Tunable-barrier Pump Research Articles

Robust operation of a GaAs tunable barrier electron pump

We demonstrate the robust operation of a gallium arsenide tunable-barrier single-electron pump operating with 1 part-per-million accuracy at a temperature of 1.3 K and a pumping frequency of 500 MHz. The accuracy of current quantisation is investigated as a function of multiple control parameters, and robust plateaus are seen as a function of three control gate voltages and RF drive power. The electron capture is found to be in the decay-cascade, rather than the thermally-broadened regime. The observation of robust plateaus at an elevated temperature which does not require expensive refrigeration is an important step towards validating tunable-barrier pumps as practical current standards.

Gigahertz single-hole transfer in Si tunable-barrier pumps

We report high-speed single-hole (SH) transfer using Si tunable-barrier pumps comprising p-type metal-oxide-semiconductor field-effect transistors. A clear SH-transfer-current plateau with the current level of about 160 pA was observed when a clock signal having a frequency of 1 GHz was applied to one of the gates. Temperature dependence measurements of the transfer current reveal that the transfer probability is dominated by non-equilibrium SH escape by thermal hopping from the electrically formed charge island. The lower bound of the relative error rate for the 1-GHz transfer is about 10−3 at a temperature of about 17 K. In addition, we investigate the frequency dependence of the transfer, where we discuss possible sources causing the change in the error rate. These results pave the way for accurate manipulation of SHs and its application to metrological current standards.

Rectification in mesoscopic alternating current-gated semiconductor devices

We analyse the rectified dc currents resulting when a three-terminal semiconductor device with gate-dependent conductance is driven with an ac gate voltage. The rectified currents exhibit surprisingly complex behaviour as the dc source-drain bias voltage, the dc gate voltage, and the amplitude of the ac gate voltage are varied. We obtain good agreement between our data and a model based on simple assumptions about the stray impedances on the sample chip, over a wide frequency range. Secondly, we evaluate the small rectified currents flowing in tunable-barrier electron pumps operated in the pinched-off regime. These currents are at most 10−12 of the pumped current for a pump current of 100 pA. This result is encouraging for the development of tunable-barrier pumps as metrological current standards. Our method is applicable to many types of experiment which involve ac gating of a non-linear device, and where an undesirable rectified contribution to the measured signal is present.

Gigahertz quantized charge pumping in graphene quantum dots

Single-electron pumps are set to revolutionize electrical metrology by enabling the ampere to be redefined in terms of the elementary charge of an electron. Pumps based on lithographically fixed tunnel barriers in mesoscopic metallic systems and normal/superconducting hybrid turnstiles can reach very small error rates, but only at megahertz pumping speeds that correspond to small currents of the order of picoamperes. Tunable barrier pumps in semiconductor structures are operated at gigahertz frequencies, but the theoretical treatment of the error rate is more complex and only approximate predictions are available. Here, we present a monolithic, fixed-barrier single-electron pump made entirely from graphene that performs at frequencies up to several gigahertz. Combined with the record-high accuracy of the quantum Hall effect and proximity-induced Josephson junctions, quantized-current generation brings an all-graphene closure of the quantum metrological triangle within reach. Envisaged applications for graphene charge pumps outside quantum metrology include single-photon generation via electron-hole recombination in electrostatically doped bilayer graphene reservoirs, single Dirac fermion emission in relativistic electron quantum optics and read-out of spin-based graphene qubits in quantum information processing.