arXiv subjects

Archana Kamal

Publications and source records attributed to Archana Kamal.

At least 19 recordsLinked to original sources

arXiv2026

Open system probes of renormalization group flow

Open system probes can provide an efficient means to characterize quantum many-body systems by employing them as engineered environments. The key idea is to map long-range spatial correlations of the environment onto dynamical correlations in the evolution of a simple quantum probe. Using the example of a qubit coupled to a transverse-field Ising model, we show how the non-Markovian rate or spectral flow can be used to identify stable and unstable fixed points, infer scaling dimensions of relevant fields, and deduce the renormalization group flow induced by deformations around any fixed point.

quant-ph↗

arXiv2026

Tutorial: Understanding quantum Zeno and anti-Zeno regimes

The quantum Zeno effect is a striking feature of quantum mechanics with foundational implications and practical applications in quantum control, error suppression, and error correction. In recent years, the effect has branched off into a variety of different interpretations, making it easy to miss its underlying unifying features. In particular, the quantum Zeno effect has been studied in the context of both selective and nonselective measurements; for both pulsed and continuous interactions; for suppression and enhancement of decay (Zeno / anti-Zeno effects); and even in the absence of measurement entirely. This tutorial presents a unified picture of these effects by examining how they all arise in the pedagogical example of a driven qubit subjected to measurements or dissipation. Zeno and anti-Zeno effects appear as regimes of a unified effect that appears when a measurement-like process competes with non-commuting evolution. The current landscape of Zeno and anti-Zeno effects is examined through this unifying lens, with a focus on experimental applications and implementations. Thus, this tutorial is ideal for new researchers interested in learning about cutting-edge tools in quantum control and measurement, while remaining highly relevant to quantum computing specialists aiming to understand the quantum Zeno effect's applications in adjacent subfields or apply it in a novel way in their own research. The quantum Zeno effect is found to be both ubiquitous and essential for the future of near-term quantum computing.

quant-ph↗

arXiv2026

Practical Limits to Single-Mode Vacuum Squeezing with a SNAIL Parametric Amplifier

We characterize single-mode vacuum squeezing generated by a SNAIL Parametric Amplifier (SPA) operated under conditions representative of practical sensing and qubit-readout experiments. Motivated by prior expectations that Kerr-induced distortion limits squeezing in degenerate parametric amplifiers, we varied external flux and pump power to explore operating points where Kerr nonlinearity is theoretically minimized. We find that for practical applications where the squeezing frequency is fixed, the Kerr was variable by about a factor of two and the achievable squeezing showed no significant dependence on Kerr. Theoretical modeling supports this observation and indicates that baseline Kerr values in state-of-the-art SPAs are already too small to impose a practical limitation. Instead, squeezing was dominated by internal resonator loss and insertion loss in the microwave chain. These results indicate that, in practical SPAs, reducing loss, rather than suppressing Kerr, is the primary route to improved squeezing performance.

quant-ph↗

arXiv2026

High-fidelity iSWAP gate with Double Transmon Coupler

Entangling operations are at the heart of all approaches to quantum information processing. Parametric gates, in particular, offer a versatile solution to strongly couple off-resonant superconducting qubits with suppressed parasitic crosstalk to spectator qubits due to frequency-selective activation. In this work, we demonstrate a parametric iSWAP gate between two transmon qubits using the recently developed double transmon coupler (DTC). The DTC supports robust internally-defined cancellation point (``off'' state) for static interactions, while simultaneously mediating a fast parametric coupling between data qubits that can be deployed for high-fidelity two-qubit operations. We use robust phase estimation to calibrate non-commuting error terms in the parametric iSWAP gate, and achieve a 99.827% gate fidelity in 40ns without any numerical optimization. The circuit architecture and calibration techniques developed here are extensible to other gate implementations and qubit modalities, paving the way towards resource-efficient quantum information processing.

