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A Metric-Compatible Path-Capacity Principle for Quantum Control Time

Y.Y.N. Li

Published Jun 18, 2026• 0 citations
Publisher

Abstract

A fixed complete ideal unitary gate can be implemented by infinitely many time-dependent control paths, which are invisible to the endpoint but may affect the system's response to environmental noise. In a two-atom Rydberg system with six-segment piecewise-constant drives, we construct distinct controls that share the same full unitary endpoint up to a global phase. We compute the interaction-picture dissipative response operator K_z = ∫_0^T U_z^{-1}(t) D U_z(t) dt, where D is the local dephasing generator. For two main lifted controls (denoted CW and CCW) we find ||K_z-K_{z0}||_F/16 ≈ 1.6×10^{-4}, and the exact noisy-channel distance D_E(γ) = (1/16)||E_z(T;γ)-E_{z0}(T;γ)||_F scales as γ^{0.9965}. The first-order prediction \hat D_E^{(1)}(γ) = γ ||K_z-K_{z0}||_F/16, with no parameters fitted to the channel differences, matches the exact distance to within 1.1% relative error for γ ≤ 0.03 μs^{-1}, while the residual scales as γ^{1.9979}. These results establish that the ideal executed path quantitatively predicts weak-noise channel separation for controls that share the same complete unitary. An exploratory non-collinear control (ALT) is also constructed and shows a smaller but similarly predictable response (e_max = 1.079%). Whether the path-response predictor can prospectively select a lower-loss control for a fixed task remains an open direction. The claims are limited to the declared two-atom model and control family; production compilation, hardware validation, and universal geometric principles remain outside the present scope.

DOI
10.5281/zenodo.20744098

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Temporal-Access Resource Theory for Continuous Process Tensors

He Wang

The uncontrolled endpoint channel can conceal how well a quantum process can protect a logical task under time-resolved control. We formulate a fixed-task resource theory of continuous process tensors with physical access interfaces, entrance-erasing free objects, and budgeted free conversions. Complete-record total-variation error measures performance; the calibrated capability is monotone under conversions that preserve a common free baseline. For unknown-state transmission the baseline is exactly $1-d^{-2}$. Two qubit processes with the same endpoint channel have optimal errors $0$ and $p=\sin^2(2gh)$ at the same finite pulse budget in an ideal-transparent system-only window architecture, forcing any endpoint-only predictor to err by at least $p/2$ on one of them. An exactly solvable family shows how the allocation of a fixed interaction phase around a control window sets the attainable error, and a nontrivial perfect echo requires and attains total control action $π$ in that architecture. The separation extends directly to every qubit unitary target at a budget that includes its boundary gate, while unitary tasks in any dimension have baseline $1-d^{-2}$ whenever a budget-feasible free realization gives the depolarizing output. Complete-record implementation bounds quantify coherent and dissipative control imperfections. Continuous-process randomized compilation supplies physical instrument pullbacks; compatible completion conditions extend their action, and certified dynamical bounds yield achievable task-error and capability guarantees. The resulting theory identifies temporal structure and control investment that endpoint characterization alone cannot resolve.

quant-pharXiv:2610.13110•Oct 12, 2026•0 citations

Fundamental Costs of Noise-Robust Quantum Control: Speed Limits and Complexity

Junkai Zeng, Xiu–Hao Deng

Noise is ubiquitous in quantum systems and is a major obstacle for the advancement of quantum information science. Noise-robust quantum control achieves high-fidelity operations by engineering the evolution path so that first-order noise contributions cancel at the final time. Such dynamical error correction typically incurs a time overhead beyond standard quantum speed limits. We derive general lower bounds on control complexity that quantify this overhead for quasi-static coherent noise under bounded control amplitude. For a single noise source, we prove a universal time lower bound for first-order robustness and give a constructive scheme that implements any target gate robustly in time 4T plus a constant time. For robustness against an entire noise space, we show dimension lower bounds on the number M of segments in any mixed-unitary schedule from two mechanism: (i) a coherent dimension bound when the error subspace contains an irreducible block isomorphic to su(q), and (ii) a projection dimension bound when the noise space contains the trace-zero span of orthogonal projectors. Under bounded speed, these bounds on number of segments imply time lower bounds. With only local controls robust against noise space defined on a graph, we obtain a graph-orthogonality time bound scales linear with graph chromatic number. We illustrate the bounds through examples. Collectively, these results establish quantitative limitations on the feasibility of first-order noise-resilient operations.

arXiv:2510.07183•Oct 8, 2025•0 citations

BKT–74–QREC. Dynamic Quasi-Closure and Relational Organization of Quantum Systems.

