arXiv subjects

Nikolay V. Vitanov

Publications and source records attributed to Nikolay V. Vitanov.

At least 19 recordsLinked to original sources

arXiv2026

Phase-Altered Interleaved Randomized Benchmarking for Compiled Non-Clifford Gates

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.

quant-ph↗

arXiv2026

Twinned Dynamical Decoupling

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.

quant-ph↗

arXiv2026

When More Becomes Less: Topology-Reversed Three-Qubit Gate Performance on IBM Quantum Processors

The exact Toffoli gate admits a six-CX decomposition, denoted by $CCX_6$, that is optimal under unrestricted two-qubit connectivity. On a linear three-qubit topology, however, $CCX_6$ contains 4 nearest-neighbor CX gates and 2 non-nearest-neighbor CX gates. By contrast, an alternative exact decomposition, denoted by $CCX_8$, uses only 8 nearest-neighbor CX gates. Because CCX is locally equivalent to CCZ, we perform the experiments using the corresponding $\cczs$ and $\cczl$ circuits. We compare these circuits on sampled linear triples of the 156-qubit IBM Quantum Heron processors \texttt{ibm\_fez} and \texttt{ibm\_kingston}. Under the compilation protocol, the nominal $\cczs$ circuit becomes a twelve-CZ implementation, whereas the linear-nearest-neighbor circuit retains eight native CZ gates. Experimentally measured ensemble-feature-selection estimates favor the eight-CZ realization on nearly all retained triples. We test the same ordering by preparing a three-qubit hypergraph state, which probes the coherent conditional phase rather than only computational-basis populations. The measured hypergraph-state infidelity is lower for the $\cczl$ circuit for most triples on both processors. Phase-altered interleaved randomized benchmarking provides a complementary comparison of Clifford surrogates preserving the two compiled entangling structures. Within the scope of the tested circuits and phase-sensitive input state, the results demonstrate that hardware connectivity can reverse the operational ranking of exact decompositions: a circuit with more abstract two-qubit gates can yield the better physical implementation.

quant-ph↗

arXiv2026

Coherent excitation of a two-state system by a Lorentzian field

We study the coherent excitation of a two-state quantum system by a field with a Lorentzian temporal envelope and constant carrier frequency. The associated differential equation admits an exact local representation in terms of confluent Heun functions. For the transition probability we develop a Dykhne-Davis-Pechukas (DDP) description based on the two relevant complex transition points and their interference. The DDP action is analyzed directly in the weak- and strong-coupling limits, yielding explicit asymptotic expressions for the oscillation phase, oscillation envelope, far-detuned line shape, and near-resonant behavior. In particular, the strong-coupling asymptotics imply a linewidth proportional to the inverse peak Rabi frequency for resonant odd-$π$ pulses, thereby exhibiting the power-narrowing characteristic of the Lorentzian pulse.

quant-ph↗

arXiv2026

Phase-Sensitive Benchmarking of Composite Quantum Gates with Chiral-Interference Circuits on Quantum Hardware

We construct and experimentally implement compact gate-native circuits that simulate the state-transfer interference underlying three- and four-level chiral-resolution protocols. Both models are encoded in a two-qubit register, with the enantiomer-dependent sign of one of the couplings simulated by a conditional-phase operation in the four-level circuit and by the sign of a final rotation in the three-level circuit. On an IBM quantum processor, the two circuits produce the expected enantiomer-dependent output states with probabilities of nearly $98\%$. We then use these circuits as physically motivated, phase-sensitive benchmarks for composite quantum gates. We introduce rotation-angle error to the single-qubit operations and replace them by several composite gates, including B5, SK1, BB1, H5s, and X5. The comparison demonstrates that single-gate robustness does not translate to equivalent whole-circuit robustness. In particular, variable-rotation sequences do not preserve the required relative phases, making them unsuitable for error correction in circuits. By contrast, the H5s/X5 sequences maintain high target-state populations for relative errors as large as $50\%$, whereas elementary rotations reach the same threshold only for approximately $8\%$. The three-level circuit exhibits a similar enhancement and additionally reveals an error-cancellation symmetry whose protection under composite replacement is exact only when the relevant full propagators satisfy an inverse relation.

quant-ph↗

arXiv2026

Optimization of multistate STIRAP by pulse shaping

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.

