The future of computing is under the spotlight at the Institute
of Physics’ Condensed Matter and Materials Physics
conference at the Royal Holloway College of the University of London on
The silicon chip, which has supplied several
decades’ worth of remarkable increases in computing power and
speed, looks unlikely to be capable of sustaining this pace for more
than another decade – in fact, in a plenary talk at the
conference, Suman Datta of Pennsylvania State University, USA, gives
the conventional silicon chip no longer than four years left to run.
As silicon computer circuitry gets ever smaller in the quest
to pack more components into smaller areas on a chip, eventually the
miniaturized electronic devices are undermined by fundamental physical
limits. They start to become leaky, making them incapable of holding
onto digital information. So if the steady increases in computing
capability that we have come to take for granted are to continue, some
new technology will have to take over from silicon.
At the conference, researchers at Leeds University in the UK
will report an important step towards one prospective replacement.
Carbon nanotubes, discovered in 1991, are tubes of pure carbon just a
few nanometres wide – about the width of a typical protein
molecule, and tens of thousands of times thinner than a human hair.
Because they conduct electricity, they have been proposed as ready-made
molecular-scale wires for making electronic circuitry.
Some nanotubes behave as semiconductors, like silicon; others
carry electric currents like metal wires. Already, fundamental elements
of computer circuits such as transistors have been made from individual
But the problem is arranging nanotubes into circuit patterns.
One particular difficulty is that they are typically made as mixtures
of metallic and semiconducting tubes, whereas just one type or the
other is needed for a specific component. These electrical properties
depend on the precise arrangement of carbon atoms in the nanotube, but
that’s hard to determine for single tubes.
Bryan Hickey and his coworkers at Leeds have now developed a
technique that will reveal an individual nanotube’s structure
(and thus its electrical properties), and then allow it to be placed in
a position on a surface with an accuracy of about 100 nanometres, a
fraction of the width of a human blood cell. The nanotubes are grown on
a perforated ceramic grid, and tubes lying across the holes are
examined in an electron microscope to deduce their atomic structures.
Then the researchers use two needle-fine tips like tweezers to pick up
a single tube under the microscope and put back down on another surface.
Chris Allen, one of the Leeds teams, says, “With
this technique we can make carbon nanotube devices of a complexity that
is not achievable by most other means.”
Two further talks at the meeting will describe an even more
dramatic way to overcome the limitations of silicon computers. Hans
Mooij of the Delft University of Technology in the Netherlands and
Raymond Simmons of the National Institute of Standards and Technology
in Boulder, Colorado, USA, will claim that superconductors –
materials that conduct electricity with zero electrical resistance
– can harness the power of quantum physics to boost computer
So-called quantum computers have become one of the hottest
items in physics over the past decade. They attempt to improve on the
power of silicon not by making components smaller but by exploiting the
counterintuitive principles of quantum mechanics, the theory generally
used to understand how objects behave at the scale of atoms and
Objects governed by quantum theory can be in several different
states at once, like a light switch being simultaneously
‘on’ and ‘off’. These
‘superposition’ states don’t correspond
to anything familiar from our everyday world, but countless experiments
have proved that they can exist so long as the quantum objects are not
disturbed by, for example, making a measurement on them.
In a quantum computer, the equivalent of
‘bits’ that hold binary information as
1’s and 0’s in today’s computers will be
quantum bits or qubits, which can also exist as superpositions of
1’s and 0’s. This massively increases the amount of
information that can be encoded in a quantum computer’s
memory. The catch is that superpositions are extremely delicate and
hard to maintain, especially in memories containing large numbers of
qubits that interact with one another.
Various candidates for making qubits are being explored, such
as magnetically trapped atoms or nanometre-scale blobs of
semiconductors. But it has long been recognized that loops of
superconducting material can also be placed in quantum superposition
states, and thus act as qubits. Here the quantum states may correspond
to an electric current circulating round the ring in one direction or
the other. (In superconductors this circulation can continue more or
less indefinitely without petering out, because there is no electrical
At the conference, Simmonds will describe the first
demonstration of information being transmitted between two such
superconducting qubits. This shows that elements of this kind can act
as a quantum-computing memory and a “bus” for
qubits to communicate with one another, an essential requirement of any
The two superconducting loops are made from thin wires of
aluminium laid down on a slice of sapphire and cooled to less than 0.1
degrees of absolute zero to make them superconducting. They sit just a
millimetre apart, but are connected by a meandering waveguide 7 mm long
– a kind of light channel, like an optical fibre, but for
microwaves. The superposition state of one qubit can be transferred
into a microwave electrical vibration of the waveguide, like plucking a
guitar string. This microwave “photon” of energy
recording the first qubit’s state can then be controllably
transferred to the other qubit – crucially, without
destroying these delicate quantum states.
Mooij was part of a group that first demonstrated in 2000 that
such superconducting loops can be placed in quantum superposition
states. He will describe the progress that he and others have made
since then, both in making practical quantum devices and in using them
to explore fundamental aspects of quantum mechanics, such as whether
and how the ‘quantum weirdness’ of superpositions
can survive when the objects concerned get much larger than atoms.
Mooij says that one of the biggest challenges in making
quantum computers this way is to progress from two to three qubits that
communicate with each other. He says that the particular approach he
and his colleagues have been developing has the advantage that, if this
can be achieved, scaling up further won’t be too difficult.
Mooij says, “With our qubit, once we have three set
up we can move on to twenty or fifty.”