Robert Kupski

Dynamic Quasi-Closure and Relational Organization of Quantum Systems Residual Certificates, Geometric Holonomies, Environmental Correlations, and Structural Transfer Conditions Robert Kupski · BKT–74–QREC · Edition 2.6 Abstract A mathematical and physical account of organizational retention in finite-dimensional quantum systems is developed within the Law of One Mechanism (LOM) research programme. Organization is specified by a distinguished family of states, while its retention depends on the dynamical generator, preparation, interaction with the environment, and observation interval. State recurrence, endpoint retention, quasi-closure of the entire trajectory, family invariance, and attraction are distinguished. Error certificates based on the residual of the equation of motion, their geometric refinements, invariance conditions, and bounds for quantum channels are derived. An exactly solvable two-coupling model relates protection of a selected sector to destructive interference between transition amplitudes. At resonance, for coupling frequencies of 0.30 and 0.40 MHz, leakage after 0.25 µs is 0 for the dark state, 0.2304 after complete dephasing of the preparation before evolution, and 0.4608 after a relative-phase shift of π. All three preparations have the same initial populations. In the ideal resonant model, keeping leakage at or below the one-percent threshold throughout evolution allows a maximum absolute phase deviation from the dark preparation of approximately 0.2087119452 rad. In an open three-qubit model, with the same Hamiltonian and preparation and local relaxation and coherence times T₁ = 80 µs and T₂ = 40 µs, replacing independent dephasing with common dephasing increases the time to one-percent total organizational loss from 0.483439 to 0.804027 µs, or by 66.31%. For these two realizations, the lower bound on the worst-case absolute error of a deterministic threshold-time prediction based solely on their shared local data is 0.1602938517 µs. An independent measurement of normalized relative-coherence decay identifies the correlation parameter within the model family and determines the coefficient of cubic leakage onset into the third qubit without fitting its subsequent trajectory. This transfer assumes an unchanged environmental generator across protocols and specified conditions of Markovian dynamics. The account is extended to the quantum geometric tensor, dark-state transport, and interferometric readout. A full coupling-phase cycle produces an adiabatic geometric phase of −0.72π and an ideal interference probability of 0.1812880051. Exact evolution determines finite-duration corrections. In the open model, return fidelity is bounded by excitation survival: for one initial excitation, no pumping, equal local relaxation times, and an excitation-preserving Hamiltonian, the upper bound is exp(−T/T₁), where T denotes the cycle duration. Geometric records and environmental memory are distinguished, and a conditional two-cycle order test is formulated. LOM postulates that fields and particles emerge from a more fundamental relational and informational organization and constitute its effective realizations. The article gives selected elements of this hypothesis an operational form through states, couplings, generators, and measurement relations. The results are conditional theorems and calculations for specified models; they are neither new experimental data nor evidence establishing the existence of a metafield. Additional constitutive content of the programme may take the form of independent constraints linking admissible generators and protocols. The study establishes a common account of retention, perturbation tolerances, environmental correlations, geometric transport, and dynamical identifiability. Introduction Persistence of organization in a quantum system is a property of the relations between the state, a distinguished class of configurations, and the dynamics. Two different, even orthogonal states may belong to the same organizational class, whereas recurrence to the initial state may follow an earlier departure from that class. Retention at one instant, control of the distance of an entire trajectory from a state family, and attraction to that family therefore describe distinct physical properties. Their quantitative separation identifies the contributions of autonomous evolution, preparation, perturbations, and measurement interventions to observed organizational retention. The study develops the LOM–GTSFC–USC–GTCW programme. The Law