Qubits are to quantum computers, just as transistors are to classical computers. However, unlike their classical cousins, qubits are fussier and more sensitive. Qubits are so fragile that any fluctuation to their environment—like an imperceptible change in temperature, magnetic fields, or even a stray cosmic ray—could disrupt the qubit from performing computations.
The delicate nature of qubits has spurred research into discovering special types of materials with quantum properties that can potentially create stable qubits. Sounds fairly simple, right? But here is the catch. These materials need to exhibit unique properties that defy the laws of classical physics. The exotic properties harnessed for quantum computing include superconductivity (conducting electricity unimpeded and without energy loss at extremely low temperatures); entanglement (ability of two particles to mirror each other’s behavior despite being separated by a large distance, without any connections, termed “spooky action at a distance”); and quantum tunneling (wherein particles are allowed to pass through otherwise impenetrable barriers) among many others. Not many quantum materials meet these esoteric requirements.
These nonintuitive properties have led to many pop culture references about a quantum world existing in a parallel universe. While Marvel Comics and movies such as Everything Everywhere All at Once hinge on a tempting, yet eccentric, proposition of a parallel universe, there is no scientific evidence of there being one, let alone a multiverse. But that brings us to question whether quantum computers fueled by quantum materials is at all possible.
Research in transistor materials has been one of the instrumental factors in advancing classical computing. This includes efforts to miniaturize silicon chips without compromising on speed or, integrating carbon nanotubes into chips to enhance computing systems. We could postulate that by extension, technological advancements in quantum materials hold the promise of faster quantum computers and fault-tolerant quantum devices.
So, where does one start with these quantum materials? Researchers could create new compounds from the elements and test whether they exhibit exotic properties. However, there are infinite ways to create new materials by mixing and matching chemical elements, which makes it challenging. Add the roadblock that even a tiny dust particle could alter these quantum material properties.
In a study published in Nature Communications, Avetik Harutyunyan and colleagues from Honda Research Institute report on the development of an innovative technology to grow super thin and narrow ribbon-shaped quantum materials. Called nanoribbons, they have a width that is 10,000 to 100,000 times smaller than a human hair. “[By changing width and thickness] we change the material property and have created completely new, so far unknown properties,” says Harutyunyan.
“This study advances quantum photonics indirectly, but [in a] very important way: By demonstrating that the emitted photons can be tuned [by varying the width of the nanoribbon],” says Boris Yacobson, professor of materials science and nano-engineering at Rice University.
Another advantage of these nanoribbons is that they are stretchable. This property enabled Harutyunyan and colleagues—in collaboration with scientists from Montana State University and Columbia University—to develop a sophisticated approach in which they stretch the nanoribbons onto special curved surfaces to stimulate single-photon light emission. This new single-photon source was used to encode information on a stream of photons. Single-photon emissions are crucial for quantum communication because attempts to eavesdrop on information encoded on a single photon will unsurprisingly interfere with the quantum state—an interference that can easily be detected. Contrast this with multiphoton emissions from the same quantum state, wherein it is easier to intercept information from one of the photons without altering the state of the other photons, thus evading detection. As a result, the current study creating a source of single photons demonstrates the potential to realize secure communication.
Scientists are working towards finding sources of quantum light that produce on-demand single photons. Such sources are crucial building blocks for quantum technologies. “Right now, our photons are not generated on demand, but we would like to have [them] on demand for efficient quantum communication,” says Harutyunyan, referring to potential extensions of their study.
One such on-demand single-photon source is quantum dots, also called artificial atoms, because, much like atoms, they exhibit discrete measurable energy levels. Quantum dots are also tiny semiconductors. Their miniscule size gives them distinct advantages over conventional heavy materials and enables them to exhibit unique and interesting optical properties like the emission of pure, bright colors, high photostability (maintaining their optical properties for extended periods of time), and superior transport (movement of charged particles like electrons) influenced by quantum tunneling effects. These exotic properties of quantum dots have been fascinating researchers for more than 40 years.
Quantum dots. Photo credit: Getty Images - atdigit.
Quantum dots have already been used to make LED lights for biomedical applications. These nanostructures are being tapped in research, potentially for quantum communication as well as to create a quantum internet. These applications hinge on the ability to use single photons to transmit, receive, and store information. But are we there yet? A recent study published in Nature Physics, which demonstrates the power of another quantum material used to make quantum dots, may hold answers.
The quantum material used in the paper is special “because [its] optical properties really are standout compared to other materials [such as silicon],” says author Dorian Gangloff, a quantum information scientist and leader of the Quantum Engineering Group at the University of Cambridge.
While photons provide immense potential to send quantum information from one place to another, it is hard to catch the photons and pin them down. “If you want to store the quantum information locally, then you need something with mass, something that stays where it is, and finding that something is proving very challenging,” says Richard Warburton, a physicist at the University of Basel.