quant-ph↗

arXiv2026

Hybrid superinductance with Al/InAs

We report microwave spectroscopy of Josephson junctions chains made from an epitaxial Al/InAs heterostructure. The chains exhibit superinductance, with characteristic wave impedance exceeding $R_{Q} = \hbar/(2e)^{2}$. The planar nature of the junctions results in a large plasma frequency, with no measurable deviations from ideal dispersion up to $12~\mathrm{GHz}$. Internal quality factors decrease sharply with frequency, which we describe with a simple loss model. The possibility of a loss mechanism intrinsic to the superconductor-semiconductor junction is considered.

cond-mat.mes-hall↗

arXiv2026

Open system dynamics in interacting quantum field theories

A quantum system that interacts with an environment generally undergoes nonunitary evolution described by a non-Markovian or Markovian master equation. In this paper, we construct the non-Markovian Redfield master equation for a quantum scalar field that interacts with a second field through a bilinear or nonlinear interaction on a Minkowski background. We use the resulting master equation to set up coupled differential equations that can be solved to obtain the equal-time two-point function of the system field. We show how the equations simplify under various approximations including the Markovian limit and argue that the Redfield equation-based solution provides a perturbative resummation to the standard second-order Dyson series result. For the bilinear interaction, we explicitly show that the Redfield solution is closer to the exact solution compared to the perturbation theory-based one. Further, the environment correlation function is oscillatory and nondecaying in this case, making the Markovian master equation a poor approximation. For the nonlinear interaction, on the other hand, the environment correlation function is sharply peaked and the Redfield solution matches that obtained using a Markovian master equation in the late-time limit.

hep-th↗

arXiv2025

Coherent and compact van der Waals transmon qubits

State-of-the-art superconducting qubits rely on a limited set of thin-film materials. Expanding their materials palette can improve performance, extend operating regimes, and introduce new functionalities, but conventional thin-film fabrication hinders systematic exploration of new material combinations. Van der Waals (vdW) materials offer a highly modular crystalline platform that facilitates such exploration while enabling gate-tunability, higher-temperature operation, and compact qubit geometries. Yet it remains unknown whether a fully vdW superconducting qubit can support quantum coherence and what mechanisms dominate loss at both low and elevated temperatures in such a device. Here we demonstrate quantum-coherent merged-element transmons made entirely from vdW Josephson junctions. These first-generation, fully crystalline qubits achieve microsecond lifetimes in an ultra-compact footprint without external shunt capacitors. Energy relaxation measurements, together with microwave characterization of vdW capacitors, point to dielectric loss as the dominant relaxation channel up to hundreds of millikelvin. These results establish vdW materials as a viable platform for compact superconducting quantum devices.

quant-ph↗

arXiv2025

Unified trajectory criterion for quantum and classical non-Markovianity

We propose a simple criterion for non-Markovianity: a quantum master equation is non-Markovian if and only if its \textit{trajectory set} contains a \textit{self-intersecting trajectory} (defined in the main text). Since self-intersection is invariant under time reversal, our criterion implies that Markovianity itself is time-reversal invariant: This property is not possessed by many existing criteria based on information flow or complete positivity. For a given quantum master equation, the set of trajectories generated from all initial states encodes its essential dynamical features. Thus, Markovianity can be determined directly from the trajectory set, reducing the problem to a general mathematical one: determing the non-Markovianity of the set itself, regardless of whether it originates from a quantum or classical process. Here, we solve this problem and classify the trajectory sets into three types: \textit{strictly Markovian}, \textit{initial-state Markovian}, and \textit{non-Markovian}. Through multiple examples, we compare our criterion with existing ones and show how they fail to capture Markovianity and non-Markovianity in various cases.