of One Mechanism (LOM; Polish: Prawo Jednego Mechanizmu, PJM) provides the overarching framework; GTSFC denotes the Global Theory of Shared Fundamental Couplings, USC the Universal Structural Code, and GTCW the Gravitational–Thermodynamic Cyclicity of Multiverses. Within this framework, the metafield is a hypothetical structure of admissible relations, couplings, and tolerance bands. USC describes their admissibility rules, while GTSFC organizes couplings and their sectoral realizations. The hypothesis of a common organizational principle permits different effective Hamiltonians and environmental channels. Structural information concerns the organization of relations that sustain a selected class of states; preservation of a bit record and state entropy do not exhaust this concept. A possible LOM principle more fundamental than the informational layer itself remains a separate source hypothesis. The article continues the BKT–74 line of research on memory, retention, and stabilization. BKT–74A distinguishes organization from instantaneous geometry; BKT–74B separates preparation and return memory from the stabilizing response; and BKT–74C relates retention and first exit to the system–environment coupling operator. BKT–74–QREC develops these distinctions through explicit state families, leakage operators, trajectory certificates, and relations between protocols. The protection analysis concerns a specified preparation and generator. The existence of an invariant subsector and the formation of such organization from general initial conditions are separate questions. Experimental context is provided by Dong and co-workers’ studies of regular trajectories in a variational description of an interacting 24-qubit system and their search and stabilization through hybrid feedback control (Dong et al., 2026; DOI: 10.1038/s41567-026-03431-z). Exact quantum evolution, dynamics on a variational manifold, and a loop comprising measurement, optimization, and re-preparation represent different levels of description. The two-coupling model considered here is a self-contained analytical construction. It provides an explicit example of interference-based protection, and its parameters and results refer to that model, without identifying it with the qubit-ladder Hamiltonian or the data of the cited experiment. Dynamic quasi-closure bounds the distance of a trajectory from a distinguished family of states at every instant of the interval considered. The residual of the equation of motion measures the mismatch between an approximate trajectory and a specified generator, yielding certificates for state and observable errors. Removing the component associated with the global phase gives an angular certificate in ray space. For organization described by a subspace, the operator K = QHP/ℏ, where P projects onto that subspace, Q = I − P, I is the identity operator, H is the Hamiltonian, and ℏ is the reduced Planck constant, identifies direct leakage directions. Full organizational retention also depends on the action of the generator within the distinguished sector. This approach connects protection properties to the dynamics and separates instantaneous cancellation of leakage, invariance, and attraction. In the resonant two-coupling model, the dark-state condition is g₁a₁ + g₂a₂ = 0. The quantities g₁ and g₂ are angular coupling frequencies, related to ordinary frequencies by gⱼ = 2πfⱼ, while a₁ and a₂ are preparation amplitudes. For f₁ = 0.30 MHz and f₂ = 0.40 MHz, the dark state is |D⟩ = 0.8|1⟩ − 0.6|2⟩, where |1⟩ and |2⟩ are orthonormal states spanning the distinguished sector. Comparing the dark preparation, its completely dephased mixture, and a preparation with reversed relative phase isolates the role of coherence at unchanged populations. Phase-error and coupling-direction tolerances quantify the robustness of this protection. Certificates based on the integrated residual norm provide sufficient conditions; loss of information about phase cancellations can limit their sharpness. The extension to open systems uses Gorini–Kossakowski–Lindblad–Sudarshan (GKLS) generators with specified jump operators. In the three-qubit model considered, identical local relaxation and coherence times T₁ and T₂ do not specify dephasing correlations and therefore do not uniquely determine retention time. The parameter c interpolates between independent noise, c = 0, and common noise, c = 1. 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Protocols of coherent motion control for an interaction-driven Rydberg gate

