arXiv subjects

Jayameenakshi Venkatraman

Publications and source records attributed to Jayameenakshi Venkatraman.

13 recordsLinked to original sources

arXiv2026

Detecting quantum noise of a solid-state spin ensemble with dispersive measurement

We theoretically explore protocols for measuring the spin polarization of an ensemble of solid-state spins, with precision at or below the standard quantum limit. Such measurements in the solid-state are challenging, as standard approaches based on optical fluorescence are often limited by poor readout fidelity. Indirect microwave resonator-mediated measurements provide an attractive alternative, though a full analysis of relevant sources of measurement noise is lacking. In this work we study dispersive readout of an inhomogeneously broadened spin ensemble via coupling to a driven resonator measured via homodyne detection. We derive generic analytic conditions for when the homodyne measurement can be limited by the fundamental spin-projection noise, as opposed to microwave-drive shot noise or resonator phase noise. By studying fluctuations of the measurement record in detail, we also propose an experimental protocol for directly detecting spin squeezing, i.e. a reduction of the spin ensemble's intrinsic projection noise from entanglement. Our protocol provides a method for benchmarking entangled states for quantum-enhanced metrology.

quant-ph↗

arXiv2025

Unraveling the switching dynamics in a quantum double-well potential

The spontaneous switching of a quantum particle between the wells of a double-well potential is a phenomenon of general interest to physics and chemistry. It was broadly believed that the switching rate decreases steadily with the size of the energy barrier. This view was challenged by a recent experiment on a driven superconducting Kerr nonlinear oscillator (often called the Kerr-cat qubit or the Kerr parametric oscillator), whose energy barrier can be increased by ramping up the drive. Remarkably, as the drive amplitude increases, the switching rate exhibits a step-like decrease termed the "staircase". The view challenged by the experiment demands a deep review of our understanding of quantum effects in double wells. In this work, we derive a semi-analytical formula for the switching rate that resolves a continuous transition between tunneling- and dissipation-dominated dynamics. These two dynamics are observed respectively in the flat and the steep parts of each step in the staircase. Our formula exposes two distinct dissipative processes that limit tunneling: dephasing and decay. This allows us to predict the critical drive amplitudes where steps occur. In addition, we show that in the regime of a few states in the well and under moderate to low temperatures, highly excited states are populated predominantly via cascaded and direct thermal heating rather than quantum heating. At very low temperatures, however, the perturbation induced by the nonhermitian Hamiltonian becomes important and facilitates a new form of quantum heating. We numerically map the activation mechanism as a function of drive amplitude, damping rate, and temperature. Our theory deepens the understanding of switching dynamics between metastable quantum states, highlights the importance of a general interplay between tunneling and dissipation, and identifies a novel quantum regime in activated transitions.

quant-ph↗

arXiv2025

A spin-embedded diamond optomechanical resonator with mechanical quality factor exceeding one million

Diamond optomechanical crystal (OMC) devices with embedded color center spins are promising platforms for a broad range of applications in quantum sensing, networking, and computing applications, offering an interface between a GHz-frequency mechanical mode and both optical photons and coherent spins. A crucial but elusive step towards realizing this platform is to engineer a device with a high-quality factor mechanical mode while preserving the bulk-like coherence of embedded spins. Here we demonstrate sideband-resolved diamond OMCs with mechanical quality factors in excess of $10^6$ at cryogenic temperatures, and find coherence times up to $T_2$ = 270 $μ$s for embedded nitrogen vacancy (NV) centers. Furthermore, we measure these devices across five orders of magnitude in intracavity optical power, demonstrating robust power handling and a high optomechanical cooperativity ($C\gg1$) at cryogenic temperatures that is essential for a broad range of quantum protocols requiring strong, coherent interactions between photons and phonons. These results are enabled by a robust, high-throughput method for forming single-crystal diamond membranes in combination with chemical vapor deposition (CVD) diamond overgrowth with nitrogen $δ$-doping. We discuss the prospects of this platform for hybrid spin-mechanical devices in the quantum regime.

quant-ph↗

arXiv2025

Integrated phononic waveguide on thin-film lithium niobate on diamond

We demonstrate wavelength-scale phononic waveguides formed by transfer-printed thin-film lithium niobate (LN) on bulk diamond (LNOD), a material stack that combines the strong piezoelectricity of LN with the high acoustic velocity and color-center compatibility of diamond. We characterize a delay line based on a 100 micron long phononic waveguide at room and cryogenic temperatures. The total insertion loss through the device at 4 kelvin is -5.8 dB, corresponding to a >50% transducer efficiency, at a frequency of 2.8 gigahertz. Our work represents a step towards phonon-mediated hybrid quantum systems consisting of strain-sensitive color centers in diamond.

