Quantum technology (QT) applies quantum mechanical properties such as quantum entanglement, quantum superposition, and No-cloning theorem to quantum systems such as atoms, ions, electrons, photons, or molecules. Quantum bit is the basic unit of quantum information. Whereas in a classical system, a bit is either in one state or the another. However, quantum qubits can exist in large number of states simultaneously, property called Superposition.

Quantum computers shall bring power of massive parallel processing, equivalent of supercomputer to a single chip. They can consider different possible solutions to a problem simultaneously, quickly converge on the correct solution without check each possibility individually. This dramatically speed up certain calculations, such as number factoring.

The power of quantum computers depends on the number of qubits and their quality measured by coherence, and gate fidelity. Qubit is very fragile, can be disrupted by things like tiny changes in temperature or very slight vibrations. Coherence measures the time during which quantum information is preserved. The gate fidelity uses distance from ideal gate to decide how noisy a quantum gate is.

### Topological Materials

The discovery of topological materials whose properties remain intact even when an object is stretched, twisted or deformed, led to the awarding of the Nobel Prize in physics in 2016 to F. Duncan M. Haldane, and others. Their topological nature means these states are resistant to change, and so stable to temperature fluctuations and physical distortion — features that could make them useful in devices.

“Imagine a rope identified by a number of knots,” Suyang Xu, assistant professor of chemical biology, said. “No matter how much the shape of the rope is changed, the number of knots — known as the topological number — cannot be changed without altering its fundamental identity by adding or undoing knots.” It is this robustness that potentially makes topological materials particularly useful.

Because topological materials are robust and resist perturbation, they could be used to build more resilient and longer-lasting qubits.

### Quantum Computer Approaches

There have been two leading approaches for building general purpose Quantum computer. One approach, adopted by Google, IBM, Rigetti and Quantum Circuits involves encoding quantum states as oscillating currents in superconducting loops. The other, pursued by IonQ and several major academic labs, is to encode qubits in single ions held by electric and magnetic fields in vacuum traps.

“Right now I think both superconductors and ion traps have shown a lot of progress and demonstrated a large number of algorithms. The advantage of trapped ions is that every ion is the same. For these small chains [of ions in the trap] you do get this advantage of basically being able to achieve communication between any pair. In superconducting devices, typically, you are only able to talk to sort of neighbor qubits. So if you have an algorithm which requires a longer distance communication between qubits, there is some cost you have to pay to get the information from one to the other,” Kenneth Brown of Duke University.

Brown, Kim, and colleague Christopher Monroe’s (University of Maryland) have written a nice paper on the topic, Co-Designing a Scalable Quantum Computer with Trapped Atomic Ions. Superconducting circuitry exploits the significant advantages of modern lithography and fabrication technologies: it can be integrated on a solid-state platform and many qubits can simply be printed on a chip. However, they suffer from inhomogeneities and decoherence, as no two superconducting qubits are the same, and their connectivity cannot be reconfigured without replacing the chip or modifying the wires connecting them within a very low temperature environment.

Trapped atomic ions, on the other hand, feature virtually identical qubits, and their wiring can be reconfigured by modifying externally applied electromagnetic fields. However, atomic qubit switching speeds are generally much slower than solid state devices, and the development of engineering infrastructure for trapped ion quantum computers and the mitigation of noise and decoherence from the applied control fields is just beginning.”

Topological qubits are protected from noise due to their values existing at two distinct points, making our quantum computer more robust against outside interference. This increased stability will help the quantum computer scale to complete longer, more complex computations, bringing the solutions we need within reach.

### Topology and quantum computing

Topology is a branch of mathematics describing structures that experience physical changes such as being bent, twisted, compacted, or stretched, yet still maintain the properties of the original form. When applied to quantum computing, topological properties create a level of protection that helps a qubit retain information despite what’s happening in the environment. The topological qubit achieves this extra protection in two different ways:

#### Electron fractionalization.

By splitting the electron, quantum information is stored in both halves, behaving similarly to data redundancy. If one half of the electron runs into interference, there is still enough information stored in the other half to allow the computation to continue.

