In a step toward a generation of ultrafast computers, physicists have used bursts of radio waves to briefly create 10 billion quantum-entangled pairs of subatomic particles in silicon. The research offers a glimpse of a future computing world in which individual atomic nuclei store and retrieve data and single electrons shuttle it back and forth.
In a paper in the journal Nature, a team led by the physicists John Morton of Oxford University and Kohei Itoh of Keio University describes bombarding a three-dimensional crystal with microwave and radio frequency pulses to create the entangled pairs. This is one of a range of competing approaches to making qubits, the quantum computing equivalent of today’s transistors.
Transistors store information on the basis of whether they are on or off. In the experiment, qubits store information in the form of the orientation, or spin, of an atomic nucleus or an electron. The storage ability is dependent on entanglement, in which a change in one particle instantaneously affects another particle even if they are widely separated. The new approach has significant potential, scientists said, because it might permit quantum computer designers to exploit low-cost and easily manufacturable components and technologies now widely used in the consumer electronics industry.
“I think this is a very neat piece of work,” said Raymond Laflamme, a physicist at the University of Waterloo in Ontario, “but I think it’s important to see it as a piece of a big puzzle. Our mecca is to build a quantum computer that could have thousands of qubits; here we have only a few.”
Indeed, there is still disagreement over whether scientific or commercially useful quantum computers will ever be built. Until now, scientists have designed prototypes based on only a handful of qubits, too few to gain meaningful speed over conventional computers.
In today’s binary computers, transistors can be in either an “on” or an “off” state, but quantum computing exploits the notion of superposition, in which a qubit can be constructed to represent both a 1 and a zero state simultaneously.
The potential power of quantum computing comes from the possibility of performing a mathematical operation on both states simultaneously. In a two-qubit system it would be possible to compute on four values at once, in a three-qubit system on eight, in a four-qubit system on 16, and so on. As the number of qubits grows, potential processing power increases exponentially.
There is, of course, a catch. The mere act of measuring or observing a qubit can strip it of its computing potential. So researchers have used quantum entanglement — in which particles are linked so that measuring a property of one instantly reveals information about the other, no matter how far apart the two particles are — to extract information. But creating and maintaining qubits in entangled states has been tremendously challenging.
The new approach is based on a purified silicon isotope doped with phosphorus atoms. The research group was able to create and measure vast numbers of quantum-entangled pairs of atomic nuclei and electrons when the crystal was cooled to about 3 kelvin. The group hopes to produce the basis for a quantum computing system by moving the entangled electrons to simultaneously entangle them with a second nucleus.
“We would move the electron from the nuclear spin it is on to the neighboring nuclear spin,” said Dr. Morton. “That shifting step is what we really now need to show works while preserving entanglement.”
One of the principal advantages of the new silicon-based approach is that the group believes that it will be able to maintain the entangled state needed to preserve quantum information as long as several seconds, far longer than competing technologies which currently measure the persistence of entanglement for billionths of a second.
“To a member of the general public, that still sounds like a lousy time for a computer memory,” Dr. Morton said. “But for quantum information, the lifetime of a second is very exciting,” because there are ways to refresh data.
The advance indicates there is an impending convergence between the subatomic world of quantum computers and today’s classical microelectronic systems, which are reaching a level of miniaturization in which wires and devices are composed of just dozens or hundreds of atoms.
“This is on a single-nucleus scale, but it isn’t that far away from what is being used today,” said Stephanie Simmons, a graduate physics researcher at Oxford and the lead author of the paper. “There are two reasons people are taking a look at quantum computing. One is its power, but the other is that the size of silicon transistors are shrinking to the point where quantum effects are becoming important.”
Advance in Quantum Computing Entangles Particles by the Billions – NYTimes.com.
By Emily Chung CBC News
The study demonstrated that billions of phosphorus atoms embedded in a silicon crystal can be put into the same quantum state. That state involves the entanglement of two data bits, represented by the spirals in this image, within each atom.The study demonstrated that billions of phosphorus atoms embedded in a silicon crystal can be put into the same quantum state. That state involves the entanglement of two data bits, represented by the spirals in this image, within each atom. (Stephanie Simmons/Nature/University of Oxford)A key step toward silicon-based quantum computers has been made by an international team of researchers.
The scientists showed that quantum bits of data, known as “qubits,” could be encoded within a type of silicon similar to that used in conventional computing.
Their findings were published online in Nature Wednesday. The lead author of the study is Stephanie Simmons, a Canadian from Ottawa and Kitchener-Waterloo, Ont., doing her D. Phil. degree in physics at the University of Oxford in the U.K.
