PORTLAND, Ore. — Quantum computers have inched closer to reality after the National Institute of Standards and Technology (NIST) researchers demonstrated the ability to read and write individual quantum bits using a combination of microwaves and polarized lasers.

The researchers said this week they are now attempting to develop their prototype into a working quantum computer.

"We use microwaves to read/write, but we use polarized laser light to select which atoms will be read and written," said Trey Porto of NIST's Joint Quantum Institute.

Quantum computers could help solve many problems that are intractable today. By encoding quantum bits, or qubits, that represent both ones and zeroes simultaneously--a technique called superposition--quantum computers bypass several steps required by normal computers to sequentially sift through each possible value.

The technical challenge is controlling a quantum computer's processing steps without disturbing the delicate superposition state that makes the qubits so useful. The situation is especially troublesome for adjacent qubits, since reading or writing one qubit often disturbs the state of adjacent ones.

An optical lattice (blue) uses lasers to isolate and control rubidium atoms (red), with polarized light used as information "bits" in future quantum computers.

NIST researchers claim to have found an technique that solves the problem by first selecting the atom with the pinpoint accuracy of a polarized laser beam. The technique allows individual qubits to be read and written with microwaves that do not disturb the state of adjacent qubits.

The researchers used individual, isolated rubidium atoms held in place by an optical lattice of laser beams to store the qubits as quantum energy states. The rubidium atoms can have eight different energy states, and the NIST team chose two "memory" states to represent the one and zero.

The team then selected a second set of "control" energy states to gate the first "memory" pair. By alternating between the "memory" and "control" qubits, the team was able to demonstrate reading and writing individual qubits without disturbing nearby qubits.

"When the atoms are transferred from the memory states to the control states--or visa versa--they can be individually controlled with microwaves," Porto said.'"This transfer between memory and working states occurs with microwaves, using the polarized light to select the atoms which will then be transferred to the working states. Once in the working states, another microwave pulse tuned to the working states will affect only them, allowing individual isolated control."

The reading, writing and alternating between memory and working qubits was enabled by the optical lattice used to hold the atoms in place, and by a polarized laser to address individual qubits. The technique is now being developed at NIST into a full-fledged quantum computer.