Active suppression of dephasing in Josephson-junction qubits

Simple majority code correcting k dephasing errors by encoding a qubit of information into 2k+1 physical qubits is studied quantitatively. We derive an equation for quasicontinuous evolution of the density matrix of encoded quantum information under the error correction procedure in the presence of correlated dephasing noise. A specific design of a Josephson-junction nanocircuit implementing this scheme is suggested.     

Decoherence of quantum information of qubits by stochastic dephasing

Decoherence of purity, distinguishability and entanglement of qubit states is investigated under the influence of stochastic dephasing which obeys the Gauss–Markov process and the two-state jump Markov process. The quantum teleportation and quantum dense coding under the influence of stochastic dephasing are also discussed.     

Spin-orbit mediated control of spin qubits

We propose to use the spin-orbit interaction as a means to control electron spins in quantum dots, enabling both single-qubit and two-qubit operations. Very fast single-qubit operations may be achieved by temporarily displacing the electrons. For two-qubit operations the coupling mechanism is based on a combination of the spin-orbit coupling and the mutual long-ranged Coulomb interaction. Compared to existing schemes using the exchange coupling, the spin-orbit induced coupling is less sensitive to random electrical fluctuations in the electrodes defining the quantum dots.     

Geometrical spin dephasing in quantum dots

We study spin-orbit mediated relaxation and dephasing of electron spins in quantum dots. We show that higher order contributions provide a relaxation mechanism that dominates for low magnetic fields and is of geometrical origin. In the low-field limit relaxation is dominated by coupling to electron-hole excitations and possibly 1/f noise rather than phonons.     

Quantum computing with spin cluster qubits

We study the low energy states of finite spin chains with isotropic (Heisenberg) and anisotropic (XY and Ising-like) 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 two-qubit 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.     

Spiraling spin structure in an exchange-coupled antiferromagnetic layer

Using trilayers of permalloy/FeMn/Co with various thicknesses t(AF) of the antiferromagnetic FeMn, we have observed evidence of a spiraling spin structure within FeMn. For t(AF)<90 A, the turn angle straight theta of the spiral varies as straight theta = (1.76 degrees /A)t(AF).     

Imaging the magnetic spin structure of exchange-coupled thin films

We have investigated the magnetic spin structure of a soft-magnetic film that is exchange-coupled to a hard-magnetic layer to form an exchange-spring layer system. The depth dependence of the magnetization direction was determined by nuclear resonant scattering of synchrotron radiation from ultrathin 57Fe probe layers. In an external field a magnetic spiral structure forms that can be described within a one-dimensional micromagnetical model. The experimental method allows one to image vertical spin structures in stratified media with unprecedented accuracy.     

Scaling of dynamical decoupling for spin qubits

We investigate the scaling of coherence time T(2) with the number of π pulses n(π) in a singlet-triplet spin qubit using Carr-Purcell-Meiboom-Gill (CPMG) and concatenated dynamical decoupling (CDD) pulse sequences. For an even numbers of CPMG pulses, we find a power law T(2) is proportional to (n(π))(γ(e)), with γ(e)=0.72±0.01, essentially independent of the envelope function used to extract T(2). From this surprisingly robust value, a power-law model of the noise spectrum of the environment, S(ω)~ω(-β), yields β=γ(e)/(1-γ(e))=2.6±0.1. Model values for T(2)(n(π)) using β=2.6 for CPMG with both even and odd n(π) up to 32 and CDD orders 3 through 6 compare very well with the experiment.     

The dynamics of quantum correlation with two controlled qubits under classical dephasing environment

We discuss the quantum correlation dynamics of two qubits controlled through the application of π$\pi$-pulses under classical dephasing non-Markovian environment. It is shown that the quantum discord (QD) and one-norm geometric quantum discord (one-norm GQD) between the two qubits, which are prepared initially in the three-parameter-X$X$-type quantum states, depend strongly on non-Markovian properties and the time difference between adjacent pulses. The freezing time of discord and one-norm GQD can be lengthened for appropriate pulse separate time and pulse numbers. And the freezing time of one-norm GQD is longer slightly than QD for both Markovian and non-Markovian cases. What is more, we find that double sudden changes of one-norm GQD can appear only for some initial parameters when π$\pi$-pulses are applied.     

Factorization law for entanglement evolution of two qubits in non-Markovian pure dephasing channels

The entanglement evolution of two qubits in local, two-sided non-Markovian pure dephasing channels is investigated. It is found that for the two-sided pure dephasing channel case, when the qubits are initially prepared in a general class of states, whether pure or mixed, the entanglement can be expressed as the products of initial entanglement and the channels? action on the maximally entangled state. This provide us a good approximation to characterize the entanglement dynamics of arbitrary states to some extent.     

Measurement efficiency and n-shot readout of spin qubits

We consider electron spin qubits in quantum dots and define a measurement efficiency e to characterize reliable measurements via n-shot readouts. We propose various implementations based on a double dot and a quantum point contact (QPC) and show that the associated efficiencies e vary between 50% and 100%, allowing single-shot readout in the latter case. We model the readout microscopically and derive its time dynamics in terms of a generalized master equation, calculate the QPC current, and show that it allows spin readout under realistic conditions.     

