Spin States In Quantum Dots Research Articles

Using Photoexcited Core/Shell Quantum Dots To Spin Polarize Appended Radical Qubits.

The synthetic tunability, flexibility, and rich spin physics of semiconductor quantum dots (QDs) make them promising candidates for quantum information science applications. However, the rapid spin relaxation observed in colloidal quantum dots limits their functionality. In the current work, we demonstrate a method to harness photoexcited spin states in QDs to produce long-lived spin polarization on an appended organic ligand molecule. We present a system composed of CdSe/CdS core/shell QDs, covalently linked to naphthalenediimide (NDI) electron-accepting molecules. The electron transfer dynamics from photoexcited QDs to the appended NDI ligands is explored as a function of both shell thickness and number of NDIs per QD. Transient EPR spectroscopy shows that the photoexcited QDs strongly spin polarize the NDI radical anion, which is interpreted in the context of both the radical pair and the triplet mechanisms of spin polarization. This work serves as an initial step toward using photoexcited QDs to strongly spin polarize organic radicals having long spin relaxation times to serve as spin qubits in quantum information science applications.

Conditional teleportation of quantum-dot spin states

Among the different platforms for quantum information processing, individual electron spins in semiconductor quantum dots stand out for their long coherence times and potential for scalable fabrication. The past years have witnessed substantial progress in the capabilities of spin qubits. However, coupling between distant electron spins, which is required for quantum error correction, presents a challenge, and this goal remains the focus of intense research. Quantum teleportation is a canonical method to transmit qubit states, but it has not been implemented in quantum-dot spin qubits. Here, we present evidence for quantum teleportation of electron spin qubits in semiconductor quantum dots. Although we have not performed quantum state tomography to definitively assess the teleportation fidelity, our data are consistent with conditional teleportation of spin eigenstates, entanglement swapping, and gate teleportation. Such evidence for all-matter spin-state teleportation underscores the capabilities of exchange-coupled spin qubits for quantum-information transfer.

Optical Fredkin gate assisted by quantum dot within optical cavity under vacuum noise and sideband leakage

We propose a deterministic Fredkin gate which can accomplish controlled-swap operation between three-qubit states. The proposed Fredkin gate consists of a photonic system (single photon) and quantum dots (QDs) confined in single-sided cavities (two electron spin states). In our scheme, the control qubit is the polarization state of the single photon, and two electron spin states in QDs play the role of target qubits (swapped states by control qubit). The interaction between a photon and an electron of QD within the cavity (QD-cavity system) significantly affects the performance of Fredkin gate. Thus, through the analysis of the QD-cavity system under vacuum noise and sideband leakage, we demonstrate that reliable interaction and performance of the QD-cavity system with photonic state (photon) can be acquired in our scheme. Consequently, the Fredkin gate proposed in this paper can be experimentally implemented with high feasibility and efficiency.

Manipulating entanglement sudden death in two coupled two-level atoms interacting off-resonance with a radiation field: an exact treatment.

We study a model of two coupled two-level atoms (qubits) interacting off-resonance (at non-zero detuning) with a single mode radiation field. This system is of special interest in the field of quantum information processing (QIP) and can be realized in electron spin states in quantum dots or Rydberg atoms in optical cavities and superconducting qubits in linear resonators. We present an exact analytical solution for the time evolution of the system starting from any initial state. Utilizing this solution, we show how the entanglement sudden death (ESD), which represents a major threat to QIP, can be efficiently controlled by tuning atom-atom coupling and non-zero detuning. We demonstrate that while one of these two system parameters may not separately affect the ESD, combining the two can be very effective, as in the case of an initial correlated Bell state. However in other cases, such as a W-like initial state, they may have a competing impacts on ESD. Moreover, their combined effect can be used to create ESD in the system, as in the case of an anti-correlated initial Bell state. A clear synchronization between the population inversion collapse-revival pattern and the entanglement dynamics is observed at all system parameter combinations. Nevertheless, only for initial states that may evolve to ESD, the population inversion revival oscillations, where exchange of energy between the atoms and the field takes place, temporally coincide with the entanglement revival peaks, whereas the population collapse periods match the ESD intervals. The variation of the radiation field intensity has a clear impact on the duration of the ESD at any combination of the other system parameters.

Adiabatically preparing quantum dot spin states in the Voigt geometry

We use mutually delayed and partially overlapping optical pulses, similar to those used in stimulated Raman adiabatic passage and its variations, for the coherent control of quantum dot spin states in the Voigt geometry. We consider the quantum dot system initially in an incoherent mixture of the two electron-spin states. We show that the application of regular delayed and partially overlapping pulses can lead to initialization. In addition, if initially delayed, partially overlapping, and simultaneously switched off pulses are applied, the initially incoherent mixture can be changed to a specifically designed coherent superposition state. We also find that due to the initial conditions of the studied quantum system, the proposed methods work for different pulse orderings.

Probing an Individual Electron Spin State in a Quantum Dot with Spin Bias.

