Semiconductor few-electron quantum dot operated as a bipolar spin filter

Abstract

We study the spin states of a few-electron quantum dot defined in a two-dimensional electron gas, by applying a large in-plane magnetic field. We observe the Zeeman splitting of the two-electron spin triplet states. Also, the one-electron Zeeman splitting is clearly resolved at both the zero-to-one and the one-to-two electron transition. Since the spin of the electrons transmitted through the dot is opposite at these two transitions, this device can be employed as an electrically tunable, bipolar spin filter. Calculations and measurements show that higher-order tunnel processes and spin-orbit interaction have a negligible effect on the polarization. The spin degree of freedom of electrons has great potential as a carrier of classical information (spintronics) 1 and quantum information (spin quantum bits). 2 Spintronics requires a device that can filter electrons by their spin orientation, i.e., a spin filter. As for quantum information, spin filters can be used for initialization and readout of spin quantum bits. 2,3 Moreover, they are an important ingredient of recent proposals to measure Bell's inequalities with entangled electron spins. 4 Much experimental progress has been made using magnetic semiconductors as spin filters. 5 However, many recent proposals focus on spin filtering in a two-dimensional electron gas (2DEG), 6,7 since this allows easy integration with other devices such as electron spin entanglers. 4 Spindependent electron transport through a 2DEG with giant Zeeman splitting of the lowest 2D subband was recently measured. 8 Also, quantum dots formed within 2DEGs have been shown to act as spin filters, by utilizing universal conductance fluctuations controlled by gate voltages, 9 and via spin-dependent coupling to the leads in a perpendicular magnetic field. 10 In the former case, the filtering efficiency (up to 70% in Ref. 9) and polarity rely on the chaotic character of the dot. In the latter case, the formation of edge channels in the leads yields reproducible spin selectivity with a high efficiency, but the polarization always corresponds to the ground-state spin orientation in the leads, and therefore the filter cannot be bipolar. The same is true for quantum point contacts, which have been used as unipolar spin filters. 9,11 For most purposes, however, a filter is required which is both bipolar and has a very high efficiency.Recher et al. 6 proposed to employ the discrete spinresolved energy levels of a quantum dot for spin filtering in a 2DEG. The low-bias electron transport through such a dot will be almost completely polarized if the Zeeman energy is much larger than the thermal energy. Furthermore, for simple pairwise spin filling of orbital states, the polarization is opposite at any two transitions with successive electron numbers. The filter polarity can thus easily be reversed by changing gate voltages.This spin filter has not been realized experimentally. The challenge is to demonstrate spin splitting of orbital levels in a quantum dot for successive electron transitions, and to sh...

Single-shot read-out of an individual electron spin in a quantum dot

“…The particular electron can have spin-↑ (shown in the lower diagram) or spin-↓ (upper diagram). (The tunnel rate for spin-↑ electrons is expected to be larger than that for spin-↓ electrons [27], i.e. Γ ↑ > Γ ↓ , but we do not assume this a priori.)…”

Effect of inelastic scattering on spin entanglement detection through current noise

“…Still, the problem of controlled spin manipulation and spin detection are two great hurdles to be tackled in the long path to spin-based quantum computation [9]. The main difficulty in the manipulation problem is that all the operations available in usual electronics address electron charge, being com- * Electronic address: pablo@tfp.uni-karlsruhe.de † Electronic address: elsa@tfp.uni-karlsruhe.de pletely independent of the electron's spin, unless some additional mechanism involving, e.g., external magnetic fields [4,10], ferromagnetic materials [11], or spin-orbit coupling [12,13] are relevant. Such mechanisms usually correlate spin states to charge states, which allows to manipulate and detect the charge states via more conventional means.…”

Filtering and analyzing mobile qubit information via Rashba–Dresselhaus–Aharonov–Bohm interferometers

“…Several proposed filters use quantum dots, in which the filtering is based on either the Coulomb blockade and the Pauli principle [17][18][19] or on the Zeeman energy splitting. 20,21 All the above filters usually generate only a partial spin polarization. For writing useful quantum information, the outgoing electrons must be fully polarized.…”