Featured Research

from universities, journals, and other organizations

Superconducting-silicon qubits: Using a bottom-up approach to make hybrid quantum devices

July 2, 2014
Joint Quantum Institute
Theorists propose a way to make superconducting quantum devices such as Josephson junctions and qubits, atom-by-atom, inside a silicon crystal. Such systems could combine the most promising aspects of silicon spin qubits with the flexibility of superconducting circuits.

Examples of superconducting-silicon quantum devices. (left) A superconducting loop interrupted at two points by junctions can form a superconducting flux qubit or a superconducting quantum interference device, or SQUID. Currents flowing in the loop can be used to measure the strength of a magnetic field threading the loop. The currents (flowing in either direction) can also be used to constitute a qubit. (middle) Separating the superconducting wires by an insulator, in this case pure, crystalline silicon, forms a Josephson junction. (right) Precisely placed, highly doped regions within the semiconductor form the superconducting wires.
Credit: LPS

Theorists propose a way to make superconducting quantum devices such as Josephson junctions and qubits, atom-by-atom, inside a silicon crystal. Such systems could combine the most promising aspects of silicon spin qubits with the flexibility of superconducting circuits. The researcher's results have now been published in Nature Communications.

High quality silicon is one of the historical foundations of modern computing. But it is also promising for quantum information technology. In particular, electron and nuclear spins in pure silicon crystals have been measured to have excellent properties as long-lived qubits, the equivalent of bits in conventional computers.

In a paper appearing this week in Nature Communications, Yun-Pil Shim and Charles Tahan from the University of Maryland and the Laboratory for Physical Sciences (on the College Park, MD campus) have shown how superconducting qubits and devices can be constructed out of silicon. Doing so can potentially combine the good quantum properties of silicon and the ubiquity of semiconductor technology with the flexibility of superconducting devices. They propose using "bottom-up" nano-fabrication techniques to construct precisely placed superconducting regions within silicon or germanium and show that such "wires" can be used to make superconducting tunnel junctions and other useful superconducting devices.

Qubits in superconductors and semiconductors

Superconducting circuits, made from superconducting metals and Josephson tunnel junctions (which allow superconducting electron pairs to tunnel between two superconductors), are exceptionally customizable and can produce devices ranging from magnetic field sensors to classical logic circuits. They are also likely to play a big role in processing quantum information, where they can be used as a platform for qubits, tiny quantum systems which reside in a superposition of quantum states.

Several types of superconducting circuits have been used to implement qubits and quantum logic gates with different properties and potential uses. For example, in one kind of circuit current can flow in either of two directions. These alternatives constitute the two superposed states needed for establishing a qubit. The two states can be labeled "0" and "1" in analogy with classical bits. Microwave pulses can drive transitions between the two levels allowing for quantum logic gates.

In general, quantum systems are delicate objects and are susceptible to noise and other environmental factors which diminish performance. Prospective quantum circuits must preserve qubits from outside interference for as long as the quantum calculation proceeds. Despite rapid progress in the quality of superconducting qubits (qubit lifetimes can now surpass 100 microseconds), qubit gate error rates are still limited by loss in the metals, insulators, substrates, and interfaces that make up the heterogeneous superconducting devices.

Spin qubits, are an example of qubits realized in a solid-state, silicon context. Spin is a quantum property of particles like an electron; physicists often think of an electron's spin as being like a small magnet, which will naturally point along the direction of an applied magnetic field. Here the 0 and the 1 states correspond to the two possible orientations of the electron spin, either up or down. Because spin is naturally decoupled from charge in some systems (meaning the information stored in the direction of the spin will not be ruined by moving the electron or by it being shaken by electric noise), spin qubits are thought to be promising candidates for a robust qubit design. Further, the use of epitaxial semiconductor devices, and the ability to bury spin qubits deep inside a semiconductor medium, far away from noise at interfaces and surfaces, has resulted in qubits that live for seconds or even hours in some situations, much longer than superconducting qubits to date.

Practical devices

Shim and Tahan propose to use the best features of superconductor and semiconductor qubits. They aim to make superconducting wires and junctions, from which qubits and sensors can be made, by placing (or "doping") acceptor atoms (such as boron or aluminum, elements which readily accept extra electrons) in silicon in precise regions within the crystal. They suggest that a recently developed technique from the silicon qubit community, "STM hydrogen lithography," can be used to do just that. Pioneered by Michelle Simmons at the University of New South Wales, a scanning tunneling microscope (STM) tip is used to selectively remove hydrogen atoms on the surface of silicon (or germanium). Doping gas such as phosphine can then be introduced, allowing the selective placement of impurities down to a single atomic site. "If acceptor atoms can be placed at sufficient density over enough layers, then superconducting regions can be fabricated within the silicon and then encapsulated with crystalline silicon," says Dr. Shim.

In some STM efforts as many as one-in-four silicon atoms have been replaced in this manner. And generally the higher the dopant density, the higher the critical superconducting temperature will be.

Scientists first learned about 10 years ago that silicon can be made superconducting if doped to sufficient density with acceptor atoms such as boron. In recent years, the quality of such superconducting silicon systems has improved greatly, yielding material with superconducting critical temperatures approaching 1 Kelvin and still leaving the crystal in good condition (in other words, it's still silicon).

