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Conference on Precision Electromagnetic Measurements

SINIS turnstile for quantum current standards

Shuji Nakamura, Yu. A. Pashkin, Jaw Shen Tsai, Nobu‐Hisa Kaneko

Published Aug 1, 2014• 1 citation
Publisher

Abstract

Single electron pumping by a SINIS turnstile is investigated up to 1 GHz in various conditions to realize a quantum current standard. A standard deviation of the pumping current at 100 MHz is about 10-4at base temperature of a dilution refrigerator. As expected, the plateau of the pumped current degrades as temperature increases because of the thermal smearing of Fermi energy of a normal metal island. On the other hand, an externally applied magnetic fields dramatically suppresses excess current on the single electron pumping plateau. This originates from the quasiparticle relaxation enhancement due to the penetration of the magnetic vortices into the superconducting leads.

DOI
10.1109/cpem.2014.6898558

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•Jan 1, 2009•0 citations
Single electron pumping by a SINIS turnstile is investigated up to 1 GHz in various conditions to realize a quantum current standard. A standard deviation of the pumping current at 100 MHz is about 10-4at base temperature of a dilution refrigerator. As expected, the plateau of the pumped current degrades as temperature increases because of the thermal smearing of Fermi energy of a normal metal island. On the other hand, an externally applied magnetic fields dramatically suppresses excess current on the single electron pumping plateau. This originates from the quasiparticle relaxation enhancement due to the penetration of the magnetic vortices into the superconducting leads.
First I discuss various candidates of single-electron current pumps for quantum metrology. Then I focus on the hybrid normal-metal-superconductor turnstile in the form of a one-island single-electron transistor with one gate. The device demonstrates robust current plateaus at multiple levels of ef at frequency f. I discuss the various error mechanisms, based on our experiments and theoretical considerations. Ultimately the quantization accuracy is expected to be limited by either two-electron tunneling or by Cooper-pair-electron co-tunneling. We predict that it should be possible to achieve the metrological accuracy of 10-8, while maintaining the quantized current on the level of more than 10 pA, just by one turnstile with realistic parameters using aluminium as a superconductor. Recently we have managed to run ten turnstiles in parallel increasing the current level to above 100 pA. Work on suppressing the harmful sub-gap leakage current is in progress with encouraging experimental results.
We perform a theoretical study of nonequilibrium effects in charge transport through a hybrid single-electron transistor based on a small normal metal (N) island with the gate-controlled number of electrons, tunnel-coupled to voltage-biased superconducting (S) electrodes (SINIS). Focusing on the turnstile mode of the transistor operation with the gate voltage driven periodically, and electrons on the island being out of equilibrium, we find that the current quantization accuracy is a nonmonotonic function of the relaxation rate ${\mathrm{\ensuremath{\Gamma}}}_{\mathcal{F}}$ of the distribution function $\mathcal{F}(\ensuremath{\epsilon})$ on the island due to tunneling, as compared to the drive frequency fff, electron-electron $1/{\ensuremath{\tau}}_{ee}$, and electron-phonon $1/{\ensuremath{\tau}}_{eph}$ relaxation rates. Surprisingly, in the strongly nonequilibrium regime, $f\ensuremath{\gg}{\mathrm{\ensuremath{\Gamma}}}_{\mathcal{F}}\ensuremath{\gg}{\ensuremath{\tau}}_{ee}^{\ensuremath{-}1},{\ensuremath{\tau}}_{eph}^{\ensuremath{-}1}$, the turnstile current plateau is recovered, similarly to the ideal equilibrium regime, ${\ensuremath{\tau}}_{eph}^{\ensuremath{-}1}\ensuremath{\gg}{\mathrm{\ensuremath{\Gamma}}}_{\mathcal{F}}$. The plateau is destroyed in the quasiequilibrium regime when the electron-electron relaxation is faster than tunneling.
We consider the influence of the electromagnetic fluctuations on the transport properties of a hybrid single electron transistor, consisting of superconducting electrodes and a normal-metal island, when operated as a turnstile. We derive the analytic expressions for the rates near the thresholds of single electron tunneling, Andreev reflection, and Cooper-pair–electron cotunneling processes. These results show that the dissipative on-chip impedance suppresses the rates of the undesirable higher-order tunneling processes much stronger than the single electron tunneling which can therefore be utilized to increase the accuracy of such a device in quantum metrological applications.
This Thesis studies opportunities to create a quantum standard for the electric current with the help of tunnel junctions. We use two types of tunnel junctions: superconducting Josephson junctions, and NIS junctions where one of the electrodes is normal (N) and the other one superconducting (S). In both cases, tunnel junctions are employed in a single-electron transistor (SET) structure, which is used to transfer a controlled number k of electrons (e) with the repetition frequency f. The magnitude of the resulting current is thus I = kef. First, we study the Cooper pair sluice, where Josephson junctions are connected as two superconducting quantum interference devices (SQUID). A gate electrode is used to control the charge state of the superconducting island formed between the SQUIDs. The sluice is based on tuning the tunneling rates through the SQUIDs with local magnetic fluxes, which allows to control the direction of the charge transfer. Weak traces of current quantization can be observed up to above 1 nA, which is large enough current for many metrological purposes. However, the accuracy of the current is still far from what is required in metrology. We study also a new type of SQUID structure, the balanced SQUID, which could be used to improve the accuracy of the sluice or in, e.g., some quantum computing applications. Second, we employ NIS junctions in the hybrid (SINIS) single-electron transistor with superconducting leads and a normal-metal island. This structure can be used as the SINIS turnstile which lets electrons to flow one by one in the direction determined by the bias voltage. We report the first experimental results on the SINIS turnstile and two methods to improve the accuracy of the current: increasing the charging energy of the island or connecting the turnstile in a resistive environment. We also show that the SINIS turnstile can be used as an electronic radio-frequency refrigerator where tunneling processes cool the electron temperature of the normal-metal island.