Excited-state spectroscopy on a quantum dot side coupled to a quantum wire

Abstract

We report excited-state spectroscopy on a quantum dot side-coupled to a quantum wire with accurate energy estimation. Our method utilizes periodic voltage pulses on the dot, and the energy calibration is performed with reference to the bias voltage across the wire. We demonstrate the observation of the orbital excited state and the Zeeman splitting in a single dot.Semiconductor quantum dots are promising candidates for quantum bits (qubits) [1]. In a conventional dot with two leads, the current through the dot as a function of the sourcedrain bias and a gate voltage provides the information of the excited states as well as the ground states [2]. However in the application to quantum information processing, the number of leads is desired to be as small as possible, because the connection to the outside circuits brings in quantum decoherence. Besides the decoherence, a dot with a single-lead has a number of advantages such as the spatial compactness, the easiness to go down to the few electron regime [3], and so on.In single-lead dots, the spectroscopic information is usually given through the interference (the Fano effect) [4,5] or the charge detection [6]. However, these measurements were limited to the spectroscopy of the ground states. To overcome the difficulty, a method combining remote charge sensing and pulsed electrostatic gating was demonstrated for excited-state spectroscopy on a nearly closed quantum dot [7]. Now the remaining difficulty is to find out a "measure" for the energy, in other words the conversion factor from the gate voltage to the energy, without applying finite voltage nor current across the dots. So far such conversion is performed by changing the effective dot configuration with gate voltages or by numerical simulation. The former inevitably causes significant variation in the electrostatic parameters, while the latter just gives approximate estimation. In order to make precise meaningful spectroscopy, a method to obtain the reliable conversion factor is indispensable.In this letter, we show that the conversion factor can be precisely obtained in a specially designed single-lead dot, namely a quantum dot side-coupled to a quantum wire. Our method is based on the one by Elzerman et al. [7], but differs in that the bias voltage can be applied across the quantum wire. When the wire length is much shorter than the mean free path, the bias on the wire creates two quasi-Fermi levels [8,9]. If the dot can detect the non-equilibrium energy distribution on the gate voltage axis, this gives the conversion factor. It is demonstrated that the present method can be applied to the measurement of spin-splittings as well as orbital excited states.Figure 1(a) shows a scanning electron micrograph of our device fabricated on a GaAs/AlGaAs heterostructure containing a 2DEG 60 nm below the surface. Applying negative voltages on gates S, P, C and W, we prepare a quantum dot which couples to a short quantum wire (QW in the figure). A quantum point contact (QPC), formed between gates S and D, is used for remo...

Nonlocal bias spectroscopy of the self-consistent bound state in quantum point contacts near pinch off

“…Our results are reproduced by a model that treats coupling of the detector to a discrete state, and are reminiscent of those in a spectroscopy study of deliberately formed quantum dots. 18 We report on the GaAs multi-QPC device of Ref. 16 ͓Fig.…”

“…To obtain this value we assumed ␤ = 1 2 , which should be reasonable since G d Ͼ 2e 2 / h. 19 ␥ is within the range reported in experiments on quantum dots formed by electrostatic gating. 18,20 In Fig. 3 we plot the differential conductance ͓g d ͑V s , V dc ͔͒ obtained by scanning V dc continuously, and in- crementing V s over the region near pinch off.…”

Detection of spin polarization with a side-coupled quantum dot

“…3(b), (c), (d), we review energy spectrum measurement reported in Ref. 17. When V amp is small, g versus V Pc shows a series of dips like Fig.…”

“…The electrostatic potential of QD is detected through the conductance of a quantum point contact (QPC) placed on the other side of QD 13,14,15 . We first explain how the energy spectrum of QD (including excited states) is obtained in this device 16,17 . For the spectroscopy, the plunger gate voltage V P on QD is driven to oscillate as a rectangular wave with the amplitude V amp and the center voltage V Pc , as illustrated in Fig.…”

Spin polarization in the vicinity of quantum point contact with spin-orbit interaction

“…21 The present measurement can therefore be applied to the case of finite source-drain bias voltage V sd of t-QPC with tuning the window to local chemical potentials. To investigate the polarization mechanism, we thus measure the variation of P versus V sd on 0.5 and 1.0 conductance plateaus.…”