Nanomechanical resonator coupling with a double quantum dot: quantum state engineering

We propose a theoretical scheme to realize quantum state engineering of a nanomechanical resonator (NAMR) through the coupling between the NAMR and a double quantum dot (DQD). Hybrid entangled states between the NAMR and the DQD and superposed coherent states of the NAMR are created explicitly. It is shown that quantum state tomography for the NAMR can be implemented through carrying out unitary operations on the NAMR and the DQD. It is indicated that the scheme is feasible at the reach of the present technology.     

Error-detected single-photon quantum routing using a quantum dot and a double-sided microcavity system

Based on a hybrid system consisting of a quantum dot coupled with a double-sided micropillar cavity, we investigate the implementation of an error-detected photonic quantum routing controlled by the other photon. The computational errors from unexpected experimental imperfections are heralded by single photon detections, resulting in a unit fidelity for the present scheme, so that this scheme is intrinsically robust. We discuss the performance of the scheme with currently achievable experimental parameters. Our results show that the present scheme is efficient. Furthermore, our scheme could provide a promising building block for quantum networks and distributed quantum information processing in the future.     

Voltage-controlled storage and retrieval of an infrared-light pulse in a quantum-dot molecule

Dynamic storage and retrieval of a weak infrared (IR)-light pulse are investigated theoretically with feasible parameters in an asymmetric double quantum dot system, a quantum dot molecule (QDM). It is shown that, with a voltage-controlled tunneling, we are able to store and retrieve the IR signal pulse in this three-subband QDM medium by slowly switching off and on the tunneling. The scheme proposed may open up the electrical controllability of quantum optical information storage and retrieval, which is expected to be useful in quantum information science in an asymmetric double quantum dot controlled by voltage.     

Differential charge sensing and charge delocalization in a tunable double quantum dot

We report measurements of a tunable double quantum dot, operating in the quantum regime, with integrated local charge sensors. The spatial resolution of the sensors allows the charge distribution within the double dot system to be resolved at fixed total charge. We use this readout scheme to investigate charge delocalization as a function of temperature and strength of tunnel coupling, demonstrating that local charge sensing can be used to accurately determine the interdot coupling in the absence of transport.     

Crystallization and quantum melting of few electron system in a spherical quantum dot: quantum Monte Carlo simulation

Spherical quantum dots with a few charged Fermi particles (electrons or holes) are studied for different total spins. Simulation by quantum path integral Monte Carlo method is performed. The dependence of the electron correlations in the quantum dot is studied at different mean interelectron separation controlled by number of electrons in the quantum dot and by steepness of electron confinement (the latter parameter can be changed by the gate voltage). The ‘cold’ melting—quantum transition from Wigner crystal-like state (i.e. from regime of strongly correlated electrons) to a Fermi liquid-like state—driven by the steepness of electron confinement is studied. The pair correlation function and radial function characterizing electron quantum delocalization are analyzed.     

Complete entanglement analysis on electron spins using quantum dot and microcavity coupled system

We present an entanglement analysis protocol on entangled electron spins using quantum dot (QD) and microcavity coupled system. Each quantum dot is placed in the microcavity and ancilla photon input-output process could be used to check the parity of the quantum dots. After the parity check process, the user only needs to measure the spin direction of the QD spin, and the state information can be readout completely. The feasibility of our scheme and the experimental challenge are discussed by considering currently available techniques.     

Bound states of Dirac electrons in a graphene-based magnetic quantum dot

We investigate the magnetically confined states of the massless Dirac fermions in a graphene quantum dot formed by the inhomogeneous distributions of the magnetic fields inside and outside the dot. The calculated energy spectrum exhibits quite different features with and without the magnetic field inside the dot. It is found that the degeneracy of the relativistic Landau level with negative angular momenta can be lifted, and this degeneracy breaking can be modulated by the magnetic field inside the dot. Moreover, such a system can form the strongly localized states within the dot and along its boundary, especially with the magnetic field inside the dot.     

The decoherence of the parabolic linear bound potential quantum dot qubit

On the condition of electric-LO phonon strong coupling in parabolic quantum dot, we obtain the eigenenergies of the ground state and the first-excited state, the eigenfuctions of the ground state and the first-excited state by using variational method of Pekar type. This system in quantum dot may be employed as a two-level quantum system—qubit. The phonon spontaneous emission causes the decoherence of the qubit. The relations between the decoherence time with the coupling strength, the confinement length, the coefficient dispersion are discussed.     

Quantum interference and population swapping in single quantum dots with V-type three-level

The quantum interference and Rabi oscillation of a V-type three-level system with two orthogonal sub-states in an elongated semiconductor quantum dot are discussed theoretically with optical Bloch equations when the system is driven by pulse-pair. Numerical calculations from the optical Bloch equations reveal that the quantum interference in the system is enhanced with the increasing of the energy decay or splitting. Furthermore, the populations swapping in two orthogonal sub-states can be realized though the direct transition is prohibited.     

