to a 1 or a 0. “When the electrons in either a real atom or our artificial atoms form a complete shell, they align their poles in opposite directions so that the total spin of the system is zero, making them useless as a qubit. But when we add one more electron to start a new shell, this extra electron has a spin that we can now use as a qubit again. “Our new work shows that we can control the spin of electrons in the outer shells of these artificial atoms to give us reliable and stable qubits. This is really important because it means we can now work with much less fragile qubits. One electron is a very fragile thing. However an artificial atom with 5 electrons, or 13 electrons, is much more robust.”
Professor Dzurak’s group was the first in the world to demonstrate quantum logic between two qubits in silicon devices in 2015, and has also published a design for a full-scale quantum computer chip architecture based on CMOS technology, which is the same technology used to manufacture all modern-day computer chips. “By using silicon CMOS technology we can significantly reduce the development time of quantum computers with the millions of qubits that will be needed to solve problems of global significance, such as the design of new medicines, or new chemical catalysts to reduce energy consumption”, says Professor Dzurak. In a continuation of this latest breakthrough, the group will explore how the rules of chemical bonding apply to these new artificial atoms, to create ‘artificial molecules’. These will be used to create improved multi-qubit logic gates needed for the realisation of a large-scale silicon quantum computer.
A world-record result in reducing errors in semiconductor electron ‘spin qubits’, a type of building block for quantum computers, has been achieved, reported in April 2019
The experimental result by University of New South Wales engineers demonstrated error rates as low as 0.043 percent, lower than any other electron spin qubit. The joint research paper by the Sydney and UNSW teams was published this week in Nature Electronics and is the journal’s cover story for April. “Reducing errors in quantum computers is needed before they can be scaled up into useful machines,” said Professor Stephen Bartlett, a corresponding author of the paper. “Once they operate at scale, quantum computers could deliver on their great promise to solve problems beyond the capacity of even the largest supercomputers. This could help humanity solve problems in chemistry, drug design and industry.”
Fully functioning quantum computers will need millions, if not billions, of qubits to operate. Designing low-error qubits now is a vital step to scaling up to such devices. Professor Raymond Laflamme is Chair of Quantum Information at the University of Waterloo in Canada and was not involved in the study. He said: “As quantum processors become more common, an important tool to assess them has been developed by the Bartlett group at the University of Sydney. It allows us to characterise the precision of quantum gates and gives physicists the ability to distinguish between incoherent and coherent errors leading to unprecedented control of the qubits.”
The joint University of Sydney-UNSW result comes soon after a paper by the same quantum theory team with experimentalists at the Niels Bohr Institute in Copenhagen. That result, published in Nature Communications, allows for the distant exchange of information between electrons via a mediator, improving the prospects for a scaled-up architecture in spin-qubit quantum computers.
The result was significant because it allows for the distance between quantum dots to be large enough for integration into more traditional microelectronics. The achievement was a joint endeavour by physicists in Copenhagen, Sydney and Purdue in the US. Professor Bartlett said: “The main problem is that to get the quantum dots to interact requires them to be ridiculously close — nanometres apart. But at this distance they interfere with each other, making the device too difficult to tune to conduct useful calculations.” The solution was to allow entangled electrons to mediate their information via a ‘pool’ of electrons, moving them further apart.
He said: “It is kind of like having a bus — a big mediator that allows for the interaction of distant spins. If you can allow for more spin interactions, then quantum architecture can move to two-dimensional layouts.” Associate Professor Ferdinand Kuemmeth from the Niels Bohr Institute in Copenhagen said: “We discovered that a large, elongated quantum dot between the left dots and right dots, mediated a coherent swap of spin states, within a billionth of a second, without ever moving electrons out of their dots. Professor Bartlett said: “What I find exciting about this result as a theorist, is that it frees us from the constraining geometry of a qubit only relying on its nearest neighbours.”
