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Register a new userDouble dot chain as a macroscopic quantum bit
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We consider an array of N quantum dot pairs interacting via Coulomb interaction between adjacent dots and hopping inside each pair. We show that at the first order in the ratio of hopping and interaction amplitudes, the array maps in an effective two level system with energy separation becoming exponentially small in the macroscopic (large N) limit. Decoherence at zero temperature is studied in the limit of weak coupling with phonons. In this case the macroscopic limit is robust with respect to decoherence. Some possible applications in quantum information processing are discussed.
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Quantum phase transitions (QPTs) in qubit systems are known to produce singularities in the entanglement, which could in turn be used to probe the QPT. Current proposals to measure the entanglement are challenging however, because of their nonlocal nature. Here we show that a double quantum dot coupled locally to a spin chain provides an alternative and efficient probe of QPTs. We propose an experiment to observe a QPT in a triple dot, based on the well-known singlet projection technique.
It was proposed that a double quantum dot can be used to be a detector of spin bias. Electron transport through a double quantum dot is investigated theoretically when a pure spin bias is applied on two conducting leads contacted to the quantum dot. It is found that the spin polarization in the left and right dots may be induced spontaneously while the intra-dot levels are located within the spin bias window and breaks the left-right symmetry of the two quantum dots. As a result, a large current emerges. For an open external circuit an charge bias instead of a charge current will be induced in equilibrium, which is believed to be measurable according to the current nanotechnology. This method may provide a practical and whole electrical approach to detect the spin bias (or the spin current) by measuring the charge bias or current in a double quantum dot.
We solve analytically the problem of a finite length Kitaev chain coupled to a quantum dot (QD), which extends the standard Kitaev chain problem making it more closely related to the quantum dot-semiconductor-superconductor (QD-SM-SC) nanowire heterostructure that is currently under intense investigation for possible occurrence of Majorana zero modes (MZMs). Our analytical solution reveals the emergence of a robust Andreev bound state (ABSs) localized in the quantum dot region as the generic lowest energy solution in the topologically trivial phase. By contrast, in the bare Kitaev chain problem such a solution does not exist. The robustness of the ABS in the topologically trivial phase is due to a partial decoupling of the component Majorana bound states (MBSs) over the length of the dot potential. As a result, the signatures of the ABS in measurements that couple locally to the quantum dot, e.g., tunneling measurements, are identical to the signatures of topologically-protected MZMs, which arise only in the topological superconducting (TS) phase of the Kitaev chain.
We introduce a model of quantum teleportation on a channel built on a quantum dot chain. Quantum dots are coupled through hopping and each dot can accept zero, one or two electrons. Vacuum and double occupation states have the same potential energy, while single occupation states are characterized by a lower potential energy. A single dot initially decoupled from the others is weakly coupled with an external element (Bob), where a pair of electrons has been previously localized. Because of hopping after a suitable time the two dots charge states become maximally entangled. Another chain dot (Alice) is put in an unknown superposition of vacuum and double occupation states, and the other dots are initially empty. The time evolution of the system involves an electron diffusive process. A post selection procedure represented by the detection of charge pairs in a region of the chain equidistant from Alice and Bob, allows, if successful, the reconstruction on the Bob site of the unknown state initially encoded by Alice. The peculiar feature of the model is that the introduction of a trapped magnetic field strongly improves the process efficiency.
A most fundamental and longstanding goal in spintronics is to electrically tune highly efficient spin injectors and detectors, preferably compatible with nanoscale electronics. Here, we demonstrate all these points using semiconductor quantum dots (QDs), individually spin-polarized by ferromagnetic split-gates (FSGs). As a proof of principle, we fabricated a double QD spin valve consisting of two weakly coupled semiconducting QDs in an InAs nanowire (NW), each with independent FSGs that can be magnetized in parallel or anti-parallel. In tunneling magnetoresistance (TMR) experiments at zero external magnetic field, we find a strongly reduced spin valve conductance for the two anti-parallel configurations, with a single QD polarization of $sim 27%$. The TMR can be significantly improved by a small external field and optimized gate voltages, which results in a continuously electrically tunable TMR between $+80%$ and $-90%$. A simple model quantitatively reproduces all our findings, suggesting a gate tunable QD polarization of $pm 80%$. Such versatile spin-polarized QDs are suitable for various applications, for example in spin projection and correlation experiments in a large variety of nanoelectronics experiments.
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