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arXiv:1204.5931

Coherent control of artificial molecules using an Aharonov-Bohm magnetic flux

Matisse Wei-Yuan Tu, Wei-Min Zhang, Franco Nori

Published Apr 26, 2012• 0 citations
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Abstract

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 $2π$ period, the quantum state of the DQD molecule has a $4π$ period in the AB flux.

DOI
10.48550/arxiv.1204.5931

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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 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.
The nonequilibrium real-time dynamics of electron coherence is explored in the quantum transport through the double-dot Aharonov-Bohm interferometers. We solve the exact master equation to find the exact quantum state of the device, from which the changes of the electron coherence through the magnetic flux in the nonequilibrium transport processes is obtained explicitly. We find that the relative phase between the two charge states of the double dot localizes to π2\fracπ{2}2π​ or −π2-\fracπ{2}−2π​ for all different magnetic flux. This nontrivial phase localization process can be manifested in the measurable occupation numbers.
We consider the phase-coherent transport of electrons passing through an Aharonov–Bohm ring while interacting with a tunnel charge in a double quantum dot (representing a charge qubit) which couples symmetrically to both arms of the ring. For Aharonov–Bohm flux Φ AB = h /2 e we find that electrons can only be transmitted when they flip the charge qubit's pseudospin parity an odd number of times. The perfect correlations of the dynamics of the pseudospin and individual electronic transmission and reflection events can be used to entangle the charge qubit with an individual passing electron.
The nonequilibrium real-time dynamics of electron decoherence is explored in the quantum transport through the nanoscale double-dot Aharonov-Bohm interferometers. We solve the exact master equation to find the exact quantum state of the device, from which the changes of the electron coherence through the magnetic flux in the nonequilibrium transport processes is obtained explicitly. We find that the relative phase between the two charge states of the degenerate double dot localizes to $\frac{\ensuremath{\pi}}{2}$ or $\ensuremath{-}\frac{\ensuremath{\pi}}{2}$ for all different magnetic fluxes, due to decoherence. This nontrivial decoherence process can be manifested in the measurable occupation numbers.
We propose a scheme to manipulate the Majorana qubit states other than braiding. By utilizing electron transport through Majorana zero modes and tuning the magnetic flux in the setup, the aim of Majorana qubit state manipulation can be fulfilled. The principal part in the setup is an Aharonov-Bohm (AB) interferometer consisting of two topological superconducting chains (TSCs) connected to two leads. We obtain the exact master equation as well as its solution and study the general properties of the real-time dynamics of the Majorana qubit states. The parity of the Majorana qubit state can be almost perfectly polarized by tuning the bias and can also be flipped by switching the sign of the cross couplings between the TSCs and the leads. Moreover, the qubit coherence shows a phase rigidity due to the intrinsic particle-hole symmetry of the Majorana AB interferometer, which is different from that in the double-quantum-dot AB interferometers without topological properties.
Recent experiments (Yamamoto et al 2012 Nature Nanotechnology 7 247 ) used the transport of electrons through an Aharonov–Bohm (AB) interferometer and two coupled channels (at both ends of the interferometer) to demonstrate a manipulable flying qubit. Results included in-phase and anti-phase (AB) oscillations of the two outgoing currents as a function of the magnetic flux, for strong and weak inter-channel coupling, respectively. Here we present new experimental results for a three terminal interferometer, with a tunnel coupling between the two outgoing wires. We show that in some limits, this system is an even simpler realization of the 'two-slit' experiment. We also present a simple tight-binding theoretical model which imitates the experimental setup. For weak inter-channel coupling, the AB oscillations in the current which is reflected from the device are very small, and therefore the oscillations in the two outgoing currents must cancel each other, yielding the anti-phase behavior, independent of the length of the coupling regime. Technically, the tight binding equations within the two coupled wires have four solutions for each electronic energy. In the 'anti-phase' region all of these solutions are wave-like, oscillating with the distance along the wires. As the coupling between the wires increases, two of these solutions become evanescent, and their amplitudes decay as the electron moves in the wires. In this regime, the amplitudes of the two remaining 'running' waves are proportional to each other, with a ratio which is practically flux-independent. As a result, the two outgoing currents are proportional to each other, yielding the 'in phase' behavior. For larger coupling all the solutions are evanescent, and the outgoing currents become very small.