#### Ground state degeneracy.

Topological qubits are engineered to have two ground states—known as ground state degeneracy—making them much more resistant to environmental noise. Normally, achieving this protection isn’t feasible because there’s no way to discriminate between the two ground states. However, topological systems can use braiding or measurement to distinguish the difference, allowing them to achieve this additional protection.

A team member proposed the use of a superconductor in conjunction with a strong magnetic field to create a topological phase of matter—an approach that has been adopted toward realizing the topological qubit. While bridging these properties has been long-taught, it had never been done in such a controlled way prior to this work.

To create the exact surface layer needed for the qubit, chemical compounds are currently being grown in Microsoft labs using a technique called “selective area growth.” Chosen for its atomic-level precision, this unique method can be described as spraying atoms in the exact arrangement needed to achieve the properties required.

The team continues testing functional accuracy through device simulation, to ensure that every qubit will be properly tuned, characterized, and validated.

### Bridging fields to advance technology

Many fields of knowledge have come together to realize the topological qubit, including mathematics, theoretical physics, solid state physics, materials science, instrumentation and measurement technology, computer science, quantum algorithms, quantum error correction, and software applications development.

#### Bridging these fields has led to breakthrough techniques across all aspects of realizing a topological qubit, including:

Theory and simulation – Turning a vision into reality by creating a rapid design, simulation, and prototyping process

Fabrication – Pioneering unique fabrication approaches and finding new ways to bridge properties

Materials growth – Developing inventive methods to create materials using special growth techniques to create the exact properties required at nanoscale

Measurement and quantum control – Tuning devices for accuracy in function and measurement

At Microsoft, the development of the topological qubit continues, bringing us closer to scalable quantum computing and finding solutions to some of the world’s most challenging problems.

### Newfound superconductor material could be the ‘silicon of quantum computers’

A potentially useful material for building quantum computers has been unearthed at the National Institute of Standards and Technology (NIST), whose scientists have found a superconductor that could sidestep one of the primary obstacles standing in the way of effective quantum logic circuits.

Newly discovered properties in the compound uranium ditelluride, or UTe2, show that it could prove highly resistant to one of the nemeses of quantum computer development—the difficulty with making such a computer’s memory storage switches, called qubits, function long enough to finish a computation before losing the delicate physical relationship that allows them to operate as a group. This relationship, called quantum coherence, is hard to maintain because of disturbances from the surrounding world.

The compound’s unusual and strong resistance to magnetic fields makes it a rare bird among superconducting (SC) materials, which offer distinct advantages for qubit design, chiefly their resistance to the errors that can easily creep into quantum computation. UTe2’s exceptional behaviors could make it attractive to the nascent quantum computer industry, according to the research team’s Nick Butch.

“This is potentially the silicon of the quantum information age,” said Butch, a physicist at the NIST Center for Neutron Research (NCNR). “You could use uranium ditelluride to build the qubits of an efficient quantum computer.”

Their paper details UTe2’s uncommon properties, which are interesting from the perspectives of both technological application and fundamental science. One of these is the unusual way the electrons that conduct electricity through UTe2 partner up. In copper wire or some other ordinary conductor, electrons travel as individual particles, but in all SCs they form what are called Cooper pairs. The electromagnetic interactions that cause these pairings are responsible for the material’s superconductivity. The explanation for this kind of superconductivity is named BCS theory after the three scientists who uncovered the pairings (and shared the Nobel Prize for doing so).

What’s specifically important to this Cooper pairing is a property that all electrons have. Known as quantum “spin,” it makes electrons behave as if they each have a little bar magnet running through them. In most SCs, the paired electrons have their quantum spins oriented in a single way—one electron’s points upward, while its partner points down. This opposed pairing is called a spin singlet.

A small number of known superconductors, though, are nonconformists, and UTe2 looks to be among them. Their Cooper pairs can have their spins oriented in one of three combinations, making them spin triplets. These combinations allow for the Cooper-pair spins to be oriented in parallel rather than in opposition. Most spin-triplet SCs are predicted to be “topological” SCs as well, with a highly useful property in which the superconductivity would occur on the surface of the material and would remain superconducting even in the face of external disturbances.