quant-ph↗

arXiv2026

Ramsey Interferometry in Wigner-Majorana Qudits

Ramsey interferometry estimates a detuning from the phase accumulated between two interaction zones, with a resolution set by the interrogation time $τ$. We propose a single-qudit extension based on Wigner-Majorana (WM) spin-$j$ dynamics, whose internal levels form a multipath interferometer. The enhancement is not generic: ideal $\mathrm{QFT}_D$ and $\sqrt{X_D}$ sequences do not densify the central fringe under the population readouts considered. Instead, in manifolds that realize the WM coupling, it arises from coherences between separated ladder states created and recombined by a single near-resonant drive per Ramsey zone. For the qutrit, preparing the central state of a spin-1 WM manifold and measuring its return probability gives $P_3(Δ)=\cos^2(Δτ)$, versus the qubit $P_2(Δ)=\cos^2(Δτ/2)$. The central fringe is thus compressed twofold at fixed $τ$ and the maximal slope doubled in the ideal limit, with contrast ideally unity. For higher-dimensional WM manifolds (odd and even $D$) the central response sharpens with dimension, but the signal spreads over several channels; we introduce a scalar readout from nearest-neighbor shoulder populations and quantify the resolution-contrast trade-off via the slope $S_D$ and contrast $C_D$. We also study robustness to diagonal phase noise from probe-shift fluctuations: under projector-type common-mode dephasing, the WM readouts are less contrast-sensitive than the qubit readout. The robustness is symmetry-selective, not universal: for linear Zeeman dephasing ($L=J_z$), the large $m$-separation that compresses the fringe also enhances dephasing, so higher-dimensional readouts become more sensitive, not protected. The WM qutrit is thus a practical operating point for enhanced Ramsey spectroscopy at fixed $τ$; higher qudits trade extra slope for reduced contrast.

quant-ph↗

arXiv2026

Non-Clifford Benchmarking via Ensemble Feature Selection

We propose an Ensemble Feature Selection (EFS) method for fast estimation of process infidelity of involutory multi-qubit gates, including non-Clifford targets, for which standard Clifford-based benchmarking does not apply. The method selects a compact set of experimentally executable circuit measurements from a candidate pool through offline training on a physically motivated ensemble of noisy channels, and combines them into a linear estimator with weights learned by ridge regression. The training ensemble is an explicit and tunable component of the protocol, incorporating prior knowledge about dominant hardware noise mechanisms. The estimator is validated on ibm_kingston using two Clifford validation benchmarks structurally related to the transpiled CCZ circuit, against independent Interleaved Randomized Benchmarking (IRB). Both show close EFS-IRB agreement across a wide range of process infidelities, with an estimation precision of approximately 0.01 over a process infidelity range of 0.02-0.2. EFS is subsequently applied directly to CCZ on the same device.

quant-ph↗

arXiv2026

DRAG-Compatible Leakage Suppression in Landau--Zener Control via Isoprobability Twins

Analytically solvable models -- particularly the Landau-Majorana-Stückelberg-Zener (LMSZ) and Allen-Eberly-Hioe (AEH) models -- underpin many quantum-gate implementations and population-transfer protocols. However, their canonical pulse shapes are incompatible with modern leakage-suppression techniques and some systems. Most notably, the constant Rabi envelope of the LMSZ pulse prevents many leakage-suppression approaches, which require smoothness. We address both limitations by developing the concept of isoprobability twin models: distinct pairs of Rabi frequency $Ω(t)$ and detuning $Δ(t)$ that yield identical post-pulse transition probabilities based on the Delos-Thorson transformation. In this work, we formalise the method by experimentally demonstrating the equivalence of multiple LMSZ and AEH twin models on IBM's ibm_kyiv processor. Finally, we show a staggering leakage reduction by more than 3 orders of magnitude using a custom DRAG implementation of a cosine LMSZ isoprobability model.

quant-ph↗

arXiv2026

Composite quantum gates simultaneously compensated for multiple errors

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.

quant-ph↗

arXiv2026

Improvement of performance of Grover's algorithm on three generations of Heron family IBM QPUs without and with topological dynamical decoupling

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.

quant-ph↗

arXiv2025

Observation of Power Superbroadening of Spectral Line Profiles on IBM Quantum

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.

quant-ph↗

arXiv2025

Robust, fast and high-fidelity composite single-qubit gates for superconducting transmon qubits

We introduce a novel quantum control method for superconducting transmon qubits that substantially outperforms conventional techniques in precision and robustness against coherent errors. Our approach leverages composite pulses (CP) to effectively mitigate system-specific errors, such as qubit frequency and anharmonicity variations. By utilizing CP, we demonstrate both complete and partial population transfers between qubit states, as well as the implementation of two essential single-qubit quantum gates. Simulations reveal substantial reductions in common error rates and gate durations. The effectiveness of our method is validated through four independent verification techniques, underscoring its potential for advancing quantum computing with superconducting qubits.

