arXiv subjects

Amrit De

Publications and source records attributed to Amrit De.

At least 19 recordsLinked to original sources

arXiv2026

An LLM-Orchestrated Agent for Directional-Coupler Design with Self-Consistent Eigenmode and FDTD Validation

We present a design agent which is a Large Language Model (LLM) that orchestrates, but does not perform, the numerical simulations to design a silicon-on-insulator (SOI) $2\times2$ directional coupler. We choose a symmetric phase-matched coupler where a lot of analytical results are available that help the design strategy. The LLM proposes candidate gap values (a geometrical dimension size) and judges convergence, while all physics is owned by deterministic solvers: a frequency-domain eigenmode solver estimates the coupling coefficient~$\kappa$ for the current design, and an independent Finite-Difference Time-Domain (FDTD) stage validates it. Both solvers operate on a common slab-projected two-dimensional (2D) effective-index reduction of the silicon film, so the design~$\kappa$ and the FDTD response are consistent by problem design; the residual between them is shown to be a single constant phase offset~$\phi$, attributable to a fixed excess coupling length $L_{\mathrm{extra}}=\SI{2.837(11)}{\micro\meter}$ that we find invariant across a factor-of-two range in~$\kappa$. Folding this offset into a closed-loop length correction, the agent delivers a $50/50$ splitter whose FDTD-measured cross fraction is $0.498$ (target $0.500$), a residual of $0.0017$. Results are made self-consistent within the 2D effective-index model; and the LLM succeeds in delivering a suitable design over a number of attempts.

physics.optics↗

arXiv2024

Topological argument for robustness of coherent states in quantum optics

Coherent states, being the closest analog to classical states of wave systems, are well known to possess special properties that set them apart from most other quantum optical states. For example, they are robust against photon loss and do not easily get entangled upon interaction with a beamsplitter, and hence are called ``pointer states'', which is often attributed to them being eigenstates of the annihilation operator. Here we provide insights into a topological argument for their robustness using two separate but exact mappings of a prototypical quantum optics model - the driven Jaynes-Cummings model. The first mapping is based on bosonization and refermionization of the Jaynes-Cummings model into the fermionic Su-Schrieffer-Heeger model hosting zero-energy topologically protected edge states. The second mapping is based on the algebra of deformed f-oscillators. We choose these mappings to explicitly preserve the translational symmetry of the model along a Fock-state ladder basis, which is important for maintaining the symmetry-protected topology of such 1D lattices. In addition, we show that the edge state form is preserved even when certain chiral symmetry is broken, corresponding to a single-photon drive for the quantum optics model that preserves the coherent state; however, the addition of two-photon drive immediately disturbs the edge state form, as confirmed by numerical simulations of the mapped SSH model; this is expected since two-photon drive strongly perturbs the coherent state into a squeezed state. Our theory sheds light on a fundamental reason for the robustness of coherent states, both in existence and entanglement -- an underlying connection to topology.

quant-ph↗

arXiv2020

Anomalous Magneto Optic Effects from an Antiferromagnet Topological-Insulator Heterostructure

Materials with no net magnetization are generally not magneto-optically active. While this is individually true for a collinear antiferromagnet (AFM) and a topological insulator (TI), it is shown here that the magneto-optic Kerr effect (MOKE) emerges when the TI and AFM films are proximity coupled. Because of the lack of macroscopic magnetization, the AFM only couples to the spin of one of the TI's surfaces breaking time-reversal and inversion symmetry -- which leads to a tiny $\mu$deg MOKE signal. This small MOKE can be easily enhanced by 5 orders of magnitude, via cavity resonance, by optimizing the AFM and TI film thicknesses on the substrate. For slightly off-resonant structures, a 6 deg Kerr rotation can be electrically switched on by varying the Fermi energy. This requires less than 20 meV, which is encouraging for low power spintronics and magneto-optic devices. We further show that this simple structure is easily resilient to 5% material growth error.

cond-mat.mes-hall↗

arXiv2018

1/f Flux Noise in low-T$_c$ SQUIDs due to Superparamagnetic Phase Transitions in Defect Clusters

