Publications (16)76.31 Total impact
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ABSTRACT: In the field of quantum information science, semiconductor quantum dots (QDs) are of particular interest for their ability to confine a single electron for use as a qubit. However, to realize the potential offered by quantum information processing, it is necessary to couple two or more qubits. In contrast to coupling individual QDs, we demonstrate the integration of two coupled electronic states within a single QD heterostructure. These chemically synthesized nanocrystals, known as quantumdot quantum wells (QDQWs), comprise concentric layers of different semiconducting materials. We investigate carrier and spin dynamics in these structures using transient absorption and timeresolved Faraday rotation measurements. By tuning the excitation and probe energies, we find that we can selectively initialize and read out spins in different coupled states within the QDQW. These results open a pathway for engineering coupled qubits within a single nanostructure.  [Show abstract] [Hide abstract]
ABSTRACT: Colloidal nanoparticles provide a flexible system for studying individual quantumconfined electrons and holes. By layering different semiconducting materials in a single nanoparticle, we can create a low bandgap (CdSe) core and surrounding shell, separated by a high bandgap (ZnS) barrier. We have studied spin dynamics in such colloidal heterostructures using twocolor timeresolved Faraday rotation (TRFR). By tuning the excitation energy, electron spins can be initialized into different states either in the core or the shell of the nanoparticle. The resulting spin dynamics show a gfactor (spin splitting) that depends on the size of the core or the shell. This gfactor dependence, as well as the spectroscopic dependence of the Faraday effect, allow electron spins in the core or the shell to be addressed independently.  [Show abstract] [Hide abstract]
ABSTRACT: We study a model for a pair of qubits which interact with a single offresonant cavity mode and, in addition, exhibit a direct interqubit coupling. Possible realizations for such a system include coupled superconducting qubits in a line resonator as well as exciton states or electron spin states of quantum dots in a cavity. The emergent dynamical phenomena are strongly dependent on the relative energy scales of the interqubit coupling strength, the coupling strength between qubits and cavity mode, and the cavity mode detuning. We show that the cavity mode dispersion enables a measurement of the state of the coupledqubit system in the perturbative regime. We discuss the effect of the direct interqubit interaction on a cavitymediated twoqubit gate. Further, we show that for asymmetric coupling of the two qubits to the cavity, the direct interqubit coupling can be controlled optically via the ac Stark effect.  [Show abstract] [Hide abstract]
ABSTRACT: Timeresolved Faraday rotation studies of CdS/CdSe/CdS quantumdot quantum wells have recently shown that the Faraday rotation angle exhibits several welldefined resonances as a function of probe energy close to the absorption edge. Here, we calculate the Faraday rotation angle from the eigenstates of the quantumdot quantum well obtained with k.p theory. We show that the large number of narrow resonances with comparable spectral weight observed in experiment is not reproduced by the level scheme of a quantumdot quantum well with perfect spherical symmetry. A simple model for broken spherical symmetry yields results in better qualitative agreement with experiment. Comment: 9 pages, 4 figures  [Show abstract] [Hide abstract]
ABSTRACT: We have characterized CdS∕CdSe∕CdS quantumdot quantum wells using timeresolved Faraday rotation (TRFR). The spin dynamics shows that the electron g factor varies as a function of quantum well width and the transverse spin lifetime of several nanoseconds is robust up to room temperature. As a function of probe energy, the amplitude of the TRFR signal shows pronounced resonances, which allow one to identify individual exciton transitions. The resonance energies in the TRFR data are consistent with different exciton transitions in which the electron occupies the conductionband ground state.  [Show abstract] [Hide abstract]
