arXiv subjects

A. Kamal

Publications and source records attributed to A. Kamal.

13 recordsLinked to original sources

arXiv2026

Long-range waveguide-quantum electrodynamics with left-handed transmission lines

While engineering long-range light-matter interactions is the principal aim in waveguide-QED, ironically most of the building blocks rest on local short-range couplings, such as nearest-neighbor-coupled cavity arrays employed in canonical models. Here, we propose a waveguide-QED system with native long-range interactions, comprising a single emitter coupled to a left-handed transmission line (LHTL). Interestingly, the LHTL emulates a synthetic photonic lattice with a slow logarithmic decay of hopping amplitudes over a distance set entirely by the ratio of UV and IR cutoffs of line dispersion. Its intrinsic long-range nature manifests both in the properties of atom-photon bound and scattering states, which exhibit algebraic localization and accelerated photon propagation respectively. Using a method of 'running exponents', we develop a unified picture connecting waveguide dispersion to bound state and light front profiles obtained in the strong long-range hopping regime. These results motivate how transmission lines can enable multi-qubit information processing with tunable-range interactions.

quant-ph↗

arXiv2025

Resonant transport and line-type resonances in tilted Dirac cone double-barrier structures

We study the transport properties of Dirac fermions in a graphene-based double-barrier structure composed of two tilted-cone regions separated by a central pristine graphene region. Using the transfer matrix method, we systematically analyze how different cone tilts affect Dirac fermion transmission. In reciprocal space, at fixed energy, the Dirac cones of distinct regions generate isoenergetic conical surfaces (Fermi surfaces). When these surfaces overlap, their intersections define ``active surfaces'' that enable fermion transmission. In the symmetric double-barrier configuration, coupling between the barriers and the central well gives rise to multiple resonance peaks, including line-type resonances, even within nominally forbidden energy zones. The number and positions of these resonances depend sensitively on the system parameters. These findings provide new insights into the role of Dirac cone tilt in complex junctions and may guide the design of nanoelectronic devices based on two-dimensional tilted-cone materials such as $\alpha$-(BEDT-TTF)$_2$I$_3$ and borophene.

cond-mat.mes-hall↗

arXiv2024

Quantifying spectral signatures of non-Markovianity beyond the Born-Redfield master equation

Memory or time-non-local effects in open quantum dynamics pose theoretical as well as practical challenges in the understanding and control of noisy quantum systems. While there has been a comprehensive and concerted effort towards developing diagnostics for non-Markovian dynamics, all existing measures rely on time-domain measurements which are typically slow and expensive as they require averaging several runs to resolve small transient features on a broad background, and scale unfavorably with system size and complexity. In this work, we propose a spectroscopic measure of non-Markovianity which can detect persistent non-Markovianity in the system steady state. In addition to being experimentally viable, the proposed measure has a direct information theoretic interpretation: a large value indicates the information loss per unit bandwidth of making the Markov approximation. In the same vein, we derive a frequency-domain quantum master equation (FD-QME) that goes beyond the standard Born-Redfield description and retains the full memory of the state of the reduced system. Using the FD-QME and the proposed measure, we are able to reliably diagnose and quantify non-Markovianity in several system-environment settings including those with environmental correlations and retardation effects.

quant-ph↗

arXiv2024

Self phase-matched broadband amplification with a left-handed Josephson transmission line

Josephson Traveling Wave Parametric Amplifiers (J-TWPAs) are promising platforms for realizing broadband quantum-limited amplification of microwave signals. However, substantial gain in such systems is attainable only when strict constraints on phase matching of the signal, idler and pump waves are satisfied -- this is rendered particularly challenging in the presence of nonlinear effects, such as self- and cross-phase modulation, which scale with the intensity of propagating signals. In this work, we present a simple J-TWPA design based on `left-handed' (negative-index) nonlinear Josephson metamaterial, which has phase matching native to its design precluding the need for any complicated circuit or dispersion engineering. The resultant efficiency of four-wave mixing process can implement gains in excess of 20 dB over few GHz bandwidths with much shorter lines than previous implementations. Furthermore, the autonomous nature of phase matching considerably simplifies the J-TWPA design than previous implementations based on `right-handed' (positive index) Josephson metamaterials, making the proposed architecture particularly appealing from a fabrication perspective. The left-handed JTL introduced here constitutes a new modality in distributed Josephson circuits, and forms a crucial piece of the unified framework that can be used to inform the optimal design and operation of broadband microwave amplifiers.