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Generating entanglement between two Rydberg atoms is at the core of neutral-atom quantum computers. Current two-qubit gates operate in the Rydberg-blockade regime, in which the full strength of the van der Waals interaction between the two Rydberg atoms is not directly exploited, to avoid sensitivity to the position noise of the tweezer-trapped atoms, at the cost of a longer time spent in the Rydberg state. Here, we propose a set of techniques based on coherent control of the atomic motion obtained by combining optical tweezers and a two-dimensional optical lattice, and a sequence of multiple on/off pulses. The protocols keep the two-qubit gate error contribution from position noise below $10^{-4}$, heat the atom by less than~$Δn = 0.01$, while being robust to alignment errors of the potential up to $50$~nm and thermal excitation up to $\bar{n} = 3$. This toolbox opens the path for new two-qubit Rydberg gates directly, or partially, driven by the interaction, in which the atoms spend only $\sim 10$~ns in the Rydberg state, minimizing the increasingly dominant error source originating from its finite lifetime.

physics.atom-phCross-listedarXiv:2607.23090•Jul 28, 2026•0 citations

A Fundamental Bound for Robust Quantum Gate Control

Robert L. Kosut, Daniel A. Lidar, Herschel Rabitz

We derive a universal performance limit for coherent quantum control in the presence of modeled and unmodeled uncertainties. For any target unitary $W$ that is implementable in the absence of error, we prove that the worst-case (and hence the average) gate fidelity obeys the lower bound $F \ge \Flb\bigl(\tf \Omeff\bigr)$, where $\tf$ is the gate duration and $\Omeff$ is a single frequency-like measure that aggregates \emph{all} bounded uncertainty sources, e.g., coherent control imperfections, unknown couplings, and residual environment interactions, without assuming an initially factorizable system-bath state or a completely positive map. The bound is obtained by combining an interaction-picture averaging method with a Bellman-Gronwall inequality and holds for any finite-norm Hamiltonian decomposition. Hence it applies equally to qubits, multi-level qudits, and ancilla-assisted operations. Because $\Flb$ depends only on the dimensionless product $\tf\Omeff$, it yields a device-independent metric that certifies whether a given hardware platform can, in principle, reach a specified fault-tolerance threshold, and also sets a quantitative target for robust-control synthesis and system identification.

arXiv:2507.01215•Jul 1, 2025•0 citations

Optimal control for Rydberg multi-qubit operations

Mohammadsadegh Khazali, Hossein Abedi, Klaus Mølmer

Quantum computing algorithms can be decomposed into a universal set of elementary one- and two-qubit gates. Different physical implementations of quantum computing, however, employ interactions that permit direct conditional dynamics on multiple qubits in a single step. In this work, we leverage quantum optimal control techniques to design single continuous laser pulses that implement multi-qubit controlled-phase and -swap (Fredkin) gates on Rydberg atom quantum processors. The identification of robust multi-qubit operations leads to reduced operation time and less decoherence, and the control field provides continuous protection of the atoms from environmental noise. Notably, we find that the controlled-SWAP (Fredkin) gate, implemented using this approach, achieves 99.88% fidelity while accounting for imperfections such as spontaneous emission, laser fluctuations, and Doppler dephasing.

