Quantum Dot Turnstile Research Articles

Nonadiabatic transport in a quantum dot turnstile

We present a theoretical study of the electronic transport through a many-level quantum dot driven by time-dependent signals applied at the contacts to the leads. If the barriers oscillate out of phase, the system operates like a turnstile pump under a finite constant bias, as observed in the experiments of Kouwenhoven et al. [Phys. Rev. Lett. 67, 1626 (1991)]. The time-dependent currents and their averages over successive pumping periods are computed from the Keldysh formalism for tight-binding models. The calculation considers a sudden application of the pumping potentials at $t=0$, which leads to transient features of the time-dependent and averaged currents during the first pumping cycles which turn out to be important in the high-frequency regime. We show that in the transient regime, the efficiency of the system as a pump is rather poor because it mainly absorbs charge from both leads in order to fill the levels located below the bias window. Under a finite bias and a low-frequency pumping signal, the charge transferred across the system depends on the number of levels located within the bias window. The internal charge dynamics and the role of energy sidebands are investigated. The so-called satellite peaks of the averaged current are also observed in the transient regime.

Acoustoelectric current transport through a double quantum dot

We present acoustoelectric current measurements through a double quantum dot. Due to background impurity potential fluctuations an unintentional quantum dot is situated next to an intentionally induced single dot. By changing the top gate voltages, each of the two dots alone can be addressed separately but also a situation can be realized where both dots are coupled and form a double quantum dot system. In the regime that is dominated by the conduction through the intentional quantum dot quantized charge transport has been realized mediated by surface acoustic waves. Based on the measurements on this device we propose a mechanism for a parallel double quantum dot turnstile that can be used as a spin entangler.

Spin pump turnstile: Parametric pumping of a spin-polarized current through a nearly closed quantum dot

We investigate parametric pumping of a spin-polarized current through a nearly-closed quantum dot in a perpendicular magnetic field. Pumping is achieved by tuning the tunnel couplings to the left and right lead - thereby operating the quantum dot as a turnstile - and changing either the magnetic field or a gate-voltage. We analyze the quantum dynamics of a pumping cycle and the limiting time scales for operating the quantum dot turnstile as a pure spin pump. The proposed device can be used as a fully controllable double-sided and bipolar spin filter and to inject spins "on demand".

Erratum: Double quantum dot turnstile as an electron spin entangler [Phys. Rev. B69, 115312 (2004)

Erratum: Double quantum dot turnstile as an electron spin entangler [Phys. Rev. B<b>69</b>, 115312 (2004)

Double quantum dot turnstile as an electron spin entangler

We study the conditions for a double quantum dot system to work as a reliable electron spin entangler, and the efficiency of a beam splitter as a detector for the resulting entangled electron pairs. In particular, we focus on the relative strengths of the tunneling matrix elements, the applied bias and gate voltage, the necessity of time-dependent input/output barriers, and the consequence of considering wavepacket states for the electrons as they leave the double dot to enter the beam splitter. We show that a double quantum dot turnstile is, in principle, an efficient electron spin entangler or entanglement filter because of the exchange coupling between the dots and the tunable input/output potential barriers, provided certain conditions are satisfied in the experimental set-up.

Conditions for quantized current in quantum dot turnstile

Competition between leak and quantized currents leads to the breakdown of the current quantization as the frequencyf of RF signal is smaller than the tunneling rate Γ leak . We find that whenf>Γ leak , the dc current is quantized, while the much larger current than quantized current flows through the dot whenf<Γ leak . We also find new fine structures in the current plateaus resulting from the quantized level spacing ΔE. These structures are washed out at the temperaturek B T∼ΔE, though the quantized plateaus remain. The quantized plateaus are washed out atk B T∼e 2/C, whereC is the capacitance of the dot.