cond-mat.mes-hall↗

arXiv2024

Symmetries of Liouvillians of squeeze-driven parametric oscillators

We study the symmetries of the Liouville superoperator of one dimensional parametric oscillators, especially the so-called squeeze-driven Kerr oscillator, and discover a remarkable quasi-spin symmetry $su(2)$ at integer values of the ratio $η=ω/K$ of the detuning parameter $ω$ to the Kerr coefficient $K$, which reflects the symmetry previously found for the Hamiltonian operator. We find that the Liouvillian of an $su(2)$ representation $\left\vert j,m_{j}\right\rangle$ has a characteristic double-ellipsoidal structure, and calculate the relaxation time $T_{X}$ for this structure. We then study the phase transitions of the Liouvillian which occur as a function of the parameters $ξ=\varepsilon _{2}/K$ and $η=ω/K$. Finally, we study the temperature dependence of the spectrum of eigenvalues of the Liouvillian. Our findings may have applications in the generation and stabilization of states of interest in quantum computing.

quant-ph↗

arXiv2024

A diagrammatic method to compute the effective Hamiltonian of driven nonlinear oscillators

In this work, we present a new diagrammatic method for computing the effective Hamiltonian of driven nonlinear oscillators. At the heart of our method is a self-consistent perturbation expansion developed in phase space, which establishes a direct correspondence between the diagram and algebra. Each diagram corresponds to a Hamiltonian term, the prefactor of which, like those in Feynman diagrams, involves a simple counting of topologically equivalent diagrams. Leveraging the algorithmic simplicity of our diagrammatic method, we provide a readily available computer program that generates the effective Hamiltonian to arbitrary order. We show the consistency of our schemes with existing perturbation methods such as the Schrieffer-Wolff method. Furthermore, we recover the classical harmonic balance scheme from our result in the limit of $\hbar\rightarrow0$. Our method contributes to the understanding of dynamic control within quantum systems and achieves precision essential for advancing future quantum information processors. To demonstrate its value and versatility, we analyze five examples from the field of superconducting circuits. These include an experimental proposal for the Hamiltonian stabilization of a three-legged Schrödinger cat, modeling of energy renormalization phenomena in superconducting circuits experiments, a comprehensive characterization of multiphoton resonances in a driven transmon, a proposal for an inductively shunted transmon circuit, and a characterization of classical ultra-subharmonic bifurcation in driven oscillators. Lastly, we benchmark the performance of our method by comparing it with experimental data and exact Floquet numerical diagonalization.

quant-ph↗

arXiv2023

Spectral kissing and its dynamical consequences in the squeeze-driven Kerr oscillator

Transmon qubits are the predominant element in circuit-based quantum information processing, such as existing quantum computers, due to their controllability and ease of engineering implementation. But more than qubits, transmons are multilevel nonlinear oscillators that can be used to investigate fundamental physics questions. Here, they are explored as simulators of excited state quantum phase transitions (ESQPTs), which are generalizations of quantum phase transitions to excited states. We show that the spectral kissing (coalescence of pairs of energy levels) experimentally observed in the effective Hamiltonian of a driven SNAIL-transmon is an ESQPT precursor. We explore the dynamical consequences of the ESQPT, which include the exponential growth of out-of-time-ordered correlators, followed by periodic revivals, and the slow evolution of the survival probability due to localization. These signatures of ESQPT are within reach for current superconducting circuits platforms and are of interest to experiments with cold atoms and ion traps.

quant-ph↗

arXiv2023

A driven quantum superconducting circuit with multiple tunable degeneracies

We present the experimental discovery of multiple simultaneous degeneracies in the spectrum of a Kerr oscillator subjected to a squeezing drive. This squeezing, in combination with the Kerr interaction creates an effective static two-well potential in the frame rotating at half the frequency of the sinusoidal driving force. Remarkably, these degeneracies can be turned on-and-off on demand, and their number is tunable. We find that when the detuning $Δ$ between the frequency of the oscillator and characteristic frequency of the drive equals an even multiple of the Kerr coefficient $K$, $Δ/K = 2m$, the oscillator displays $m + 1$ exact, parity-protected, spectral degeneracies, insensitive to the drive amplitude. The degeneracies stem from the unusual destructive interference of tunnel paths in the classically forbidden region of the double well static effective potential that models our experiment. Exploiting this interference, we measure a peaked enhancement of the incoherent well-switching lifetime creating a super-protected cat qubit in the ground state manifold of our oscillator. {Our results demonstrate the relationship between degeneracies and noise protection in quantum systems.

