Vitanov group
Sofia University, Sofia, Bulgaria
- Tracked papers
- 66 (2002–2026)
Topics
The most frequent field-map topics among the PI' tracked papers.
- Composite pulses 28
- Adiabatic passage 24
- State preparation 10
- Gates 10
- Trapped ions 10
- Dynamical decoupling 9
- Photonics 8
- Molecules 8
Recent papers
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Optimization of multistate STIRAP by pulse shaping
Abstract
We propose a pulse-shaping method for improving population transfer in multistate stimulated Raman adiabatic passage (STIRAP) with nearest-neighbour couplings only. The method extends the quasiparallel-eigenenergy approach, previously used for two- and three-state systems, to multistate chains without introducing additional shortcut fields. The pump and Stokes fields are shaped so that the eigenenergies closest to the dark state remain nearly parallel to it during the central part of the interaction, where nonadiabatic transitions are most likely to occur. We derive analytic prescriptions for the required pulse shapes and apply them to five-, seven-, and nine-state chains formed by magnetic sublevels of degenerate manifolds with angular momenta $J_g\rightarrow J_e=J_g$ and $J_g\rightarrow J_e=J_g-1$ driven by a pair of right- and left-circularly polarized pulses. Numerical simulations show that the optimized pulses can reduce the population transfer error by several orders of magnitude compared to standard Gaussian pulses. The same shapes also improve robustness against variations of the peak Rabi frequency, the single-photon detuning, and the multiphoton detuning. The comparison between the two angular-momentum families shows that the $J_g\rightarrow J_g$ chains are generally more favourable, because their Clebsch-Gordan coefficients keep the relevant bright-state gap larger in the region where the dark state changes most rapidly. Finally, simulations with spontaneous emission show that the optimized resonant pulses remain advantageous in the lossy regime as well.
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Phase-Altered Interleaved Randomized Benchmarking for Compiled Non-Clifford Gates
Abstract
Interleaved randomized benchmarking (IRB) provides a scalable estimate of a gate's error rate, but its standard guarantees require the interleaved gate to be Clifford~\cite{Magesan2012Interleaved,magesan2012characterizing}. In superconducting processors, many non-Clifford phase gates in compiled circuits are implemented virtually as software-defined frame updates rather than as additional control pulses~\cite{mckay2017efficient}. This raises the question of whether inserting or removing such virtual phases measurably changes IRB error estimates. We introduce \emph{phase-altered interleaved randomized benchmarking} (PA-IRB), a paired-IRB diagnostic protocol comparing phase-stripped and phase-dressed Clifford interleaving gates derived from the same compiled implementation. PA-IRB reports $Δr=r_d-r_s$ with combined uncertainty to test whether virtual phase gates affect the extracted IRB decay beyond statistical error. As a case study, we apply PA-IRB to a compiled Toffoli gate executed on IBM superconducting processors, where the constituent $T/T^\dagger$ gates are implemented as virtual $Z$ rotations. Across tested calibration runs, $Δr$ is consistent with zero within uncertainty, indicating that virtual phase addition or removal does not measurably alter the IRB-derived error estimate under the employed compilation and execution stack. More generally, PA-IRB provides a lightweight, abstraction-aware diagnostic for benchmarking workflows involving software-defined phase operations. The same paired comparison can also be used to place operational bounds on the contribution of non-Clifford components to the compiled gate error, even when those components are physically executed rather than implemented virtually.
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Improvement of performance of Grover's algorithm on three generations of Heron family IBM QPUs without and with topological dynamical decoupling
Abstract
We investigate the performance of Grover's algorithm on three different generations of IBM Heron QPUs. On Heron family of IBM QPUs the success probabilities for three, four and five qubits without dynamical decoupling is better than results reported for previous generations of QPUs. The success probability as function of number of iterations of Grover operator is considered. A study of the improvement of results of Grover's algorithm for five qubit case with the help of topological dynamical decoupling is considered. For a six qubit case on Heron r3 QPU a clear result for finding the sought-after bitstring is reported for theoretically suboptimal number of iterations of Grover operator with the help of dynamical decoupling.
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Composite quantum gates simultaneously compensated for multiple errors
Abstract
Systematic control errors remain a primary obstacle to realizing high-fidelity single-qubit gates. We introduce composite pulse sequences that implement X and Hadamard gates while simultaneously compensating amplitude (Rabi-frequency), detuning (frequency), and duration errors. Our construction uses two complementary strategies: (i) derivative-based cancellation of error terms in the full unitary (not just the transition probability), formulated via the Cayley-Klein parametrization, and (ii) direct minimization of the average gate infidelity over prescribed error ranges. We derive symmetric five-pulse solutions with closed-form phases that cancel all first-order terms (including the mixed derivative), and numerically optimize longer sequences -- up to 15 pulses -- to achieve higher-order suppression. We also show that standard ``universal'' five-pulse sequences (U5a/U5b) emerge as simple phase-shifted instances of our symmetric solutions, yielding broad robustness to both detuning and amplitude errors. Finally, we construct variable-area sequences for $R_x(π/2)$, which, up to virtual Z rotations, benchmark the Hadamard gate. Across all families we observe the expected trade-off between sequence length and robustness window, with substantial boosts in fidelity over large error domains.
