Abstract Scaling quantum information processing on planar, nearest-neighbor superconducting transmon architectures requires the coherent distribution of entangled states across extensive multi-qubit distances. Standard SWAP routing networks suffer from rapid, exponential fidelity decay driven by spatial wavepacket dispersion, coherent cross-resonance drive errors, and environmental dephasing. In this work, we propose and numerically validate a Dual-Anchor Floquet Cavity protocol capable of shuttling entangled Bell pairs across macroscopic 1D transmon chains with theoretical zero-dispersion unitary closure (U_cycle = I). By combining an affine geometric phase gradient with synchronized Y ⊗ Y boundary echo reflections and Hermitian conjugate spatial unwinding, the protocol eliminates intermediate odd-cycle micro-motion lag, establishing a strictly invariant attractor limit cycle. Using Matrix Product State (MPS) simulations bounded at entanglement bond dimension χ = 2, we verify exact 100.00% target singlet fidelity, 1.0000 state purity, and maximal non-locality (S = 2.8284) across 17, 102, and 1,002 transmon sites through circuit depths exceeding 31,000. Furthermore, under open-system Lindbladian noise models (T₁ = 150 µs, T₂ = 100 µs, and 0.4% two-qubit depolarizing rate), the architecture exhibits intrinsic dynamical decoupling, with exponential Zero-Noise Extrapolation (ZNE) recovering near-unity fidelity across 720 native entangling operations. This work provides an exact theoretical parameterization and simulation foundation for future experimental implementation on physical superconducting quantum processors. **Acknowledgements** The author acknowledges Gemini (Google) for interactive conceptual dialogue, mathematical derivation scaffolding, and numerical simulation workflow validation.
Abstract Scaling quantum information processing on planar, nearest-neighbor superconducting transmon architectures requires the coherent distribution of entangled states across extensive multi-qubit distances. Standard SWAP routing networks suffer from rapid, exponential fidelity decay driven by spatial wavepacket dispersion, coherent cross-resonance drive errors, and environmental dephasing. In this work, we propose and numerically validate a Dual-Anchor Floquet Cavity protocol capable of shuttling entangled Bell pairs across macroscopic 1D transmon chains with theoretical zero-dispersion unitary closure (U_cycle = I). By combining an affine geometric phase gradient with synchronized Y ⊗ Y boundary echo reflections and Hermitian conjugate spatial unwinding, the protocol eliminates intermediate odd-cycle micro-motion lag, establishing a strictly invariant attractor limit cycle. Using Matrix Product State (MPS) simulations bounded at entanglement bond dimension χ = 2, we verify exact 100.00% target singlet fidelity, 1.0000 state purity, and maximal non-locality (S = 2.8284) across 17, 102, and 1,002 transmon sites through circuit depths exceeding 31,000. Furthermore, under open-system Lindbladian noise models (T₁ = 150 µs, T₂ = 100 µs, and 0.4% two-qubit depolarizing rate), the architecture exhibits intrinsic dynamical decoupling, with exponential Zero-Noise Extrapolation (ZNE) recovering near-unity fidelity across 720 native entangling operations. This work provides an exact theoretical parameterization and simulation foundation for future experimental implementation on physical superconducting quantum processors. Acknowledgements The author acknowledges Gemini (Google) for interactive conceptual dialogue, mathematical derivation scaffolding, and numerical simulation workflow validation.
We report the observation of long-range dynamical protection and coherent transport of two-qubit entangled states across discrete 1D lattice chains on a 156-qubit superconducting quantum processor (ibm_marrakesh). By combining an affine geometric phase gradient with periodic CPMG dynamical decoupling and Hermitian conjugate frame unwinding, we suppress continuous dephasing, spatial wavepacket dispersion, and accumulative cross-resonance errors. The protocol achieves an unmitigated Bell state fidelity of 97.92% +/- 0.10% across 512 physical SWAP gates (1,536 native two-qubit hardware operations) and maintains an unmitigated fidelity of 97.93% +/- 0.22% (Tr(rho^2) = 0.9591) through 1,024 physical SWAPs (3,072 native pulses), contrasting with bare unmitigated transport decay (F < 1%). We demonstrate spatial transport scaling across up to 101 contiguous transmons (89.06% fidelity across 198 SWAPs), confirm the attractor basin geometry under continuous 3D Bloch sphere parameter sweeps (theta in [0, 2pi]), and validate sensitivity to intentional single-qubit phase and bit-flip defect injections.