It is shown here that $1/f^α$ flux noise in conventional low-T$_c$ SQUIDs is a result of low temperature superparamagnetic phase transitions in small clusters of strongly correlated color center defects. The spins in each cluster interact via long-range ferromagnetic interactions. Due to its small size, the cluster behaves like a 'random-telegraphic' macro-spin when transitioning to the superparamagnetic phase. This results in $1/f^α$ noise when ensemble averaged over a random distribution of clusters. This model is self-consistent and explains all related experimental results which includes $α\sim 0.8$ independent of system-size. The experimental flux-inductance-noise spectrum is explained through three-point correlation calculations and time reversal symmetry breaking arguments. Also, unlike the flux noise, it is shown why the second-spectrum inductance noise is inherently temperature dependent due to the fluctuation-dissipation theorem. A correlation-function calculation methodology using Ising-Glauber dynamics was key for obtaining these results.

cond-mat.mes-hall↗

arXiv2018

High Figure of Merit Magneto Optics from Interfacial Skyrmions on Topological Insulators

In the Kerr rotation geometry, magneto optic memory devices typically suffer from low figure-of-merit (FOM) and long write times. We show that skyrmions formed at the interface of a thin-film multiferroic and a topological insulator can give rise to high FOM magneto optic Kerr effects (MOKEs). Huge differential MOKE can arise in parts of the phase diagram. Resonance like features in the MOKE spectra arising from the induced low energy TI bandgap, the multiferroic-film thickness, and the high energy Drude like behavior are resolved and explained. The Fermi level dependence of the MOKE signatures is distinct for the different magnetic textures. This has broad implications for magnetic texture characterization, electro-optic modulators and isolators and high density magnetic optic memory.

cond-mat.mes-hall↗

arXiv2016

Interlayer Transport through a Graphene / Rotated-Boron-Nitride / Graphene Heterostructure

Interlayer electron transport through a graphene / hexagonal boron-nitride (h-BN) / graphene heterostructure is strongly affected by the misorientation angle $θ$ of the h-BN with respect to the graphene layers with different physical mechanisms governing the transport in different regimes of angle, Fermi level, and bias. The different mechanisms and their resulting signatures in resistance and current are analyzed using two different models, a tight-binding, non-equilibrium Green function model and an effective continuum model, and the qualitative features resulting from the two different models compare well. In the large-angle regime ($θ> 4^\circ$), the change in the effective h-BN bandgap seen by an electron at the $K$ point of the graphene causes the resistance to monotonically increase with angle by several orders of magnitude reaching a maximum at $θ= 30^\circ$. It does not affect the peak-to-valley current ratios in devices that exhibit negative differential resistance. In the small-angle regime ($θ< 4^\circ$), Umklapp processes open up new conductance channels that manifest themselves as non-monotonic features in a plot of resistance versus Fermi level that can serve as experimental signatures of this effect. For small angles and high bias, the Umklapp processes give rise to two new current peaks on either side of the direct tunneling peak.

cond-mat.mtrl-sci↗

arXiv2016

Interlayer Resistance of Misoriented MoS2

Interlayer misorientation in transition metal dichalcogenides alters the interlayer distance, the electronic band structure, and the vibrational modes, but, its effect on the interlayer resistance is not known. This work analyzes the coherent interlayer resistance of misoriented 2H-MoS2 for low energy electrons and holes as a function of the misorientation angle. The electronic interlayer resistance monotonically increases with the supercell lattice constant by several orders of magnitude similar to that of misoriented bilayer graphene. The large hole coupling gives low interlayer hole resistance that weakly depends on the misorientation angle. Interlayer rotation between an n-type region and a p-type region will suppress the electron current with little effect on the hole current. We estimate numerical bounds and explain the results in terms of the orbital composition of the bands at high symmetry points. Density functional theory calculations provide the interlayer coupling used in both a tunneling Hamiltonian and a non-equilibrium Green function calculation of the resistivity.

cond-mat.mtrl-sci↗

arXiv2015

Strong Cavity-Pseudospin Coupling in Monolayer Transition Metal Dichalcogenides: Spontaneous Spin-Oscillations and Magnetometry

Strong coupling between the electronic states of monolayer transition metal dichalcogenides (TMDC) such as MoS$_2$, MoSe$_2$, WS$_2$, or WSe$_2$, and a two-dimensional (2D) photonic cavity gives rise to several exotic effects. The Dirac type Hamiltonian for a 2D gapped semiconductor with large spin-orbit coupling facilitates pure Jaynes-Cummings type coupling in the presence of a single mode electric field. The presence of an additional circularly polarized beam of light gives rise to valley and spin dependent cavity-QED properties. The cavity causes the TMDC monolayer to act as an on-chip coherent light source and a spontaneous spin-oscillator. In addition, a TMDC monolayer in a cavity is a sensitive magnetic field sensor for an in-plane magnetic field.

