Kurzbeschreibung
(Englisch)
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There is a clear trend in micro-electronic industry towards increasingly reduced electronic circuits, which are used in high performance, hand-held, battery operated and low power consumer electronics. A well known example is the development in cellular phones. In such devices, the electrical currents flowing through both active and passive circuit elements become increasingly small. This induces a growing demand for instrumentation that is able to measure sub-nanoampere currents accurately and traceable.
The objectives of the project are to develop instrumentation for the accurate generation and measurement of small electrical currents, and in addition to establish a fundamental measurement technique for the SI base unit of electrical current.
The technological approach to the solution of the problem is to exploit single electron transport (SET) devices. More specifically, two complementary SET instruments will be developed. The first is a SET current source, based on the so-called single electron pump. The second one is a SET current meter or single electron counter. These devices are able to manipulate or detect individual electrons one by one, making it possible to count electrons one by one.
The project is carried out within the 5th European framework programme. The main tasks of METAS are:
-Development of cryogenic switches which are an important part of the SET pump circuitry;
-Development of a cryogenic capacitor which will be used to check the accuracy of the SET pump.
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Abstract
(Englisch)
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In 2000, the European project COUNT started. One of the goals of the project was to realise a quantum primary standard of capacitance with an relative uncertainty on the order of 1 ppm by charging a cryo-capacitance electron by electron. This project stopped end of 2003 just before the final goal was achieved. In February 2004, we succeeded to manipulate electrons one at a time. This breakthrough - we are the first laboratory to achieve such a result in Europe- indicates that the realisation of the quantum capacitance standard in now possible in the near future. Our tasks in the project were the following:
1) Design and fabrication of cryoswitches
An improved design for the needle switches was realised. The fabrication was completed in September 2000. The switches were tested and showed perfect operation at 4.2K. During the first annual meeting, a set of 2 switches was delivered to PTB, LCIE, NMI and NPL. These switches are now routinely used at METAS to connect the electron pumps.
Degree of reaching: 100%
2) Adaptation of existing set up
A new sample holder that integrates the electron pump, the cryocapacitor and the cryoswitches has been designed and constructed. In addition, the wiring of the dilution fridge was completely redesigned. The base temperature of the mixing chamber with the new cables was found to be around 10mK, using a nuclear orientation thermometer.
Degree of reaching: 100%
3) Testing of the 3 junctions R-pump
Three junction R-pumps are particularly simple to operate: A dc offset has to be applied to the two gate voltages in order to select a triple point of the stability diagram. Then two sinusoidal signals with a fixed phase shift between the two gates allow turning around the triple point on an elliptical trajectory. In every cycle, one electron is transferred through the pump. Thereby a current of I = ef is generated. The sign of the current can be reversed simply by reversing the phase shift between the two gate signals. The I-V curves obtained clearly showed the current plateau as a function of Vbias. A critical point to consider for metrological applications of the pump is the slope of the current plateau which should be zero. An analysis of the data showed that a precision of the pumping current in the range of 100 µA/A seems possible. Calculations of zero temperature cotunneling errors indicate that pumps with a larger number of tunnel junctions will have to be realised to reach the accuracy required for our primary standard of capacitance.
Degree of reaching: 100%
4) Link single electron pump to cryogenic capacitor
The experimental set-up now comprises a tunable cryogenic capacitor, a set of 2 cryoswitches and the single electron chip. The chips comprise a single electron pump connected to a needle pad which is capacitively coupled to an electrometer. The feedback circuitry which will be required to charge the cryo-capacitor was also assembled. In February 2004, we succeeded to manipulate electrons one at a time. This success shows that it will soon be possible to charge the cryogenic capacitor electron by electron.
Degree of reaching: 90%
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Umsetzung und Anwendungen
(Englisch)
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The direct use of the project results will be mainly by metrology and research institutes. However its indirect relevance will be much larger, since industry leans heavily on traceability, precision measurements and technological knowledge. Manufacturers of high precision, low current sources and meters will have their equipment calibrated against the quantum standards developed in the project.
Traceability of small electrical current is of importance for other reference standards. The common denominator is "detectors for small signals". For instance, light detectors (trap detectors) used in telecommunications generate small electrical currents. Another example that requires a practical method for traceability is the measurement of radioactive radiation by means of ionisation dosimetry (radiation is transferred to a small electrical current).
The use of SET instrumentation is limited by the need for cryogenic cooling below 50 milli-Kelvin (mK). However, a range of applications may be foreseen in fields where commercially available pico-ampere meters fail. An example is for the high quality testing of silicon wafers. In the longer term, SET devices may be used as elements of quantum computers.
In the short term, the realisation of this quantum primary standard of capacitance will lead to a significant improvement in the realisation of the capacitance scale. In addition, this primary capacitance standard is a key component to close the metrological triangle.
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Publikationen / Ergebnisse
(Englisch)
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Publications in scientific journals and conference contributions
1 F. Overney, B. Jeanneret and M. Furlan, "A Tunable Vacuum-Gap Cryogenic Coaxial Capacitor", IEEE Trans. Instrum. Meas. 49/6, 1326-1330, (2000).
2 M. Furlan, T. Heinzel, B. Jeanneret, S. V. Lotkhov, and K. Ensslin, "Non-Gaussian distribution of nearest-neighbour Coulomb peak spacings in metallic single-electron transistors", Europhys. Letters 49, 369-375 (2000).
3 M. Furlan, T. Heinzel, B. Jeanneret, and S. V. Lotkhov, "Coulomb Blockade Peak Statistics Influenced by Background Charge Configuration", J. Low Temp. Phys. 118, 297-306 (2000).
4 M. Furlan, A. L. Eichenberger, T. Heinzel, B. Jeanneret, and S. V. Lotkhov, "Realistic and relevant models for the description of SET transistors", Physica B 284-288, 1798-1799 (2000).
5 C. Hof, B. Jeanneret, A. Eichenberger and F. Overney, “First step towards a quantum capacitance standard at METAS”, IEEE Trans. Instrum. Measure. 52/2, 604-607 (2003).
6 H.E. van den Brom et al., “Counting electrons one by one – Overview of a joint European research project”, IEEE Trans. Instrum. Measure. 52/2, 584-589 (2003).
7 C. Hof, B. Jeanneret, A. Eichenberger and F. Overney, M. W. Keller and M. J. Dalberth, “Manipulating single electrons with a seven-junction pump", submitted to IEEE Trans. Instrum. Measure.
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