However, what if we could develop an interface between light and matter that can exchange quantum information? In the study, scientists from Johannes Kepler University grew quantum dots. Gangloff, physicist Mete Atatüre, and colleagues from the University of Cambridge used an interface between photons emitted by these quantum dots and spins of electrons trapped inside the quantum dots. While ideally one could store quantum information between this local light-matter interface, it comes with a catch. The electronic spin states of quantum dots are ephemeral. “That’s been a problem since the beginning of research into the quantum dots,” says Warburton.
The novelty of Gangloff and colleagues’ study is that their work manages to hand off information from the fleeting electronic spin to the much longer-lived nuclear spin of atoms in a quantum dot and then transfer the information back to the electron spin. “This is the big breakthrough,” says Warburton.
In essence, “We can transfer the information from the electron qubit to the collective state of these nuclei in the quantum dots,” says Gangloff. This study sets the stage for quantum dots to be potentially transformed into storage systems or quantum memory that can preserve quantum information. These results could have implications in quantum information tasks including information processing, securing communications, and performing complex calculations. One of the immediate implications of this work that researchers foresee is in the reliable exchange of quantum information.
Such an exchange of quantum information over long distances could be accomplished, in theory, via entanglement. However, the ability to physically realize entanglement is limited by the exponential loss of information over long distances when transmitted via photons. To prevent this loss, quantum repeaters, or devices that can act as intermediate nodes in a quantum entangled network, are needed. With this ability to store information within quantum dots for a long time, quantum dots could be used as repeaters to transmit information without loss over long distances.
While the scientific advances in these studies underscore the power of quantum materials, are we any closer to finding our magic quantum material? Though silicon is the clear favorite for making integrated circuits for classical computers, Warburton notes, “If you want to build a quantum internet, what material do you use? It’s an open question.”
To answer this open question, one must look at the myriad technologies used by scientists to solve a problem. While light signals have been used in classical communication, another property—magnetism—has been used to both transmit signals and store information. Researchers are now exploring whether magnetism has the potential to revolutionize quantum computing.
In a study published in Nature Materials, researchers hailed a semiconductor material, chromium sulfide bromide, with the ability to connect the very disparate worlds of optics and magnetism. It is a miracle material, says Mack Kira, professor of electrical and computer engineering at the University of Michigan and one of the paper’s authors.
Illustration of perovskite quantum dot technology. Quantum dots, which are based on perovskite materials, exhibit excellent photoluminescence quantum yields and high color purity. Photo credit: Getty Images - Thom Leach/Science Photo Library.
While the optics world has the ability to precisely tune light pulses to control electrical excitations, they do not last long enough to be useful for quantum-related applications. In the magnetic world, however, this isn’t a concern. Researchers have been trying to investigate how to leverage ultrafast light pulses for speedy quantum information processing coupled with magnetization for long-term processes like information storage. Using chromium sulfide bromide, Kira and colleagues have theoretically and experimentally demonstrated the interaction between optical and magnetic features to potentially develop applications for quantum technology.
The quantum particles studied by Kira and colleagues are bound electron-hole pairs called excitons. They are created when light shines on a solid semiconductor material. Excitons show immense potential in quantum technology because they have been shown to behave as solid-state qubits. However, excitons also can be transient as they crash into each other and lose information.
How does the quantum material used in this study help to retain quantum information in excitons longer? The secret lies in the layered structure of this material, which is akin to sheets stacked one on top of the other, with each sheet having a crucial feature. The magnetic spins of the material in each sheet are aligned in the same direction—pointing up or down. However, when you compare the magnetic spins in one sheet with that of its adjacent layer, they are switched in opposite directions.
As these spins are coupled to excitons, the opposing magnetic spins do not allow the excitons to escape between the sheets, essentially trapping these excitons in each layer. In a quantum device, confining excitons to a single atomic layer (a sheet in this case) allows for information to last longer. “When the excitations are limited to one layer, they get way stronger, and it changes how [excitons behave] in a quantum way,” says Kira.
Kira and colleagues also went on to successfully un-trap the excitons from within the sheets by turning off magnetism and freeing them to move in any direction in three dimensions. Using magnetic manipulations to control excitons is like “adding another knob into [a] toolbox.”
All of the studies show the tantalizing possibilities of quantum materials, paving the way for further innovations to bring more exotic quantum properties to the fore. As evident from these research studies, different quantum materials may need to be optimized for different quantum applications. “There is not going to be a single material for everything we want to do,” says Gangloff.
Hunting for new quantum materials remains an immense scientific challenge, but existing materials may provide surprising clues and solutions. While the paper by Harutyunyan and colleagues shows how changing the dimensionality of an existing material could create single-photon sources, the spirit of the other two works is to demonstrate how quantum materials could help process and store quantum information captured in these light sources.
What stands out in these advancements are the research collaborations between scientific fields, as well as academic and industry partnerships. “These multidisciplinary collaborations are hard, but when it happens, it always brings beautiful results and that’s crucial,” says Harutyunyan.
Lakshmi Chandrasekaran is a freelance science and technology writer based in Chicago, whose work has appeared in major popular science outlets.
