arXiv subjects

Manthan Badbaria

Publications and source records attributed to Manthan Badbaria.

3 recordsLinked to original sources

arXiv2026

Rigorous characterization of continuous-variable quantum states via optical parametric amplifiers

Characterizing non-Gaussian quantum states is of paramount importance for continuous-variable quantum information processing, yet conventional homodyne-measurement-based state tomography remains limited by optical loss, detector efficiency, and measurement bandwidth. Here, we introduce an integrated framework for loss-tolerant characterization and certification using high-gain phase-sensitive optical parametric amplification and power measurements. Our computationally efficient semidefinite programming approach enables faithful reconstruction of parity-symmetric quantum states from amplified quadrature measurements while substantially relaxing detector-efficiency requirements and increasing the measurement bandwidth. We further develop a certification framework that directly quantifies non-Gaussianity via stellar-rank witnesses and Wigner negativity, using the same quadrature-power measurements. We demonstrate the efficacy of the proposed framework through both simulated and experimental data for representative quantum states, including single-photon, Schrodinger cat, and Gottesman-Kitaev-Preskill (GKP) states. By unifying loss-tolerant measurements, state tomography, and nonclassical-state certification within a single experimentally accessible framework, our approach provides a practical pathway toward verifying increasingly complex states and can be readily implemented with current quantum photonic technologies.

quant-ph↗

arXiv2026

Hardware-Efficient Erasure Qubits With Superconducting Transmon Qutrits

Quantum error correction using erasure qubits offers higher fault-tolerant thresholds and improved scaling by converting dominant physical errors into detectable erasures. In superconducting circuits, erasure qubits can be constructed using the dual-rail approach, which, however, requires additional qubit-count overhead and tailored coupling elements. Here, we demonstrate a hardware-efficient scheme that operates transmon qutrits as erasure qubits, which is compatible with standard superconducting circuit-QED hardware. The logical states $\ket{0_\text{L}}$ and $\ket{1_\text{L}}$ are represented by the ground and second excited states, while the dominant relaxation errors can be detected via an ancilla qubit using a microwave-activated two-qutrit SWAP gate. We demonstrate a logical qubit $T_1$ lifetime exceeding $500\,μ\mathrm{s}$, post-selected with repeated mid-circuit erasure detection, which is ten times longer than the $T_1$ time of the transmon physical qubit. Coherence times beyond $300\,μ\mathrm{s}$ are achieved using dynamical decoupling. Single-qubit gate operations reach average Clifford gate infidelity on the order of $10^{-4}$. We further demonstrate dual-purposing an ancilla qubit for both erasure detection and parity checking, showing heralded generation of Bell states between erasure qubits. These results suggest that mainstream architectures of transmon qubit arrays may already be capable of implementing erasure-based QEC strategies for hardware-efficient fault-tolerant quantum computing.

quant-ph↗

arXiv2024

State-dependent mobility edge in kinetically constrained models

In this work, we show that the kinetically constrained quantum East model lies between a quantum scarred and a many-body localized system featuring an unconventional type of mobility edge in the spectrum. We name this scenario $\textit{state-dependent}$ mobility edge: while the system does not exhibit a sharp separation in energy between thermal and non-thermal eigenstates, the abundance of non-thermal eigenstates results in slow entanglement growth for $\textit{many}$ initial states, such as product states, below a finite energy density. We characterize the state-dependent mobility edge by looking at the complexity of classically simulating dynamics using tensor network for system sizes well beyond those accessible via exact diagonalization. Focusing on initial product states, we observe a qualitative change in the dynamics of the bond dimension needed as a function of their energy density. Specifically, the bond dimension typically grows $\textit{polynomially}$ in time up to a certain energy density, where we locate the state-dependent mobility edge, enabling simulations for long times. Above this energy density, the bond dimension typically grows $\textit{exponentially}$ making the simulation practically unfeasible beyond short times, as generally expected in interacting theories. We correlate the polynomial growth of the bond dimension to the presence of many non-thermal eigenstates around that energy density, a subset of which we compute via tensor network. The outreach of our findings encompasses quantum sampling problems and the efficient simulation of quantum circuits beyond Clifford families.

cond-mat.stat-mech↗