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Decoupling of the STIRAP and Microwave-Dressing paths in Trapped Rydberg Ion Gates
Abstract
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.
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Twinned Dynamical Decoupling
Abstract
Systematic pulse-area errors limit the fidelity of quantum control across many qubit platforms. We introduce twinned dynamical decoupling (TDD), an analytic family of sequences $T2n$ in which a pulse sequence is paired with its $π$-phase-shifted twin. This $π$-phase step cancels common-mode systematic pulse-area errors to all orders on exact resonance. Then the phases of the pulses in each of the constituent twins are determined in such a manner that detuning errors are suppressed to the highest possible order as well. We have derived a simple analytic formula for these phases applicable to arbitrary sequence length. We demonstrate the sequences with superconducting transmon qubits on the IBM Quantum processor ibm$\_$torino and the IQM Quantum processor Garnet. The measured population plateaus agree closely with theory and show enhanced robustness compared to the most frequently used dynamical decoupling protocols.
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Fault-tolerant hyper-Ramsey spectroscopy of optical clock transitions with dynamical decoupling
Abstract
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.
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Observation of Power Superbroadening of Spectral Line Profiles on IBM Quantum
Abstract
Power broadening refers to the widening of the spectral line profile in a two-state quantum transition as the strength of the driving field increases. This phenomenon commonly arises in continuous-wave driving when the radiation field's intensity exceeds the transition's saturation intensity and it has been extensively studied in spectroscopy. For pulsed-field excitation, the spectral response of the quantum system may differ significantly: while a rectangular-shaped pulse leads to a linear power broadening, pulses with smooth shapes show significantly reduced power broadening, for instance, logarithmic for the Gaussian shape and none for the hyperbolic-secant shape. Recently [Phys. Rev. Lett. 132, 020802 (2024)], in a dramatic paradigm shift, we have demonstrated experimentally that for Lorentzian-shaped pulses, the opposite effect - power narrowing - takes place: the width of the spectral profile decreases when the driving pulse amplitude increases, with a narrowing factor of as much as 10 observed. While in high-resolution spectroscopy the push is for eliminating or even inverting the power broadening, there are applications where it is used to an advantage for it facilitates off-resonance excitation. Here, we present a number of shaped pulses that exhibit power broadening much greater than that of the rectangular pulse of the same pulse area. They are grouped in two families of pulse shapes. In particular, in regard to the width of the second Rabi oscillation maximum, the quadratic pulse family shows an increase by a factor of 3.3 whereas the even-exponent pulse family exhibits an increase by a factor of more than 3.5.
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Motional state analysis of a trapped ion by ultra-narrowband composite pulses
Abstract
In this work, we present a method for measuring the motional state of a two-level system coupled to a harmonic oscillator. Our technique uses ultra-narrowband composite pulses on the blue sideband transition to scan through the populations of the different motional states. Our approach does not assume any previous knowledge of the motional state distribution and is easily implemented. It is applicable both inside and outside of the Lamb-Dicke regime. For higher phonon numbers especially, the composite pulse sequence can be used as a filter for measuring phonon number ranges. We demonstrate this measurement technique using a single trapped ion and show good detection results with the numerically evaluated pulse sequence.
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Deterministic generation of arbitrary ultrasmall excitation of quantum systems by composite pulse sequences
Abstract
In some applications of quantum control, it is necessary to produce very weak excitation of a quantum system. Such an example is presented by the concept of single-photon generation in cold atomic ensembles or doped solids, e.g. by the DLCZ protocol, for which a single excitation is shared among thousands and millions atoms or ions. Another example is the possibility to create huge Dicke state of $N$ qubits sharing a single or a few excitations. Other examples are using tiny rotations to tune high-fidelity quantum gates or using these tiny rotations for testing high-fidelity quantum process tomography protocols. Ultrasmall excitation of a quantum transition can be generated by either a very weak or far-detuned driving field. However, these two approaches are sensitive to variations in the experimental parameters, e.g. the transition probability varies with the square of the pulse area. Here we propose a different method for generating a well-defined pre-selected very small transition probability -- of the order of $10^{-2}$ to $10^{-8}$ -- by using composite pulse sequences. The method features high fidelity and robustness to variations in the pulse area and the pulse duration.