arXiv subjects

Govind Unnikrishnan

Publications and source records attributed to Govind Unnikrishnan.

6 recordsLinked to original sources

arXiv2024

Coherent Control of the Fine-Structure Qubit in a Single Alkaline-Earth Atom

We report on the first realization of a novel neutral atom qubit encoded in the metastable fine-structure states ${^3\rm{P}_0}$ and ${^3\rm{P}_2}$ of single $^{88}$Sr atoms trapped in an optical tweezer. Raman coupling of the qubit states promises rapid single-qubit rotations on par with the fast Rydberg-mediated two-body gates. We demonstrate preparation, read-out, and coherent control of the qubit. In addition to driving Rabi oscillations bridging an energy gap of more than 17 THz using a pair of phase-locked clock lasers, we also carry out Ramsey spectroscopy to extract the transverse qubit coherence time $T_2$. When the tweezer is tuned into magic trapping conditions, which is achieved in our setup by tuning the tensor polarizability of the ${^3\rm{P}_2}$ state via an external control magnetic field, we measure $T_2 = 1.2$ ms. A microscopic quantum mechanical model is used to simulate our experiments including dominant noise sources. We identify the main constraints limiting the observed coherence time and project improvements to our system in the immediate future. Our work opens the door for a so far unexplored qubit encoding concept for neutral atom based quantum computing.

quant-ph↗

arXiv2021

Long distance optical transport of ultracold atoms: A compact setup using a Moir\'e lens

We present a compact and robust setup to optically transport ultracold atoms over long distances. Using a focus-tunable Moir\'e lens that has recently appeared on the market, we demonstrate transport of up to a distance of 465 mm. A transfer efficiency of 70% is achieved with negligible temperature change at 11 $\mu$K. With its high thermal stability and low astigmatism, the Moir\'e lens is superior to fluid-based varifocal lenses. It is much more compact and stable than a lens mounted on a linear translation stage, allowing for simplified experimental setups.

cond-mat.quant-gas↗

arXiv2020

Interaction-Enhanced Group Velocity of Bosons in the Flat Band of an Optical Kagome Lattice

Geometric frustration of particle motion in a kagome lattice causes the single-particle band structure to have a flat s-orbital band. We probe this band structure by exciting a Bose-Einstein condensate into excited Bloch states of an optical kagome lattice, and then measuring the group velocity through the atomic momentum distribution. We find that interactions renormalize the band structure of the kagome lattice, greatly increasing the dispersion of the third band that, according to non-interacting band theory, should be nearly non-dispersing. Measurements at various lattice depths and gas densities agree quantitatively with predictions of the lattice Gross-Pitaevskii equation, indicating that the observed distortion of band structure is caused by the disortion of the overall lattice potential away from the kagome geometry by interactions.

cond-mat.quant-gas↗

arXiv2019

Sub-Doppler laser cooling of $^{39}$K via the 4S$\to$5P transition

We demonstrate sub-Doppler laser cooling of $ ^{39} $K using degenerate Raman sideband cooling via the 4S$_{1/2} \rightarrow $5P$ _{1/2} $ transition at 404.8 nm. By using an optical lattice in combination with a magnetic field and optical pumping beams, we obtain a spin-polarized sample of up to $5.6 \times 10^{7}$ atoms cooled down to a sub-Doppler temperature of 4 $μ$K, reaching a peak density of $3.9 \times 10^{9}$ atoms/cm$ ^{3} $, a phase-space density greater than $ 10^{-5} $, and an average vibrational level of $ \langle ν\rangle=0.6 $ in the lattice. This work opens up the possibility of implementing a single-site imaging scheme in a far-detuned optical lattice utilizing shorter wavelength transitions in alkali atoms, thus allowing improved spatial resolution.

physics.atom-ph↗

arXiv2017

Spectral investigation of the noise influencing multi-qubit states

Characterizing and understanding noise affecting quantum states has immense benefits in spectroscopy as well as in realizing quantum devices. Transverse relaxation times under a set of dynamical decoupling (DD) sequences with varying interpulse delays were earlier used for obtaining the noise spectral densities of single-qubit coherences. In this work, using a pair of homonuclear spins and NMR techniques, we experimentally characterize noise in certain decoherence-free subspaces. We also explore the noise of similar states in a heteronuclear spin pair. Further, using a 10-qubit system, we investigate noise profiles of various multiqubit coherences and study the scaling of noise with respect to the coherence order. Finally, using the experimentally obtained noise spectrum of the 10-qubit NOON state, we predict the performance of a Uhrig DD sequence and verify it experimentally.

quant-ph↗

arXiv2016

Engineering and Suppression of Decoherence in Two Qubit Systems

In this work, two experimentally feasible methods of decoherence engineering-one based on the application of stochastic classical kicks and the other based on temporally randomized pulse sequences are combined. A different coupling interaction is proposed, which leads to amplitude damping as compared to existing methods which model phase damping, utilizing the $zz$ coupling interaction. The decoherence process on combining the stochastic kick method and the randomized pulse sequence method and the effectiveness of dynamical decoupling under these coupling interactions are analyzed. Finally, a counter-intuitive result where decoherence is suppressed in the presence of two noise sources under certain resonant conditions is presented.

quant-ph↗