“These parallel spin pairs could help the computer remain functional,” Butch said. “It can’t spontaneously crash because of quantum fluctuations.” All quantum computers up until this point have needed a way to correct the errors that creep in from their surroundings. SCs have long been understood to have general advantages as the basis for quantum computer components, and several recent commercial advances in quantum computer development have involved circuits made from superconductors. A topological SC’s properties—which a quantum computer might employ—would have the added advantage of not needing quantum error correction.

“We want a topological SC because it would give you error-free qubits. They could have very long lifetimes,” Butch said. “Topological SCs are an alternate route to quantum computing because they would protect the qubit from the environment.”

The team stumbled upon UTe2 while exploring uranium-based magnets, whose electronic properties can be tuned as desired by changing their chemistry, pressure or magnetic field—a useful feature to have when you want customizable materials. (None of these parameters are based on radioactivity. The material contains “depleted uranium,” which is only slightly radioactive. Qubits made from UTe2 would be tiny, and they could easily be shielded from their environment by the rest of the computer.) The team did not expect the compound to possess the properties they discovered.

The NIST team started exploring UTe2 with specialized tools at both the NCNR and the University of Maryland. They saw that it became superconducting at low temperatures (below -271.5 degrees Celsius, or 1.6 kelvin). Its superconducting properties resembled those of rare superconductors that are also simultaneously ferromagnetic—acting like low-temperature permanent magnets. Yet, curiously, UTe2 is itself not ferromagnetic.

“That makes UTe2 fundamentally new for that reason alone,” Butch said.It is also highly resistant to magnetic fields. Typically a field will destroy superconductivity, but depending on the direction in which the field is applied, UTe2 can withstand fields as high as 35 tesla. This is 3,500 times as strong as a typical refrigerator magnet, and many times more than most low-temperature topological SCs can endure.

While the team has not yet proved conclusively that UTe2 is a topological SC, Butch says this unusual resistance to strong magnetic fields means that it must be a spin-triplet SC, and therefore it is likely a topological SC as well. This resistance also might help scientists understand the nature of UTe2 and perhaps superconductivity itself.

“Exploring it further might give us insight into what stabilizes these parallel-spin SCs,” he said. “A major goal of SC research is to be able to understand superconductivity well enough that we know where to look for undiscovered SC materials. Right now we can’t do that. What about them is essential? We are hoping this material will tell us more.”

### New material shows high potential for quantum computing

A joint team of scientists at the University of California, Riverside, and the Massachusetts Institute of Technology is getting closer to confirming the existence of an exotic quantum particle called Majorana fermion, crucial for fault-tolerant quantum computing—the kind of quantum computing that addresses errors during its operation.

Quantum computing uses quantum phenomena to perform computations. Majorana fermions exist at the boundary of special superconductors called topological superconductors, which have a superconducting gap in their interiors and harbor Majorana fermions outside, at their boundaries. Majorana fermions are one of the most sought-after objects in quantum physics because they are their own antiparticles, they can split the quantum state of an electron in half, and they follow different statistics compared to electrons. Though many have claimed to have identified them, scientists still cannot confirm their exotic quantum nature.

The UCR-MIT team overcame the challenge by developing a new heterostructure material system, based on gold, that could be potentially used to demonstrate the existence and quantum nature of Majorana fermions. Heterostructure materials are made up of layers of drastically dissimilar materials that, together, show completely different functionalities when compared to their individual layers.

“It is highly nontrivial to find a material system that is naturally a topological superconductor,” said Peng Wei, an assistant professor of physics and astronomy and a condensed matter experimentalist, who co-led the study, appearing in Physical Review Letters, with Jagadeesh Moodera and Patrick Lee of MIT. “A material needs to satisfy several stringent conditions to become a topological superconductor.”

The Majorana fermion, considered to be half of an electron, is predicted to be found at the ends of a topological superconductor nanowire. Interestingly, two Majorana fermions can combine with each other to make up one electron, allowing the quantum states of the electron to be stored nonlocally—an advantage for fault-tolerant quantum computing.