quant-ph↗

arXiv2022

On the static effective Lindbladian of the squeezed Kerr oscillator

We derive the static effective Lindbladian beyond the rotating wave approximation (RWA) for a driven nonlinear oscillator coupled to a bath of harmonic oscillators. The associated dissipative effects may explain orders of magnitude differences between the predictions of the ordinary RWA model and results from recent superconducting circuits experiments on the Kerr-cat qubit. The higher-order dissipators found in our calculations have important consequences for quantum error-correction protocols and parametric processses.

quant-ph↗

arXiv2022

The squeezed Kerr oscillator: spectral kissing and phase-flip robustness

By applying a microwave drive to a specially designed Josephson circuit, we have realized an elementary quantum optics model, the squeezed Kerr oscillator. This model displays, as the squeezing amplitude is increased, a cross-over from a single ground state regime to a doubly-degenerate ground state regime. In the latter case, the ground state manifold is spanned by Schrödinger-cat states, i.e. quantum superpositions of coherent states with opposite phases. For the first time, having resolved up to the tenth excited state in a spectroscopic experiment, we confirm that the proposed emergent static effective Hamiltonian correctly describes the system, despite its driven character. We also find that the lifetime of the coherent state components of the cat states increases in steps as a function of the squeezing amplitude. We interpret the staircase pattern as resulting from pairwise level kissing in the excited state spectrum. Considering the Kerr-cat qubit encoded in this ground state manifold, we achieve for the first time quantum nondemolition readout fidelities greater than 99%, and enhancement of the phase-flip lifetime by more than two orders of magnitude, while retaining universal quantum control. Our experiment illustrates the crucial role of parametric drive Hamiltonian engineering for hardware-efficient quantum computation.

quant-ph↗

arXiv2022

Fast Universal Control of an Oscillator with Weak Dispersive Coupling to a Qubit

A controlled evolution generated by nonlinear interactions is required to perform full manipulation of a quantum system, and such control is only coherent when the rate of nonlinearity is large compared to the rate of decoherence. As a result, engineered quantum systems typically rely on a bare nonlinearity much stronger than all decoherence rates, and this hierarchy is usually assumed to be necessary. In this work, we challenge this assumption by demonstrating the universal control of a quantum system where the relevant rate of bare nonlinear interaction is comparable to the fastest rate of decoherence. We do this by introducing a novel noise-resilient protocol for the universal quantum control of a nearly-harmonic oscillator that takes advantage of an in-situ enhanced nonlinearity instead of harnessing a bare nonlinearity. Our experiment consists of a high quality-factor microwave cavity with weak-dispersive coupling to a much lower quality superconducting qubit. By using strong drives to temporarily excite the oscillator, we realize an amplified three-wave-mixing interaction, achieving typical operation speeds over an order of magnitude faster than expected from the bare dispersive coupling. Our demonstrations include preparation of a single-photon state with $98\pm 1(\%)$ fidelity and preparation of squeezed vacuum with a squeezing level of $11.1$ dB, the largest intracavity squeezing reported in the microwave regime. Finally, we also demonstrate fast measurement-free preparation of logical states for the binomial and Gottesman-Kitaev-Preskill (GKP) quantum error-correcting codes.

quant-ph↗

arXiv2022

On the static effective Hamiltonian of a rapidly driven nonlinear system

We present a recursive formula for the computation of the static effective Hamiltonian of a system under a fast-oscillating drive. Our analytical result is well-suited to symbolic calculations performed by a computer and can be implemented to arbitrary order, thus overcoming limitations of existing time-dependent perturbation methods and allowing computations that were impossible before. We also provide a simple diagrammatic tool for calculation and treat illustrative examples. By construction, our method applies directly to both quantum and classical systems; the difference is left to a low-level subroutine. This aspect sheds light on the relationship between seemingly disconnected independently developed methods in the literature and has direct applications in quantum engineering.

quant-ph↗

arXiv2017

Resonator reset in circuit QED by optimal control for large open quantum systems

We study an implementation of the open GRAPE (Gradient Ascent Pulse Engineering) algorithm well suited for large open quantum systems. While typical implementations of optimal control algorithms for open quantum systems rely on explicit matrix exponential calculations, our implementation avoids these operations leading to a polynomial speed-up of the open GRAPE algorithm in cases of interest. This speed-up, as well as the reduced memory requirements of our implementation, are illustrated by comparison to a standard implementation of open GRAPE. As a practical example, we apply this open-system optimization method to active reset of a readout resonator in circuit QED. In this problem, the shape of a microwave pulse is optimized such as to empty the cavity from measurement photons as fast as possible. Using our open GRAPE implementation, we obtain pulse shapes leading to a reset time over four times faster than passive reset.

quant-ph↗