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N. V. Vitanov

Publications and source records attributed to N. V. Vitanov.

At least 19 recordsLinked to original sources
arXiv2026

Fault-tolerant hyper-Ramsey spectroscopy of optical clock transitions with dynamical decoupling

A probe laser field that interrogates an atomic transition also shifts it. Although Hyper-Ramsey (HR) spectroscopy was developed to mitigate these light-induced frequency shifts, the technique remains sensitive to probe intensity fluctuations. In this work, we introduce a novel class of dynamically decoupled hyper-Ramsey (DDHR) sequences that utilize a modified refocusing pulse to effectively eliminate this residual sensitivity. This three-pulse interferometric protocol significantly enhances the contrast of quantum interference fringes while providing robust immunity against probe-induced frequency shifts, even in the presence of external field inhomogeneities. We experimentally validate both HR and DDHR protocols on the NQCH and \textcolor{blue}{IQM} superconducting quantum processor, demonstrating an error scaling that is rigorously consistent with theoretical simulations of the optical clock regime. DDHR spectroscopy yields superior suppression of residual probe-induced frequency shifts compared to the standard HR scheme under probe amplitude fluctuation and decoherence. Furthermore, we demonstrate that the implementation of composite refocusing pulses, derived from a Pascal binomial tree architecture, gives substantial resilience against technical pulse area imperfections. Ultimately, fault-tolerant, dynamically decoupled hyper-clocks will offer a promising experimental platform for the evaluation of advanced, NMR-inspired DDHR sequences of multiple refocusing pulses holding significant potential for high-precision matter/antimatter quantum sensing and rigorous tests of fundamental physics within harsh electromagnetic environments.

physics.atom-ph↗
arXiv2026

Level crossings and superradiant quantum phase transition for a two-qutrit quantum Rabi model

A two-qutrit extension of the quantum Rabi model is studied. Despite its increased complexity, the model results to be integrable under specific, physically relevant conditions. This feature allows for the emergence of analytically tractable subdynamics. In this framework, the ground-state phase diagram can be derived, and the analysis reveals critical phenomena linked to both level crossings and quantum phase transitions.

quant-ph↗
arXiv2026

Decoupling of the STIRAP and Microwave-Dressing paths in Trapped Rydberg Ion Gates

The strong dipole-dipole interaction of trapped Rydberg ions offers the possibility of sub-microsecond entanglement gates. For example a two-qubit Control-Phase gate in 88 Sr + ions can be realized, by simultaneous excitation to the Rydberg states via stimulated Raman adiabatic passage (STIRAP) with simultaneous microwave induced dipole-dipole interaction. We show that this excitation protocol distorts the dark-state of the STIRAP stage and is prone to decay from the intermediate state. Here, we propose a novel pulse ordering, in which the STIRAP and the microwave dressing of the Rydberg states occurs in separate stages, preventing mutual interference effects that are detrimental to the gate fidelity. We show that, for experimentally feasible parameters, the proposed excitation scheme can achieve a fidelity of 99.93%, surpassing the experimentally demonstrated gate. In addition, we demonstrate a non-adiabatic speed-up to 400 ns by employing asymmetric pulse shapes in the STIRAP stage. The entangling phase is then controlled solely through the interaction strength by nonresonant asymmetric chirping of the microwave field.

quant-ph↗
arXiv2025

Composite Mølmer-Sørensen gate

The Mølmer-Sørensen (MS) gate is a two-qubit controlled-phase gate in ion traps that is highly valued due to its ability to preserve the motional state of the ions. However, its fidelity is obstructed by errors affecting the motion of the ions as well as the rotation of the qubits. In this work, we propose an amplitude-modulated composite MS gate which features high fidelity robust to gate timing, detuning and coupling errors and is also tolerant of a.c. Stark shifts and drifting detuning errors.

quant-ph↗
arXiv2022

Controlled Quantized Adiabatic Transport in a superlattice Wannier-Stark ladder

The Born-Fock theorem is one of the most fundamental theorems of quantum mechanics and forms the basis for reliable and efficient navigation in the Hilbert space of a quantum system with a time-dependent Hamiltonian by adiabatic evolution. In the absence of level crossings, i.e. without degeneracies, and under adiabatic time evolution all eigenstates of the Hamiltonian keep their energetic order, labelled by a conserved integer quantum number. Thus controlling the eigenstates of the Hamiltonian and their energetic order in asymptotic limits allows to engineer a perfect adiabatic transfer between a large number of initial and target states. The fidelity of the state transfer is only limited by adiabaticity and the selection of target states is controlled by the integer invariant labelling the order of eigenstates. We here show for the example of a finite superlattice Wannier-Stark ladder, i.e. a one-dimensional lattice with alternating hopping amplitudes and constant potential gradient, that such an adiabatic control of eigenstates can be used to induce perfectly quantized single-particle transport across a pre-determined number of lattice sites. We dedicate this paper to the memory of our late friend and colleague Bruce Shore, who was an expert in adiabatic processes and taught us much about this field.

