Available online at www.sciencedirect.com Physica E 18 (2003) 37 – 38 www.elsevier.com/locate/physe A seven-junction Cooper pair pump J. Aumentado *; 1 , Mark W. Keller, John M. Martinis National Institute of Standards and Technology, Boulder, CO 80305-3337, USA Abstract We have measured current–voltage curves and individual charge transfer events in single-Cooper-pair pumps. We observe clear charge pumping in units of 2e, but this behavior is accompanied by signicant eects from unwanted quasiparticles. ? 2003 Elsevier Science B.V. All rights reserved. Keywords: Superconductivity; Tunneling; SET A single-electron tunneling (SET) pump consists of a chain of tunnel junctions with gate electrodes capacitively coupled to the islands between junctions. Electrons are pumped by pulsing the gates to allow tunneling at each junction in sequence, producing a current proportional to the repetition frequency, Ip = ef [1]. Pumps with seven junctions have produced such a current with metrologi- cal accuracy [2], but only at f∼10 7 Hz, giving Ip∼1 pA. The maximum rate for tunneling of electrons is limited by the stochastic tunneling time, Rj Cj , where Rj and Cj are respectively the junction resistance and capacitance. One must wait ∼100Rj Cj to reduce missed tunneling errors to ∼1 ppb. The current generated by existing SET pumps is sucient for certain metrological applications such as a capacitance standard [3], but other applications such as the quantum metrology triangle [4] require ∼1 nA or more. One promising scheme for larger current is to op- erate a charge pump in the superconducting state. A single-Cooper-pair tunneling (SCPT) pump transfers charge via coherent Josephson tunneling processes that are not subject to the probabilistic tunneling time limit of the normal state. Past studies of three-junction SCPT pumps [5] did not show clear pumping behavior, but the ratio of the Josephson energy for a single junction to the charging energy for a single electron was small: EJ =EC∼0:03. We are investigating SCPT pumps with more junctions, larger ∗ Corresponding author. E-mail address: jose.aumentado@boulder.nist.gov (J. Aumentado). 1 Contribution of NIST; not subject to copyright in the U.S. EJ =EC, and an integrated SET electrometer for detecting in- dividual charges in order to understand the conditions nec- essary for controlled pumping of Cooper pairs. Each pump consists of six m-scale Al islands linked by Al2O3 tunnel barriers (junction areas ∼100 nm × 100 nm), patterned by e-beam lithography and deposited using two-angle evaporation with an intermediate in situ oxida- tion step. The lower inset of Fig. 1 shows a schematic of the measurement circuit. The arrays terminate in an island (120 m × 120 m) capacitively coupled to a codeposited SET electrometer that measures the charge transferred through the pump [2,6]. A cryogenic needle switch contacts the island to allow current–voltage (IV ) measurements. All measurements are performed at ∼25 mK with cold mi- crowave ltering on all lines. We focus here on one of four pumps that we have measured, all of which showed similar behavior. We employed two methods to characterize our pumps: IV measurements and single-charge measurements. The latter uses the SET electrometer to monitor individual charges transferred through the pump and is able to detect much smaller levels of error or leakage than the IV measurement [2,6]. In the normal state, the IV curves with and without pumping show plateaus in the current near zero voltage bias (not shown). As expected, the pumping curves are oset by I p = ±ef with well-dened plateaus ∼200 eV wide. Shuttle error measurements in the normal state showed an error per electron of ∼10 -5 and a leakage rate in the “hold mode” (i.e., while not pumping) of ∼0:1 Hz. From the IV curve at large V we nd EC∼143 eV and Rj ∼43 k. The latter value implies EJ ∼15 eV from the 1386-9477/03/$ - see front matter ? 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1386-9477(02)00947-5