While Hewlett-Packard is not among the top chipmakers, it is doing pretty interesting physics in its labs. We have already mentioned the crossbar latch technology a few years ago.

Now, the team of Stan Williams has experimentally realized an old theoretical idea. They have described their breakthrough in Wednesday's issue of Nature, in the article called

The Missing Memristor Found (podcast, go to the last 1/3; Nature's summary).It seems like the beginning of the birth of

memory chips based on memristors.What is a memristor? Well, verbally speaking, it is a (non-linear) resistor with memory. Visually speaking, it is the gadget on the picture above. Two titanium layers connected into a 150-atoms-thick wire: there are 17 of them on the picture. The resistance of one layer can be modified by a certain amount of current that has flown through the other layer.

This trick could lead to more efficient memories - denser, more energy efficient, and persistent after you turn off your PC! - and HP claims that the physical issues have been solved and now it is all about engineering. Engineers should draw and produce better circuits.

**A theorist who was 37 years ahead of experiments**

The existence of microgadgets that should behave like that was theoretically proposed by Leon Chua (UC Berkeley) in a paper from 1971. He decided that resistors, capacitors, and inductors should be supplemented by the fourth passive circuit element, the memristors, that are able to integrate the total current passed through them. It's the fourth mathematically natural possibility.

For memristors, the

**total charge**- the integral of the current - is proportional to (or a fixed function of) the total magnetic flux (more precisely,

**flux linkeage**, also including the factor of the number of turns) - which is essentially the integrated voltage.

If you omitted the integrals above, a memristor would be just like a resistor. For a linear relationship between the charge and the flux linkeage, the coefficient would therefore be nothing else than the ratio of the voltage and the current i.e. the resistance (the "∫dt" factor cancels). But the integrals add a new twist to the whole game, namely the constants of integration i.e. the memory. Moreover, the relation between the charge and the flux linkeage is nonlinear.