Manipulation of elementary charge in a silicon charge-coupled device

Surpassing the classical limit in magic square game with distant quantum dots coupled to optical cavities

The emergence of quantum technologies is heating up the debate on quantum supremacy, usually focusing on the feasibility of looking good on paper algorithms in realistic settings, due to the vulnerability of quantum systems to myriad sources of noise. In this vein, an interesting example of quantum pseudo-telepathy games that quantum mechanical resources can theoretically outperform classical resources is the Magic Square game (MSG), in which two players play against a referee. Due to noise, however, the unit winning probability of the players can drop well below the classical limit. Here, we propose a timely and unprecedented experimental setup for quantum computation with quantum dots inside optical cavities, along with ancillary photons for realizing interactions between distant dots to implement the MSG. Considering various physical imperfections of our setup, we first show that the MSG can be implemented with the current technology, outperforming the classical resources under realistic conditions. Next, we show that our work gives rise to a new version of the game. That is, if the referee has information on the physical realization and strategy of the players, he can bias the game through filtered randomness, and increase his winning probability. We believe our work contributes to not only quantum game theory, but also quantum computing with quantum dots.

Electron aspirator using electron-electron scattering in nanoscale silicon

Current enhancement without increasing the input power is a critical issue to be pursued for electronic circuits. However, drivability of metal-oxide-semiconductor (MOS) transistors is limited by the source-injection current, and electrons that have passed through the source unavoidably waste their momentum to the phonon bath. Here, we propose the Si electron-aspirator, a nanometer-scaled MOS device with a T-shaped branch, to go beyond this limit. The device utilizes the hydrodynamic nature of electrons due to the electron-electron scattering, by which the injected hot electrons transfer their momentum to cold electrons before they relax with the phonon bath. This momentum transfer induces an electron flow from the grounded side terminal without additional power sources. The operation is demonstrated by observing the output-current enhancement by a factor of about 3 at 8 K, which reveals that the electron-electron scattering can govern the electron transport in nanometer-scaled MOS devices, and increase their effective drivability.

Three-waveform bidirectional pumping of single electrons with a silicon quantum dot

Semiconductor-based quantum dot single-electron pumps are currently the most promising candidates for the direct realization of the emerging quantum standard of the ampere in the International System of Units. Here, we discuss a silicon quantum dot single-electron pump with radio frequency control over the transparencies of entrance and exit barriers as well as the dot potential. We show that our driving protocol leads to robust bidirectional pumping: one can conveniently reverse the direction of the quantized current by changing only the phase shift of one driving waveform with respect to the others. We anticipate that this pumping technique may be used in the future to perform error counting experiments by pumping the electrons into and out of a reservoir island monitored by a charge sensor.

A molecular quantized charge pump

A novel single electron pump based on individual molecules (a single wall carbon nanotube) is discussed in terms of the hybrid superconducting-normal conducting pumping principle. A concept demonstration device has been built based on a carbon nanotube contacted by Nb-Ti leads. Charge current quantization is achieved through rf modulation of the back gate voltage. The device is able to transfer a given number of electrons per pumping cycle. Single electron pumping is achieved for pumping frequencies up to 80 MHz.

Silicon-on-insulator-based radio frequency single-electron transistors operating at temperatures above 4.2 K

A radio frequency single-electron transistor (RF-SET) based on a silicon-on-insulator (SOI) substrate is demonstrated to operate successfully at temperatures above 4.2 K. The SOI SET was fabricated by inducing lateral constrictions in doped SOI nanowires. The device structure was optimized to overcome the inherent drawback of high resistance with the SOI SETs. We performed temperature variation measurements after five thermal cyclings of the same sample to 4.2 K and found that the single-dot device transport characteristics are highly stable. The charge sensitivity was measured to be 36 microe(rms) Hz(-1/2) at 4.2 K, and the RF-SET operation was demonstrated up to 12.5 K for the first time. This work is an important prerequisite to realizing operation of RF-SETs at noncryogenic temperatures.

Valley polarization in Si(100) at zero magnetic field

The valley splitting, which lifts the degeneracy of the lowest two valley states in a SiO(2)/Si(100)/SiO(2) quantum well, is examined through transport measurements. We demonstrate that the valley splitting can be observed directly as a step in the conductance defining a boundary between valley-unpolarized and -polarized regions. This persists to well above liquid helium temperature and shows no dependence on magnetic field, indicating that single-particle valley splitting and valley polarization exist in (100) silicon even at zero magnetic field.

An Experimental Look into Subelectron Charge Flow

The prediction and measurement of charge distribution among interacting chemical entities in complex environments is a major challenge for modern chemistry. It encompasses information concerning fundamental quantities such as the electronic chemical potential and hardness of molecular fragments as well as their interactions with the surroundings. Although a wealth of theoretical work has been accumulated from the days of Pauling to the present, a specific molecular model system that allows quantitative and direct measurement of these properties has not yet been reported. Because atomic charges are not quantum mechanical observables, they cannot be derived from first principles, but rather they rely on the availability of high-precision experimental data and the interpretation of related experimental observables. Here, we demonstrate, for the first time, that a fragmental charge flow between a chelated metal center and reversibly bound molecules can be accurately monitored experimentally.

Hydrogenic spin quantum computing in silicon: a digital approach

We suggest an architecture for quantum computing with spin-pair encoded qubits in silicon. Electron-nuclear spin-pairs are controlled by a dc magnetic field and electrode-switched on and off hyperfine interaction. This digital processing is insensitive to tuning errors and easy to model. Electron shuttling between donors enables multiqubit logic. These hydrogenic spin qubits are transferable to nuclear spin-pairs, which have long coherence times, and electron spin-pairs, which are ideally suited for measurement and initialization. The architecture is scalable to a highly parallel operation.