Suppose a single qubit is repeatedly prepared and evolved under imperfectly-controlled conditions. A drunk model represents uncontrolled interactions on each experimental trial as random or stochastic terms in the qubit's Hamiltonian operator. Time evolution of states is generated by a stochastic differential equation whose sample paths evolve according to the Schrödinger equations. For models with Gaussian white noise which is independent of the qubit's state, the expectation value of the solution obeys a master equation which is idential to the high-temperature limit of the Bloch equation. Drunk models predict that experimental data can appear consistent with decoherence even if qubit states evolve by unitary transformations. Examples are shown in which reversible evolution appears to cause irreversible information loss. This paradox is resolved by distinguishing between the true state of a system and the estimated state inferred from an experimental dataset.
Suppose a quantum experiment includes one or more random processes. Then the results of repeated measurements may appear consistent with irreversible decoherence even if the system's evolution prior to measurement is reversible and unitary. Two thought experiments are constructed as examples.
The Josephson junction phase qubit has been shown to be a viable candidate for quantum computation. In recent years, the two coupled phase system has been extensively studied theoretically and experimentally. We have analysed the quantum behavior of three and four capacitively-coupled phase qubits with different possible configurations, using a two-level system model. Energy levels and eigenstates have been calculated as a function of bias current and detuning. The properties of these simple networks are discussed.
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Thrailkill, Zechariah E., Samuel T. Kennerly, and Roberto C. Ramos. "Modeling Three and Four Coupled Phase Qubits." IEEE Transactions on Applied Superconductivity 19, no. 3 (2009): 968-972.
The superconducting Josephson junction has been demonstrated to be a strong candidate for building quantum bits or "qubits" which are the components of a future quantum computer. In recent years, considerable theoretical and experimental effort have been focused on studying quantum properties of single qubits and two coupled solid-state qubits. We present results of numerical simulations of the energy spectra of more three phase qubits that are capacitively-coupled in different configurations. We discuss the ensuing entanglement between component qubits as manifested in avoided crossings and how these may play a role in building gates and transmitting qubit state information.
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Thrailkill, Z., S. Kennerly, A. Tyler, and R. C. Ramos. "Energies and entanglement in multiply-coupled phase qubit systems." In Journal of Physics: Conference Series, vol. 150, no. 5, p. 052268. IOP Publishing, 2009.
