A trimodal resonator for three mutually perpendicular magnetic fields

We describe a trimodal resonator for the simultaneous delivery of three perpendicular magnetic fields. The resonator consists of a shielded loop single-mode radio frequency (rf) resonator placed inside of a bimodal waveguide microwave resonator. The microwave modes are nondegenerate, tunable over a range of 100 MHz, and have typical Q factors of 150 and 200. The rf mode is tunable over a range of 125 MHz, and has a typical Q of 45. Control of the relative phase between the three fields is demonstrated. The resonator will be used to drive three magnetic dipole transitions coherently between Zeeman states in the ground state of 87Rb.87Rb. © 1999 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71204/2/RSINAK-70-3-1780-1.pd

Measurement of relaxation between polarization eigenstates in single quantum dots

Low temperature relaxation of excitons between polarization eigenstates in single interface fluctuation quantum dots is studied using copolarized and cross-polarized transient differential transmission spectroscopy. The measured spin relaxation times are on the order of ∼100 ps. Such a spin relaxation time is longer than the reported times for thin quantum wells, but considerably shorter than the predicted times for interface fluctuation quantum dots. © 2002 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70166/2/APPLAB-81-22-4251-1.pd

Wavelength modulation spectroscopy of single quantum dots

We demonstrate that external cavity diode lasers with large mode-hop-free tuning ranges (up to 80 GHz) together with wavelength modulation spectroscopy can be used to study excitonic transitions in semiconductor nanostructures. Such transitions are characterized by homogeneous linewidths typically on the order of a few GHz. Wavelength modulation spectroscopy offers a high signal-to-noise method for the determination of resonance line shapes. We have used this technique to accurately measure dipole moments and dephasing rates of single semiconductor quantum dot eigenstates. These measurements are important for the use of quantum dots in semiconductor cavities and quantum logic gates, and for an improved understanding of the physics of exciton confinement. © 2002 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70029/2/APPLAB-80-11-1876-1.pd

Optimal quantum control in nanostructures: Theory and application to generic three-level system

Coherent carrier control in quantum nanostructures is studied within the framework of Optimal Control. We develop a general solution scheme for the optimization of an external control (e.g., lasers pulses), which allows to channel the system's wavefunction between two given states in its most efficient way; physically motivated constraints, such as limited laser resources or population suppression of certain states, can be accounted for through a general cost functional. Using a generic three-level scheme for the quantum system, we demonstrate the applicability of our approach and identify the pertinent calculation and convergence parameters.Comment: 7 pages; to appear in Phys. Rev.

Strong-field terahertz-optical mixing in excitons

Driving a double-quantum-well excitonic intersubband resonance with a terahertz (THz) electric field of frequency \omega_{THz} generated terahertz optical sidebands \omega=\omega_{THz}+\omega_{NIR} on a weak NIR probe. At high THz intensities, the intersubband dipole energy which coupled two excitons was comparable to the THz photon energy. In this strong-field regime the sideband intensity displayed a non-monotonic dependence on the THz field strength. The oscillating refractive index which gives rise to the sidebands may be understood by the formation of Floquet states, which oscillate with the same periodicity as the driving THz field.Comment: 4 pages, 6 figure

Universal quantum gates based on both geometric and dynamic phases in quantum dots

A large-scalable quantum computer model, whose qubits are represented by the subspace subtended by the ground state and the single exciton state on semiconductor quantum dots, is proposed. A universal set of quantum gates in this system may be achieved by a mixed approach, composed of dynamic evolution and nonadibatic geometric phase.Comment: 4 pages, to appear in Chin. Phys. Let

Spectrum of qubit oscillations from Bloch equations

We have developed a formalism suitable for calculation of the output spectrum of a detector continuously measuring quantum coherent oscillations in a solid-state qubit, starting from microscopic Bloch equations. The results coincide with that obtained using Bayesian and master equation approaches. The previous results are generalized to the cases of arbitrary detector response and finite detector temperature.Comment: 8 page

Theory of Fast Quantum Control of Exciton Dynamics in Semiconductor Quantum Dots

Optical techniques for the quantum control of the dynamics of multiexciton states in a semiconductor quantum dot are explored in theory. Composite bichromatic phase-locked pulses are shown to reduce the time of elementary quantum operations on excitons and biexcitons by an order of magnitude or more. Analytic and numerical methods of designing the pulse sequences are investigated. Fidelity of the operation is used to gauge its quality. A modified Quantum Fourier Transform algorithm is constructed with only Rabi rotations and is shown to reduce the number of operations. Application of the designed pulses to the algorithm is tested by a numerical simulation.Comment: 11 pages,5 figure

Size-dependent decoherence of excitonic states in semiconductor microcrystallites

The size-dependent decoherence of the exciton states resulting from the spontaneous emission is investigated in a semiconductor spherical microcrystallite under condition $a_{B}\ll R_{0}\leq\lambda$. In general, the larger size of the microcrystallite corresponds to the shorter coherence time. If the initial state is a superposition of two different excitonic coherent states, the coherence time depends on both the overlap of two excitonic coherent states and the size of the microcrystallite. When the system with fixed size is initially in the even or odd coherent states, the larger average number of the excitons corresponds to the faster decoherence. When the average number of the excitons is given, the bigger size of the microcrystallite corresponds to the faster decoherence. The decoherence of the exciton states for the materials GaAs and CdS is numerically studied by our theoretical analysis.Comment: 4 pages, two figure

Spin-based all-optical quantum computation with quantum dots: understanding and suppressing decoherence

We present an all-optical implementation of quantum computation using semiconductor quantum dots. Quantum memory is represented by the spin of an excess electron stored in each dot. Two-qubit gates are realized by switching on trion-trion interactions between different dots. State selectivity is achieved via conditional laser excitation exploiting Pauli exclusion principle. Read-out is performed via a quantum-jump technique. We analyze the effect on our scheme's performance of the main imperfections present in real quantum dots: exciton decay, hole mixing and phonon decoherence. We introduce an adiabatic gate procedure that allows one to circumvent these effects, and evaluate quantitatively its fidelity