quant-ph↗

arXiv2023

Readout-induced suppression and enhancement of superconducting qubit lifetimes

It has long been known that the lifetimes of superconducting qubits suffer during readout, increasing readout errors. We show that this degradation is due to the anti-Zeno effect, as readout-induced dephasing broadens the qubit so that it overlaps 'hot spots' of strong dissipation, likely due to two-level systems in the qubit's bath. Using a flux-tunable qubit to probe the qubit's frequency dependent loss, we accurately predict the change in lifetime during readout with a new self-consistent master equation that incorporates the modification to qubit relaxation due to measurement-induced dephasing. Moreover, we controllably demonstrate both the Zeno and anti-Zeno effects, which explain suppression and the rarer enhancement of qubit lifetimes during readout.

quant-ph↗

arXiv2023

Modular tunable coupler for superconducting qubits

The development of modular and versatile quantum interconnect hardware is a key next step in the scaling of quantum information platforms to larger size and greater functionality. For superconducting quantum systems, fast and well-controlled tunable circuit couplers will be paramount for achieving high fidelity and resource efficient connectivity, whether for performing two-qubit gate operations, encoding or decoding a quantum data bus, or interfacing across modalities. Here we propose a versatile and internally-tunable double-transmon coupler (DTC) architecture that implements tunable coupling via flux-controlled interference in a three-junction dcSQUID. Crucially, the DTC possesses an internally defined zero-coupling state that is independent of the coupled data qubits or circuit resonators. This makes it particular attractive as a modular and versatile design element for realizing fast and robust linear coupling in several applications such as high-fidelity two-qubit gate operations, qubit readout, and quantum bus interfacing.

quant-ph↗

arXiv2021

Trade off-Free Entanglement Stabilization in a Superconducting Qutrit-Qubit System

Quantum reservoir engineering is a powerful framework for autonomous quantum state preparation and error correction. However, traditional approaches to reservoir engineering are hindered by unavoidable coherent leakage out of the target state, which imposes an inherent trade off between achievable steady-state state fidelity and stabilization rate. In this work we demonstrate a protocol that achieves trade off-free Bell state stabilization in a qutrit-qubit system realized on a circuit-QED platform. We accomplish this by creating a purely dissipative channel for population transfer into the target state, mediated by strong parametric interactions coupling the second-excited state of a superconducting transmon and the engineered bath resonator. Our scheme achieves a state preparation fidelity of 84% with a stabilization time constant of 339 ns, leading to the lowest error-time product reported in solid-state quantum information platforms to date.

quant-ph↗

arXiv2020

Engineering Framework for Optimizing Superconducting Qubit Designs

Superconducting quantum technologies require qubit systems whose properties meet several often conflicting requirements, such as long coherence times and high anharmonicity. Here, we provide an engineering framework based on a generalized superconducting qubit model in the flux regime, which abstracts multiple circuit design parameters and thereby supports design optimization across multiple qubit properties. We experimentally investigate a special parameter regime which has both high anharmonicity ($\sim\!1$GHz) and long quantum coherence times ($T_1\!=\!40\!-\!80\,\mathrm{μs}$ and $T_\mathrm{2Echo}\!=\!2T_1$).

quant-ph↗

arXiv2020

Quantum versus classical switching dynamics of driven-dissipative Kerr resonators

We report a first-principles study of the driven dissipative dynamics for Kerr oscillators in the mesoscopic regime. This regime is characterized by large Kerr nonlinearity, realized here using the nonlinear kinetic inductance of a large array of Josephson junctions. The experimentally measured nonlinear resonance lineshapes of the junction array modes show significant deviations from steady-state numerical predictions, and necessitate time-dependent numerical simulations indicative of strong measurement-induced dephasing in the system arising from the large cross-Kerr effect between array modes. Analytical and numerical calculations of switching rate corroborate this by showing the emergence of a slow time scale, which is much longer than the linear decay rate and is set by fluctuation-induced switching times in the bistable regime. Furthermore, our analysis shows that the usual quantum-activated escape treatment is inadequate for prediction of the switching rates at large frequency shifts caused by strong nonlinearities, necessitating a quantum treatment that utilizes the full system Liouvillian. Based on our analysis, we identify a universal crossover parameter that delineates the regimes of validity of semiclassical and quantum descriptions, respectively. Our work shows how dynamical switching effects in strongly nonlinear systems provide a platform to study quantum-to-classical transitions.

