We propose a method for controlling the decay and decoherence of a driven qubit that is strongly coupled to a reservoir, when the qubit resonance frequency is close to a continuum edge of the reservoir spectrum. This strong-coupling regime is outside the scope of existing methods of decay or decoherence control. We demonstrate that an appropriate sequence of nearly abrupt changes of the resonance frequency can protect the qubit state from decay and decoherence more effectively than the intuitively obvious alternative, which is to fix the resonance well within a forbidden band gap of the reservoir spectrum, as far as possible from the continuum edge. The ``counterintuitive'' nonadiabatic method outlined here can outperform its adiabatic counterparts in maintaining a high fidelity of quantum logic operations. The remarkable effectiveness of the proposed method, which requires much lower rates of frequency changes than previously proposed control methods, is due to the ability of appropriately alternating detunings from the continuum edge to augment the interference of the emitted and backscattered quanta, thereby helping to stabilize the qubit state against decay. Applications to the control of decay and decoherence near the edge of radiative, vibrational, and photoionization continua are discussed.
A unified theory is given of dynamically modified decay and decoherence in driven two-level and multilevel quantum systems that are weakly coupled to arbitrary finite-temperature reservoirs and undergo random phase fluctuations. Criteria for the optimization of decoherence suppression and the limitations of this approach are obtained. For a driven qubit that is strongly coupled to the continuum edge of reservoir's spectrum, we demonstrate that only an appropriately ordered sequence of abrupt changes of the resonance frequency, near the continuum edge, can effectively protect the qubit state from decoherence.
Novel methods are discussed for the state control of atoms coupled to multi-mode reservoirs with non-Markovian spectra: 1) Excitation decay control : we point out that the quantum Zeno effect, i.e., inhibition of spontaneous decay by frequent measurements, is observable in open cavities and waveguides using a sequence of evolution- interrupting pulses or randomly-modulated CW fields. 2) Location-dependent interference of decay channels - nonadiabatic (resonant) control : We show that the control of populations and coherences of two metastable states is feasible via resonant single-photon absorption to an intermediate state, by controlled spontaneous emission in a cavity. 3) Decoherence control by conditionally interfering parallel evolutions: We demonstrate that an arbitrary internal atomic state can be completely protected from decoherence by interference of its interactions with the reservoir over many different time interals in parallel . Such interference is conditional upon the detection of appropriate atomic-momentum observables. Realization in cavities is suggested. The rich arsenal of control methods described above can improve the performance of single-atom devices. It can also advance the state-of-the-art of quantum information encoding and processing.
Preventing quantum entanglement from decoherence effect is of theoretical and practical importance in the quantum information processing technologies. In this regard, we consider the entanglement dynamics of two identical qubits where the qubits which are coupled to two independent (Markovian and/or non-Markovian) as well as a common reservoir at zero temperature are further interacted with a classical driving laser field. Then, we study the preservation of generated two-qubit entanglement in various situations using the concurrence measure. It is shown that by applying the classical driving field and so the possibility of controlling the Rabi frequency, the amount of entanglement of the two-qubit system is improved in the off-resonance condition between the qubit and the central cavity frequencies (central detuning) in both non-Markovian and Markovian reservoirs. While the central detuning has a constructive role, the detuning between the qubit and the classical field (laser detuning) affects negatively on the entanglement protection. The obtained results show that long-living entanglement in the non-Markovian reservoir is more accessible than in the Markovian reservoir. We demonstrate that, in a common reservoir non-zero stationary entanglement is achievable whenever the two-qubit system is coupled to the reservoir with appropriate values of relative coupling strengths.
We demonstrate the coherent control of spontaneous emission for a three-level atom located within a photonic band-gap structure with one resonant frequency near the edge of the photonic band gap. Spontaneous emission from the three-level atom can be totally suppressed or strongly enhanced depending on the relative phase between the steady-state control laser coupling the two upper levels and the pump laser pulse used to create an excited state of the atom in the form of a coherent superposition of the two upper levels. Unlike the free-space case, the steady-state inversion of the atomic system is strongly dependent on the externally prescribed initial conditions. This nonzero steady-state population is achieved by virtue of the localization of light in the vicinity of the emitting atom. It is robust to decoherence effects provided that the Rabi frequency of the control laser field-atom interaction exceeds the rate of dephasing interactions. As a result, such a system may be relevant for a single-atom, phase-sensitive, optical memory device on the atomic scale. The protected electric dipole within the photonic band gap provides a basis for a qubit to encode information for quantum computations.
We study the absorption spectrum of a weak probe field near resonant to an atomic transition, the upper level of which is strongly coupled to a third level by the interaction with a Lorentzian bosonic reservoir, such as, e.g., a mode of a high-
Q cavity or a local vibration in a solid. The reservoir coupling is approximated by the interaction with a classical complex Gaussian-Markovian random process (control field), which is justified when the reservoir temperature exceeds significantly the mode frequency or when the high-
Q cavity is pumped by broadband incoherent radiation. The present theory is applicable also when the control field is chaotic laser light. We assume that the rms control-field Rabi frequency
V0 is much greater than the field detuning ${\ensuremath{\Delta}}_{c}$, which, in turn, is much greater than the material relaxation constants. We reveal and describe analytically all qualitatively different regimes of the spectrum modification and obtain their validity conditions. The analytical results are verified by numerical calculations using the exact continued-fraction solution. The analytical formulas obtained allow one to perform fast computer calculations for arbitrarily small values of the reservoir (control-field) bandwidth $\ensuremath{\nu}$, in contrast to the known numerical methods, which require sharply increasing computational resources with a decrease of $\ensuremath{\nu}$. In the most interesting case $\ensuremath{\nu}⪡{V}_{0}$, the spectrum consists of two peaks, the nonvanishing bandwidth and material relaxation affecting mainly the dip between the peaks. The results obtained in the static limit (i.e., a very narrow reservoir) are independent of the reservoir band shape. We reveal reservoir-induced transparency (RIT)---i.e., absorption reduction due to the reservoir coupling. Moreover, two unexpected, remarkable features are uncovered in a range of intermediate values of $\ensuremath{\nu}$ and
V0, ${\ensuremath{\Gamma}}^{2}\ensuremath{\mid}{\ensuremath{\Delta}}_{c}\ensuremath{\mid}⪡{V}_{0}^{2}\ensuremath{\nu}⪡{\ensuremath{\mid}{\ensuremath{\Delta}}_{c}\ensuremath{\mid}}^{3}$ ($\ensuremath{\Gamma}$ is the spectral width in the absence of the control field): an extra peak in the dip and a resonance-absorption decrease by a half (i.e., an increase of RIT) relative to the static limit. The existence of the above spectral features depends on the statistical details of the control-field temporal behavior.
The fluxonium qubit is a promising platform for quantum operations due to its large anharmonicity and long coherence time. However, conventional resonant driving methods often require long operation times and complex implementation. As an alternative, nonadiabatic transitions induced by a time-dependent external flux ϕ ext ( t ) can enable faster and simpler control by exploiting quantum interference between multiple transition paths. Existing approaches typically impose periodicity or symmetry on ϕ ext ( t ) , which can limit control efficiency. We propose a design strategy for ϕ ext ( t ) that relaxes these constraints. We demonstrate the effectiveness of this approach in two key scenarios: population inversion and adiabatic passage. Numerical simulations show that our method achieves higher control efficiency than existing schemes, emphasizing the advantage of breaking periodicity and symmetry. Furthermore, by incorporating decoherence effects using the Lindblad master equation, we confirm that the proposed scheme remains robust under realistic conditions.