Quantum processors based on superconducting circuitry have reached the scale of hundreds of qubits and demonstrated computational capabilities that challenge state-of-the-art classical methods for carefully designed problems. Despite these advances, fundamental questions persist over how to best improve and scale the computational power of this emerging technology. Most transmon-based quantum processors encode information in two-level systems, or qubits, leaving the rich multilevel structure of the transmon largely unexploited. In this thesis, we demonstrate how the capabilities of these devices can be meaningfully extended by harnessing this intrinsic multilevel structure to encode qudits — d-level quantum systems that access a larger and more connected Hilbert space than their qubit counterparts. We begin by demonstrating universal single-qudit operation before turning to microwave-activated entanglement to realize scalable two-qudit gates. We present a high-fidelity, fully entangling two-qutrit CZ gate based on a novel tunable cross-Kerr mechanism. We further explore two-qudit gates that selectively entangle subspaces of the full Hilbert space via two-photon multipartite Raman transitions, employing these operations to generate large entangled states across a transmon array. To address the noisy nature of near-term qudit computation, we introduce and characterize error suppression techniques for qudit circuits, including dynamical decoupling and quantum error mitigation. We then synthesize these directions by exploring simulations of quantum systems that have innate multilevel interactions and are therefore well tailored to qudit operation. First we investigate digital quantum simulations of collective neutrino oscillations and the spin-1 truncated Abelian Higgs model, finding error mitigation methods critical for improving circuit performance. Finally, out of equilibrium --- or Floquet --- spin-1 systems are realized using an array of 15 superconducting qutrits to probe a novel Z3 discrete time crystalline (DTC) phase of matter with intrinsically chiral interactions. This DTC result demonstrates that native qudit hardware provides powerful access to non-equilibrium phases rooted in higher discrete symmetries, opening a route to a broader landscape of dynamical orders beyond the Z2 Ising paradigm.
Deterministic Fock-state generation and inter-mode SWAP are foundational primitives for bosonic quantum computing, yet most implementations rely on numerically optimized pulses, per-state calibration, strong dispersive coupling, or higher transmon levels, each adding control overhead that grows with system size. We present an analytical, calibration-light protocol operating entirely within the two-level g-e manifold of a weakly dispersively coupled transmon. A Rabi drive on the qubit, combined with a single sideband tone per mode, synthesizes an on-demand Jaynes-Cummings interaction whose entire family of pulse times follows the closed-form scaling τn=τ1/n. Once the single base time τ1 is set, every higher-n operation is fixed analytically, with no per-state retuning, shelving, or numerical optimization. On a superconducting flute cavity with two high-Q modes, we deterministically prepare Fock states through ∣n=5⟩, realize an inter-mode SWAP characterized on vacuum, single-photon, and coherent-state inputs, and generate and coherently swap the dual-rail Bell state (∣1,0⟩+∣0,1⟩)/2, confirming that the operation preserves inter-mode coherence. Because the pulses are constant-amplitude and free of per-state optimization, the achievable fidelity is set directly by ancilla coherence and drive-ramp duration; a master-equation analysis isolates these hardware factors and shows that the analytical scaling itself imposes no obstacle to high-fidelity operation at high n. Requiring only one sideband line per mode and a single Rabi drive, the protocol is well suited to weakly coupled, high-Q 3D architectures where calibration economy and analytical pulse design are at a premium.
