Jerzy Dajka

Distance growth of quantum states due to initial system-environment correlations

Intriguing features of the distance between two arbitrary states of an open quantum system are identified that are induced by initial system-environment correlations. As an example, we analyze a qubit dephasingly coupled to a bosonic environment. Within tailored parameter regimes, initial correlations are shown to substantially increase the distance between two qubit states evolving to long-time-limit states according to exact non-Markovian dynamics. It exemplifies the breakdown of the distance contractivity of the reduced dynamics.

Flux qubit on a mesoscopic nonsuperconducting ring

The possibility of making a flux qubit on a nonsuperconducting mesoscopic ballistic quasi-one-dimensional ring is discussed. We showed that such a ring can be effectively reduced to a two-state system with two external control parameters. The two states carry opposite persistent currents and are coupled by tunneling, which leads to a quantum superposition of states. The qubit states can be manipulated by resonant microwave pulses. The flux state of the sample can be measured by a superconducting quantum interference device magnetometer. Two or more qubits can be coupled by the flux the circulating currents generate. The problem of decoherence is also discussed.

New symmetry in the Rabi model

It is recognized that, apart from the total energy conservation, there is a nonlocal and a somewhat hidden symmetry in this model. Conditions for the existence of this observable, its form and its explicit construction are presented.

Geometric phase as a determinant of a qubit--- environment coupling

We investigate the qubit geometric phase and its properties in dependence on the mechanism for decoherence of a qubit weakly coupled to its environment. We consider two sources of decoherence: dephasing coupling (without exchange of energy with environment) and dissipative coupling (with exchange of energy). Reduced dynamics of the qubit is studied in terms of the rigorous Davies Markovian quantum master equation, both at zero and non–zero temperature. For pure dephasing coupling, the geometric phase varies monotonically with respect to the polar angle (in the Bloch sphere representation) parameterizing an initial state of the qubit. Moreover, it is antisymmetric about some points on the geometric phase-polar angle plane. This is in distinct contrast to the case of dissipative coupling for which the variation of the geometric phase with respect to the polar angle typically is non-monotonic, displaying local extrema and is not antisymmetric. Sensitivity of the geometric phase to details of the decoherence source can make it a tool for testing the nature of the qubit–environment interaction.

Negativity and quantum discord in Davies environments

We investigate the time evolution of negativity and quantum discord for a pair of non-interacting qubits with one being weakly coupled to a decohering Davies-type Markovian environment. At initial time of preparation, the qubits are prepared in one of the maximally entangled pure Bell states. In the limiting case of pure decoherence (i.e. pure dephasing), both the quantum discord and negativity decay to zero in the long time limit. In the presence of a manifest dissipative dynamics, the entanglement negativity undergoes a sudden death at finite time, while the quantum discord relaxes continuously to zero with increasing time. We find that in dephasing environments, the decay of the negativity is more propitious with increasing time; in contrast, the evolving decay of the quantum discord proceeds more weakly for dissipative environments. Particularly, the slowest decay of the quantum discord emerges when the energy relaxation time matches the dephasing time.

Entanglement persistence in contact with the environment: exact results

The evolution of quantum entanglement of a pair of non-interacting qubits is studied for the case where one of them is non-dissipatively but dephasingly coupled to the environment. The reduced non-Markovian dynamics of the qubits is exact for an arbitrary strength of coupling to the environment and the arbitrary frequency spectrum of environment fluctuations. While for the subohmic and ohmic environments the entanglement diminishes, for the superohmic zero-temperature environment it survives for a long time.

Dephasing of qubits by the Schrodinger cat

We study the dephasing of a single qubit coupled to a bosonic bath. In particular, we investigate the case when the bath is initially prepared in a pure state known as the Schrodinger cat. In clear contradistinction to the time evolution of an initial coherent state, the time evolutions of the purity and the coherence factor now depend on the particular choice of the Schrodinger cat state. We also demonstrate that the evolution of the entanglement of a two-qubit system depends on the initial conditions in a similar way.

Geometric phase of a qubit in dephasing environments

The geometric phase of a qubit coupled dephasingly to ohmic or non-ohmic bosonic environment exhibits a strong dependence on both an initial state of the system and spectral properties of the environment. Analysing the exact reduced dynamics we show that the geometric phase can be more easily maintained for the subohmic and ohmic environments than for the superohmic environment. We show the existence of the initial states for which the geometric phase is robust against coupling to the environment.

Bifurcations of the geometric phase of a qubit asymmetrically coupled to the environment

The geometric phase of a qubit asymmetrically coupled to the outer bosonic environment is studied. We demonstrate that with the change of the coupling asymmetry, the geometric phase can exhibit a cascade of bifurcations and therefore can be useful for testing the asymmetry coupling.

Temperature-independent teleportation of qubits in Davies environments

A quantum channel teleporting qubits from Alice to Bob is considered in which the bipartite resource state shared by Alice and Bob is affected by bilocal thermal noise treated in rigorous Davis approximation. Specific conditions are identified where the fidelity of the teleportation channel can be either independent of temperature or even enhanced by the temperature of the environment.