An international team of researchers including scientists at Princeton University have achieved a 100-fold increase in the ability to maintain control the spins of electrons in a solid material, a key step in the development of ultrafast quantum computers.
Until recently, the best attempts at such control lasted for only a fraction of a second. But researchers Stephen Lyon and Alexei Tyryshkin have found a way to extend the control over the spins of billions of electrons in a silicon chip for up to 10 seconds, far longer than any previous attempt.
Lyon, an electrical engineering professor, said the key to the new results lies in a highly purified sample of silicon. The experiment uses a small silicon chip the size of a pencil lead made almost entirely of a particular isotope of silicon: silicon-28. The researchers, part of an international team, reported their results online Dec. 4 in Nature Materials.
"Partly, it is an improvement in our measurements, but it is mainly the material," Lyon said. "This is the purest sample we have ever used."
In an experiment conducted in the basement of Princeton's Hoyt laboratory, the researchers suspended the sample of pure silicon inside a cylinder filled with liquid helium, dropping its temperature to 2 kelvin, or just above absolute zero. They locked the cylinder between two donut-shaped rings about the size of pizza boxes that control the magnetic field around the sample. A click of a computer mouse sent microwaves pulsing across the silicon, and coordinated the spins of about 100 billion electrons.
"The first pulse twists them, the second reverses them, and at some point the sample itself produces a microwave pulse, and we call that the echo," Lyon said. "By doing the second pulse, getting everything to reverse, we get the electrons into phase."
In describing electrons, scientists use the term spin. But like a lot of things in quantum mechanics, the meaning is a little bit tricky. For subatomic particles like electrons, spin is a fundamental characteristic that can make them behave like incredibly tiny magnets. Lyon's team uses this magnetic signature in their observations.
Maintaining that phase is what scientists call "coherence." Unlike objects in the everyday world, subatomic particles, which operate under the rules of quantum mechanics, can be in more than one place at the same time. Electrons' spin, for example, can be classified as up, down, or in superposition, a state that is both up and down simultaneously. It is this superposition state that allows for the highly complex mathematics at the heart of quantum computing.
A standard computer uses transistors either switched off or on to represent the 0's and 1's that are the bits that make up the basis of all computer programs. But instead of this binary language, a quantum computer would incorporate the uncertainty of quantum mechanics into its programming. Instead of bits, the computers will use quantum bits or qubits - a value that is inherently indeterminate.
Mathematicians are still working on ways to take advantage of such a machine. They believe it could be used to factor incredibly large numbers, break cryptographic codes or to simulate the behavior of molecules.
Although mathematically fascinating, keeping electrons in this indeterminate state is fantastically difficult for engineers. In a 2003 report in Physical Review B, Lyon's group reported a breakthrough when they maintained coherence for 60 milliseconds (a millisecond is one thousandth of a second.)
To understand why it is so hard, imagine circus performers spinning plates on the top of sticks. Now imagine a strong wind blasting across the performance space, upending the plates and sending them crashing to the ground. In the subatomic realm, that wind is magnetism, and much of the effort in the experiment goes to minimizing its effect. By using a magnetically calm material like silicon 28, the researchers are able to keep the electrons spinning together for much longer.
"The project started ten years ago," said Tyryshkin, a research scientist in the electrical engineering department. "Steve came into my office saying let's try a sample that is clean of other isotopes."
Lyon said the experiment is a successor to one performed in 1958 at Bell Labs by James Gordon, one of the co-inventors of the maser (the predecessor to the laser). Using technology available at the time, Gordon was able to maintain coherence for 600 microseconds (a microsecond is one millionth of second.) Lyon and Tyryshkin began following a similar path after they ran early tests in their lab and began seeing similar numbers to those reported by Gordon.
The researchers tried a series of different samples, each of increasingly purified silicon. The answer finally came through the Avogadro Project, an international effort to create a pure kilogram of silicon. Michael Thewalt, a physics professor at Simon Fraser University and Kohei Itoh, a professor at Keio University, co-authors of the recent paper with Lyon, had been working with the Avogadro Project and requested a special sample for use in the electron spinning experiment.
Elements are identified by the number of protons inside their nucleus: carbon has 6 protons, silicon has 14. But most elements come in different version - called isotopes - determined by the number of neutrons. Some isotopes, like silicon-28, have no magnetism, while others create a strong magnetic effect at atomic level. A relatively common isotope of silicon, silicon-29, has a very strong magnetic presence and, therefore it was a prime target for elimination.
The combined effort largely eliminated isotopes of silicon that create a strong magnetic field - like silicon-29 - and other impurities. The scientists managed to clean the sample to an amazing degree: silicon-29, which usually makes up nearly 50,000 parts per million of a typical sample, was reduced to 50 parts per million in the chip sent to the Princeton team. In fact, the final silicon crystal was so pure that the scientists in Berlin added a trace amount of phosphorous so the sample would be electrically active enough to respond to the microwave pulse in the laboratory. That response, which researchers call "the echo" is how the researchers read the electrons' spin state. So calibrating the correct amount of phosphorous was critical - too much would create the magnetic noise that the team was trying to eliminate, but too little would leave the echo too faint to detect.
"A lot of the work boils down to getting the phosphorous far enough apart," Lyon said. Temperature is also critical. At room temperature, the electrons from the phosphorous are too active, and prevent the control that the team is trying to exert. But once the researchers use liquid helium to drop the sample's temperature to 2 Kelvin (-455.8 degrees Fahrenheit,) everything calms down.
"It has taken quite a bit of work to get to this point," Lyon said. "Nine years of refining measurements and materials."
Besides Tyryshkin, Lyon, Thewalt and Itoh, the team that contributed to the work described in the Nature Materials article included Shinichi Tojo and of Keio University; John Morton of Oxford University; Helge Riemann and Nikolai Abrosimov of the Institut fur Kristallzuchtung; Peter Becker of Physikalisch-Technische Bundesanstalt; Hans-Joachim Pohl, of VITCON Projectconsult GMBH; and Thomas Schenkel of Lawrence Berkeley National Laboratory.
Extending coherence time is an important step toward building a working quantum computer. The key is to maintain coherence long enough for programs to correct and maintain data before the spins lose their coherence. It is hard to set a threshold for the length of coherence needed for a practical computer, because it depends on the type of program and size of the computer.
"The bottom line is, you want it as long as possible," Tyryshkin said.
Other researchers have managed to attain extended coherence using ions in a vacuum instead of a solid material like silicon. But Lyon's team chose to work with silicon because they believe it is more practical to attempt to scale up the material for use in a computer than to try to do so with vacuum tubes.
"It would be far easier to build devices out of silicon, but we still have to do many other things before we can get to that point," he said.
The researchers stressed that their results were one step on a long road toward a working computer. The electrons on their sample represent one quantum bit, or qubit, and many such qubits would be needed for a working computer. How many is difficult to say.
"Right now, we are using one," Tyryshkin said "If we could come up with a thousand, that would be a very interesting machine."
Note: An expanded version is available on the University website