npj Quantum Information•Jun 25, 2026•0 citations
A fixed complete ideal unitary gate can be implemented by infinitely many time-dependent control paths, which are invisible to the endpoint but may affect the system's response to environmental noise. In a two-atom Rydberg system with six-segment piecewise-constant drives, we construct distinct controls that share the same full unitary endpoint up to a global phase. We compute the interaction-picture dissipative response operator K_z = ∫0^T U_z^{-1}(t) D U_z(t) dt, where D is the local dephasing generator. For two main lifted controls (denoted CW and CCW) we find ||K_z-K{z0}||F/16 ≈ 1.6×10^{-4}, and the exact noisy-channel distance D_E(γ) = (1/16)||E_z(T;γ)-E{z0}(T;γ)||F scales as γ^{0.9965}. The first-order prediction \hat D_E^{(1)}(γ) = γ ||K_z-K{z0}||_F/16, with no parameters fitted to the channel differences, matches the exact distance to within 1.1% relative error for γ ≤ 0.03 μs^{-1}, while the residual scales as γ^{1.9979}. These results establish that the ideal executed path quantitatively predicts weak-noise channel separation for controls that share the same complete unitary. An exploratory non-collinear control (ALT) is also constructed and shows a smaller but similarly predictable response (e_max = 1.079%). Whether the path-response predictor can prospectively select a lower-loss control for a fixed task remains an open direction. The claims are limited to the declared two-atom model and control family; production compilation, hardware validation, and universal geometric principles remain outside the present scope.
The uncontrolled endpoint channel can conceal how well a quantum process can protect a logical task under time-resolved control. We formulate a fixed-task resource theory of continuous process tensors with physical access interfaces, entrance-erasing free objects, and budgeted free conversions. Complete-record total-variation error measures performance; the calibrated capability is monotone under conversions that preserve a common free baseline. For unknown-state transmission the baseline is exactly 1−d−21-d^{-2}1−d−2. Two qubit processes with the same endpoint channel have optimal errors 000 and p=sin⁡2(2gh)p=\sin^2(2gh)p=sin2(2gh) at the same finite pulse budget in an ideal-transparent system-only window architecture, forcing any endpoint-only predictor to err by at least p/2p/2p/2 on one of them. An exactly solvable family shows how the allocation of a fixed interaction phase around a control window sets the attainable error, and a nontrivial perfect echo requires and attains total control action πππ in that architecture. The separation extends directly to every qubit unitary target at a budget that includes its boundary gate, while unitary tasks in any dimension have baseline 1−d−21-d^{-2}1−d−2 whenever a budget-feasible free realization gives the depolarizing output. Complete-record implementation bounds quantify coherent and dissipative control imperfections. Continuous-process randomized compilation supplies physical instrument pullbacks; compatible completion conditions extend their action, and certified dynamical bounds yield achievable task-error and capability guarantees. The resulting theory identifies temporal structure and control investment that endpoint characterization alone cannot resolve.
Noise is ubiquitous in quantum systems and is a major obstacle for the advancement of quantum information science. Noise-robust quantum control achieves high-fidelity operations by engineering the evolution path so that first-order noise contributions cancel at the final time. Such dynamical error correction typically incurs a time overhead beyond standard quantum speed limits. We derive general lower bounds on control complexity that quantify this overhead for quasi-static coherent noise under bounded control amplitude. For a single noise source, we prove a universal time lower bound for first-order robustness and give a constructive scheme that implements any target gate robustly in time 4T plus a constant time. For robustness against an entire noise space, we show dimension lower bounds on the number M of segments in any mixed-unitary schedule from two mechanism: (i) a coherent dimension bound when the error subspace contains an irreducible block isomorphic to su(q), and (ii) a projection dimension bound when the noise space contains the trace-zero span of orthogonal projectors. Under bounded speed, these bounds on number of segments imply time lower bounds. With only local controls robust against noise space defined on a graph, we obtain a graph-orthogonality time bound scales linear with graph chromatic number. We illustrate the bounds through examples. Collectively, these results establish quantitative limitations on the feasibility of first-order noise-resilient operations.
Dynamic Quasi-Closure and Relational Organization of Quantum Systems Residual Certificates, Geometric Holonomies, Environmental Correlations, and Structural Transfer Conditions Robert Kupski · BKT–74–QREC · Edition 2.6 Abstract A mathematical and physical account of organizational retention in finite-dimensional quantum systems is developed within the Law of One Mechanism (LOM) research programme. Organization is specified by a distinguished family of states, while its retention depends on the dynamical generator, preparation, interaction with the environment, and observation interval. State recurrence, endpoint retention, quasi-closure of the entire trajectory, family invariance, and attraction are distinguished. Error certificates based on the residual of the equation of motion, their geometric refinements, invariance conditions, and bounds for quantum channels are derived. An exactly solvable two-coupling model relates protection of a selected sector to destructive interference between transition amplitudes. At resonance, for coupling frequencies of 0.30 and 0.40 MHz, leakage after 0.25 µs is 0 for the dark state, 0.2304 after complete dephasing of the preparation