quant-ph↗

arXiv2023

Scalable entanglement stabilization with modular reservoir engineering

Dissipation engineering is a powerful framework for quantum state preparation and autonomous error correction in few-qubit systems. In this work, we examine the scalability of this approach and give three criteria which any dissipative state stabilization protocol should satisfy to be truly scalable as the number of qubits grows. Besides the requirement that it can be constructed in a resource-efficient manner from simple-to-engineer building blocks, a scalable protocol must also exhibit favorable scaling of the stabilization time with the increase in system size. We present a family of protocols which employ fixed-depth qubit-qubit interactions alongside engineered linear dissipation to stabilize an $N$-qubit W state. We find that a modular approach to dissipation engineering, with several overlapping few-qubit dissipators rather than a single $N$-qubit dissipator, is essential for our protocol to be scalable. With this approach, as the number of qubits increases our protocol exhibits low-degree polynomial scaling of the stabilization time and linear growth of the number of control drives in the best case. While the proposed protocol is most easily accessible with current state-of-the-art circuit-QED architectures, the modular dissipation engineering approach presented here can be readily adapted to other platforms and for stabilization of other interesting quantum states.

quant-ph↗

arXiv2022

Perturbative diagonalization for time-dependent strong interactions

We present a systematic method to implement a perturbative Hamiltonian diagonalization based on the time-dependent Schrieffer-Wolff transformation. Applying our method to strong parametric interactions we show how, even in the dispersive regime, full Rabi model physics is essential to describe the dressed spectrum. Our results unveil several qualitatively new results including realization of large energy-level shifts, tunable in magnitude and sign with the frequency and amplitude of the pump mediating the parametric interaction. Crucially Bloch-Siegert shifts, typically thought to be important only in the ultra-strong or deep-strong coupling regimes, can be rendered large even for weak dispersive interactions to realize points of exact cancellation of dressed shifts (`blind spots') at specific pump frequencies. The framework developed here highlights the rich physics accessible with time-dependent interactions and serves to significantly expand the functionalities for control and readout of strongly-interacting quantum systems.

quant-ph↗

arXiv2021

Strong parametric dispersive shifts in a statically decoupled multi-qubit cavity QED system

Cavity quantum electrodynamics (QED) with in-situ tunable interactions is important for developing novel systems for quantum simulation and computing. The ability to tune the dispersive shifts of a cavity QED system provides more functionality for performing either quantum measurements or logical manipulations. Here, we couple two transmon qubits to a lumped-element cavity through a shared dc-SQUID. Our design balances the mutual capacitive and inductive circuit components so that both qubits are highly decoupled from the cavity, offering protection from decoherence processes. We show that by parametrically driving the SQUID with an oscillating flux it is possible to independently tune the interactions between either of the qubits and the cavity dynamically. The strength and detuning of this cavity QED interaction can be fully controlled through the choice of the parametric pump frequency and amplitude. As a practical demonstration, we perform pulsed parametric dispersive readout of both qubits while statically decoupled from the cavity. The dispersive frequency shifts of the cavity mode follow the expected magnitude and sign based on simple theory that is supported by a more thorough theoretical investigation. This parametric approach creates a new tunable cavity QED framework for developing quantum information systems with various future applications, such as entanglement and error correction via multi-qubit parity readout, state and entanglement stabilization, and parametric logical gates.

quant-ph↗

arXiv2020

Strong coupling diagnostics for multi-mode open systems

We present a new method to diagnose strong coupling in multi-mode open systems. Our method presents a non-trivial extension of exceptional point (EP) analysis employed for such systems; specifically, we show how eigenvectors can not only reproduce all the features predicted by EPs but are also able to identify the physical modes that hybridize in different regions of the strong coupling regime. As a demonstration, we apply this method to study hybridization physics in a three-mode optomechanical system and determine the parameter regime for efficient sideband cooling of the mechanical oscillator in the presence of reservoir correlations.

