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New Topological Material Could Enable Fault-Tolerant Quantum Computing

A new topological material which could enable fault-tolerant quantum computing has been discovered by researchers from the University of Pennsylvania, in collaboration with Johns Hopkins University and Goucher College.

A false color image of one of the Researchers' samples. Credit: University of Pennsylvania

It is a type of computing that taps into the power of atoms and subatomic phenomena in order to carry out calculations considerably faster than present computers. This new material could also bring about improvements in drug development and various other complex systems.​​​​​​​

The research, published in ACS Nano, was headed by Jerome Mlack, a Postdoctoral Researcher in the Department of Physics & Astronomy in Penn’s School of Arts & Sciences, and his mentors Nina Marković, presently an Associate Professor at Goucher, and Marija Drndić, Fay R. and Eugene L. Langberg Professor of Physics at Penn. Penn Grad Students Gopinath Danda and Sarah Friedensen, who received an NSF fellowship for this work, and Johns Hopkins Associate Research Professor Natalia Drichko and Postdoc Atikur Rahman, currently an Assistant Professor at the Indian Institute of Science Education and Research, Pune, also contributed to the research.

The research started while Mlack was a Ph.D. candidate at Johns Hopkins. Mlack along with other Researchers were focusing on growing and developing devices out of topological insulators, a particular kind of material that does not conduct current via the bulk of the material but is capable of carrying current along its surface. ​​​​​​​​​​​​​​

While working with these materials, one of their devices used by the Researchers blew up, similar to what will happen when there is a short circuit.

It kind of melted a little bit and what we found is that, if we measured the resistance of this melted region of one of these devices, it became superconducting. Then, when we went back and looked at what happened to the material and tried to find out what elements were in there, we only saw bismuth selenide and palladium.

Jerome Mlack, a Postdoctoral Researcher, the Department of Physics & Astronomy, Penn’s School of Arts & Sciences

When cooled, superconducting materials will be able to carry a current with zero electrical resistance without losing any energy.

It has been predicted that topological insulators with superconducting properties will have great potential for developing a fault-tolerant quantum computer. However, it is not easy to develop good electrical contact between the topological insulator and superconductor and also to scale such device for manufacture, using exiting techniques. This new material will be able to potentially overcome both of these difficulties if it could be recreated.

In traditional computing, the smallest unit of data that actually makes up the computer and stores information, the binary digit, or bit, can in fact have either 0, for off, or 1, for on. Quantum computing takes advantage of superposition, a phenomenon in which the bits, in this case called qubits, can be 0 and 1 at the same time.

Schrödinger's cat is a thought experiment that is considered to be a famous way of illustrating this phenomenon. In this experiment, a cat is placed in a box, however one will not know if the cat is alive or dead until the box is opened. Prior to opening the box, the cat can be considered both dead and alive, existing in two states simultaneously, but, immediately after opening the box, the state of the cat, or in the case of qubits, the system’s configuration, falls into one: the cat is either dead or alive and the qubit is either 1 or 0.

“The idea is to encode information using these quantum states,” Marković said, “but in order to use it in needs to be encoded and exist long enough for you to read.”

One major problem present in the field of quantum computing refers to the fact that the qubits are not extremely stable and it is extremely easy to destroy the quantum states. These topological materials provide a way for allowing these states to live long enough in order to read them off and do something with them, Marković stated.

It's kind of like if the box in Schrödinger's cat were on the top of a flag pole and the slightest wind could just knock it off. The idea is that these topological materials at least widen the diameter of the flag pole so the box is sitting on more a column than a flag pole. You can knock it off eventually, but it's otherwise very hard to break the box and find out what happened to the cat.

Jerome Mlack, a Postdoctoral Researcher, the Department of Physics & Astronomy, Penn’s School of Arts & Sciences

The Researchers were able to develop a process for recreating the material in a controlled way even though their initial discovery of the material was an accident.

Marković, who was Mlack’s advisor at Johns Hopkins at the time, proposed that, this material can be recreated, without the need for constantly blowing up devices, by thermally annealing it. This indeed refers to a process in which the material is placed into a furnace and then heated to a certain temperature.

Using this method, the Researchers wrote, “the metal directly enters the nanostructure, providing good electrical contact and can be easily patterned into the nanostructure using standard lithography, allowing for easy scalability of custom superconducting circuits in a topological insulator.”

Despite the fact that Researchers already have the potential to produce a superconducting topological material, there is indeed a big problem, referring to the aspect that when two materials are put together a crack appears in between, which reduces the electrical contact. This results in ruining the measurements that they make and also the physical phenomena that lead to developing devices that will enable quantum computing.

Patterning it directly into the crystal, allows the superconductor to be embedded and none of these contact problems appear. The resistance is extremely low, and it is possible for them to pattern devices for quantum computing in just a single crystal.

The material’s superconducting properties are tested by placing it in two very cold refrigerators, one of which cools down to almost absolute zero. A magnetic field was also swept across it, which would kill the topological nature and the superconductivity of the material, in order to discover the material’s limitations. Standard electrical measurements were also carried out by running a current through and observing the voltage that is developed.

I think what is also nice in this paper is the combination of the electrical transport performance and the direct insights from the actual device materials characterization. We have good insights on the composition of these devices to support all these claims because we did elemental analysis to understand how these two materials join.

Marija Drndić, Mentor

One of the advantages of the Researchers' device refers to the fact that it is potentially scalable and has the potential to fit onto a chip similar to the ones that are presently used on computers.

“Right now the main advances in quantum computing involve very complicated lithography methods,” Drndić said. “People are doing it with nanowires which are connected to these circuits. If you have single nanowires that are very, very tiny and then you have to put them in particular places, it’s very difficult. Most of the people who are on the forefront of this research have multimillion-dollar facilities and lots of people behind them. But this, in principle, we can do in one lab. It allows for making these devices in a simple way. You can just go and write your device any way you want it to be.”

Mlack explains that even though there is still a fair amount of limitation on it; there is an entire field that has been developed and which focuses on producing new and interesting ways that will help in leveraging these quantum states and quantum information. If successful, quantum computing will make room for several things.

“It will allow for much faster decryption and encryption of information,” he said, “which is why some of the big defense contractors in the NSA, as well as companies like Microsoft, are interested in it. It will also allow us to model quantum systems in a reasonable amount of time and is capable of doing certain calculations and simulations faster than one would typically be able to do.”

It is specifically good for a wide range of problems, such as those that need massive parallel computations, Marković stated. Quantum computing speeds things up tremendously in situations where one needs to perform too many things simultaneously.

“There are problems right now that would take the age of the universe to compute,” she said.

“With quantum computing, you'd be able to do it in minutes.”

This could also eventually lead to improvements in drug development and various other complex systems, besides enabling new technologies.

The Researchers plan to commence with the building up of more improved devices that are geared towards essentially constructing a qubit out of the systems that they have, and also try out varied metals in order study the possibility of changing the properties of the material.

“It really is a new potential way of fabricating these devices that no one has done before,” Mlack said. “In general, when people make some of these materials by combining this topological material and superconductivity, it is a bulk crystal, so you don't really control where everything is. Here we can actually customize the pattern that we're making into the material itself. That’s the most exciting part, especially when we start talking about adding in different types of metals that give it different characteristics, whether those be ferromagnetic materials or elements that might make it more insulating. We still have to see if it works, but there's a potential for creating these interesting customized circuits directly into the material.”

The National Science Foundation supported this work through grants DGE-1232825, DMR-1507782 and EFRI 2-DARE 1542707.

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