A traditional “bit” in computing can exist in one of two states, “0” or “1.”
Qubits are the quantum computing equivalent of bits, but the laws of quantum physics mean one quantum bit can simultaneously exist in both “0” and “1” states. That means each quantum bit is the equivalent of two conventional bits, but it is not simply an arithmetical doubling. Each qbit would exponentially add computing power to a system.
Just 20 qubits would offer the computing power of 220 conventional bits, or about one million bytes — one megabyte — of processing ability. A 30-qubit system would have the power of 230 conventional bits, or about one billion bytes — a gigabyte — of power; 40 qubits are equivalent to 240 bits — a trillion bytes — or a terabyte system; and 50 qubits would be akin to 250 bits or – one quintillion bytes — one petabyte.
“Nobody’s entangled quantum bits in a solid state system before,” said Mike Thewalt, a physics professor at Simon Fraser University in Burnaby, B.C., who co-authored the paper.
“One of the reasons that people are looking at silicon specifically is if we can think of a way of doing that, then you inherit all of that technology that’s been used for silicon electronics.”
Quantum computers have the potential for exponentially greater computing power than conventional computers. They are based on laws of physics that apply to very small particles like electrons and are very different from the classical laws of physics that we are familiar with in daily life.
Such computers encode data using a phenomenon called entanglement, which permanently links two objects so that each is affected by the experience of the other, no matter how far apart they are.
Up until now, the entangled systems that researchers have been able to observe and control have involved mainly photons — two particles of light — or atomic gases.
But Simmons and her colleagues demonstrated that billions of phosphorus atoms embedded in a silicon crystal can be put into the same quantum state. That state involves the entanglement of two data bits within each atom.
The laboratory of Mike Thewalt at Simon Fraser University prepared the tiny crystal of isotopically pure silicon that was used in the experiment. The crystal is shown here on a penny, for scale. The laboratory of Mike Thewalt at Simon Fraser University prepared the tiny crystal of isotopically pure silicon that was used in the experiment. The crystal is shown here on a penny, for scale. (Stephanie Simmons/University of Oxford/Nature)While that sounds impressive, Thewalt said, it would actually be a bigger deal if researchers had shown entanglement in a single phosphorus atom.
“If you want to build a quantum computer, what you’re going to have to do is entangle a single phosphorus and do something with it and then measure what happened at the end,” he said.
Many different approaches to quantum computing are currently being researched, and “they’re all a long way from fruition,” said Thewalt.
Atoms consist of a core called a nucleus surrounded by a cloud of electrons. Both the nucleus and the electrons have a property called spin. Their spins can flip between states that could be used to represent data bits.
In the Oxford experiment, the researchers entangled an electron spin with a nuclear spin within each phosphorus atom by hitting it with radiofrequency magnetic fields — one targeted at the electron and one targeted at the nucleus — in sequence.
The large signal generated by billions of atoms in the same quantum state allowed them to measure the entanglement.
Thewalt’s contribution was figuring out what type of material could be used for the experiment — a special type of silicon called isotopically enriched, or istopically “pure,” silicon.
Thewalt had commissioned a sample of the special silicon to use in studies involving light. He later contacted Simmons’s research advisor, John Morton, with the idea that a leftover chunk could be used for a quantum entanglement experiment.
His lab analyzed it and “picked out the best parts,” then sent the sample to Morton.
The material was key because normal silicon generates a lot of background signals that swamp the signals that researchers are trying to measure, Thewalt said. The background signals also make the information stored in the phosphorus atoms fade far more quickly, making it more difficult to conduct experiments and measurements. Isotopically enriched silicon overcomes those difficulties.
“It’s like a perfect host material,” Thewalt said.
Read more: http://www.cbc.ca/technology/story/2011/01/19/science-silicon-quantum-computing.html?ref=rss#ixzz1Bz41ummW
- A Step Towards Quantum Computing: Entangling 10 Billion Particles | 80beats (blogs.discovermagazine.com)
- Study marks spooky step towards quantum computers (reuters.com)
- Ultrafast quantum computer closer: Ten billion bits of entanglement achieved in silicon (sciencedaily.com)
- Scientists create 10 billion qubits in silicon, get us closer than ever to quantum computing (engadget.com)
- Silicon quantum computing leap made (cbc.ca)
- Silicon quantum computer a possibility (nature.com)
- First silicon entanglement will aid quantum computing (newscientist.com)
- 10 billion pairs of particles entangled with single radio wave (arstechnica.com)
- Oxford scientists edge toward quantum PC with 10b qubits (electronista.com)