A quantum dot coupled to two electrodes with spin-dependent splitting of chemical potentials (spin bias) is proposed as a detector of an individual electron spin. Spin polarized transport properties through the quantum dot have been investigated theoretically by means of the nonequilibrium Green's function formalism. We found that the direction of current flow is dependent on the electronic spin state in quantum dot. Measuring the direction of the current flow through the devices, we can determine the direction of the electronic spin state in quantum dot. This proposed detector provides a practical and all electrical approach to detect the electronic spin state in quantum dot structure.

Chirality of nanophotonic waveguide with embedded quantum emitter for unidirectional spin transfer.

Scalable quantum technologies may be achieved by faithful conversion between matter qubits and photonic qubits in integrated circuit geometries. Within this context, quantum dots possess well-defined spin states (matter qubits), which couple efficiently to photons. By embedding them in nanophotonic waveguides, they provide a promising platform for quantum technology implementations. In this paper, we demonstrate that the naturally occurring electromagnetic field chirality that arises in nanobeam waveguides leads to unidirectional photon emission from quantum dot spin states, with resultant in-plane transfer of matter-qubit information. The chiral behaviour occurs despite the non-chiral geometry and material of the waveguides. Using dot registration techniques, we achieve a quantum emitter deterministically positioned at a chiral point and realize spin-path conversion by design. We further show that the chiral phenomena are much more tolerant to dot position than in standard photonic crystal waveguides, exhibit spin-path readout up to 95±5% and have potential to serve as the basis of spin-logic and network implementations.

Landau–Zener spin transitions in Fe2+–Fe2+ quantum dots controlling dislocation mobility in NaCl:Fe crystals

Magnetic field induces transition in the distorted Fe2+–Fe2+ pairs (quantum dots) from the initial bonding singlet state to the high spin antibonding state providing decay of the pairs for two separated Fe2+ ions. Dislocations moving under internal stresses easily overcome separated Fe2+ ions in comparison with Fe2+–Fe2+ pairs lying close to the glide plane. Non-monotonous field dependence of dislocation displacements under internal stresses governed by short (100 μs) impulse of high magnetic fields up to 31 T was revealed in NaCl:Fe crystals. This non-typical dependence is the fingerprint of the Landau–Zener non-adiabatic spin transition between singlet and high spin states in quantum dots distorted by mechanical stresses of moving dislocations.

Proposal for a Phonon Laser Utilizing Quantum-Dot Spin States

We propose a nanoscale realization of a phonon laser utilizing phonon-assisted spin flips in quantum dots to amplify sound. Owing to a long spin relaxation time, the device can be operated in a strong pumping regime, in which the population inversion is close to its maximal value allowed under Fermi statistics. In this regime, the threshold for stimulated emission is unaffected by spontaneous spin flips. Considering a nanowire with quantum dots defined along its length, we show that a further improvement arises from confining the phonons to one dimension, and thus reducing the number of phonon modes available for spontaneous emission. Our work calls for the development of nanowire-based, high-finesse phonon resonators.

Exchange and spin states in quantum dots under strong spatial correlations. Computer simulation by the Feynman path integral method

The fundamental laws in the behavior of electrons in model quantum dots that are caused by exchange and strong Coulomb correlations are studied. The ab initio path integral method is used to numerically simulate systems of two, three, four, and six interacting identical electrons confined in a three-dimensional spherical potential well with a parabolic confining potential against the background of thermal fluctuations. The temperature dependences of spin and collective spin magnetic susceptibility are calculated for model quantum dots of various spatial sizes. A basically exact procedure is proposed for taking into account the permutation symmetry and the spin state of electrons, which makes it possible to perform numerical calculations using modern computer facilities. The conditions of applicability of a virial energy estimator and its optimum form in exchange systems are determined. A correlation estimator of kinetic energy, which is an alternative to a basic estimator, is suggested. A fundamental relation between the kinetic energy of a quantum particle and the character of its virtual diffusion in imaginary time is demonstrated. The process of natural “pairing” of electron spins during the compression of a quantum dot and cooling of a system is numerically reproduced in terms of path integrals. The temperature dependences of the spin magnetic susceptibility of electron pairs with a characteristic maximum caused by spin pairing are obtained.

DECOHERENCE STUDY OF ELECTRON SPIN STATES IN QUANTUM DOTS USING A SIMPLISTIC MODEL

Using a simple model of central spin interacting with a spin bath, we study the decoherence of single electron spin states in quantum dots within one-, two- and three-dimensional nanostructure geometries. We consider a one-component hyperfine interaction term and express its strength as a smooth function of position vector. Decoherence measures are evaluated and plotted for various initial states, and we notice a decrease in decoherence time with the reduction of spin-bath dimension.

Electrically tunedgtensor in an InAs self-assembled quantum dot

We investigate the all-electrical tuning of the Land\'e $g$ tensor in a single uncapped InAs quantum dot contacted with a nanogap electrode technique and electrically gated with both back- and side-gate electrodes. Magnetotransport measurements allow extraction of the $g$ tensor components from measurements of the Zeeman energy for magnetic fields applied in the plane of the sample. The side-gate electrode allows tuning of the anisotropy of the in-plane $g$ tensor components and is suitable for the manipulation of the quantum-dot spin states using $g$ factor modulation resonance schemes.