quant-ph↗

arXiv2024

Tunable broadband polarization retarders

We theoretically propose a type of tunable polarization retarder, which is composed of sequences of half-wave and quarter-wave polarization retarders, allowing operation at broad spectral bandwidth. The constituent retarders are composed of stacked standard half-wave retarders and quarter-wave retarders rotated at designated angles relative to their fast-polarization axes. The proposed composite retarder can be tuned to an arbitrary value of the retardance by varying the middle retarder alone, while maintaining its broadband spectral bandwidth intact.

physics.optics↗

arXiv2024

Qubit dynamics driven by smooth pulses of finite duration

We present a study of the dynamics of a qubit driven by a pulsed field of finite duration. The pulse shape starts and ends linearly in time. The most typical example of such a shape is the sine function between two of its nodes, but several other pulse shapes are also studied. All of them present smooth alternatives to the commonly used rectangular pulse shape, resulting in much weaker power broadening, much faster vanishing wings in the excitation line profile and hence much reduced sidebands. In the same time, such shapes with a well-defined finite duration do not suffer from the spurious effects arising when truncating a pulse of infinite duration, e.g. Gaussian. We derive two approximate analytic solutions which describe the ensuing quantum dynamics. Both approximations assume that the field changes linearly at the beginning and the end of the driving pulse, and adiabatically in between. The first approximation matches the linear and adiabatic parts at an appropriate instant of time and is expressed in terms of Weber's parabolic cylinder functions. The second, much simpler, approximation uses the asymptotics of the Weber function in order to replace it by simpler functions, and some additional transformations. Both approximations prove highly accurate when compared to experimental data obtained with two of the IBM Quantum processors. Both the greatly reduced power broadening and the greatly suppressed sidebands are observed for all pulse shapes, in a nearly complete agreement between theory and experiment.

quant-ph↗

arXiv2024

Motional state analysis of a trapped ion by ultra-narrowband composite pulses

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.

quant-ph↗

arXiv2024

Multipass Quantum Process Tomography: Precision and Accuracy Enhancement

We introduce a method to enhance the precision and accuracy of Quantum Process Tomography (QPT) by mitigating the errors caused by state preparation and measurement (SPAM), readout and shot noise. Instead of performing QPT solely on a single gate, we propose performing QPT on a sequence of multiple applications of the same gate. The method involves the measurement of the Pauli transfer matrix (PTM) by standard QPT of the multipass process, and then deduce the single-process PTM by two alternative approaches: an iterative approach which in theory delivers the exact result for small errors, and a linearized approach based on solving the Sylvester equation. We examine the efficiency of these two approaches through simulations on IBM Quantum using ibmq_qasm_simulator. Compared to the Randomized Benchmarking type of methods, the proposed method delivers the entire PTM rather than a single number (fidelity). Compared to standard QPT, our method delivers PTM with much higher accuracy and precision because it greatly reduces the SPAM, readout and shot noise errors. We use the proposed method to experimentally determine the PTM and the fidelity of the CNOT gate on the quantum processor ibmq_manila (Falcon r5.11L).

quant-ph↗

arXiv2024

Defying Conventional Wisdom in Spectroscopy: Power Narrowing on IBM Quantum

Power broadening $-$ the broadening of the spectral line profile of a two-state quantum transition as the amplitude of the driving field increases $-$ is a well-known and thoroughly examined phenomenon in spectroscopy. It typically occurs in continuous-wave driving when the intensity of the radiation field increases beyond the saturation intensity of the transition. In pulsed-field excitation, linear power broadening occurs for a pulse of rectangular temporal shape. Pulses with smooth shapes are known to exhibit much less power broadening, e.g. logarithmic for a Gaussian pulse shape. It has been predicted, but never experimentally verified, that pulse shapes which vanish in time as $\sim |t|^{-λ}$ should exhibit the opposite effect $-$ power narrowing $-$ in which the post-pulse transition line width decreases as the amplitude of the driving pulse increases. In this work, power narrowing is demonstrated experimentally for a class of powers-of-Lorentzian pulse shapes on the IBM Quantum processor ibmq_manila. Reduction of the line width by a factor of over 10 is observed when increasing the pulse area from $π$ to $7π$, in a complete reversal of the power broadening paradigm. Moreover, thorough theoretical and experimental study is conducted on the truncation of the pulse wings which introduces a (small) power-broadened term which prevents power narrowing from reaching extreme values $-$ a hitherto unknown cut-off broadening effect for which an explicit analytical formula is derived. In the absence of other power broadening mechanisms, Lorentzian pulses truncated at sufficiently small values can achieve as narrow line profiles as desired.

quant-ph↗