before evolution, and 0.4608 after a relative-phase shift of π. All three preparations have the same initial populations. In the ideal resonant model, keeping leakage at or below the one-percent threshold throughout evolution allows a maximum absolute phase deviation from the dark preparation of approximately 0.2087119452 rad. In an open three-qubit model, with the same Hamiltonian and preparation and local relaxation and coherence times T₁ = 80 µs and T₂ = 40 µs, replacing independent dephasing with common dephasing increases the time to one-percent total organizational loss from 0.483439 to 0.804027 µs, or by 66.31%. For these two realizations, the lower bound on the worst-case absolute error of a deterministic threshold-time prediction based solely on their shared local data is 0.1602938517 µs. An independent measurement of normalized relative-coherence decay identifies the correlation parameter within the model family and determines the coefficient of cubic leakage onset into the third qubit without fitting its subsequent trajectory. This transfer assumes an unchanged environmental generator across protocols and specified conditions of Markovian dynamics. The account is extended to the quantum geometric tensor, dark-state transport, and interferometric readout. A full coupling-phase cycle produces an adiabatic geometric phase of −0.72π and an ideal interference probability of 0.1812880051. Exact evolution determines finite-duration corrections. In the open model, return fidelity is bounded by excitation survival: for one initial excitation, no pumping, equal local relaxation times, and an excitation-preserving Hamiltonian, the upper bound is exp(−T/T₁), where T denotes the cycle duration. Geometric records and environmental memory are distinguished, and a conditional two-cycle order test is formulated. LOM postulates that fields and particles emerge from a more fundamental relational and informational organization and constitute its effective realizations. The article gives selected elements of this hypothesis an operational form through states, couplings, generators, and measurement relations. The results are conditional theorems and calculations for specified models; they are neither new experimental data nor evidence establishing the existence of a metafield. Additional constitutive content of the programme may take the form of independent constraints linking admissible generators and protocols. The study establishes a common account of retention, perturbation tolerances, environmental correlations, geometric transport, and dynamical identifiability. Introduction Persistence of organization in a quantum system is a property of the relations between the state, a distinguished class of configurations, and the dynamics. Two different, even orthogonal states may belong to the same organizational class, whereas recurrence to the initial state may follow an earlier departure from that class. Retention at one instant, control of the distance of an entire trajectory from a state family, and attraction to that family therefore describe distinct physical properties. Their quantitative separation identifies the contributions of autonomous evolution, preparation, perturbations, and measurement interventions to observed organizational retention. The study develops the LOM–GTSFC–USC–GTCW programme. The Law of One Mechanism (LOM; Polish: Prawo Jednego Mechanizmu, PJM) provides the overarching framework; GTSFC denotes the Global Theory of Shared Fundamental Couplings, USC the Universal Structural Code, and GTCW the Gravitational–Thermodynamic Cyclicity of Multiverses. Within this framework, the metafield is a hypothetical structure of admissible relations, couplings, and tolerance bands. USC describes their admissibility rules, while GTSFC organizes couplings and their sectoral realizations. The hypothesis of a common organizational principle permits different effective Hamiltonians and environmental channels. Structural information concerns the organization of relations that sustain a selected class of states; preservation of a bit record and state entropy do not exhaust this concept. A possible LOM principle more fundamental than the informational layer itself remains a separate source hypothesis. The article continues the BKT–74 line of research on memory, retention, and stabilization. BKT–74A distinguishes organization from instantaneous geometry; BKT–74B separates preparation and return memory from the stabilizing response; and BKT–74C relates retention and first exit to the system–environment coupling operator. BKT–74–QREC develops these distinctions through explicit state families, leakage operators, trajectory certificates, and relations between protocols. The protection analysis concerns a specified preparation and generator. The existence of an invariant subsector and the formation of such organization from general initial conditions are separate questions. Experimental context is provided by Dong and co-workers’ studies of regular trajectories in a variational description of an interacting 24-qubit system and their search and stabilization through hybrid feedback control (Dong et al., 2026; DOI: 10.1038/s41567-026-03431-z). Exact quantum evolution, dynamics on a variational manifold, and a loop comprising measurement, optimization, and re-preparation represent different levels of description. The two-coupling model considered here is a self-contained analytical construction. It provides an explicit example of interference-based protection, and its parameters and results refer to that model, without identifying it with the qubit-ladder Hamiltonian or the data of the cited experiment. Dynamic quasi-closure bounds the distance of a trajectory from a distinguished family of states at every instant of the interval considered. The residual of the equation of motion measures the mismatch between an approximate trajectory and a specified generator, yielding certificates for state and observable errors. Removing the component