Floquet engineering—the periodic driving of quantum systems—offers a powerful pathway to realize exotic nonequilibrium phases, including topological Floquet insulators and discrete time crystals. However, computing the quasienergy band structure of interacting, many-body Floquet systems remains a formidable challenge for classical computers due to the exponential growth of the Hilbert space and the difficulty of diagonalizing the non-Hermitian effective Floquet Hamiltonian. In this work, we introduce and experimentally demonstrate a variational quantum algorithm (VQA) tailored specifically for the near-term computation of Floquet quasienergy bands. Unlike standard variational eigensolvers that minimize static energy expectations, our approach employs a parameterized quantum circuit to prepare variational trial states and directly extracts the quasienergy spectrum via a Hadamard-test-based cost function that measures the complex overlap with the single-period Floquet unitary propagator. By scanning the conserved crystal momentum, we reconstruct the full quasienergy dispersion for both one- and two-dimensional lattice models. We benchmark the algorithm against exact diagonalization for system sizes up to N=8N=8 qubits and subsequently deploy it on superconducting transmon processors, systematically characterizing the impact of decoherence, gate infidelity, and readout errors on the resolved band structure. To mitigate hardware noise, we integrate zero-noise extrapolation and measurement-error mitigation, recovering quasienergy features with fidelities exceeding 90% for shallow circuit depths. Our results establish that variational phase-estimation protocols offer a scalable, hardware-efficient route to exploring driven quantum matter, providing a practical toolkit for simulating Floquet topological phenomena on current-generation quantum devices well before the advent of fault-tolerant quantum computation.
Abstract Superconducting quantum circuits are a leading platform for quantum-information processing, but their development requires expertise spanning device design, microfabrication, cryogenic operation, microwave control, and quantum characterization. In this work, we report the realization of a flux-tunable superconducting transmon platform and present an integrated workflow encompassing design, fabrication, measurement, and numerical modeling. The device consists of an Al/AlOx/Al transmon coupled to a three-dimensional microwave cavity operating in the circuit quantum electrodynamics (cQED) dispersive regime. Two-tone spectroscopy reveals a fundamental transition frequency of ω01=5.0122GHz and an anharmonicity of α/2π=−202.4MHz , corresponding to EJ/EC≈82.9 . Flux spectroscopy yields a cavity-qubit coupling strength of g/2π=260MHz , while the cavity linewidth gives a loaded quality factor of QL=6127 and a Purcell limit of T1Purcell≈82μs . Time-domain measurements yield coherence times of T1=17.4μs and T2∗=5.8μs . To investigate coherence and dissipation in the device, the experimental observations are analyzed within a Lindblad master-equation framework describing cavity photon loss and thermal excitation, qubit energy relaxation, and pure dephasing processes. Using a set of experimentally extracted parameters, the model reproduces the measured spectroscopic and time-domain observables, providing a quantitative description of the open-system dynamics of the transmon-cavity platform. The analysis shows that the measured coherence is primarily limited by pure dephasing and energy-relaxation processes, whereas thermal excitation plays a minor role under the experimental conditions. Inspired by the broader framework of quantum dissipation in macroscopic quantum systems pioneered by Caldeira and collaborators, this approach enables a quantitative analysis of relaxation and dephasing processes in superconducting quantum circuits. Beyond the characterization of a single device, the present work demonstrates the establishment of the expertise, infrastructure, and experimental workflow required for the design, fabrication, measurement, and modeling of superconducting quantum circuits, contributing to the development of quantum technologies in Brazil and related applications.
The existing scalable superconducting quantum processors have only nearest-neighbor coupling. This leads to a reduced circuit depth, requiring a large series of gates to perform an arbitrary unitary operation in such systems. Recently, multi-modal devices have been demonstrated as a promising candidate for small quantum processor units. Always on longitudinal coupling in such circuits leads to implementation of native high fidelity multi-qubit gates. We propose an architecture using such devices as building blocks for a highly connected larger quantum circuit. To demonstrate a quantum operation between such blocks, a standard transmon is coupled to the multi-modal circuit using a 3D bus cavity giving rise to small exchange interaction between the transmon and one of the modes. We study the cross-resonance interaction in such systems and characterize the entangling operation and the unitary imperfections and crosstalk as a function of device parameters. Finally, we tune up the cross-resonance drive to implement multi-qubit gates in this architecture.