quant-ph↗

arXiv2018

A cosmological open quantum system

We derive the evolution equation for the density matrix of a UV- and IR- limited band of comoving momentum modes of the canonically normalized scalar degree of freedom in two examples of nearly de Sitter universes. Including the effects of a cubic interaction term from the gravitational action and tracing out a set of longer wavelength modes, we find that the evolution of the system is non-Hamiltonian and non-Markovian. We find linear dissipation terms for a few modes with wavelength near the boundary between system and bath and nonlinear dissipation terms for all modes. The non-Hamiltonian terms persist to late times when the scalar field dynamics is such that the curvature perturbation continues to evolve on super-Hubble scales.

hep-th↗

arXiv2016

Suppressing relaxation in superconducting qubits by quasiparticle pumping

Dynamical error suppression techniques are commonly used to improve coherence in quantum systems. They reduce dephasing errors by applying control pulses designed to reverse erroneous coherent evolution driven by environmental noise. However, such methods cannot correct for irreversible processes such as energy relaxation. In this work, we investigate a complementary, stochastic approach to reducing errors: instead of deterministically reversing the unwanted qubit evolution, we use control pulses to shape the noise environment dynamically. In the context of superconducting qubits, we implement a pumping sequence to reduce the number of unpaired electrons (quasiparticles) in close proximity to the device. We report a 70% reduction in the quasiparticle density, resulting in a threefold enhancement in qubit relaxation times, and a comparable reduction in coherence variability.

quant-ph↗

arXiv2016

Improved superconducting qubit coherence with high-temperature substrate annealing

We assess independently the impact of high-temperature substrate annealing and metal deposition conditions on the coherence of transmon qubits in the standard 2D circuit-QED architecture. We restrict our study to devices made with aluminum interdigital capacitors on sapphire substrates. We record more than an order-of-magnitude improvement in the relaxation time of devices made with an annealed substrate, independent of whether a conventional evaporator or molecular beam epitaxy chamber was used to deposit the aluminum. We also infer similar levels of flux noise and photon shot noise through dephasing measurements on these devices. Our results indicate that substrate annealing plays a primary role in fabricating low-loss qubits, consistent with the hypothesis that substrate-air and substrate-metal interfaces are essential factors limiting the qubit lifetimes in superconducting circuits.

cond-mat.mes-hall↗

arXiv2015

Heisenberg-limited qubit readout with two-mode squeezed light

We show how to use two-mode squeezed light to exponentially enhance cavity-based dispersive qubit measurement. Our scheme enables true Heisenberg-limited scaling of the measurement, and crucially, is not restricted to small dispersive couplings or unrealistically long measurement times. It involves coupling a qubit dispersively to two cavities, and making use of a symmetry in the dynamics of joint cavity quadratures (a so-called quantum-mechanics-free subsystem). We discuss the basic scaling of the scheme and its robustness against imperfections, as well as a realistic implementation in circuit quantum electrodynamics.

quant-ph↗

arXiv2014

Coherence and Decay of Higher Energy Levels of a Superconducting Transmon Qubit

We present measurements of coherence and successive decay dynamics of higher energy levels of a superconducting transmon qubit. By applying consecutive $π$-pulses for each sequential transition frequency, we excite the qubit from the ground state up to its fourth excited level and characterize the decay and coherence of each state. We find the decay to proceed mainly sequentially, with relaxation times in excess of 20 $μ$s for all transitions. We also provide a direct measurement of the charge dispersion of these levels by analyzing beating patterns in Ramsey fringes. The results demonstrate the feasibility of using higher levels in transmon qubits for encoding quantum information.

quant-ph↗