Showing papers by "Jason R. Petta published in 2005"

Coherent manipulation of coupled electron spins in semiconductor quantum dots.

Abstract: We demonstrated coherent control of a quantum two-level system based on two-electron spin states in a double quantum dot, allowing state preparation, coherent manipulation, and projective readout. These techniques are based on rapid electrical control of the exchange interaction. Separating and later recombining a singlet spin state provided a measurement of the spin dephasing time, T2*, of E10 nanoseconds, limited by hyperfine interactions with the gallium arsenide host nuclei. Rabi oscillations of two-electron spin states were demonstrated, and spin-echo pulse sequences were used to suppress hyperfine-induced dephasing. Using these quantum control techniques, a coherence time for two-electron spin states exceeding 1 microsecond was observed.

Triplet-singlet spin relaxation via nuclei in a double quantum dot.

Abstract: The GaAs double quantum dot is the classic spin qubit widely studied for its potential as information carrier in quantum computers. The discovery that electron spin flips in this system are governed by nuclear interactions, and slowed dramatically by a weak magnetic field, is promising in terms of the control and manipulation of spin-based memory. The spin of a confined electron, when oriented originally in some direction, will lose memory of that orientation after some time. Physical mechanisms leading to this relaxation of spin memory typically involve either coupling of the electron spin to its orbital motion or to nuclear spins1,2,3,4,5,6,7. Relaxation of confined electron spin has been previously measured only for Zeeman or exchange split spin states, where spin-orbit effects dominate relaxation8,9,10; spin flips due to nuclei have been observed in optical spectroscopy studies11. Using an isolated GaAs double quantum dot defined by electrostatic gates and direct time domain measurements, we investigate in detail spin relaxation for arbitrary splitting of spin states. Here we show that electron spin flips are dominated by nuclear interactions and are slowed by several orders of magnitude when a magnetic field of a few millitesla is applied. These results have significant implications for spin-based information processing12.

Singlet-triplet spin blockade and charge sensing in a few-electron double quantum dot

Abstract: Singlet-triplet spin blockade in a few-electron lateral double quantum dot is investigated using simultaneous transport and charge-sensing measurements. Transport from the (1,1) to the (0,2) electron occupancy states is strongly suppressed relative to the opposite bias [(0,2)--(1,1)]. At large bias, spin blockade ceases as the (0,2) triplet state enters the transport window, giving a direct measure of exchange splitting of the (0,2) state as a function of magnetic field. A simple model for current and steady-state charge distribution in spin-blockade conditions is developed and found to be in excellent agreement with experiment. Three other transitions [(1,1)--(2,0), (1,3)--(2,2), and (1,3)--(0,4)] exhibit spin blockade while other nearby transitions and opposite bias configurations do not, consistent with simple even-odd shell filling.

Pulsed-gate measurements of the singlet-triplet relaxation time in a two-electron double quantum dot

Abstract: A pulsed-gate technique with charge sensing is used to measure the singlet-triplet relaxation time for nearly degenerate spin states in a two-electron double quantum dot. Transitions from the (1,1) charge occupancy state to the (0,2) state, measured as a function of pulse cycle duration and magnetic field, allow the (1,1) singlet-triplet relaxation time $(\ensuremath{\geqslant}70\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s})$ and the (0,2) singlet-triplet splitting to be measured. This technique can be readily applied to read out a spin-qubit operating in a singlet-triplet basis.

Pulsed-gate measurements of the singlet-triplet relaxation time in a two-electron double quantum dot

Abstract: A pulsed-gate technique with charge sensing is used to measure the singlet-triplet relaxation time for nearly degenerate spin states in a two-electron double quantum dot. Transitions from the 1,1 charge occupancy state to the 0,2 state, measured as a function of pulse cycle duration and magnetic field, allow the 1,1 singlettriplet relaxation time 70 s and the 0,2 singlet-triplet splitting to be measured. This technique can be readily applied to read out a spin-qubit operating in a singlet-triplet basis.

Relaxation of Single Electron Spins by Nuclei in a Double Quantum Dot

Abstract: The spin of a confined electron, when oriented originally in some direction, will lose memory of that orientation after some time. Physical mechanisms leading to this relaxation of spin memory typically involve either coupling of the electron spin to its orbital motion or to nuclear spins. Relaxation of confined electron spin has been previously measured only for Zeeman or exchange split spin states, where spin-orbit effects dominate relaxation, while relaxation due to nuclei has been observed in optical spectroscopy studies. Using an isolated GaAs double quantum dot defined by electrostatic gates and direct time domain measurements, we investigate in detail spin relaxation for arbitrary splitting of spin states. Results demonstrate that the relaxation time is dominated by nuclear interactions and increases by several orders of magnitude when a magnetic field of a few millitesla is applied. These results have significant implications for spin-based information processing.