19 research outputs found
Microscopy of spin-charge dynamics in Fermi-Hubbard chains
Obtaining a microscopic understanding of the dynamics in strongly correlated electronic systems has remained a challenge for many decades.
The interplay between the spin and charge degrees of freedom in these materials at different temperatures and dopant concentrations is not well understood and is still an area of intense scientific research.
Recently, quantum simulators based on ultracold atoms in optical lattices have emerged as a promising platform to probe strongly correlated fermionic systems.
This thesis reports on the work carried out with a quantum gas microscope of ultracold fermionic Li-6, where Fermi-Hubbard systems are prepared and imaged with single site spin and density resolution.
The main results of the thesis explore the microscopic dynamics underlying one-dimensional materials, where individual constituents such as the electron with charge e and spin-1/2 are not relevant to the description of the system anymore and are instead replaced by spin and charge excitations that can propagate independent of one another - a phenomenon called spin-charge separation.
In our quantum simulator, we use analogous one-dimensional Fermi-Hubbard chains of Li-6, to perform time- and space-resolved microscopy of the spin and charge excitations following a local quench.
By extracting their strikingly different velocities and showing an absence of binding between the excitations, we demonstrate spin-charge separation.
Our microscopic technique also allows us to quantitatively extract the excess spin carried by the spin excitiation, connecting our results to the phenomenon of fractionalization.
In another set of experiments, Fermi-Hubbard chains are probed at equilibrium and incommensurate spin correlations arising in the presence of both density doping and spin polarization are observed.
The wavevector of these incommensurate correlations are found to have a linear dependence on doping and polarization.
Finally, the effect of the spin-charge interplay is probed in the crossover from one to two dimensions.
The spin correlations across dopants are seen to be dramatically different in two dimensions, and the strong antiferromagnetic correlations across dopants present in one dimension disappear.
For a single dopant in a fully two dimensional system, the spin-charge interplay manifests as a distorted spin cloud surrounding the dopant, indicating the formation of a magnetic polaron.
The experiments reported here demonstrate the power of a quantum simulator; by probing the physics of strongly correlated systems in real space with unprecedented resolution, we can zoom into emergent phenomena, validate theories and access regimes that are not possible in other experimental settings
Microscopy of spin-charge dynamics in Fermi-Hubbard chains
Obtaining a microscopic understanding of the dynamics in strongly correlated electronic systems has remained a challenge for many decades.
The interplay between the spin and charge degrees of freedom in these materials at different temperatures and dopant concentrations is not well understood and is still an area of intense scientific research.
Recently, quantum simulators based on ultracold atoms in optical lattices have emerged as a promising platform to probe strongly correlated fermionic systems.
This thesis reports on the work carried out with a quantum gas microscope of ultracold fermionic Li-6, where Fermi-Hubbard systems are prepared and imaged with single site spin and density resolution.
The main results of the thesis explore the microscopic dynamics underlying one-dimensional materials, where individual constituents such as the electron with charge e and spin-1/2 are not relevant to the description of the system anymore and are instead replaced by spin and charge excitations that can propagate independent of one another - a phenomenon called spin-charge separation.
In our quantum simulator, we use analogous one-dimensional Fermi-Hubbard chains of Li-6, to perform time- and space-resolved microscopy of the spin and charge excitations following a local quench.
By extracting their strikingly different velocities and showing an absence of binding between the excitations, we demonstrate spin-charge separation.
Our microscopic technique also allows us to quantitatively extract the excess spin carried by the spin excitiation, connecting our results to the phenomenon of fractionalization.
In another set of experiments, Fermi-Hubbard chains are probed at equilibrium and incommensurate spin correlations arising in the presence of both density doping and spin polarization are observed.
The wavevector of these incommensurate correlations are found to have a linear dependence on doping and polarization.
Finally, the effect of the spin-charge interplay is probed in the crossover from one to two dimensions.
The spin correlations across dopants are seen to be dramatically different in two dimensions, and the strong antiferromagnetic correlations across dopants present in one dimension disappear.
For a single dopant in a fully two dimensional system, the spin-charge interplay manifests as a distorted spin cloud surrounding the dopant, indicating the formation of a magnetic polaron.
The experiments reported here demonstrate the power of a quantum simulator; by probing the physics of strongly correlated systems in real space with unprecedented resolution, we can zoom into emergent phenomena, validate theories and access regimes that are not possible in other experimental settings
Imaging magnetic polarons in the doped Fermi-Hubbard model
Polarons are among the most fundamental quasiparticles emerging in
interacting many-body systems, forming already at the level of a single mobile
dopant. In the context of the two-dimensional Fermi-Hubbard model, such
polarons are predicted to form around charged dopants in an antiferromagnetic
background in the low doping regime close to the Mott insulating state.
