arXiv subjects

Gurudev Dutt

Publications and source records attributed to Gurudev Dutt.

3 recordsLinked to original sources

arXiv2026

Variational Joint Magnetometry and Gradiometry on Dipolar Spin Chains

Estimating a uniform magnetic field B0 and its spatial gradient g on a dipolar-coupled spin chain calls for a multiparameter figure of merit. The GHZ state, optimal for single-parameter Heisenberg-limited sensing, has a rank-one quantum Fisher information matrix with det(Q^GHZ) = 0 at every chain length N, ruling it out for the two-parameter problem. We present a variational framework that takes det(F) as the objective and a hardware-motivated layered dipolar circuit as the ansatz. Both encoding generators are diagonal in the computational basis, which reduces the search for the quantum Fisher information benchmark to a probability-simplex optimization and yields a tractable best-found benchmark det(Q*) against which variational performance is compared. The same diagonal structure makes the classical Fisher information depend only on basis-state probabilities under any single-qubit decoder, so encoder and decoder parameters are co-trained with CMA-ES in a single run. Decoder optimization past fixed Ramsey adds at most a few percentage points across the grid, in contrast to the persistent decoder gains seen in our prior single-parameter work. Variational probes at L = 3 reach 0.92 of the best-found benchmark at N = 5, a 4.2x SQL advantage in det(F), and concentrate on a four-string motif of the two GHZ extrema and two half-chain-flip strings whose structure follows from the Dicke-sector decomposition of the two generators.

quant-ph↗

arXiv2025

Variational Probe and Measurement Optimization for Structured Phase Estimation

We present a proof-of-principle study of variational quantum sensing for estimating a structured linear function of local phase parameters, in which each qubit in a spin-1/2 array accumulates a phase phi_i = alpha_i theta with known weights alpha and a global parameter theta. In a hardware-motivated regime of shallow circuits and shallow decoding measurements, we optimize the probe state with respect to the classical Fisher information (CFI) using the Covariance Matrix Adaptation Evolution Strategy. The variational ansatz is built from dipolar-interacting gates and global rotations on a polygon-centered qubit layout. To assess whether the standard Ramsey readout extracts all available information, we introduce a shallow global decoder and optimize it independently with the encoder frozen. For uniform (alpha_i = 1/N) and weighted-central (alpha_c = 1, alpha_i = 0.5) encodings with N = 2-8 qubits and depths L = 1-3, the optimized probes approach the respective entanglement-enhanced precision bounds, which reduce to the Heisenberg limit only for uniform encoding. The decoder provides systematic but modest CFI gains. For uniform encoding, these gains are smallest at the deepest circuits, confirming that fixed Ramsey readout is near-optimal for well-converged probes. For weighted encoding, a persistent component remains, reflecting the broken permutation symmetry of the generator under unequal weights. At large N, the weighted-encoding CFI also exhibits non-monotonic growth with system size, revealing an expressivity limit of the polygon-symmetric ansatz under asymmetric encoding.

quant-ph↗

arXiv2018

Multiple-photon excitation of nitrogen vacancy center in diamond

We report the first observation of multi-photon photoluminescence excitation (PLE) below the resonant energies of nitrogen vacancy (NV) centers in diamond. The quadratic and cubic dependence of the integrated fluorescence intensity as a function of excitation power indicate a two-photon excitation pathway for the NV- charge state and a three-photon process involved for the neutral NV0 charge state respectively. Comparing the total multi-photon energy with its single-photon equivalent, the PLE spectra follows the absorption spectrum of single photon excitation. We also observed that the efficiency of photoluminescence for different charge states, as well as the decay time constant, was dependent on the excitation wavelength and power.

cond-mat.mtrl-sci↗