quant-ph↗
arXiv2022

Extension of the Morris-Shore transformation to arbitrary time-dependent driving fields

The treatment of time-dependent dynamics of quantum systems involving multiple states poses considerable technical challenges. One of the most efficient approaches in treating such systems is the Morris-Shore (MS) transformation which decomposes the multistate dynamics to a set of independent systems of simpler interaction pattern and uncoupled spectator states. The standard MS transformation imposes restrictions on the time dependence of the external fields addressing the states, as it requires that both Rabi frequencies have the same time profile. In this work we treat the case of the time-dependent MS transformation, which opens prospects for a variety of physically interesting processes wherein the fields may have different time dependences. We explore the adiabatic and the double-adiabatic limit, in which we demonstrate population transfer between the MS states that results in population transfer from one set of states onto another. We demonstrate the generation of superposition states between the MS states by the techniques of half adiabatic passage and fractional stimulated Raman adiabatic passage, which translate to superpositions of all the states of the involved levels.

quant-ph↗
arXiv2021

Numerical optimization of amplitude-modulated pulses in microwave-driven entanglement generation

Microwave control of trapped ions can provide an implementation of high-fidelity two-qubit gates free from errors induced by photon scattering. Furthermore, microwave conductors may be embedded into a scalable trap structure, providing the chip-level integration of control that is desirable for scaling. Recent developments have demonstrated how amplitude modulation of the gate drive can permit a two-qubit entangling operation to become robust against motional mode noise and other experimental imperfections. Here, we discuss a method for the numerical optimization of the microwave pulse envelope to produce gate pulses with improved resilience, faster operation and higher energy efficiency.

quant-ph↗
arXiv2021

Chiral resolution based on laser-induced continuum structure

In this paper we propose a chiral resolution technique based on laser-induced continuum structure (LICS). We show that the two enantiomers can have a different ionization profile based on implemented LICS excitation. We treat a cyclic excitation between two bound states and a continuum state and show how asymmetric ionization can be achieved based on different trapping of the enantiomers due to the additional coupling between the bound states. Alternatively, when multiple states are involved in the interaction we investigate a multilevel LICS strategy of asymmetric ionization based on dark-bright state trapping. In this latter case one of the enantiomers is trapped in a dark state, which is immune to ionization, while the other is driven in a bright state, from which the population leaks into the continuum.

quant-ph↗
arXiv2021

Multilevel Laser Induced Continuum Structure

Laser-induced-continuum-structure(LICS) allows for coherent control techniques to be applied in a Raman type system with intermediate continuum state. To this end the standard LICS problem treats two bound states coupled to one or more continua states. In this paper we discuss the simplest non-trivial multilevel generalization of LICS coupling two bound levels, each composed of two degenerate states through a common continuum state. We reduce the complexity of the effective system of bound states by switching to a rotated basis in which different sub-systems of lower dimension evolve independently. We derive a trapping condition and explore the dynamics of the sub-systems under different initial conditions.

quant-ph↗
arXiv2021

Morris-Shore transformation for non-degenerate systems

The Morris-Shore (MS) transformation is a powerful tool for decomposition of the dynamics of multistate quantum systems to a set of two-state systems and uncoupled single states. It assumes two sets of states wherein any state in the first set can be coupled to any state in the second set but the states within each set are not coupled between themselves. Another important condition is the degeneracy of the states in each set, although all couplings between the states from different sets can be detuned from resonance by the same detuning. The degeneracy condition limits the application of the MS transformation in various physically interesting situations, e.g. in the presence of electric and/or magnetic fields or light shifts, which lift the degeneracy in each set of states, e.g. when these sets comprise the magnetic sublevels of levels with nonzero angular momentum. This paper extends the MS transformation to such situations, in which the states in each of the two sets are nondegenerate. To this end, we develop an alternative way for the derivation of Morris-Shore transformation, which can be applied to non-degenerate sets of states. We present a generalized eigenvalue approach, by which, in the limit of small detunings from degeneracy, we are able to generate an effective Hamiltonian that is dynamically equivalent to the non-degenerate Hamiltonian. The effective Hamiltonian can be mapped to the Morris-Shore basis with a two-step similarity transformation. After the derivation of the general framework, we demonstrate the application of this technique to the popular Lambda three-state system, and the four-state tripod, double-Lambda and diamond systems. In all of these systems, our formalism allows us to reduce their quantum dynamics to simpler two-state systems even in the presence of various detunings, e.g. generated by external fields of frequency drifts.

quant-ph↗
arXiv2020

Two-qubit entanglement generation through non-Hermitian Hamiltonians induced by repeated measurements on an ancilla

