Solid-state Quantum Systems Research Articles

Optically induced phase transition of excitons in coupled quantum dots

The weak classical light excitations in many semiconductor quantum dots have been chosen as important solid-state quantum systems for processing quantum information and implementing quantum computing. For strong classical light we predict theoretically a novel phase transition as a function of magnitude of this classical light from the deformed to the normal phases in resonance case, and the essential features of criticality such as the scaling behaviour, critical exponent and universality are also present in this paper.

Swap action in a solid-state controllable anisotropic Heisenberg model

Correct swap action can be realized via the control of the anisotropic Heisenberg interaction in solid-state quantum systems. The conditions of performing a swap are derived by the dynamics of arbitrary bipartite pure state. It is found that swap errors can be eliminated in the presence of symmetric anisotropy. In realistic quantum computers with unavoidable fluctuations, the gate fidelity of swap action is estimated. The scheme of quantum computation via the anisotropic Heisenberg interaction is implemented in a one-dimensional quantum dots. The slanting and static magnetic field can be used to adjust the anisotropy.

Microwave-Induced Cooling of a Superconducting Qubit

We demonstrated microwave-induced cooling in a superconducting flux qubit. The thermal population in the first-excited state of the qubit is driven to a higher-excited state by way of a sideband transition. Subsequent relaxation into the ground state results in cooling. Effective temperatures as low as approximately 3 millikelvin are achieved for bath temperatures of 30 to 400 millikelvin, a cooling factor between 10 and 100. This demonstration provides an analog to optical cooling of trapped ions and atoms and is generalizable to other solid-state quantum systems. Active cooling of qubits, applied to quantum information science, provides a means for qubit-state preparation with improved fidelity and for suppressing decoherence in multi-qubit systems.

Quantum effects in small-capacitance high temperature superconducting tunneling junctions

We investigated the effects of single electron charging energy in high temperature superconductors. Various phenomena originating from Coulomb blockade were observable in superconducting tunnel junctions. High quality tunneling junctions were fabricated from c-axis oriented NdBa2Cu3O7−δ∕PrBa2Cu3O7−δ∕NdBa2Cu3O7−δ thin film multilayers by the pulsed laser deposition method. The current-voltage characteristics (CVCs) show a Coulomb gap for Cooper pair tunneling when the charging energy exceeds the Josephson coupling energy. We found a regime in which the CVCs exhibit sharply defined Coulomb steps due to single electron dynamics and nonlinear tunneling rates. From the obtained Coulomb staircase, the tunneling resistance shows a quantum effect: It is modulated by the tunneling current in the form h∕4e2RT∼[sin(πI∕I0)2∕(πI∕I0)]. We suggest an interpretation involving the quantum resistance h∕e2 and the competition between the charging, Josephson, and thermal energies of the system. Our results give a perspective on a solid-state quantum system with considerable interest for direct application in quantum computing.

Near-field coherent spectroscopy and microscopy of a quantum dot system.

We combined coherent nonlinear optical spectroscopy with nano-electron volt energy resolution and low-temperature near-field microscopy with subwavelength resolution (<lambda/2) to provide direct and local access to the excitonic dipole in a semiconductor nanostructure quantum system. Our technique allows the ability to address, excite, and probe single eigenstates of solid-state quantum systems with spectral and spatial selectivity while simultaneously providing a measurement of all the various time scales of the excitation including state relaxation and decoherence rates. In analogy to scanning tunneling microscopy measurements, we can now map the optical local density of states of a disordered nanostructure. These measurements lay the groundwork for studying and exploiting spatial and temporal coherence in the nanoscopic regime of solid-state systems.

Potentialities for solid state maser excitation by dynamic high pressure

During rapid transitions of solid state quantum systems between different high pressure equilibrium states, deviations from thermodynamical equilibrium, like in an excited maser system, are presumed.