ABSTRACT: Coherent Rabi oscillations between quantum states of superconducting microcircuits have been observed in a number of experiments, albeit with a visibility which is typically much smaller than unity. Here, we show that the coherent coupling to background charge fluctuators [R.W. Simmonds et al., Phys. Rev. Lett. 93, 077003 (2004)] leads to a significantly reduced visibility if the Rabi frequency is comparable to the coupling energy of microcircuit and fluctuator. For larger Rabi frequencies, transitions to the second excited state of the superconducting microcircuit become dominant in suppressing the Rabi oscillation visibility. We also calculate the probability for Bogoliubov quasiparticle excitations in typical Rabi oscillation experiments. Comment: 5 pages, 2 figures  [Show abstract] [Hide abstract]
ABSTRACT: A quantum dot interacting with two resonant cavity modes is described by a twomode JaynesCummings model. Depending on the quantum dot energy level scheme, the interaction of a singly doped quantum dot with a cavity photon generates entanglement of electron spin and cavity states or allows one to implement a SWAP gate for spin and photon states. An undoped quantum dot in the same structure generates pairs of polarization entangled photons from an initial photon product state. For realistic cavity loss rates, the fidelity of these operations is of order 80%. Comment: 6 pages, 4 figures; extended discussion of experimental implementation  [Show abstract] [Hide abstract]
ABSTRACT: Timeresolved Faraday rotation has recently demonstrated coherent transfer of electron spin between quantum dots coupled by conjugated molecules. Using a transfer Hamiltonian ansatz for the coupled quantum dots, we calculate the Faraday rotation signal as a function of the probe frequency in a pumpprobe setup using neutral quantum dots. Additionally, we study the signal of one spinpolarized excess electron in the coupled dots. We show that, in both cases, the Faraday rotation angle is determined by the spin transfer probabilities and the Heisenberg spin exchange energy. By comparison of our results with experimental data, we find that the transfer matrix element for electrons in the conduction band is of order 0.08 eV and the spin transfer probabilities are of order 10%. Comment: 13 pages, 6 figures; minor changes  [Show abstract] [Hide abstract]
ABSTRACT: We analyze transport of magnetization in insulating systems described by a spin Hamiltonian. The magnetization current through a quasionedimensional magnetic wire of finite length suspended between two bulk magnets is determined by the spin conductance which remains finite in the ballistic limit due to contact resistance. For ferromagnetic systems, magnetization transport can be viewed as transmission of magnons, and the spin conductance depends on the temperature T. For antiferromagnetic isotropic spin1/2 chains, the spin conductance is quantized in units of order (gmu(B))(2)/h at T=0. Magnetization currents produce an electric field and, hence, can be measured directly. For magnetization transport in electric fields, phenomena analogous to the Hall effect emerge.  [Show abstract] [Hide abstract]
ABSTRACT: Molecular magnetic clusters with antiferromagnetic exchange interaction and easyaxis anisotropy belong to the most promising candidate systems for the observation of coherent spin quantum tunneling on the mesoscopic scale. We point out that both nuclear magnetic resonance and electron spin resonance on doped rings are adequate experimental techniques for the detection of coherent spin quantum tunneling in antiferromagnetic molecular rings. Although challenging, the experiments are feasible with presentday techniques.  [Show abstract] [Hide abstract]
ABSTRACT: We show that a wide range of spin clusters with antiferromagnetic intracluster exchange interaction allows one to define a qubit. For these spin cluster qubits, initialization, quantum gate operation, and readout are possible using the same techniques as for single spins. Quantum gate operation for the spin cluster qubit does not require control over the intracluster exchange interaction. Electric and magnetic fields necessary to effect quantum gates need only be controlled on the length scale of the spin cluster rather than the scale for a single spin. Here, we calculate the energy gap separating the logical qubit states from the next excited state and the matrix elements which determine quantum gate operation times. We discuss spin cluster qubits formed by one and twodimensional arrays of s=1/2 spins as well as clusters formed by spins s>1/2. We illustrate the advantages of spin cluster qubits for various suggested implementations of spin qubits and analyze the scaling of decoherence time with spin cluster size. Comment: 15 pages, 7 figures; minor changes  [Show abstract] [Hide abstract]
