The construction of a completely new kind of computer that dispenses with transistors might be made easier by findings now described in Applied Physics Letters¹. Researchers in America say that they have figured out how to make logic circuits without wires.

Islamshah Amlani and colleagues at the University of Notre Dame, Indiana, USA, have constructed an electronic device that would form the basis of a new computer based on so-called 'quantum-dot cellular automata' (QCA).

The QCA idea was proposed in the early 1990s, by a second team of Notre Dame researchers led by Douglas Tougaw. They envisaged a computer rather like a vast binary memory, made of cells encoding the '1's and '0's of binary data. Each cell comprises four 'quantum dots' -- tiny devices capable of holding electrons, the charged particles that carry electrical current.

In this imaginary cell of computer memory, the four quantum dots are arranged at the corners of a square, and can pass electrons to one another. Introduce two 'spare' electrons to the square and they will position themselves at opposite corners because their mutual repulsion keeps them as far apart as possible.

The electrons could sit on either of the two diagonals of the square, and so the four-quantum-dot cell would have two stable configurations, which can encode either a 1 or a 0 of binary data. But crucially, cells placed side by side in an array will also influence one another because of the repulsion between electrons in different cells. This makes it possible for cells to communicate and perform logic operations (in traditional computers these are conducted by transistors sending voltage pulses to one another).

Cell switching can be extremely fast. And in principle the quantum dots can be made as small as single molecules. So QCA computers would be very fast and compact.

Amlani's group reported the first experimental demonstration of a quantum-dot cell in 1997². Their quantum dots were tiny blobs of aluminium deposited on a silicon wafer. Electrons were supplied to, and removed from, each dot through tiny wires that connected each dot to 'electron reservoir' electrodes.

But while leads were needed to demonstrate the basic functioning of their cell, the researchers said that a true QCA computer should have isolated cells, without wires attached.

And this is what they have now made. The devices are more or less the same as before: four aluminium quantum dots connected together in two pairs by 'tunnel junctions' through which electrons can pass if they have enough energy.

But there is no way to get electrons into the cell from the outside: all the switching involves the electrons already present in the metal dots. The whole cell measures just a few thousandths of a millimetre across.

The researchers push an electron from one dot to its partner by applying voltage to an adjacent electrode. In turn, this pushes another electron across the junction between the other pair, so that it sits in the opposite corner: the fundamental switching process of the QCA idea.

In practice, the 'switching' of a neighbouring cell would throw the same switch in its neighbour, dispensing with the electrode.

Not only are the new cells without leads easier to make than their earlier counterparts, the researchers say, but they work better because extra junctions that degrade the 'signal' are removed. That's to say, in a wireless QCA array, the cells are more responsive to each other. 

• References

  1. Amlani, I. et al. Experimental demonstration of a leadless quantum-dot cellular automata cell. Applied Physics Letters 77, 738 - 740 2000. | Article | ISI | ChemPort |
  2. Orlov, A.O., Amlani, I., Bernstein, G.H., Lent, C.S. & Snider, G.L. Realization of a functional cell for quantum-dot cellular automata. Science 277, 928 - 930 1997. | Article | ISI | ChemPort |