Łukasz Cywiński

Spin-based logic in semiconductors for reconfigurable large-scale circuits

Research in semiconductor spintronics aims to extend the scope of conventional electronics by using the spin degree of freedom of an electron in addition to its charge. Significant scientific advances in this area have been reported, such as the development of diluted ferromagnetic semiconductors, spin injection into semiconductors from ferromagnetic metals and discoveries of new physical phenomena involving electron spin. Yet no viable means of developing spintronics in semiconductors has been presented. Here we report a theoretical design that is a conceptual step forward—spin accumulation is used as the basis of a semiconductor computer circuit. Although the giant magnetoresistance effect in metals has already been commercially exploited, it does not extend to semiconductor/ferromagnet systems, because the effect is too weak for logic operations. We overcome this obstacle by using spin accumulation rather than spin flow. The basic element in our design is a logic gate that consists of a semiconductor structure with multiple magnetic contacts; this serves to perform fast and reprogrammable logic operations in a noisy, room-temperature environment. We then introduce a method to interconnect a large number of these gates to form a ‘spin computer’. As the shrinking of conventional complementary metal-oxide–semiconductor (CMOS) transistors reaches its intrinsic limit, greater computational capability will mean an increase in both circuit area and power dissipation. Our spin-based approach may provide wide margins for further scaling and also greater computational capability per gate.

How to enhance dephasing time in superconducting qubits

We theoretically investigate the influence of designed pulse sequences in restoring quantum coherence lost due to background noise in superconducting qubits. We consider both 1/f noise and Random Telegraph Noise, and show that the qubit coherence time can be substantially enhanced by carefully engineered pulse sequences. Conversely, the time dependence of qubit coherence under external pulse sequences could be used as a spectroscopic tool for extracting the noise mechanisms in superconducting qubits, i.e. by using Uhrigs pulse sequence one can obtain information about moments of the spectral density of noise. We also study the effect of pulse sequences on the evolution of the qubit affected by a strongly coupled fluctuator, and show that the non-Gaussian features in decoherence are suppressed by the application of pulses.

Electron Spin Dephasing due to Hyperfine Interactions with a Nuclear Spin Bath

We investigate pure dephasing decoherence (free induction decay and spin echo) of a spin qubit interacting with a nuclear spin bath. While for infinite magnetic field B the only decoherence mechanism is spectral diffusion due to dipolar flip-flops of nuclear spins, with decreasing B the hyperfine-mediated interactions between the nuclear spins become important. We give a theory of decoherence due to these interactions which takes advantage of their long-range nature. For a thermal uncorrelated bath we show that our theory is applicable down to B approximately 10 mT, allowing for comparison with recent experiments in GaAs quantum dots.

Ultrafast magneto-optics in ferromagnetic III–V semiconductors

We investigate various ultrafast optical processes in ferromagnetic (III,Mn)V semiconductors induced by femtosecond laser pulses. Two-colour timeresolved magneto-optical spectroscopy has been developed, which allows us to observe a rich array of dynamical phenomena. We isolate several distinct temporal regimes in spin dynamics, interpreting the fast (< 1p s) dynamics as spin heating through sp–d exchange interaction between photo-carriers and Mn ions while the ∼100 ps component is interpreted as a manifestation of spin– lattice relaxation. Charge carrier and phonon dynamics were also carefully studied, showing an ultrashort charge lifetime of photo-injected electrons (∼ 2p s) and propagating coherent acoustic phonon wavepackets with a strongly probe energy dependent oscillation period, amplitude and damping. (Some figures in this article are in colour only in the electronic version)

Pure quantum dephasing of a solid-state electron spin qubit in a large nuclear spin bath coupled by long-range hyperfine-mediated interactions

We investigate decoherence due to pure dephasing of a localized spin qubit interacting with a nuclear spin bath. Although in the limit of a very large magnetic field the only decoherence mechanism is spectral diffusion due to dipolar flip-flops of nuclear spins, with decreasing field the hyperfine-mediated interactions between the nuclear spins become important. We take advantage of their long-range nature and resum the leading terms in an

Exchange coupling in silicon quantum dots: Theoretical considerations for quantum computation

1/N

Quantum decoherence of the central spin in a sparse system of dipolar coupled spins

expansion of the decoherence time-evolution function (

Realizing singlet-triplet qubits in multivalley Si quantum dots

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Femtosecond demagnetization and hot-hole relaxation in ferromagneticGa1−xMnxAs

, being the number of nuclear spins interacting appreciably with the electron spin, is large). For the case of the thermal uncorrelated bath we show that our theory is applicable down to low magnetic fields (

Dynamical-decoupling noise spectroscopy at an optimal working point of a qubit

ensuremath{sim}10text{ }text{mT}