Tool
reads quantum bits
By
Kimberly Patch,
Technology Research NewsThe key to quantum computing is being able to use the spins of subatomic particles such as electrons to represent the ones and zeros of computing. A particle can be spin-up or spin-down in a way similar to a top spinning either clockwise or counterclockwise.
If you could reliably distinguish between spin-up and spin-down energy in large numbers of particles, the spin possibilities in each particle could serve as a quantum bit, or qubit, representing a one or a zero, and you could build a fantastically powerful computer in very little space.
The trouble is, it's difficult to measure spin. Scientists have done so by trapping isolated atoms and using lasers to measure spin states, but they are still a long way from being able to read the millions of quantum bits required to form a practical quantum computer.
Researchers at the University of California at Berkeley have taken a step towards that goal by showing that it is possible to measure the spin of a quantum state of an electron in a nickel atom embedded in a copper oxide crystal. The development has the potential to make a promising quantum computer scheme considerably more practical.
There are four major problems to be solved in making a quantum computer: its qubits must be able to represent a one or zero long enough for the computer to perform logic operations on them; the qubits must be able to interact with each other to carry out those operations; there must be some way to read the information contained in a qubit in order to see the results of the operations; and the system must contain a lot of qubits to do useful computing.
By measuring the spin of a single atom, the Berkeley researchers have found a way to read the information contained in a certain type of qubit. This type of qubit -- a single atom embedded in a solid made of other atoms -- has already shown potential for solving the other three problems associated with quantum computing.
A theoretical proposal by University of Maryland researcher Bruce Kane shows that qubits made from phosphorus atoms embedded in silicon could hold their spin states for a long enough time to do computing, could be placed closely enough to interact with each other, and could be made in a large quantity.
The Berkeley method addresses the key missing piece in that plan by showing that it is possible to measure the spin of a single electron within an impurity, or atom of one material embedded in another.
The researchers used a scanning tunneling microscope (STM) to measure the spin of an electron associated with a nickel impurity embedded in copper oxide, but they had to make some modifications to do so.
Scanning tunneling microscopes use tips that resemble needles, but are so sharp that they taper to a single atom. The tip hovers over the surface of a material and maps the changes the material's electron energy makes to the electron current flowing through the tip similar to the way a seismograph maps movement.
Because spin-up and spin-down states have different energy, the researchers were able to distinguish between them. "We are trying to get an electron to jump into one of the quantum states from a nearby metal tip. The spin-down state exists at a lower energy than the spin-up state at the atom we studied, so by measuring the rate at which the electrons jump into the state as a function of their energy we can tell which is which," said Davis.
To make the scheme work, however, the researchers had to solve a pair of problems.
First, the spin energy of an electron can only be split into discernible spin-up or spin-down states under certain conditions, said Davis. "In each [impurity] atom there's a single wave function of the electron... you can split that wave function into a spin-up and spin-down state if you're in a high magnetic field at low temperatures," he said.
The amount by which the two energy levels are split is proportional to the strength of the magnetic field, so the stronger the magnetic field, the easier it is to distinguish the two levels.
Second, heat energy easily drowns out spin energy. "The amount of energy associated with the temperature has to be smaller than the splitting between the two levels [otherwise] thermal energy would just be knocking electrons up and down from the bottom [energy level] to the top one all the time," Davis said.
The researchers solved the problems by measuring electron spin in a nickel impurity embedded in a superconductor at a relatively low temperature.
Copper oxide is a high-temperature superconductor, meaning its electrons are free to travel without resistance at 85 degrees Kelvin, or -188 degrees Celsius, which, though very cold, is less cold than the temperatures of 4 degrees Kelvin, or -269 degrees Celsius required by low-temperature superconductors.
Because nickel is magnetic, it exerts a magnetic force that is very strong at distances of 10 or 20 nanometers away from the atom. "The effective field at the nickel atom is hundreds of Tesla. So we didn't need a big external magnet, we got it for free by putting a magnetic atom into the solid," said Davis.
The researchers next plan to use the same technique to measure electron spin in a phosphorus atom embedded in a silicon chip, which is the setup required in the Kane quantum computer proposal.
Because phosphorus is not magnetic, the Berkeley researchers need to generate a large magnetic field in order to measure the spins of its quantum particles. The researchers are planning to build an STM that can generate an eight Tesla field at temperatures as low as 20 millikelvin in order to carry out the measurements, said Davis.
If the researchers are able to measure spin states in phosphorus atoms, "then that's really big news because that was the really big problem of the Kane proposal," said Paul Kwiat, a physics professor at the University of Illinois at Urbana-Champaign.
"The main reason people were skeptical about [the Kane proposal] was the need for reading out single spins, which seemed like it was not going to be very easy, and it still may not be very easy. But certainly this is an experiment in the right direction," Kwiat said.
The Kane proposal is probably the most promising model so far for quantum computing, largely because it is based on silicon, Kwiat added. "If you can do something in silicon... and you get it to work, you can hand it to the silicon industry," he said.
Researchers in the quantum field generally agree that practical quantum computers are at least two decades away, if they can be built at all. "It's like asking when fusion will generate cheap energy. It's a possible but technically hard challenge," said Davis.
Davis' research colleagues were Eric W. Hudson of the University of California at Berkeley and the National Institute of Standards and Technology, Christine M. Lang and Vidya Madhavan of the University of California at Berkeley, Shuheng H. Pan of the University of California at Berkeley and Boston University, Hiroshi Eisaki from the University of Tokyo in Japan and Stanford University, and Shin-ichi Uchida of the University of Tokyo.
They published the research in the June 21, 2001 issue of the journal Nature. The research was funded by the Office of Naval Research (ONR) and the Department of Energy (DOE).
Timeline: > 20 years
Funding: Government
TRN Categories: Quantum Computing
Story Type: News
Related Elements: Technical paper, "Interplay of Magnesium and High Tc Superconductivity at individual Ni impurity atoms in Bi2Sr2CaCu2O8+ d," Nature, June 21, 2000; Additional images at the Davis group website: http://Socrates.Berkeley.edu/~davisgrp/stm/results/nickel/index.htm
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August 1/8, 2001
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