Physicists set guidelines for qubit candidates

May 4, 2010 By Lisa Zyga, feature

( -- To build a quantum computer, it's essential to be able to quickly and efficiently manipulate the quantum states of qubits. The qubits, which are the basic unit of quantum information, can be composed of many different kinds of materials, although some work much better than others. With the goal of identifying which physical entities make the best qubits, a team of physicists from the University of California, Santa Barbara, has developed a list of characteristics and qualities that a material defect called deep centers should have in order to exhibit superior quantum mechanical properties.

“The qubits in a quantum computer must be able to retain the they are given long enough to perform quantum across them,” coauthor David Awschalom of UCSB told “In general, this means that the qubits must not interact strongly with the other particles that make up their surroundings. However, if the qubits interact with their environment too weakly, you won’t able to controllably manipulate them in the first place. Therefore, it is important to find quantum systems that interact strongly with their environments in the ways we can control (so that we can manipulate them), but that don’t interact strongly in the other, more random ways that are not easily controllable (so that they don’t lose the information they are given). These kinds of quantum systems will make the best qubit candidates.”

During the past two decades, qubits have been implemented in a range of materials including atoms, liquids, and solids (such as semiconductors, , and insulators). However, a deep center defect in diamond called the nitrogen vacancy center has emerged as a leading qubit candidate due to its attractive quantum . Specifically, the defect enables high quality at that persists for milliseconds. This means that the diamond defect’s quantum state can be easily controlled and measured, making it a good candidate for a qubit.

“In many cases, a defect in a crystalline material (such as diamond) can cause a single quantum state to form that does not interact strongly with the surrounding atoms that make up the remainder of the material,” Awschalom said. “This is the case for the nitrogen-vacancy (NV) center in diamond, which can retain the quantum information imparted to it for many milliseconds even at room temperature. In addition, the of the NV center in diamond can be easily controlled with a laser and microwave source, even though it does not interact strongly with the huge numbers of atoms that surround the defect. While the quantum states of most defects can not be controlled in the same way, the NV center owes these promising properties to its origin as a defect in diamond.”

However, since growing and fabricating devices from diamond is difficult from an engineering perspective, finding a similar defect in another material is desirable. As the physicists explain, defects in a more technologically mature host material could allow for more sophisticated single- and multiqubit devices and innovations in device functionality.

In their study, the physicists based their criteria for qubit candidates on the diamond deep center defect, with the hope of finding defects with similar properties in other materials. The scientists explained that the diamond defect has two important features that distinguish it from other qubit systems. First, the defect is well isolated from possible sources of decoherence, enabling it to have long coherence times. Second, the defect’s excited state manifold has a structure that allows it to be optically initialized and measured at room temperature, whereas many other solid state systems require cryogenic operating temperatures. The physicists identified several physical characteristics that a candidate defect should have in order to reproduce these two important features.

Finally, the scientists compared the diamond defects with deep center defects in another material, 4H silicon carbide. They noted that future work is needed to determine which other classes of deep centers follow the guidelines presented here, which could eventually lead to the qubits in tomorrow’s quantum computers.

“We have several goals,” Awschalom said. “For instance, we would like to identify other defects that can be used as qubits so that we can better understand what combination of material properties lead to the most robust and easily controllable defect qubits. In addition, we’d like to find defect qubits in other materials that can be more easily grown and fabricated than diamond. We are also hopeful that this research will lead us to discover defects with quantum properties that have useful applications beyond those of .”

Explore further: Scientists look beyond diamond for quantum computing

More information: J.R. Weber, et al. “Quantum computing with defects.” PNAS. To be published. Doi:10.1073/pnas.1003052107


Related Stories

Scientists look beyond diamond for quantum computing

April 30, 2010

A team of scientists at UC Santa Barbara that helped pioneer research into the quantum properties of a small defect found in diamonds has now used cutting-edge computational techniques to produce a road map for studying defects ...

Turning down the noise in quantum data storage

January 19, 2010

Researchers who hope to create quantum computers are currently investigating various methods to store data. Nitrogen atoms embedded in diamond show promise for encoding quantum bits (qubits), but the process of reading the ...

UCSB physicists move one step closer to quantum computing

November 20, 2009

Physicists at UC Santa Barbara have made an important advance in electrically controlling quantum states of electrons, a step that could help in the development of quantum computing. The work is published online today on ...

Straightening messy correlations with a quantum comb

November 23, 2009

Quantum computing promises ultra-fast communication, computation and more powerful ways to encrypt sensitive information. But trying to use quantum states as carriers of information is an extremely delicate business. Now ...

12-qubits reached in quantum information quest

May 8, 2006

In the drive to understand and harness quantum effects as they relate to information processing, scientists in Waterloo and Massachusetts have benchmarked quantum control methods on a 12-Qubit system. Their research was performed ...

Recommended for you

A new model of frequency combs in optical microresonators

January 24, 2018

A team from the Faculty of Physics of the Lomonosov Moscow State University, together with scientists from the Russian Quantum Center, have developed a new mathematical model that describes the process of soliton occurrence ...

Retrospective test for quantum computers can build trust

January 24, 2018

Tech companies are racing to make commercial quantum computers. A new scheme from researchers in Singapore and Japan could help customers establish trust in buying time on such machines—and protect companies from dishonest ...

Scientists achieve high power with new smaller laser

January 24, 2018

An international team of scientists has produced the first high-powered, randomly polarised laser beam with a "Q switch" laser, which typically emits pulses of light so brief that they're measured in nanoseconds. Lasers are ...


Adjust slider to filter visible comments by rank

Display comments: newest first

May 07, 2010
This comment has been removed by a moderator.
not rated yet May 09, 2010

A diamond quantum crystal computer, just like the Kryptonians used in Super Man!

Isn't this what I said a few months ago, and the forum moderator deleted and banned me for it?

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.