Steering quantum nonlocalities of quantum dot system suffering from decoherence

The important applications of quantum dot system are to implement logic operations and achieve universal quantum computing based on different quantum nonlocalities. Here, we characterize the quantum steering, Bell nonlocality, and nonlocal advantage of quantum coherence (NAQC) of quantum dot system suffering nonunital and unital channels. The results reveal that quantum steering, Bell nonlocality, and NAQC can display the traits of dissipation, enhancement, and freezing. One can achieve the detections of quantum steering, Bell nonlocality, and NAQC of quantum dot system in different situations. Among these quantum nonlocalities, NAQC is the most fragile, and it is most easily influenced by different system parameters. Furthermore, considering quantum dot system coupling with amplitude damping channel and phase damping channel, these quantum nonlocalities degenerate with the enlargement of the channel parameters $t$ and $\varGamma$. Remarkably, measurement reversal can effectively control and enhance quantum steering, Bell nonlocality, and NAQC of quantum dot system suffering from decoherence, especially in the scenarios of the amplitude damping channel and strong operation strength.     

Tunneling-induced ultraslow solitons in triangular quantum dot molecules

We theoretically investigate the propagation of a weak probe laser pulse in a triangular quantum dot molecules scheme based on the tunneling induced transparency. We find that the ultraslow optical solitons can be realized due to the destructive quantum interference induced by the interdot tunneling coupling which can be adjusted by the gate voltage appropriately. This work may provide practical applications such as electro-optic modulated devices and other information processes in semiconductor quantum dots structure.     

Onion like growth and inverted many-particle energies in quantum dots

Use of surfactants like antimony in MOCVD growth enables novel growth regimes for quantum dots (QDs). The quantum dot ensemble luminescence no longer appears as a single inhomogeneously broadened peak but shows a multi-modal structure. Quantum dot subensembles are forming which differ in height by exactly one monolayer. For the first time the systematic dependence of excitonic properties on quantum dot size and shape can be investigated in detail. Both biexcitonic binding energy and excitonic fine-structure splitting vary from large positive through zero to negative values. Correlation and piezoelectric effects explain the observations.     

Equilibrium spin current induced by spin-orbital interaction in a quantum dot system

We report a theoretical study of the equilibrium spin current flowing in a quantum dot system. Two electrodes are the two-dimensional electron gas with Rashba or Dresselhaus spin-orbital interaction. By using the Keldysh Green's function technique, we demonstrated that a nonzero spin current can flow in the system without bias. At the weak coupling between electrodes and the quantum dot, the spin current is approximately proportional to the cross product of two average pseudo-magnetizations in two electrodes, which agrees with the result of the linear response theory; whereas at the opposite case, the strong coupling between the quantum dot and electrodes can lead to a non-sinusoidal behavior of the equilibrium spin current. These behaviors of the equilibrium spin current are similar to the Josephson current.     

Disorder-mediated electron valley resonance in carbon nanotube quantum dots

We propose a scheme for coherent rotation of the valley isospin of a single electron confined in a carbon nanotube quantum dot. The scheme exploits the ubiquitous atomic disorder of the nanotube crystal lattice, which induces time-dependent valley mixing as the confined electron is pushed back and forth along the nanotube axis by an applied ac electric field. Using experimentally determined values for the disorder strength we estimate that valley Rabi oscillations with a period on the nanosecond time scale are feasible. The valley resonance effect can be detected in the electric current through a double quantum dot in the single-electron transport regime.     

Dipole-allowed optical absorption in a parabolic quantum dot with two electrons

Dipole-allowed optical absorption in a parabolic quantum dot with two electrons are studied by using the exact diagonalization techniques and the compact density-matrix approach. Numerical results are presented for typical GaAs parabolic quantum dots. The results show that the total optical absorption coefficient of two electrons in quantum dot is about five times smaller than that of one electron in quantum dot.     

Tunable quantum dot resonator embedded in a quantum wire

Combined quantum wire and quantum dot system is theoretically predicted to show unique conductance properties associated with Coulomb interactions. We use a split gate technique to fabricate a quantum wire containing a quantum dot with two tunable potential barriers in a two-dimensional electron gas. We observe the effects of the quantum dot cavity on the electron transport through the quantum wire, such as Coulomb oscillations near the pinch-off voltage and periodic conductance oscillations on the first conductance plateau.     

Chemisorption on a quantum dot

The quantum kinetic equations for a “quantum dot-adatom” system are derived. It is demonstrated that the inclusion of the interaction between a quantum dot and an adatom leads to an increase in the quantum dot radius. The perturbations of the electron density of the quantum dot and the adatom upon chemisorption are calculated.     

Magneto-absorption in cylindrical quantum dots

The absorption of light in an ensemble of non-interacting cylindrical quantum dots in the presence of a magnetic field is discussed using a model consisting of dots with rectangular infinitely-high potential barriers. The ensemble’s absorption coefficient is calculated — as well as the threshold frequency of absorption — as a function of the applied magnetic field and the quantum dot size. Theoretical results are compared with experimental data on magneto-luminescence in an In_0.53Ga_0.47As/InP cylindrical quantum dot system. In addition, using a perturbation theory framework, the influence of excitonic effects on the behaviour of the electron-hole energetic spectrum of said system is discussed.     

A bound polaron in a spherical quantum dot

The binding energy of a bound polaron in a spherical quantum dot has been investigated by using the variational method. The influence of LO and SO phonons have taken into consideration. Result shows that the phonon contribution to the binding energy is dependent on the size of the quantum dot as well as the position of the impurity in the quantum dot. Numerical calculation on the ZnSe quantum dot shows that such contribution is about 5% to 20% of the total binding energy. Received: 13 October 1997 / Revised: 4 March 1998 / Accepted: 26 May 1998