Multiple-pulse coherence enhancement of solid state spin qubits

We describe how the spin coherence time of a localized electron spin in solids, i.e., a solid state spin qubit, can be prolonged by applying designed electron spin resonance pulse sequences. In particular, the spin echo decay due to the spectral diffusion of the electron spin resonance frequency induced by the non-Markovian temporal fluctuations of the nuclear spin flip-flop dynamics can be strongly suppressed using multiple-pulse sequences akin to the Carr-Purcell-Meiboom-Gill pulse sequence in nuclear magnetic resonance. Spin coherence time can be enhanced by factors of 4-10 in GaAs quantum-dot and Si:P quantum computer architectures using composite sequences with an even number of pulses.     

Chirality in quantum computation with spin cluster qubits

We study corrections to the Heisenberg interaction between several lateral, single-electron quantum dots. We show, using exact diagonalization, that three-body chiral terms couple triangular configurations to external sources of flux rather strongly. The chiral corrections impact single-qubit encodings utilizing loops of three or more Heisenberg coupled quantum dots.     

Universal pulse sequence to minimize spin dephasing in the central spin decoherence problem

We present a remarkable finding that a recently discovered [G. S. Uhrig, Phys. Rev. Lett. 98, 100504 (2007)10.1103/PhysRevLett.98.100504] series of pulse sequences, designed to optimally restore coherence to a qubit in the spin-boson model of decoherence, is in fact completely model independent and generically valid for arbitrary dephasing Hamiltonians given sufficiently short delay times between pulses. The series maximizes qubit fidelity versus number of applied pulses for sufficiently short delay times because the series, with each additional pulse, cancels successive orders of a time expansion for the fidelity decay. The "magical" universality of this property, which was not appreciated earlier, requires that a linearly growing set of "unknowns" (the delay times) must simultaneously satisfy an exponentially growing set of nonlinear equations that involve arbitrary dephasing Hamiltonian operators.     

Effective spin dephasing mechanism in confined two-dimensional topological insulators

A Kramers pair of helical edge states in quantum spin Hall effect (QSHE) is robust against normal dephasing but not robust to spin dephasing. In our work, we provide an effective spin dephasing mechanism in the puddles of two-dimensional (2D) QSHE, which is simulated as quantum dots modeled by 2D massive Dirac Hamiltonian. We demonstrate that the spin dephasing effect can originate from the combination of the Rashba spin-orbit coupling and electron-phonon interaction, which gives rise to inelastic backscattering in edge states within the topological insulator quantum dots, although the time-reversal symmetry is preserved throughout. Finally, we discuss the tunneling between extended helical edge states and local edge states in the QSH quantum dots, which leads to backscattering in the extended edge states. These results can explain the more robust edge transport in InAs/GaSb QSH systems.     

Quantum computation with quantum-dot spin qubits inside a cavity

Universal set of quantum gates are realized from quantum-dot spin qubits inside a cavity via two-channel Raman interactions. Individual addressing and effective switch of the cavity mediated interaction are directly possible here. This simple realization of all wanted interaction for selective qubits makes current scenario more suitable for scalable quantum computation.     

Graphene antidot lattices: designed defects and spin qubits

Antidot lattices, defined on a two-dimensional electron gas at a semiconductor heterostructure, are a well-studied class of man-made structures with intriguing physical properties. We point out that a closely related system, graphene sheets with regularly spaced holes ("antidots"), should display similar phenomenology, but within a much more favorable energy scale, a consequence of the Dirac fermion nature of the states around the Fermi level. Further, by leaving out some of the holes one can create defect states, or pairs of coupled defect states, which can function as hosts for electron spin qubits. We present a detailed study of the energetics of periodic graphene antidot lattices, analyze the level structure of a single defect, calculate the exchange coupling between a pair of spin qubits, and identify possible avenues for further developments.     

Quantum information transfer between photonic and quantum-dot spin qubits

We propose schemes for the efficient information transfer between a propagating photon and a quantum-dot(QD) spin qubit in an optical microcavity that have no auxiliary particles required. With these methods, the information transfer between two photons or two QD spins can also be achieved. All of our proposals can work with high fidelity, even with a high leakage rate. What is more, each information transfer process above can also be seen as a controlled-NOT(CNOT) operation. It is found that the information transfer can be equivalent to a CNOT gate. These proposals will promote more efficient quantum information networks and quantum computation.     

A novel mechanism for spin dephasing due to spin-conserving scatterings

A new spin dephasing mechanism is proposed for semiconductors with carrier momentum-dependent transition energies (inhomogeneous broadening) between spin states. In the presence of this inhomogeneous broadening of the spin transitions, spin-conserving (SC) scatterings lead to irreversible spin dephasing in a complete analogy to the optical dephasing of inhomogeneously broadened optical transitions. This phenomenon is demonstrated for the case when the g-factor becomes electron-energy dependent. Received 29 June 2000