ABSTRACT: We study the low energy states of finite spin chains with isotropic (Heisenberg) and anisotropic (XY and Isinglike) antiferromagnetic exchange interaction with uniform and nonuniform coupling constants. We show that for an odd number of sites a spin cluster qubit can be defined in terms of the ground state doublet. This qubit is remarkably insensitive to the placement and coupling anisotropy of spins within the cluster. One and twoqubit quantum gates can be generated by magnetic fields and intercluster exchange, and leakage during quantum gate operation is small. Spin cluster qubits inherit the long decoherence times and short gate operation times of single spins. Control of single spins is hence not necessary for the realization of universal quantum gates.  [Show abstract] [Hide abstract]
ABSTRACT: The detailed theoretical understanding of quantum spin dynamics in various molecular magnets is an important step on the roadway to technological applications of these systems. Quantum effects in both ferromagnetic and antiferromagnetic molecular clusters are, by now, theoretically well understood. Ferromagnetic molecular clusters allow one to study the interplay of incoherent quantum tunneling and thermally activated transitions between states with different spin orientation. The Berry phase oscillations found in Fe_8 are signatures of the quantum mechanical interference of different tunneling paths. Antiferromagnetic molecular clusters are promising candidates for the observation of coherent quantum tunneling on the mesoscopic scale. Although challenging, applications of molecular magnetic clusters for data storage and quantum data processing are within experimental reach already with present day technology.  [Show abstract] [Hide abstract]
ABSTRACT: We present detailed calculations of lowenergy spin dynamics in the ``ferric wheel'' systems Na:Fe_6 and Cs:Fe_8 in a magnetic field. We compute by exact diagonalisation the lowenergy spectra and matrix elements for totalspin and N'eelvector components, and thus the timedependent correlation functions of these operators. Comparison of our results with the semiclassical theory of coherent quantum tunnelling of the N'eel vector demonstrates the validity of a twostate description for the lowenergy dynamics of ferric wheels. We discuss the implications of our results for mesoscopic quantum coherent phenomena, and for the experimental techniques to observe them, in molecular magnetic rings.  [Show abstract] [Hide abstract]
ABSTRACT: We study theoretically the thermodynamic properties and spin dynamics of a class of magnetic rings closely related to ferric wheels, antiferromagnetic ring systems, in which one of the Fe (III) ions has been replaced by a dopant ion to create an excess spin. Using a coherentstate spin path integral formalism, we derive an effective action for the system in the presence of a magnetic field. We calculate the functional dependence of the magnetization and tunnel splitting on the magnetic field and show that the parameters of the spin Hamiltonian can be inferred from the magnetization curve. We study the spin dynamics in these systems and show that quantum tunneling of the Neel vector also results in tunneling of the total magnetization. Hence, the spin correlation function shows a signature of Neel vector tunneling, and electron spin resonance (ESR) techniques or AC susceptibility measurements can be used to measure both the tunneling and the decoherence rate. We compare our results with exact diagonalization studies on small ring systems. Our results can be easily generalized to a wide class of nanomagnets, such as ferritin. Comment: 15 pages, 5 figures  [Show abstract] [Hide abstract]
ABSTRACT: We study theoretically the spin dynamics of antiferromagnetic molecular rings, such as the ferric wheel Fe10. For a single nuclear or impurity spin coupled to one of the electron spins of the ring, we calculate nuclear and electronic spin correlation functions and show that nuclear magnetic resonance (NMR) and electron spin resonance (ESR) techniques can be used to detect coherent tunneling of the Néel vector in these rings. The location of the NMR/ESR resonances gives the tunnel splitting and its linewidth an upper bound on the decoherence rate of the electron spin dynamics. We illustrate the experimental feasibility of our proposal with estimates for Fe10 molecules.
Publication Stats
755  Citations  
76.31  Total Impact Points  
Top Journals
Institutions

20042006

University of California, Santa Barbara
 Department of Physics
Santa Barbara, California, United States


20012004

Universität Basel
 Department of Physics
Basel, BS, Switzerland