Quantum Photonics Advance Optical Circuits, Computing Capabilities, reported in Feb 2021
Researchers at the University of Southern California (USC) developed a method that emits uniform single photons from precisely arranged quantum dots. The researchers said the development is expected to enable the production of optical circuits and, as a result, advancements in quantum computing and communications technologies. Quantum optical circuits use light sources that generate individual photons, acting as qubits, on demand and one at a time.
The research was led by Jiefei Zhang, a research assistant professor in the Mork Family Department of Chemical Engineering and Materials Science, with corresponding author Madhukar, the Kenneth T. Norris Professor in Engineering and professor of chemical engineering, electrical engineering, materials science, and physics.
In some ways, photonic circuits function similarly or analogously to electronic circuits. Whereas electronic circuits guide electrons carrying data, photonic circuits use light sources generating individual photons carrying data. The new technology, Zhang said, paves the way toward moving from lab demonstrations of single-photon physics to chip-scale fabrication of quantum photonic circuits. “The current technology that is allowing us to communicate online, for instance using a technological platform such as Zoom, is based on the silicon integrated electronic chip. If the transistors on that chip are not placed in exact designed locations, there would be no integrated electrical circuit,” Madhukar said. “It is the same requirement for photon sources such as quantum dots to create quantum optical circuits.”
Similarly, the quantum dots must be of a uniform shape and size, something that current manufacturing techniques are unable to achieve. Without a uniform shape and size, the photons the dots release do not have uniform wavelengths. To make a uniform arrangement of quantum dots, the team used Madhukar’s method developed in the early 1990s called SESRE (substrate-encoded size-reducing epitaxy). The team fabricated regular arrays of nanoscale mesas with a defined edge-orientation, shape, and depth on a flat semiconductor substrate composed of gallium arsenide (GaAs). Then, the quantum dots are created on top of the mesas by adding appropriate atoms.
In optical circuits, nano-size semiconductor quantum dots function as light sources. First, in the new system, incoming gallium (Ga) atoms gather on top of the nanoscale mesas attracted by surface energy forces where they deposit GaAs. Then the incoming flux is switched to indium (In) atoms, which in turn deposit indium arsenide (InAs), followed back by Ga atoms to form GaAs, and then creating the desired individual quantum dots that release single photons.
To be useful in optical circuits, the space between the pyramid-shape nano-mesas must filled by material that flattens the surface. In the final chip, opaque GaAs is depicted as a translucent overlayer under which the quantum dots are located. “This work also sets a new world record of ordered and scalable quantum dots in terms of the simultaneous purity of single-photon emission greater than 99.5%, and in terms of the uniformity of the wavelength of the emitted photons, which can be as narrow as 1.8 nm, which is a factor of 20 to 40 better than typical quantum dots,” Zhang said.
According to Zhang, this uniformity makes it possible to use established methods, such as local heating or electric fields, to fine-tune the wavelengths of the photons emitted by the quantum dots in order to create exact matches. That process is necessary to develop the required interconnections between different quantum dots for circuits. With the advancements from this research, it becomes possible to apply well-established semiconductor processing techniques to create scalable photonic chips. With that in mind, the researchers are now focused on determining exactly how identical the emitted photons are from the same, and different, quantum dots.
“We now have an approach and a material platform to provide scalable and ordered sources generating potentially indistinguishable single photons for quantum information applications,” Zhang said. “The approach is general and can be used for other suitable material combinations to create quantum dots emitting over a wide range of wavelengths preferred for different applications — for example fiber-based communication or the mid-infrared regime, suited for environmental monitoring and medical diagnostics.”
Quantum dot based photon switch for application in quantum technology, reported in Feb 2020
An international team led by the Institute of Materials Science (ICMUV) of the University of Valencia has developed an Quantum dot based optical (quantum) switch that modifies the emission properties of photons, the particles of electromagnetic radiation. The new device works with ultra-fast switching times and very low energy consumption and, in comparison to other designs, it can be implemented in a variety of semiconductor platforms and is of great application in current quantum technologies.