Tuning Electron Spin States in Quantum Dots by Spin-Orbit Interactions

We theoretically investigate the influence of both Rashba spin-orbit interaction (RSOI) and Dresselhaus spin-orbit interaction (DSOI) on electron spin states, electron distribution and the optical absorption of a quantum dot. Our theoretical results show that the interplay between RSOI and DSOI results in an effective periodic potential, which consequently breaks the rotational symmetry and makes the quantum dot behave like two laterally coupled quantum dots. In the presence of RSOI and/or DSOI the spin is no longer a conserved quantity and its magnitude can be tuned by changing the strength of RSOI and/or DSOI. By reversing the direction of the perpendicular electric field, we can rotate the spatial distribution. This property provides us with a new way to control quantum states in a quantum dot by electrical means.

The influence of spin-flip scattering on the preparation and detection of a single spin state in a quantum dot attached to a spin battery

Recently, the possibility of an all electrical scheme of preparation and readout for a single spin state in a single quantum dot attached to spin biased leads has been shown [F. Chi et al. Phys. Rev. B 81, 075310 (2010)]. However, spin scattering mechanisms have been omitted. To remedy this lack we consider the influence of the spin-flip scattering process on the proposed preparation and readout scheme.

Decoherence of two-electron spin states in quantum dots

The time evolution of spin states of two electrons interacting with a nuclear spin bath in a quantum dot system is studied. The hyperfine interaction between the electrons and the nuclear spins is modeled by an isotropic Heisenberg interaction, and the interaction between the electron spins by Heisenberg exchange. Depending on the extent of the overlap between the spatial wave functions of the electrons, there are two physically different cases, namely, the two qubits either interact with the same set of nuclear spins or they see different nuclear spin environments. In the two cases, the decoherence of the two-qubit state is studied analytically. We have identified a class of two-qubit states that have a rich dynamics when the exchange interaction between the qubits becomes large in comparison to the hyperfine interaction strengths. The decoherence time scale is determined as a function of the bath-spin distribution and the polarizations of the initial two-qubit state. States with large decoherence times are identified by performing a minimization over all the two-qubit pure states.

Decoherence and fidelity of single-electron spin states in quantum dots: effects of nuclearhyperfine coupling and double occupancy

The decoherence and fidelity of spin states in a localized single-electron quantum dot in thepresence of a dc magnetic field, arising either from the nuclear hyperfine interaction withinthe dot or due to its coupling with another localized quantum dot, are examined in detail.A general framework for determining the time evolution of the reduced density matrixρ for a single dot is presented, which is exact up to the second order in interaction with anyreservoir. In particular, it is applied to the problem of nuclear hyperfine coupling, andapproximate estimates of coherence decay time are made when the nuclear spins are eitherpolarized or unpolarized and the internal dynamics of nuclear spins is determinedmainly by the nuclear magnetic dipole–dipole interaction. The fidelity of a pureunperturbed electronic one-qubit spin state is obtained as a function of time,which is exact even on a very short timescale of logic gate operations. The timevariation of the fidelity of the same one-qubit state on the localized dot as a part ofthe direct product with another one-qubit state on another localized dot arisingbecause of coupling between these quantum dots is also calculated in this paper.In this case, we include both the single-particle tunnelling between the dots aswell as the direct and exchange Coulomb interactions, including on-site Coulombrepulsion. This allows for the double occupation of a single dot. It is found that theloss of fidelity of such two-qubit states due to double occupancy and additionalphase errors in the presence of appreciable dot–dot coupling can become a moresevere limiting factor than that due to the hyperfine interaction in individual dots.

Stability of spin states in quantum dots

We have investigated electronic transport through a Coulomb-blockaded quantum dot in which interactions are strong. Linear changes in conductance peak spacings with in-plane magnetic field are observed and interpreted in terms of Zeeman splitting of single-particle levels. Thereby, the measurements allow tracking changes in the dot's ground-state spin as the dot is gradually opened to the leads and the electron number is changed. Spin states have been identified in the weak- $(kTg\ensuremath{\Gamma}),$ intermediate- $(\ensuremath{\Gamma}\ensuremath{\approx}kT),$ and strong- $(\ensuremath{\Gamma}gkT)$ coupling regime. It is found that ground states with spin $S=0$ or $S=1/2$ are most likely, while larger total spins $Sg~1$ can occasionally occur, despite the large number of 50--100 electrons. A g factor close to the bare bulk GaAs value has been determined experimentally for the majority of the spin states. A perpendicular magnetic field applied to the dot in the same state allows the investigation of spin-pair candidates under conditions where orbital effects dominate the evolution of conductance peaks. Strong correlations in the position and in the amplitude of neighboring peaks allow the final identification of spin pairs. The method of combining parallel and perpendicular magnetic fields for identifying spin states and spin-pairs works well for intermediate and strong coupling of dot states to the leads while the data in the weak-coupling regime is less conclusive. Our results indicate that the spin degree of freedom is remarkably stable and the spin states are well described within a single-particle picture.