associated with the global phase gives an angular certificate in ray space. For organization described by a subspace, the operator K = QHP/ℏ, where P projects onto that subspace, Q = I − P, I is the identity operator, H is the Hamiltonian, and ℏ is the reduced Planck constant, identifies direct leakage directions. Full organizational retention also depends on the action of the generator within the distinguished sector. This approach connects protection properties to the dynamics and separates instantaneous cancellation of leakage, invariance, and attraction. In the resonant two-coupling model, the dark-state condition is g₁a₁ + g₂a₂ = 0. The quantities g₁ and g₂ are angular coupling frequencies, related to ordinary frequencies by gⱼ = 2πfⱼ, while a₁ and a₂ are preparation amplitudes. For f₁ = 0.30 MHz and f₂ = 0.40 MHz, the dark state is |D⟩ = 0.8|1⟩ − 0.6|2⟩, where |1⟩ and |2⟩ are orthonormal states spanning the distinguished sector. Comparing the dark preparation, its completely dephased mixture, and a preparation with reversed relative phase isolates the role of coherence at unchanged populations. Phase-error and coupling-direction tolerances quantify the robustness of this protection. Certificates based on the integrated residual norm provide sufficient conditions; loss of information about phase cancellations can limit their sharpness. The extension to open systems uses Gorini–Kossakowski–Lindblad–Sudarshan (GKLS) generators with specified jump operators. In the three-qubit model considered, identical local relaxation and coherence times T₁ and T₂ do not specify dephasing correlations and therefore do not uniquely determine retention time. The parameter c interpolates between independent noise, c = 0, and common noise, c = 1. At T₁ = 80 µs and T₂ = 40 µs, the difference between the times to one-percent total loss yields a lower error bound for predictions based on local data that do not distinguish these realizations. Total loss includes both leakage into the third qubit and relaxation to the state with no excitation. This non-identifiability concerns the scope of the calibration data, while the predictions of a fully specified model remain unambiguous. An additional measurement of relative coherence provides information about environmental correlations. With the couplings switched off, the decay rate of population-normalized coherence determines c within the family considered, provided that the pure-dephasing rate is positive. The calibration result then determines the coefficient A₃ in the expansion P₀₀₁(t) = A₃t³ + O(t⁴), where P₀₀₁ denotes the population of the state with the third qubit excited, and t is the time since dark-state preparation and activation of the couplings. The relation holds for th
Generating entanglement between two Rydberg atoms is at the core of neutral-atom quantum computers. Current two-qubit gates operate in the Rydberg-blockade regime, in which the full strength of the van der Waals interaction between the two Rydberg atoms is not directly exploited, to avoid sensitivity to the position noise of the tweezer-trapped atoms, at the cost of a longer time spent in the Rydberg state. Here, we propose a set of techniques based on coherent control of the atomic motion obtained by combining optical tweezers and a two-dimensional optical lattice, and a sequence of multiple on/off pulses. The protocols keep the two-qubit gate error contribution from position noise below 10−410^{-4}10−4, heat the atom by less than~Δn=0.01Δn = 0.01Δn=0.01, while being robust to alignment errors of the potential up to 505050~nm and thermal excitation up to nˉ=3\bar{n} = 3nˉ=3. This toolbox opens the path for new two-qubit Rydberg gates directly, or partially, driven by the interaction, in which the atoms spend only ∼10\sim 10∼10~ns in the Rydberg state, minimizing the increasingly dominant error source originating from its finite lifetime.
We derive a universal performance limit for coherent quantum control in the presence of modeled and unmodeled uncertainties. For any target unitary WWW that is implementable in the absence of error, we prove that the worst-case (and hence the average) gate fidelity obeys the lower bound $F \ge \Flb\bigl(\tf \Omeff\bigr)$, where $\tf$ is the gate duration and $\Omeff$ is a single frequency-like measure that aggregates \emph{all} bounded uncertainty sources, e.g., coherent control imperfections, unknown couplings, and residual environment interactions, without assuming an initially factorizable system-bath state or a completely positive map. The bound is obtained by combining an interaction-picture averaging method with a Bellman-Gronwall inequality and holds for any finite-norm Hamiltonian decomposition. Hence it applies equally to qubits, multi-level qudits, and ancilla-assisted operations. Because $\Flb$ depends only on the dimensionless product $\tf\Omeff$, it yields a device-independent metric that certifies whether a given hardware platform can, in principle, reach a specified fault-tolerance threshold, and also sets a quantitative target for robust-control synthesis and system identification.
Quantum computing algorithms can be decomposed into a universal set of elementary one- and two-qubit gates. Different physical implementations of quantum computing, however, employ interactions that permit direct conditional dynamics on multiple qubits in a single step. In this work, we leverage quantum optimal control techniques to design single continuous laser pulses that implement multi-qubit controlled-phase and -swap (Fredkin) gates on Rydberg atom quantum processors. The identification of robust multi-qubit operations leads to reduced operation time and less decoherence, and the control field provides continuous protection of the atoms from environmental noise. Notably, we find that the controlled-SWAP (Fredkin) gate, implemented using this approach, achieves 99.88% fidelity while accounting for imperfections such as spontaneous emission, laser fluctuations, and Doppler dephasing.