NIST uncovers clue to successful manipulation of paired electrons. (General Developments).

Nanoscale devices that manipulate single electrons of charge e with metrological accuracy are being used at NIST to develop new fundamental electrical standards, such as a capacitance standard based on counting electrons. The superconducting analogs of these devices, in which electrons are bound in "Cooper pairs" of charge 2 e, are attractive for two reasons. First, the coherent transfer of Cooper pairs is expected to be much faster than the incoherent transfer of unpaired electrons, providing much larger currents with metrological accuracy. Second, several schemes for quantum computation, in which information is processed using the unique properties of quantum bits or "qubits," rely on the precise manipulation of Cooper pairs. Past attempts at manipulation of single Cooper pairs for both metrology and quantum computation have been hampered by the existence of a small number of residual unpaired electrons in the superconducting state. Furthermore, the conditions needed to avoid these unpaired electrons in a g iven device were not clearly understood because different research groups have reported conflicting results on this question.

Researchers at NIST uncovered an important clue to this mystery by showing that a previously unappreciated factor has a strong effect on the amount of unpaired electrons in Cooper pair devices. Each device consists of two layers of aluminum, and the strength of the pairing of electrons in each layer can be different. This slight difference has generally been thought to be unimportant. However, a study of more than a dozen devices in which this difference was varied in a controlled way, and independently measured in each device, shows that it directly affects device performance. In every device made the right way, unpaired electrons were very rare, allowing manipulation of single Cooper pairs over long time scales and over a wide range of temperature. In all devices made the wrong way, unpaired electrons dominated the device operation and the desired manipulation of single Cooper pairs was not possible. These results, which may also explain previous reports that seemed to be contradictory, point the way to a r obust recipe for devices that can realize the promise of superconducting charge transfer for both metrology and quantum computation.

CONTACT: Jose Aumentado, [phone]; [e-mail].