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Europhysics Letters (EPL)

A scheme for accurate detection of molecular bonds in quantum dots

Y. X. Deng, Xin Yan

Published Dec 1, 2009• 1 citation
Publisher

Abstract

We propose a closed two-terminal Aharonov-Bohm (AB) interferometer which contains a double quantum dot to detect the coherent molecule-like states. This scheme is based on the measurement of AB oscillation properties by changing interdot coupling strength. By comparison with an open multiterminal AB setup, we find that due to the phase locking effect the initial phase of the AB oscillations changes from π/2 to 3π/2 with the increase of the interdot coupling. The crossover implies the transition from the nearly decoupled states with the increase of the to the merged artificial molecular states.

DOI
10.1209/0295-5075/88/67002

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We propose a closed two-terminal Aharonov-Bohm (AB) interferometer which contains a double quantum dot to detect the coherent molecule-like states. This scheme is based on the measurement of AB oscillation properties by changing interdot coupling strength. By comparison with an open multiterminal AB setup, we find that due to the phase locking effect the initial phase of the AB oscillations changes from π/2 to 3π/2 with the increase of the interdot coupling. The crossover implies the transition from the nearly decoupled states with the increase of the to the merged artificial molecular states.
Bonding and anti-bonding states of artificial molecules have been realized in experiments by directly coupling two quantum dots. Without a direct coupling between two nearby quantum dots, here we show that a continuous crossover, from symmetric to anti-symmetric molecular state, can be achieved by changing the flux through a double quantum dot Aharonov-Bohm (AB) interferometer. We explicitly present the flux-dependent real-time processes of molecular-state formation. In contrast to the transport current, which has a 2π2π2π period, the quantum state of the DQD molecule has a 4π4π4π period in the AB flux.
We study complementarity in a two-terminal closed-loop Aharonov-Bohm interferometer with a quantum dot embedded in an arm of the ring and a nearby charge detector capacitively coupled to the quantum dot. We show that perfect charge detection does riot give full information of the electronic path, so it does not completely suppress the interference. We propose that this property can be verified even in the limit of weak charge detection by investigating the dephasing of the first and the second harmonics of the Aharonov-Bohm oscillation. The first harmonic of the AB oscillation is reduced monotonically as a function of the charge detection strength. In contrast, the second harmonic interference remains unaffected due to the fact that charge detection is not able to detect the electronic path responsible for the second harmonic. Our theory confirms that the particle-like behavior of an electron emerges from the path information itself rather than from the disturbance.
We report on a study of complementarity in a two-terminal closed-loop Aharonov-Bohm interferometer. In this interferometer, the simple picture of two-path interference cannot be applied. We introduce a nearby quantum point contact to detect the electron in a quantum dot inserted in the interferometer. We found that charge detection reduces but does not completely suppress the interference even in the limit of perfect detection. We attribute this phenomenon to the unique nature of the closed-loop interferometer. That is, the closed-loop interferometer cannot be simply regarded as a two-path interferometer because of multiple reflections of electrons. As a result, there exist indistinguishable paths of the electron in the interferometer and the interference survives even in the limit of perfect charge detection. This implies that charge detection is not equivalent to path detection in a closed-loop interferometer. We also discuss the phase rigidity of the transmission probability for a two-terminal conductor in the presence of a detector.
To investigate the decoherence of Kondo singlet, we once again check a model, an Aharonov–Bohm interferometer with a quantum dot coupling to left–right electrodes, which is designed by Yacoby to measure phase-sensitive of a quantum dot. By employing the cluster expansion, the equations of motion of Green's functions are transformed into the corresponding equations of connected Green's functions, which contain the correlation of two conducting electrons. With the method, we show that the Kondo singlet is suppressed by phase-sensitive detection of Aharonov–Bohm interferometer. Our numerical results have provided a qualitative explanation with the anomalous features observed in an experiment by Avinun-Kalish et al. [Phys. Rev. Lett.92 (2004) 156801].
The Aharonov-Bohm effect is measured in a four-terminal open ring geometry. Two quantum dots are embedded in the structure, one in each of the two interfering paths. The number of electrons in the two dots can be controlled independently. The transmission phase is measured as electrons are added to or taken away from the individual quantum dots. Although the measured phase shifts are in qualitative agreement with theoretical predictions, the phase evolution exhibits unexpected dependence on the magnetic field. Phase lapses are found only in certain ranges of the magnetic field.
Spin entanglement between two spatially separated electrons can be generated in nonequilibrium interacting quantum dots, coherently coupled to a common lead. In this system entangled two-electron states develop which are Werner states with an imbalance between singlet and triplet probabilities. We propose a multi-terminal, multiply connected set-up for the generation and detection of this imbalance. In particular, we identify a regime in which the formation of spin entanglement leads to a cancellation of Aharonov–Bohm oscillations.