By calculating the properties of these superconducting-semiconductor regions, Shim and Tahan show that wires with sufficient critical temperature can be constructed practically with the bottom-up hydrogen lithography approach. They also show that Josephson tunnel junctions and weak links, the fundamental non-linearity from which superconducting circuits can be constructed, can also be made. Finally they show that the previously-demonstrated superconducting qubit types (seen in metal samples) can be constructed in this silicon system as well and provide the geometric requirements needed for fabrication.

"There is ongoing effort to make the tunneling barrier epitaxial to improve its quality," said Charles Tahan, "but no previous work to make the whole device out of a single semiconductor crystal. As far as we know, this is the first proposal on the feasibility of SC silicon for Josephson junctions and qubits. I'm also excited about these systems' potential for other devices such as sensors and particle detectors."

Beyond the possibility of superconducting circuits built inside a homogeneous silicon crystal, engineered superconducting-semiconductor devices like these could be used to build other types of exotic quantum many-body systems, at the atomic scale, and even act as testbeds for our understanding of superconductivity itself.

Story Source:

The above story is based on materials provided by Joint Quantum Institute. Note: Materials may be edited for content and length.

Journal Reference:

  1. Yun-Pil Shim, Charles Tahan. Bottom-up superconducting and Josephson junction devices inside a group-IV semiconductor. Nature Communications, 2014; 5 DOI: 10.1038/ncomms5225

Cite This Page:

Joint Quantum Institute. "Superconducting-silicon qubits: Using a bottom-up approach to make hybrid quantum devices." ScienceDaily. ScienceDaily, 2 July 2014. <www.sciencedaily.com/releases/2014/07/140702093612.htm>.
Joint Quantum Institute. (2014, July 2). Superconducting-silicon qubits: Using a bottom-up approach to make hybrid quantum devices. ScienceDaily. Retrieved October 21, 2014 from www.sciencedaily.com/releases/2014/07/140702093612.htm
Joint Quantum Institute. "Superconducting-silicon qubits: Using a bottom-up approach to make hybrid quantum devices." ScienceDaily. www.sciencedaily.com/releases/2014/07/140702093612.htm (accessed October 21, 2014).

Share This

More Matter & Energy News

Tuesday, October 21, 2014

Featured Research

from universities, journals, and other organizations

Featured Videos

from AP, Reuters, AFP, and other news services

Thanks, Marty McFly! Hoverboards Could Be Coming In 2015

Thanks, Marty McFly! Hoverboards Could Be Coming In 2015

Newsy (Oct. 21, 2014) If you've ever watched "Back to the Future Part II" and wanted to get your hands on a hoverboard, well, you might soon be in luck. Video provided by Newsy
Powered by NewsLook.com
Robots to Fly Planes Where Humans Can't

Robots to Fly Planes Where Humans Can't

Reuters - Innovations Video Online (Oct. 21, 2014) Researchers in South Korea are developing a robotic pilot that could potentially replace humans in the cockpit. Unlike drones and autopilot programs which are configured for specific aircraft, the robots' humanoid design will allow it to fly any type of plane with no additional sensors. Ben Gruber reports. Video provided by Reuters
Powered by NewsLook.com
Graphene Paint Offers Rust-Free Future

Graphene Paint Offers Rust-Free Future

Reuters - Innovations Video Online (Oct. 21, 2014) British scientists have developed a prototype graphene paint that can make coatings which are resistant to liquids, gases, and chemicals. The team says the paint could have a variety of uses, from stopping ships rusting to keeping food fresher for longer. Jim Drury reports. Video provided by Reuters
Powered by NewsLook.com
China Airlines Swanky New Plane

China Airlines Swanky New Plane

Buzz60 (Oct. 21, 2014) China Airlines debuted their new Boeing 777, and it's more like a swanky hotel bar than an airplane. Enjoy high-tea, a coffee bar, and a full service bar with cocktails and spirits, and lie-flat in your reclining seats. Sean Dowling (@SeanDowlingTV) has the details. Video provided by Buzz60
Powered by NewsLook.com

Search ScienceDaily

Number of stories in archives: 140,361

Find with keyword(s):
Enter a keyword or phrase to search ScienceDaily for related topics and research stories.


Breaking News:

Strange & Offbeat Stories

Space & Time

Matter & Energy

Computers & Math

In Other News

... from NewsDaily.com

Science News

Health News

Environment News

Technology News


Free Subscriptions

Get the latest science news with ScienceDaily's free email newsletters, updated daily and weekly. Or view hourly updated newsfeeds in your RSS reader:

Get Social & Mobile

Keep up to date with the latest news from ScienceDaily via social networks and mobile apps:

Have Feedback?

Tell us what you think of ScienceDaily -- we welcome both positive and negative comments. Have any problems using the site? Questions?
Mobile: iPhone Android Web
Follow: Facebook Twitter Google+
Subscribe: RSS Feeds Email Newsletters
Latest Headlines Health & Medicine Mind & Brain Space & Time Matter & Energy Computers & Math Plants & Animals Earth & Climate Fossils & Ruins