The operation principle of the device is based on the nanostructured semiconductor quantum confinement technology, which are small structures of nanometric size capable of absorbing and emitting light. The optical properties of these materials, called quantum dots, are similar to those of isolated atoms and their emission of light occurs photon to photon. They are very interesting for developing quantum technologies, since isolated photons or pairs of photons can be used to reproduce overlapping or entanglement conditions.
At present, one of the scientific and technological challenges in this field is directed towards the development of logic gates and optical circuits that can perform operations with photons, and in this way work and modify the information under the quantum description. Therefore, tools and materials that can affect the emission of photons individually are necessary. Of all of them, those who manipulate and control photons using light are very interesting, since chained systems can be built or they can represent large reductions in energy consumption. This is the case of all-optical devices.
The main idea of the work came about through a collaboration with researcher Massimo Gurioli, from the University of Florence and the European Nonlinear Spectroscopy Laboratory. Under this collaboration the processes of accumulation and saturation of the charge in quantum dots of indium arsenide (InAs) were studied according to the power and color of the lighting laser.
One of the outstanding properties of the new device is that next to the temporary switching, a switching of the color of the emitted photon (its wavelength) can be added if two different lasers are used. This quality allows us to think of devices for multiplexing photons by wavelength (combining two or more channels of information in a transmission medium), so that each color of the photon is associated with one of these channels. Finally, the physical principle by which the device operates is fulfilled by many other quantum confinement nanostructures, so this new design represents a general scheme that can be implemented in a wide variety of semiconductor platforms.
The UCF-developed, new photonic material overcomes drawbacks of contemporary topological designs that offered less features and control, while supporting much longer propagation lengths for information packets by minimizing power losses, reported in May 2022
niversity of Central Florida researchers are developing new photonic materials that could one day help enable low power, ultra-fast, light-based computing. The unique materials, known as topological insulators, are like wires that have been turned inside out, where the current runs along the outside and the interior is insulated.
In their latest work, published in the journal Nature Materials, the researchers demonstrated a new approach to create the materials that uses a novel, chained, honeycomb lattice design. The researchers laser etched the chained, honeycombed design onto a sample of silica, the material commonly used to make photonic circuits.
Nodes in the design allow the researchers to modulate the current without bending or stretching the photonic wires, an essential feature needed for controlling the flow of light and thus information in a circuit.
The new photonic material overcomes drawbacks of contemporary topological designs that offered less features and control, while supporting much longer propagation lengths for information packets by minimizing power losses. The researchers envision that the new design approach introduced by the bimorphic topological insulators will lead to a departure from traditional modulation techniques, bringing the technology of light-based computing one step closer to reality.
Topological insulators could also one day lead to quantum computing as their features could be used to protect and harness fragile quantum information bits, thus allowing processing power hundreds of millions of times faster than today’s conventional computers. The researchers confirmed their findings using advanced imaging techniques and numerical simulations.
“Bimorphic topological insulators introduce a new paradigm shift in the design of photonic circuitry by enabling secure transport of light packets with minimal losses,” says Georgios Pyrialakos, a postdoctoral researcher with UCF’s College of Optics and Photonics and the study’s lead author.
Next steps for the research include the incorporation of nonlinear materials into the lattice that could enable the active control of topological regions, thus creating custom pathways for light packets, says Demetrios Christodoulides, a professor in UCF’s College of Optics and Photonics and study co-author.
The research was funded by the Defense Advanced Research Projects Agency; the Office of Naval Research Multidisciplinary University Initiative; the Air Force Office of Scientific Research Multidisciplinary University Initiative; the U.S. National Science Foundation; The Simons Foundation’s Mathematics and Physical Sciences division; the W. M. Keck Foundation; the US–Israel Binational Science Foundation; U.S. Air Force Research Laboratory; the Deutsche Forschungsgemein-schaft; and the Alfried Krupp von Bohlen and Halbach Foundation.
References and Resources also include: