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 pass茅 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 鈥� the famous qubits. But Weijun Yao of Brown University in Providence, Rhode Island, wants to replace atoms with curious things called electron bubbles.
Advertisement
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鈥� worth of helium around it (New 杏吧原创, 14 October 2000, p 24).
鈥淚 see no major technical obstacles to electron bubbles carrying out 1000 billion billion billion operations at once鈥�
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. 鈥淲hat could be more isolated than an electron in a bubble?鈥� asks Yao. 鈥淭he electron inside each bubble interacts very weakly with the background helium atoms.鈥�
Yao says 0s or 1s could be encoded in the electrons鈥� spins. In the presence of a magnetic field, the spin can either be parallel or anti-parallel to the field. Crucially, an electron鈥檚 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 鈥渧alley鈥� 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 logic-gate operations (). 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鈥檚 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. 鈥淚 see no major technical obstacles to the system I envisage working with 100 qubits,鈥� says Yao. 鈥淭hat means it could do 1000 billion billion billion operations all at once.鈥�
