Quantum bubbles are the key

November 23, 2005

As futuristic as quantum computers seem, what with all those qubits and entangled atoms, here is an idea that promises to make atom-based quantum computers look passé even before anyone has built a full-sized version.

It seems that bubbles of electrons lined up in ultracold liquid helium could be used to build a quantum computer capable of carrying out a staggering 1030 simultaneous calculations.

To carry out these simultaneous calculations, quantum computers normally exploit entities such as atoms and molecules, which can be in several quantum states at once, to encode bits in those quantum states - the famous qubits. But Weijun Yao of Brown University in Providence, Rhode Island, wants to replace atoms with curious things called electron bubbles.

To make an electron bubble, start with liquid helium that has been cooled below 2.17 kelvin so that it behaves like a superfluid, a state of matter with zero viscosity. Now inject a fast-moving electron into the superfluid. When the electron eventually slows to a halt after numerous collisions with helium atoms, it creates a cavity about 3.8 nanometres across by repelling nearly 700 atoms' worth of helium around it (New Scientist, 14 October 2000, p 24).

It is this cavity that makes the electron bubble so very valuable. In a quantum computer, the quantum entities need to be isolated from their surroundings to preserve their fragile states. "What could be more isolated than an electron in a bubble?" asks Yao. "The electron inside each bubble interacts very weakly with the background helium atoms."

Yao says 0s or 1s could be encoded in the electrons' spins. In the presence of a magnetic field, the spin can either be parallel or anti-parallel to the field. Crucially, an electron's spin can exist in both states at the same time, enabling the qubit to be both 0 and 1.

According to Yao, large numbers of electrons, each in its own bubble, can be neatly caged using a combination of a device called a linear quadrupole trap, which traps the electrons in a line, and a set of conducting rings, which create a voltage "valley" for each bubble (see Diagram).

All the spins can be initialised to the same value by cooling the apparatus to 0.1 kelvin. You can then manipulate the electrons by applying a combination of a magnetic field gradient along the line and varying the frequency of the voltages in the quadrupole trap. This changes the spin of individual electrons and makes them interact to perform logicgate operations (www.arxiv.org/ cond-mat/0510757). To read the spin of an electron, the voltage at the end of the electron chain can be lowered so that each bubble drifts in the magnetic field gradient at a velocity that depends on the electron's spin. This drift velocity can be read using lasers.

Because each qubit carries two values, a quantum computer with two qubits could carry out four parallel calculations, one with three qubits eight calculations, and so on. "I see no major technical obstacles to the system I envisage working with 100 qubits," says Yao. "That means it could do 1000 billion billion billion operations all at once."
Written by Marcus Chown


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