PTB News 2/2002 (149 kB) PTB News 2/2002, English edition, Issue August 2002Computing with quantum states is a fascinating idea which is, however, still far from being put into practice. One step forward is now being made with a concept under development at PTB. The new concept exploits special properties of a novel electrical component. Its basic unit of storage — the quantum bit (qubit) — is located on a superconducting ring. Its outstanding features: The qubit is infallible and its state can be easily read out.
Quantum computers are intended to exploit the fact that conventional logic is not applicable in the world of quanta: While a classical bit either has the value zero or the value one, a qubit is in a coherent superposition of the two states. Arithmetic operations which must be carried out sequentially in the classical case could be performed in a single operation by a quantum computer – thanks to the qubits. Furthermore, it is thought that – in principle – quantum computing will be the only way to solve specific tasks, such as large prime number factorizations. For some years superconducting circuits have been discussed to make qubits. In these circuits, quantum-mechanical effects (due to Bose-Einstein condensation of the charge carriers, i. e. of the Cooper pairs) can arise on a macroscopic scale. The behaviour of the macroscopic system is determined by a Schrödinger wave function depending only on a few collective variables.
The novel electrical unit is based on experience and results gained at PTB during the development of quantum standards for the electrical units. It consists of a superconducting ring with two closely adjacent Josephson tunnel barriers with an area of less than 0,01 µm2. The ring carries the qubit. Even at a ring diameter of about 1 mm, its states are quantized and depend on the charge q induced on the island and the magnetic flux F through the ring. On the basis of the two states of lowest energy, the “zero” and the “one” state and coherent mixed states – the qubits proper – can be formed. The qubit is controlled via the two electrical parameters of charge q and flux F.
Particular properties of this qubit are its low liability to electrical interferences as well as the possibility to read out the state with almost no loss of coherence. For the readout, the change in the resonant frequency of an inductively coupled resonant circuit can be determined. Conceptionally, the qubit has thus been trapped. Experimental realization will be the next step.