The quantum computer, i.e. a computer that obeys the laws of quantum mechanics, is one of the “hot” topics in physics. If it can be made to work some day, this kind of quantum processor would be able to carry out countless calculations simultaneously, putting conventional computers in the shade as far as speed is concerned. However, quantum computers are still a long way off.

However, researchers led by physics professor Andreas Wallraff at the Quantum Device Lab of ETH Zurich have now taken what is possibly an important step towards the quantum computer. They have successfully created a very special state of a quantum bit on a microchip. Their paper was published last week in “Science Express”.

Geometry helps

This state is based on a geometric operation that can be replicated on one’s own body by extending the right arm above the head with the thumb pointing towards the left. The arm is now lowered forwards until horizontal, moved to the side and raised above the head again – without twisting it. Now the thumb points forwards instead of to the left, i.e. it has performed a geometrical transformation. Comparable effects also occur in quantum physics and are regarded as promising opportunities for future quantum computers.

Physicists in the Quantum Device Lab of ETH Zurich have now recreated this state in a solid object. They built a superconducting circuit on a microchip in an architecture called Circuit Quantum Electrodynamics, abbreviated Circuit QED. Built into this circuit is a tiny rectangular “box” called an island, in which electrons can be stored. The number of electrons in it can be controlled individually with electrical and magnetic fields to obtain the qubit states 0 and 1 or various superimpositions of the two alternatives.

The number of charges on the island can be kept stable for a prolonged time. This enabled high frequency microwave signals fed into the circuit to be used to exercise geometrical control over information stored in this way, as described above for the arm. This manipulation of the state of a qubit is called “Berry’s Phase” after the British physicist Michael Berry, who postulated it in the eighties.

This controlled phase change that the ETH Zurich researchers have now created can be used to construct logical control elements in a superconducting quantum computer.

Ultra-low temperatures are needed

However, observing this effect was anything but easy. Whereas atoms or photons show “inherent” quantum-mechanical behaviour, different laws apply in an electronic circuit. Very great efforts by the ETH Zurich researchers were needed to observe such small changes. First of all the lifetime of a quantum state, once generated, had to last long enough to manipulate it in a geometrically controlled way. Therefore the ETH Zurich physicists protected the quantum circuit in their experiments from “ambient noise” such as electrical noise and interference, e.g. from mobile phones. A special chip and careful manufacture of the circuit enabled them to do experiments with coherence times never previously achieved in solid-state qubits. Because of the extremely small energy difference between the 0 and 1 states, the qubit circuit needed to be cooled down to -273°C (0.02 K), almost absolute zero.

The ETH Zurich scientists see numerous advantages in the approach using solid-state qubits. As soon as the fundamental physics is well understood and it has become possible to develop robust fabrication processes, it should also be possible to expand the number of circuits to a large number of qubits – an advantage that other present-day technologies not based on integrated electronics do not possess.

The quantum computer: the dream of science

A quantum computer processes information using the laws of quantum mechanics. Like conventional computers, such computers process information in binary code as 0 or 1, with the difference that a quantum bit can be both 0 and 1 at the same time. However, to achieve this, the coding of the bit takes place in energy quanta stored in individual atoms, photons or even electronic circuits. A quantum computer exploits these capabilities of quantum mechanics to carry out a large number of calculations simultaneously, for example factorising large numbers of the kind that occur in encryption, when searching through data bases or in the simulation of complicated molecules or other quantum-mechanical systems.