In contrast to classical systems, actual implementation of non-Hermitian Hamiltonian dynamics for quantum systems is a challenge because the processes of energy gain and dissipation are based on the underlying Hermitian system-environment dynamics that is trace preserving. Recently, a scheme for engineering non-Hermitian Hamiltonians as a result of repetitive measurements on an anicillary qubit has been proposed. The induced conditional dynamics of the main system is described by the effective non-Hermitian Hamiltonian arisng from the procedure. In this paper we demonstrate the effectiveness of such a protocol by applying it to physically relevant multi-spin models, showing that the effective non-Hermitian Hamiltonian drives the system to a maximally entangled stationary state. In addition, we report a new recipe to construct a physical scenario where the quantum dynamics of a physical system represented by a given non-Hermitian Hamiltonian model may be simulated. The physical implications and the broad scope potential applications of such a scheme are highlighted.

quant-ph↗
arXiv2019

Landau-Majorana-Stuckelberg-Zener dynamics driven by coupling for two interacting qutrit systems

A time-dependent two interacting spin-qutrit model is analysed and solved. The two interacting qutrits are subjected to a longitudinal field linearly varying over time as in the Landau-Majorana-Stückelberg-Zener (LMSZ) scenario. Although a transverse field is absent, we show the occurrence of LMSZ transitions assisted by the coupling between the two spin-qutrits. Such a physical effects permits to estimate experimentally the coupling strength between the spins and allows the generation of entangled states of the two qutrits by appropriately setting the slope of the ramp. Furthermore, the possibility of local and non-local control as well as the existence of dark states of the two qutrits have been brought to light. Effects stemming from a noisy surrounding environment are also taken into account by introducing a random fluctuating field component as well as non-Hermitian terms in the Hamiltonian model.

quant-ph↗
arXiv2019

Coupling-assisted Landau-Majorana-Stuckelberg-Zener transition in two-interacting-qubit systems

We analyse a system of two interacting spin-qubits subjected to a Landau-Majorana-Stückelberg-Zener (LMSZ) ramp. We prove that LMSZ transitions of the two spin-qubits are possible without an external transverse static field since its role is played by the coupling between the spin-qubits. We show how such a physical effect could be exploited to estimate the strength of the interaction between the two spin-qubits and to generate entangled states of the system by appropriately setting the slope of the ramp. Moreover, the study of effects of the coupling parameters on the time-behaviour of the entanglement is reported. Finally, our symmetry-based approach allows us to discuss also effects stemming from the presence of a classical noise or non-Hermitian dephasing terms.

quant-ph↗
arXiv2018

Generation of high-fidelity quantum control methods for multi-level systems

In recent decades there has been a rapid development of methods to experimentally control individual quantum systems. A broad range of quantum control methods has been developed for two-level systems, however the complexity of multi-level quantum systems make the development of analogous control methods extremely challenging. Here, we exploit the equivalence between multi-level systems with SU(2) symmetry and spin-1/2 systems to develop a technique for generating new robust, high-fidelity, multi-level control methods. As a demonstration of this technique, we develop new adiabatic and composite multi-level quantum control methods and experimentally realise these methods using an $^{171}$Yb$^+$ ion system. We measure the average infidelity of the process in both cases to be around $10^{-4}$, demonstrating that this technique can be used to develop high-fidelity multi-level quantum control methods and can, for example, be applied to a wide range of quantum computing protocols including implementations below the fault-tolerant threshold in trapped ions.

quant-ph↗
arXiv2016

Spin-1/2 sub-dynamics nested in the quantum dynamics of two coupled qutrits

In this paper we investigate the quantum dynamics of two spin-1 systems, $\vec{\textbf{S}}_1$ and $\vec{\textbf{S}}_2$, adopting a generalized $(\vec{\textbf{S}}_1+\vec{\textbf{S}}_2)^2$-nonconserving Heisenberg model. We show that, due to its symmetry property, the nine-dimensional dynamics of the two qutrits exactly decouples into the direct sum of two sub-dynamics living in two orthogonal four- and five-dimensional subspaces. Such a reduction is further strengthened by our central result consisting in the fact that in the four-dimensional dynamically invariant subspace, the two qutrits quantum dynamics, with no approximations, is equivalent to that of two non interacting spin 1/2's. The interpretative advantages stemming from such a remarkable and non-intuitive nesting are systematically exploited and various intriguing features consequently emerging in the dynamics of the two qutrits are deeply scrutinised. The possibility of exploiting the dynamical reduction brought to light in this paper for exactly treating as well time-dependent versions of our Hamiltonian model is briefly discussed.

quant-ph↗
arXiv2014

Morris-Shore transformation with unequal detunings

The aim of this paper is to analyze the MS transformation in the case of weak deviation of the condition for equal detunings, which in necessary for factorization towards set of two-state systems and set of decouple states. Some elements concerning the general solution of the MS transformation will be also discussed.

quant-ph↗