In 2012, MIT theorists, led by Lee, predicted that heterostructures of gold can become a topological superconductor under strict conditions. Experiments done by the UCR-MIT team have achieved all the needed conditions for heterostructures of gold. “Achieving such heterostructure is highly demanding because several material physics challenges needed to be addressed first,” said Wei, a UCR alum who returned to campus in 2016 from MIT.

Wei explained that the research paper shows superconductivity, magnetism, and electrons’ spin-orbit coupling can co-exist in gold—a difficult challenge to meet—and be manually mixed with other materials through heterostructures. “Superconductivity and magnetism ordinarily do not coexist in the same material,” he said. Gold is not a superconductor, he added, and neither are the electron states on its surface.

“Our paper shows for the first time that superconductivity can be brought to the surface states of gold, requiring new physics,” he said. “We show that it is possible to make the surface state of gold a superconductor, which has never been shown before.”

The research paper also shows the electron density of superconductivity in the surface states of gold can be tuned. “This is important for future manipulation of Majorana fermions, required for better quantum computing,” Wei said. “Also, the surface state of gold is a two-dimensional system that is naturally scalable, meaning it allows the building of Majorana fermion circuits.” Besides Wei, Moodera, and Lee, the research team also includes Sujit Manna and Marius Eich of MIT.

### Microsoft building quantum computer based on topological qubit

Researchers at Microsoft are working on an entirely new topological quantum computer, which uses exotic materials to limit errors. To fully embrace the power and potential of quantum computing, the system must be engineered to meet the demands of the solutions the world needs most, says microsoft. The fragile nature of qubits is well-known as one of the most significant hurdles in quantum computing. Even the slightest interference can cause qubits to collapse, making the solutions we’re pursuing impossible to identify because the computations cannot be completed.

At the same time as Microsoft is working to build a quantum computer, it is also creating the software that could run on it. The goal is to have a system that can begin to efficiently solve complex problems from day one. “Similar to classical high-performance computing, we need not just hardware but also optimized software,” Troyer said.

To the team, that makes sense: The two systems can work together to solve certain problems, and the research from each can help the other side. “A quantum computer is much more than the qubits,” Reilly said. “It includes all of the classical hardware systems, interfaces and connections to the outside world.”

Microsoft’s approach to building a quantum computer is based on a type of qubit – or unit of quantum information – called a topological qubit. The Microsoft team believes that topological qubits are better able to withstand challenges such as heat or electrical noise, allowing them to remain in a quantum state longer. That, in turn, makes them much more practical and effective. “A topological design is less impacted by changes in its environment,” Holmdahl said.

Microsoft’s Azure Quantum program has developed devices that can create quantum properties which scientists have imagined for nearly a century but have not been able to unambiguously produce in the real world — until now.

Building on two decades of scientific research and recent investments in simulation and fabrication, the Azure Quantum team has engineered devices that allow them to induce a topological phase of matter bookended by a pair of Majorana zero modes. These quantum excitations don’t normally exist in nature and must be coaxed into appearing under incredibly precise conditions.

The Azure Quantum team has also been able to produce what is known as a topological phase and to measure the topological gap, which quantifies the stability of the phase. The ability to create and sustain a quantum phase with Majorana zero modes and a measurable topological gap removes the biggest obstacle to producing a unique type of qubit, which Microsoft’s quantum machine will use to store and compute information, called a topological qubit. It’s the foundation for Microsoft’s approach to building a quantum computer that is expected to be more stable than machines built with other types of known qubits, and therefore scale like no other.

“Microsoft has taken this very risky but high reward approach in trying to make a qubit which on the theory side looks like the very best qubit you can get. But the challenge was that nobody has really seen these Majorana zero modes in real life,” said Peter Krogstrup, scientific director of Microsoft’s Quantum Materials Lab in Lyngby, Denmark. “But we have done that now, and that’s super exciting. We have to continue to evolve our engineering capabilities, but it really looks like there is a path towards scalable quantum computing now.”

### References and Resources also include:

https://news.microsoft.com/innovation-stories/azure-quantum-majorana-topological-qubit/