Role of Displacement Current in Quantum-Dot Turnstile Devices

The dynamics of the current and the electron number in quantum-dot turnstile devices, in which the heights of the tunneling barriers are modulated by external rf signals, is studied theoretically within the adiabatic approximation. The displacement current is more accurately quantized than the tunneling currents flowing through both the left and the right barriers, since the leakage current is compensated. We find numerically that an electron tunnels through the barrier before its height reaches the minimum value. The tunneling phases of rf signals, at which an electron can actually tunnel, change as the amplitude of rf signals and the barrier height and width are varied. We also discuss the conditions for the current quantization. The dc I-V curve which we obtain shows a plateau at each quantized current nef with width e2/C+ΔE and sharp steps between neighboring plateaus. Whenever the dc current is quantized, the change of the number of electrons in the quantum dot during one half-period of rf signal is an integer.

Dynamics of Quantum-Dot Turnstile in Adiabatic Approximation

We derive an expression for the current in a quantum-dot turnstile device in the lowest order of tunnel processes. The electrochemical potential µ in the quantum dot is determined from the current continuity across both barriers. By the use of the adiabatic approximation, the quantized current plateaus in the I - V curve are obtained for the device with symmetric barriers oscillating with a phase difference π. We find that both the height and the position of the steps change as the gate voltage is varied. We also show how the current and the electron number in a quantum dot vary as a function of time.

Time and radiation time quantization and their application to the transport through a quantum dot turnstile

In this paper, we analyse the time in the time evolution operator and then give time a reasonable definition. We realize the radiation time quantization by supposing that a radiation-matter interaction process is an eigenstate of the time operator, which results in the quantized resonance time. We discuss the application to the transport through a quantum dot turnstile in an rf field.

Radiation Phase Quantization and Its Application to the Transport Through a Quantum Dot Turnstile

Following Schommers' viewpoint on time, we give radiation time a reasonable definition. We realize the radiation phase quantization by supposing that a radiation-matter interaction process is an eigen state of radiation time operator. The successful application to the transport through a quantum dot turnstile in a radiation field leads to quantized current formula.

Quantized current in a quantum dot turnstile

We have observed a quantized current in a lateral quantum dot defined by metal gates in the two-dimensional electron gas (2DEG) of a GaAs/AlGaAs heterostructure. By modulating the tunnel barriers in the 2DEG with two phase shifted RF signals, and employing the Coulomb blockade of electron tunneling, we produced quantized current plateaus in the current-voltage characteristics at integer multiples of ef, where f is the RF frequency. This demonstrates that an integer number of electrons pass through the quantum dot each RF cycle.

Quantized current in a quantum dot turnstile

We have observed a quantized current in a lateral quantum dot defined by metal gates in the two dimensional electron gas (2DEG) of a GaAs/AlGaAs heterostructure. By modulating the tunnel barriers in the 2DEG with two phase shifted RF signals, and employing the Coulomb blockade of electron tunneling, we produced quantized current plateaus in the current-voltage characteristics at integer multiples of ef, where f is the RF frequency. This demonstrates that an integer number of electrons pass through the quantum dot each RF cycle.

Quantized current in a quantum dot turnstile

We have performed RF experiments on a lateral quantum dot defined in the two dimensional electron gas (2DEG) of a GaAs/AlGaAs heterostructure. The small capacitance of the quantum dot gives rise to single-electron charging effects, which we employed to realize a quantum dot turnstile device. By modulating the tunnel barriers between the quantum dot and the 2DEG leads with two phase-shifted RF signals, we pass an integer number of electrons through the quantum dot per RF cycle. This is demonstrated by the observation of quantized current plateaus at multiples ofef in current-voltage characteristics, wheref is the frequency of the RF signals. When an asymmetry is induced by applying unequal RF voltages, our quantum dot turnstile operates as a single-electron pump producing a quantized current at zero bias voltage.

Quantized current in a quantum-dot turnstile using oscillating tunnel barriers.

We have observed a quantized current in a lateral quantum dot, defined by metal gates in the two-dimensional electron gas (2DEG) of a GaAs/AlGaAs heterostructure. By modulating the tunnel barriers in the 2DEG with two phase-shifted rf signals, and employing the Coulomb blockade of electron tunneling, we produced quantized current plateaus in the current-voltage characteristics at integer multiples of ef, where f is the rf frequency. This demonstrates that an integer number of electrons pass through the quantum dot each rf cycle.