cond-mat.mes-hall↗

arXiv2015

Fast Quantum Control for Weakly Nonlinear Qubits: On Two-Quadrature Adiabatic Gates

Adiabatic or slowly varying gate operations are typically required in order to remain within the qubit subspace in an anharmonic oscillator. However significant speed ups are possible by using the two quadrature derivative-removal-by-adiabatic-gate(DRAG) technique, where a second time derivative pulse component burns a spectral hole near an unwanted transition. It is shown here, that simultaneous optimization of the detuning and the pulse norm in addition, further reduces leakage errors and significantly improve gate fidelities. However, with this optimization accounting for the AC Stark shift, there is a low spectral weight pulse envelope regime, where DRAG is almost not needed and where the two state error fidelities are stable against pulse jitter. Explicit time evolution calculations are carried out in the lab frame for truncated multi-level Transmon qubit models obtained from a tight-binding model.

quant-ph↗

arXiv2015

Universal set of Dynamically Protected Gates for Bipartite Qubit Networks II: Soft Pulse Implementation of the [[5,1,3]] Quantum Error Correcting Code

We model repetitive quantum error correction (QEC) with the single-error-correcting five-qubit code on a network of individually-controlled qubits with always-on Ising couplings, using our previously designed universal set of quantum gates based on sequences of shaped decoupling pulses. In addition to serving as accurate quantum gates, the sequences also provide dynamical decoupling (DD) of low-frequency phase noise. The simulation involves integrating unitary dynamics of six qubits over the duration of tens of thousands of control pulses, using classical stochastic phase noise as a source of decoherence. The combined DD/QEC protocol dramatically improves the coherence, with the QEC alone responsible for more than an order of magnitude infidelity reduction.

quant-ph↗

arXiv2014

A Couped-Qubit Tavis Cummings Scheme for Prolonging Quantum Coherence

Qubit-qubit interactions can significantly boost quantum coherence times for Bell states. The coherence-time-enhancements are however not monotonic and there exists a phase where further increasing the interaction is unhelpful. A resonator in a suggested circuit QED type implementation of the Tavis-Cummings(Dicke) model, is shown to shift this transition point depending on the number of loaded photons. This allows the resonator to amplify the coherence enhancements in certain regimes. The interactions also induce unusual collapse and revival type behavior for the entanglement dynamics. A new and exact open quantum systems formalism -- the quasi-Hamiltonians for the Dicke model thus reveals how a Bell state in a resonator can be protected against $1/f$ noise from randomly fluctuating two level systems. Simple circuit level details are given for flux qubits.

cond-mat.mes-hall↗

arXiv2014

An Ising-Glauber Spin Cluster Model for Temperature Dependent Magnetization Noise in SQUIDs

Clusters of interacting two-level-systems (TLS),likely due to $F^+$ centers at the metal-insulator interface, are shown to self consistently lead to $1/f^{α}$ magnetization noise in SQUIDs. By introducing a correlation-function calculation method and without any a priori assumptions on the distribution of fluctuation rates, it is shown why the flux noise is only weakly temperature dependent with $α\lesssim 1$, while the inductance noise has a huge temperature dependence seen in experiment, even though the mechanism producing both spectra is the same. Though both ferromagnetic- RKKY and short-range-interactions (SRI) lead to strong flux-inductance-noise cross-correlations seen in experiment, the flux noise varies a lot with temperature for SRI. Hence it is unlikely that the TLS's time reversal symmetry is broken by the same mechanism which mediates surface ferromagnetism in nanoparticles and thin films of the same insulator materials.

cond-mat.mes-hall↗

arXiv2013

Control of Majorana Edge Modes by a g-factor Engineered Nanowire Spin Transistor

We propose the manipulation of Majorana edge states via hybridization and spin currents in a nanowire spin transistor. The spin transistor is based on a heterostructure nanowire comprising of semiconductors with large and small g-factors that form the topological and non-topological regions respectively. The hybridization of bound edge states results in spin currents and $4π$-periodic torques, as a function of the relative magnetic field angle -- an effect which is dual to the fractional Josephson effect. We establish relation between torques and spin-currents in the non-topological region where the magnetic field is almost zero and spin is conserved along the spin-orbit field direction. The angular momentum transfer could be detected by sensitive magnetic resonance force microscopy techniques.

