We report on high-accuracy measurements of quantized current, sourced by a tunable-barrier single-electron pump at frequencies f up to 1 GHz. The measurements were performed with a new picoammeter instrument, traceable to the Josephson and quantum Hall effects. Current quantization according to I = ef with e the elementary charge was confirmed at f = 545 MHz with a total relative uncertainty of 0.2 ppm, improving the state of the art by about a factor of 5. For the first time, the accuracy of a possible future quantum current standard based on single-electron transport was experimentally validated to be better than the best realization of the ampere within the present SI.

We report on characterizations of single-electron pumps at the highest accuracy level, enabled by improvements of the small-current measurement technique. With these improvements a new accuracy record in measurements on single-electron pumps is demonstrated: 0.16 µA • A −1 of relative combined uncertainty was reached within less than 1 d of measurement time. Additionally, robustness tests of pump operation on a sub-ppm level revealed a good stability of tunable-barrier single-electron pumps against variations in the operating parameters.

We study single-parameter quantized charge pumping via a semiconductor quantum dot in high magnetic fields. The quantum dot is defined between two top gates in an AlGaAs/GaAs heterostructure. Application of an oscillating voltage to one of the gates leads to pumped current plateaus in the gate characteristic, corresponding to controlled transfer of integer multiples of electrons per cycle. In a perpendicular-to-plane magnetic field the plateaus become more pronounced indicating an improved current quantization. Current quantization is sustained up to magnetic fields where full spin polarization of the device can be expected. PACS numbers:Generating well defined currents by manipulating single charges has attracted considerable interest in the last two decades from both fundamental and applied points of view [1]. A particular potential of application lies in the field of metrology to provide a link between time and current units [2]. Different approaches have been studied, such as arrays of tunnel-connected metallic islands controlled by a number of phase shifted ac signals [3,4,5,6] or semiconducting channels along which the potential can be modulated continuously [7,8,9,10,11]. The pumping mechanism demonstrated in Ref.[10] allows gigahertz pumping comparable to surface-acoustic-wave pumps [8] while promising a higher degree of control. It employs three electrodes of which two are modulated at a fixed phase shift and with different amplitudes. In Ref. [11] it was shown that a single modulated voltage signal is sufficient to operate the pump and a numerical investigation indicated the importance of the tunnel barrier shape for improving the accuracy. A possible way to manipulate these tunnel couplings might be the application of a magnetic field owing to its influence on the wave function and the corresponding rearrangement of electrons between quantum states (see for instance [12]). Therefore the operation of such a single-parameter charge pump has been realized in the presented work when a perpendicular-to-plane magnetic field was applied.Two devices have been investigated which were both realized in an AlGaAs/GaAs heterostructure with a carrier concentration of 2.1 × 10 15 m −2 and a mobility of 97 m 2 /Vs in the dark. A 700 nm wide wire connected to two-dimensional electron gases was created by wetetching the doped AlGaAs layer. This channel is crossed by two 100 nm wide Ti-Au finger gates of 250 nm separation. A schematic is shown in the inset of Fig. 1(a). A quantum dot (QD) with discrete quasibound states between the gates can be created by applying sufficiently large negative voltages V 1 and V 2 to gate 1 and gate 2, * Electronic address: Bernd.Kaestner@ptb.de respectively. An additional radiofrequency (rf) signal is coupled to gate 1. The resulting voltages are thereforeat gate 1 and gate 2, respectively. If the oscillation amplitude is high enough, then the bound state drops below the Fermi level during the first half-cycle of the periodic signal and can be loaded with electrons from source. During th...

We operate an on-demand source of single electrons in high perpendicular magnetic fields up to 30 T, corresponding to a filling factor ν below 1/3. The device extracts and emits single charges at a tunable energy from and to a two-dimensional electron gas, brought into well defined integer and fractional quantum Hall (QH) states. It can therefore be used for sensitive electrical transport studies, e.g. of excitations and relaxation processes in QH edge states.

We study a mesoscopic circuit of two quantized current sources, realized by nonadiabatic single-electron pumps connected in series with a small micron-sized island in between. We find that quantum transport through the second pump can be locked onto the quantized current of the first one by a feedback due to charging of the mesoscopic island. This is confirmed by a measurement of the charge variation on the island using a nearby charge detector. Finally, the charge feedback signal clearly evidences loading into excited states of the dynamic quantum dot during single-electron pump operation. DOI: 10.1103/PhysRevB.83.193306 PACS number(s): 73.23.−b, 72.10.−d, 73.22.Dj, 73.63.Kv Quantized current sources (e.g., see Refs. 1-4) have interesting applications, e.g., as an on-demand electron source for quantum information processing 5 or as a current source for metrology. 6 In contrast to turnstiles, 7,8 the current can be driven against a voltage applied across the pump, allowing their use as a current source in mesoscopic quantum electronics. Yet the further suppression of still present current fluctuations, as desired for such applications, remains challenging. Now Brandes has proposed a new method to stabilize quantum transport in mesoscopic devices against fluctuations, named mesoscopic feedback. 9 In this paper, we experimentally realize and investigate a quantum transport device with mesoscopic feedback. The device under study is a semiconductor nonadiabatic quantized charge pump. A mesoscopic feedback loop is realized using two quantized charge pumps P1, P2 connected in series and separated by a mesoscopic island in between. In this circuit, any momentary difference between the currents through the two charge pumps immediately leads to a charge accumulation on the mesoscopic island. This in turn acts as feedback mainly onto pump P2, locking it to the nominal current set by pump P1. The charge on the mesoscopic island is monitored by a nearby capacitively coupled detector allowing us to verify the feedback mechanism. Furthermore, the highly sensitive feedback signal reveals a fine structure within the quantized current plateau due to loading into excited states during the initial phase of the pumping cycle, which is not observable in measurements of the pumped current. This demonstrates the use of mesoscopic feedback control as a characterization tool for dynamic processes in nanostructures and, furthermore, it opens the possibility of non-groundstate initialization of dynamic quantum dots with possible applications in quantum information processing. Moreover, this realization of a mesoscopic circuit of quantized charge pumps demonstrates their usability for future applications in integrated mesoscopic electronics.The device is shown in Fig. 1: Two dynamically driven quantum dots, each formed by titanium Schottky gates (colored lines) across a narrow semiconducting channel, act as singleelectron pumps. They are connected by a few-micron-wide mesoscopic island that is capacitively coupled to a charge detector,...