Different rules – new applicationsMay 2017
Quantum mechanics opens the door to new technologies
In April, SNI deputy director Professor Daniel Loss was awarded the 2017 King Faisal International Prize for Science in recognition of his groundbreaking theoretical work on spin physics, a field with potential applications that include the development of a quantum computer. Back in 1998, Daniel Loss and his colleague Professor David DiVincenzo proposed using the intrinsic angular momentum – or spin – of electrons as the smallest memory unit in the computer of the future, inspiring researchers worldwide to turn their attention to this field. The work being done by Loss and his colleagues makes use of the laws of the quantum universe. These laws, which are quite distinct from those governing our everyday lives, hold the key to game-changing applications – not just in computing, but also in fields such as communications technology and sensors. Several teams in the SNI network are focussing their research on these areas of quantum technology.
What are quanta?
The term “quanta” is often used to describe elementary particles, i.e. particles that cannot be divided into smaller components, as well as the smallest possible transferable units of energy. Quanta can act both as particles and waves, changing their position without traveling through space, surmounting obstacles in ways that are not fully understood, or appearing in different places at once. The laws of quantum mechanics were defined in the early 20th century by physicists including Albert Einstein, Niels Bohr and Erwin Schrödinger. However, it is only in the last decade that researchers have found ways to work with individual quanta to experimentally test and apply different theories.
Different rules apply in the nanoworld
There have been a number of approaches to developing a quantum computer. All of these rely on the laws of quantum mechanics, a branch of physics dating back some 80 years which seeks to describe the properties and governing principles of matter – i.e. molecules, atoms and elementary particles. The realm of minute particles is subject to its own set of rules, which are different to those we know from everyday life. For example, entities observed at the quantum scale can behave as both particles and waves. The famous double-slit experiment demonstrated that an electron traveling towards a screen with two tiny slits in it can pass through both slits at once as a result of its wave-like nature, creating an interference pattern on the other side of the screen. In the quantum world, tiny particles can exist in a state of superposition. In the case of an electron’s spin – which can be compared to the needle of a compass – this means that the spin direction is initially undetermined, pointing in different directions at once.
Particles only manifest these strange properties when they are in a system that is isolated from outside influences. As soon as researchers interfere, for example by taking a measurement, their state becomes determined – almost as if the particles wished to conceal their diversity and variability from us. But it gets even more amazing: Researchers are able to entangle two or more particles, creating a new kind of link between them. The state of entangled particles remains correlated, even if they are physically separated after entanglement. This phenomenon was known to Einstein, who described it as “spooky action at a distance”.
In the macroworld we live in, we do not realize the outlandish rules of quantum mechanics – they describe phenomena that we cannot see or experience. Nevertheless, they are there, controlling the interactions of photons, electrons, atoms and molecules.
Electron spin as a memory unit
Daniel Loss and colleagues all over the world are now seeking to harness the laws of the quantum realm to research fundamental physical principles and develop new technologies. One of their most ambitious goals is the creation of a quantum computer. A model for a quantum computer proposed by Loss uses electron spins as its smallest memory unit, the qubit – analogous to a bit (0 or 1) in digital computing. Similarly to a compass needle, an electron spin can point up or down, or in other directions as well, meaning that a qubit does not just have two different states, but several, each associated with a specific probability. It is only when a measurement is taken that the spin state is determined as up or down.
In Loss’s model, electrons are trapped in semiconductor materials to create devices known as quantum dots. Quantum dots are nanoscale objects measuring between 10 and 100 nanometers. They behave in a similar way to atoms, but are around 1000 times larger. Even so, they are still small enough that 100 million of these electrons can be arranged on a single square centimeter and controlled electrically. The spin of an electron can be entangled with that of neighboring electrons. Manipulating one spin state alters the other entangled spin states. Whereas in digital computing, operations can only be completed sequentially, these phenomena would enable a quantum computer to perform them in parallel, accessing the results simultaneously. Accordingly, a quantum computer would be able to carry out calculations and simulations involving vast amounts of data that are beyond the scope of current computers.
Spin-based computing has yet to become a reality. One of the problems it faces is that interference from the surrounding environment in the solid-state device immediately determines an electron’s spin state. The group led by Professor Dominik Zumbühl of the University of Basel’s Department of Physics is exploring ways to delay this process, known as decoherence, for as long as possible. “My colleagues have made huge progress in this regard in recent years,” reports Loss in an interview. Whereas coherence times – the length of time for which different states can be kept stable – were initially measured in billionths of a second, Zumbühl’s group currently holds the world record of one minute.
A further as yet unsolved problem is scaling the computer. In order to compete with a current conventional computer, a quantum computer would need around 108 spin qubits. Based on current knowledge, each qubit would have to be controlled by a wire, posing a major challenge in terms of space. Accordingly, new ideas are needed before the first working quantum computer can be built. Another open question concerns the choice of material. While most research groups worldwide work with gallium arsenide (GaAs), Intel continues to use silicon.
Despite the many unresolved issues, Loss remains convinced that his model can succeed. “Theoretically, the spin-based quantum computer ticks all the most important boxes,” he explains. “It is fast, small and integrable.” That said, he makes no predictions as to when the first prototype will become available.
Wide range of applications
Computing is not the only field in which quantum mechanics has a great deal to offer. Other applications include more secure data encryption, new developments in electronics and optics, or revolutionary sensor technology. Several working groups at the University of Basel’s Department of Physics within the SNI network are actively researching these areas in collaboration with Loss. For example, the team led by Professor Richard Warburton is dedicated to producing hardware for the transmission of quantum information, which involves emitting and receiving single photons. The team is working on a single photon source, as well as the use of individual spin qubits as optically addressable memory units. Their research relies primarily on quantum dots. Quantum dots, as described above, are self-organizing nanostructures, made from semiconductor materials, in which the mobility of charge carriers is restricted. They are also sometimes referred to as artificial atoms due to their similarities with the real thing. However, unlike natural atoms, they allow fine-tuning of their properties, making them ideal for research purposes. Here too, quantum effects play a key role, enabling new electronic and optical applications.
The research groups let by Professor Patrick Maletinsky and Argovia Professor Martino Poggio are busy developing groundbreaking sensors. Patrick Maletinsky’s team focuses primarily on nitrogen vacancy (NV) centers. NV centers contain single electrons that can be excited or manipulated, and react to magnetic or electrical fields. Their spin also changes according to the surrounding fields, a process that can be detected easily using various measurement methods. Maletinsky’s team leverages these principles to develop quantum sensors for nanoscale magnetic imaging, which it uses to investigate new materials. This innovative quantum-based measurement technique offers enhanced sensitivity, enabling imaging of previously invisible magnetic fields. In Poggio’s lab, meanwhile, the main focus is on using nanowires as sensors. These tiny filamentary crystals, with an almost defect-free crystal lattice structure, can serve as a robust source of quantum light, and are able to measure both the size and direction of forces. Nanowires offer great potential as sensors for biological and chemical samples, and can be used as pressure or load sensors.
Examining minute structures such as electrons or their spin is a painstaking process, and the laws that govern them are highly complex. However, they open the door to entirely new applications, leading some experts to speak of a second quantum revolution. The first quantum revolution took place decades ago, when the laws of quantum mechanics were first described. Today, researchers at the SNI and worldwide have reached the stage of being able to apply them in technological breakthroughs.