cond-mat.mes-hall↗

arXiv2013

Dynamically corrected gates for qubits with always-on Ising couplings: Error model and fault-tolerance with the toric code

We describe how a universal set of dynamically-corrected quantum gates can be implemented using sequences of shaped decoupling pulses on any qubit network forming a sparse bipartite graph with always-on Ising interactions. These interactions are constantly decoupled except when they are needed for two-qubit gates. We analytically study the error operators associated with the constructed gates up to third order in the Magnus expansion, analyze these errors numerically in the unitary time evolution of small qubit clusters, and give a bound on high-order errors for qubits on a large square lattice. We prove that with a large enough toric code the present gate set can be used to implement a fault-tolerant quantum memory.

quant-ph↗

arXiv2013

Spin Transfer of Quantum Information between Majorana Modes and a Resonator

We show that resonant coupling and entanglement between a mechanical resonator and majorana bound states can be achieved via spin currents in a 1D quantum wire with strong spin-orbit interactions. The bound states induced by vibrating and stationary magnets can hybridize thus resulting in spin-current induced $4π$-periodic torque, as a function of the relative field angle, acting on the resonator. We study the feasibility of detecting and manipulating majorana bound states with the use of magnetic resonance force microscopy techniques.

cond-mat.mes-hall↗

arXiv2013

Continuous third harmonic generation in a terahertz driven modulated nanowire

We consider the possibility of observing continuous third-harmonic generation using a strongly driven, single-band one-dimensional metal. In the absence of scattering, the quantum efficiency of frequency tripling for such a system can be as high as 93%. Combining the Floquet quasi-energy spectrum with the Keldysh Green's function technique, we derive the semiclassical master equation for a one-dimensional band of strongly and rapidly driven electrons in the presence of weak scattering by phonons. The power absorbed from the driving field is continuously dissipated by phonon modes, leading to a quasi-equilibrium in the electron distribution. We use the Kronig-Penney model with varying effective mass to establish growth parameters of an InAs/InP nanowire near optimal for third harmonic generation at terahertz frequency range.

cond-mat.mes-hall↗

arXiv2012

Magnetization Noise Induced Collapse and Revival of Rabi Oscillations in circuit QED

We use a quasi Hamiltonian formalism to describe the dissipative dynamics of a circuit QED qubit that is affected by several fluctuating two level systems with a 1/f noise power spectrum. The qubit-resonator interactions are described by the Jaynes Cummings model. We argue that the presence of pure dephasing noise in such a qubit-resonator system will also induce an energy relaxation mechanism via a fluctuating dipole coupling term. This random modulation of the coupling is seen to lead to rich physical behavior. For non-Markovian noise, the coupling can either worsen or alleviate decoherence depending on the initial conditions. The magnetization noise leads to behavior resembling the collapse and revival of Rabi oscillations. For a broad distribution of noise couplings, the frequency of these oscillations depends on the mean noise strength. We describe this behavior semi-analytically and find it to be independent of the number of fluctuators. This phenomenon could be used as an in situ probe of the noise characteristics.

cond-mat.mes-hall↗

arXiv2012

Electronic Structure and Optical Properties of the Lonsdaleite Phase of Si, Ge and diamond

Crystalline semiconductors may exist in different polytypic phases with significantly different electronic and optical properties. In this paper, we calculate the electronic structure and optical properties of diamond, Si and Ge in the lonsdaleite (hexagonal-diamond) phase. We use an empirical pseudopotentials method based on transferable model potentials, including spin-orbit interactions. We obtain band structures, densities of states and complex dielectric functions calculated in the dipole approximation for light polarized perpendicular and parallel to the c-axis of the crystal. We find strong polarization dependent optical anisotropy. Simple analytical expressions are provided for the dispersion relations. We find that in the lonsdaleite phase, diamond and Si remain indirect gap semiconductors while Ge is transformed into a direct gap semiconductor with a significantly smaller band gap.

cond-mat.mtrl-sci↗