Exisiting models for solid state scalable quantum computers often encode qubits in spin degrees of freedom of single electrons or nuclei [1-6]. We are, in fact, one of the earliest proponents of such ideas [1]. These models, while being most promising, often provide no scheme for reading or writing the qubit (spin), or else provide schemes that are very difficult, or nearly impossible, to implement. Even these difficult schemes do not guarantee accurate reading of qubits.
In our laboratory, we are developing a scheme to read/write single spins in quantum dots (or any other host for single electrons) using ferromagnetic “nanocontacts” that act as spin polarizers and analyzers. The structures are self-assembled by electrodepositing Fe, CdS and Ni sequentially in 50-nm pores of an anodic alumina film produced by the anodization of an Al foil in oxalic acid. This results in a 2-d vertical array of heterolayered quantum wires with diameter 50 nm. The thickness of each layer is approximately 0.5 mm. The ferromagnetic layers have been well characterized by magnetization measurements which show that the coercivities of the ferromagnets increase dramatically, possibly because of enhanced magnetic anisotropy, as well as near single domain behavior [7-9]. The semiconductor layer has been well characterized using optical and transport experiments which show strong quantum confinement effects for electrons and holes [10, 11].
Using a sequence of steps, we are able to contact only a few (10-100) nanowires in the 2-d array using large-area contacts (100 mm X 100 mm) defined by optical lithography. This translates to electrically contacting only about one in a million wire between the contact pads. As a result, we are able to observe Coulomb blockade at room temperature [12].
In the heterolayered nanowires, the Fe layer injects spin polarized electrons into the CdS layer, while the Ni layer transmits or reflects these electrons depending on whether the spins of electrons arriving at the Ni layer are aligned along or against the magnetization of this layer. Fe is therefore the spin polarizer and Ni is the spin analyzer. We have measured the magnetoresistance of these heterolayered nanowires and found tell-tale magnetoresistance peaks indicative of coherent spin injection and detection across the semiconductor-ferromagnetic interfaces. From the measured magnetoresistance peaks, we estimate the spin coherence length in the CdS layer to be 111 nm at 4.2 K.
We have also produced two-dimensional structures of Fe-Si-Ni by e-beam sputtering where the Si layer is only 10 nm thick. These structures also show a magnetoresistance peak indicative of spin coherent transport, but the measured spin coherence length is only about 10 nm at 4.2 K. The small coherence length is possibly due to strong spin flip scattering in the Si layer caused by diffusing Fe and Ni atoms that act as magnetic impurities in Si. Even though the same impurities are also present in the 1-d structure, they are not as effective in relaxing the spin since the 1-d confinement suppresses elastic impurity scattering [13]. This bodes well for “spin-in-a-quantum-dot” paradigms since elastic (as well as inelastic) scattering are strongly suppressed in a quantum dot because of the constriction of phase space for scattering.
In conclusion, we are able to inject and detect spin in heterolayered nanowires using large-area ferromagnetic contacts that effectively act as nanocontacts. By combining this feature with Coulomb blockade (which has been observed in these structures at room temperature), we might be able to measure few electron spins and ultimately single electron spins. These structures therefore have a potential to become simple qubit readers and writers.
This work is supported by NSF under grant ECS0196554 and by the Nebraska Research Initiative on Quantum Information Science.
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