Decoherence of a charge qubit embedded inside a suspended phonon cavity

Abstract

This study elucidates the theory of phonon-induced decoherence of a double dot charge qubit that is embedded inside a suspended semiconductor slab. The influences of the lattice temperature, width of the slab, and positions of the dots on the decoherence are analyzed. Numerical results indicate that the decoherence in the slab system is weaker than that in a bulk environment. In particular, the decoherence is markedly suppressed by the inhibition of the electron-phonon coupling. Such a system with low decoherence may be useful for manipulating the qubits.Various candidates for building blocks of quantum computers have been proposed 1-4 and demonstrated 5-8 using nanoscale solid state structures. Among them, the semiconductor double quantum dot ͑DQD͒ is a prototype of a solid state qubit, which comprises a pair of quantum dots connected through an interdot tunneling barrier. 9 The DQD acts as a two-level system when at most one excess electron is allowed in the double dot potential. 10 The two logical states correspond to the localized states of the excess electron in one or the other dot. Accordingly, the state of the charge qubit can be expressed as a linear superposition of the two localized states, and has recently been demonstrated in a GaAs/ AlGaAs heterostructure. 6,7 An important advantage of this system is that all of the qubit parameters can be controlled by varying the external gate voltages. Furthermore, it may have great potential for scalability and integration with current microelectronic technologies.However, various mechanisms may destroy the coherence of the system since the charge qubit can never be isolated completely from its environment. 6 The electron-phonon interaction is one possible decoherence channel and has been theoretically investigated. It ranges from a single charge qubit 11-16 to two coupled charge qubits. 17 Although many works have focused on bulk cases, the decoherence of a charge qubit in a confined structure has received little attention. Unlike bulk material, the confined structure supports the tailoring of the phonon density of states by altering the dimensions. 18-21 Therefore, the charge qubit in a confined structure is expected to exhibit some interesting characteristics.This work investigates the decoherence of a DQD qubit that is embedded inside a suspended semiconductor slab. To study the decoherence of the qubit, the master equation is solved under the Born-Markov approximation. 22-24 The qubit of higher quality is found in the slab system. In particular, the decoherence is dramatically suppressed by the inhibition of the electron-phonon coupling, suggesting that a robust DQD qubit can be achieved in a suspended structure.Consider the setup shown in Fig. 1, in which a single electron is confined in a GaAs double dot structure that is embedded inside a freestanding slab. With advances in novel nanofabrication technology, quantum dots can now be embedded into the suspended structure. 20,21 Most of the structure is spatially separated from most of the substrate such tha...

Experimental Observation of Enhanced Electron–Phonon Interaction in Suspended Si Double Quantum Dots

“…Together with peak broadening due to the small slab width, the overlap between the flexural mode and the dilatational phonon mode in cylindrical DQDs 13) may be responsible for the electron-phonon interaction not being suppressed in our experiments. However, we did observe tunneling peaks that are related to the various branches in the phonon dispersion 11) and distinctive phonon modes (the dilatational and flexural modes).…”

“…However, we did observe tunneling peaks that are related to the various branches in the phonon dispersion 11) and distinctive phonon modes (the dilatational and flexural modes). 13) These results as well as the specific properties of silicon provide significant advantage towards the realization of engineered Si SDQDs in which the electron-phonon interaction could be suppressed. For example, the use of a thinner film may increase specific phonon energy and consequently decrease the extent of relative peak broadening, or the use of cylindrical nanowires could decrease the absolute peak broadening by reducing the phonon-energy dissipation due to its symmetric shape.…”

“…4(a). The model was similar to that used by Liao et al 13) for disklike DQDs embedded in a thin-film monolayer. However, our model takes into account the finite thickness of the cylindrical DQDs, the asymmetry of the DQDs, the multilayered films, and the finite width of the films.…”

“…In DQDs embedded in a suspended film, information about electron-phonon interaction is contained in phonon spectral density 12,13) and thus can be accessed via conductivity measurement.…”

“…Thus, it is expected that a system composed of DQDs embedded in a suspended nanostructure (suspended DQDs, SDQDs) can substantially reduce the decoherence in charge qubits. 13) On the other hand, in spin-based qubits, the decoherence is expected to be smaller than that in the charge qubits 14) because of the absence of intrinsic nuclear spin in natural silicon-28.…”