quant-ph↗

arXiv2020

On Multifractal Spectra and Renyi Dimensions in 14.5A GeV/c $^{28}$Si-nucleus Collisions

A systematic analysis of the data on 14.5A GeV/c $^{28}$Si-nucleus collisions is carried out to investigate the behaviours of Renyi dimensions, Dq and Multifractal Spectral Function, $f(α_q)$. The Renyi dimensions, $D_q$, are observed to decrease with increasing order of the moments, $q$. However, the Multifractal Spectra are concave downwards with their maxima occurring around $α_q = 1.21 \pm 0.01$. A continuous curve representing Multifractal Spectral Function, $f(α_q)$, characterizes manifestation of fluctuations in the rapidity space.

hep-ex↗

arXiv2019

High-fidelity dissipative engineering using parametric interactions

Established methods for dissipative state preparation typically rely on resolving resonances, limiting the target state fidelity due to competition between the stabilization mechanism and uncontrolled dissipation. We propose a protocol devoid of such constraints, using parametric couplings to engineer dissipation for preparation of any maximally entangled two-qubit state. Our scheme allows high-fidelity entanglement generation with short convergence time, continuous control of the target state in the stabilized manifold, and is realizable with state-of-the-art superconducting qubit technology.

quant-ph↗

arXiv2016

The Flux Qubit Revisited to Enhance Coherence and Reproducibility

The scalable application of quantum information science will stand on reproducible and controllable high-coherence quantum bits (qubits). Here, we revisit the design and fabrication of the superconducting flux qubit, achieving a planar device with broad frequency tunability, strong anharmonicity, high reproducibility, and relaxation times in excess of $40\,μ$s at its flux-insensitive point. Qubit relaxation times $T_1$ across 22 qubits are consistently matched with a single model involving resonator loss, ohmic charge noise, and 1/f flux noise, a noise source previously considered primarily in the context of dephasing. We furthermore demonstrate that qubit dephasing at the flux-insensitive point is dominated by residual thermal photons in the readout resonator. The resulting photon shot noise is mitigated using a dynamical decoupling protocol, resulting in $T_2\approx 85\,μ$s, approximately the $2T_1$ limit. In addition to realizing an improved flux qubit, our results uniquely identify photon shot noise as limiting $T_2$ in contemporary qubits based on transverse qubit-resonator interaction.

quant-ph↗

arXiv2016

Minimal models for nonreciprocal amplification using biharmonic drives

We present a generic system of three harmonic modes coupled parametrically with a time-varying coupling modulated by a combination of two pump harmonics, and show how this system provides the minimal platform to realize nonreciprocal couplings that can lead to gainless photon circulation, and phase-preserving or phase-sensitive directional amplification. Explicit frequency-dependent calculations within this minimal paradigm highlight the separation of amplification and directionality bandwidths, universal in such schemes. We also study the influence of counter-rotating interactions that can adversely affect directionality and associated bandwidth; we find that these effects can be mitigated by suitably designing the properties of the auxiliary mode that plays the role of an engineered reservoir to the amplification mode space.

cond-mat.mes-hall↗

arXiv2015

Thermal and Residual Excited-State Population in a 3D Transmon Qubit

We present a systematic study of the first excited-state population in a 3D transmon qubit mounted in a dilution refrigerator with a variable temperature. Using a modified version of the protocol developed by Geerlings et al. [1], we observe the excited-state population to be consistent with a Maxwell-Boltzmann distribution, i.e., a qubit in thermal equilibrium with the refrigerator, over the temperature range 35-150 mK. Below 35 mK, the excited-state population saturates to 0.1%, near the resolution of our measurement. We verified this result using a flux qubit with ten-times stronger coupling to its readout resonator. We conclude that these qubits have effective temperature T_{eff} = 35 mK. Assuming T_{eff} is due solely to hot quasiparticles, the inferred qubit lifetime is 108 us and in plausible agreement with the measured 80 us.

quant-ph↗