Macroscopic transport and spectroscopy measurements related to high
materials have yielded strong evidence for the existence of such quasiparticles
in these systems. Here we report the first microscopic observation of magnetic
polarons in a doped Fermi-Hubbard system, harnessing the full single-site spin
and density resolution of our ultracold-atom quantum simulator. We reveal the
dressing of mobile doublons by a local reduction and even sign reversal of
magnetic correlations, originating from the competition between kinetic and
magnetic energy in the system. The experimentally observed polaron signatures
are found to be consistent with an effective string model at finite
temperature. We demonstrate that delocalization of the doublon is a necessary
condition for polaron formation by contrasting this mobile setting to a
scenario where the doublon is pinned to a lattice site. Our work paves the way
towards probing interactions between polarons, which may lead to stripe
formation, as well as microscopically exploring the fate of polarons in the
pseudogap and bad metal phase
Robust Bilayer Charge-Pumping for Spin- and Density-Resolved Quantum Gas Microscopy
Quantum gas microscopy has emerged as a powerful new way to probe quantum
many-body systems at the microscopic level. However, layered or efficient
spin-resolved readout methods have remained scarce as they impose strong
demands on the specific atomic species and constrain the simulated lattice
geometry and size. Here we present a novel high-fidelity bilayer readout, which
can be used for full spin- and density-resolved quantum gas microscopy of
two-dimensional systems with arbitrary geometry. Our technique makes use of an
initial Stern-Gerlach splitting into adjacent layers of a highly-stable
vertical superlattice and subsequent charge pumping to separate the layers by
m. This separation enables independent high-resolution images of each
layer. We benchmark our method by spin- and density-resolving two-dimensional
Fermi-Hubbard systems. Our technique furthermore enables the access to advanced
entropy engineering schemes, spectroscopic methods or the realization of
tunable bilayer systems
Direct observation of incommensurate magnetism in Hubbard chains
The interplay between magnetism and doping is at the origin of exotic
strongly correlated electronic phases and can lead to novel forms of magnetic
ordering. One example is the emergence of incommensurate spin-density waves
with a wave vector that does not match the reciprocal lattice. In one dimension
this effect is a hallmark of Luttinger liquid theory, which also describes the
low energy physics of the Hubbard model. Here we use a quantum simulator based
on ultracold fermions in an optical lattice to directly observe such
incommensurate spin correlations in doped and spin-imbalanced Hubbard chains
using fully spin and density resolved quantum gas microscopy. Doping is found
to induce a linear change of the spin-density wave vector in excellent
agreement with Luttinger theory predictions. For non-zero polarization we
observe a decrease of the wave vector with magnetization as expected from the
Heisenberg model in a magnetic field. We trace the microscopic origin of these
incommensurate correlations to holes, doublons and excess spins which act as
delocalized domain walls for the antiferromagnetic order. Finally, when
inducing interchain coupling we observe fundamentally different spin
correlations around doublons indicating the formation of a magnetic polaron
Learning-Based Quantum Control for Optimal Pure State Manipulation
In this paper, we propose an adaptive critic learning approach for two classes of optimal pure state transition problems for closed quantum systems: i) when the target state is an eigenstate, and ii) when the target state is a superposition pure state. First, we describe a finite-dimensional quantum system based on the Schrodinger equation with the action of control fields. Then, we consider the target state to be i) an eigenstate of the internal Hamiltonian and ii) an arbitrary pure state via a unitary transformation. Meanwhile, the quantum state manipulation is formulated as an optimal control problem for solving the complex partial differential Hamilton-Jacobi-Bellman (HJB) equation, of which the control solution is found using continuous-time Q-learning of an adaptive critic. Finally, numerical simulation for a spin-1/2 particle system demonstrates the effectiveness of the proposed approach
Cavity-mediated long-range interactions in levitated optomechanics
The ability to engineer cavity-mediated interactions has emerged as a powerful tool for the generation of non-local correlations and the investigation of non-equilibrium phenomena in many-body systems. Levitated optomechanical systems have recently entered the multi-particle regime, with promise for using arrays of massive strongly coupled oscillators for exploring complex interacting systems and sensing. Here, by combining advances in multi-particle optical levitation and cavity-based quantum control, we demonstrate, for the first time, programmable cavity-mediated interactions between nanoparticles in vacuum. The interaction is mediated by photons scattered by spatially separated particles in a cavity, resulting in strong coupling () that does not decay with distance within the cavity mode volume. We investigate the scaling of the interaction strength with cavity detuning and inter-particle separation, and demonstrate the tunability of interactions between different mechanical modes. Our work paves the way towards exploring many-body effects in nanoparticle arrays with programmable cavity-mediated interactions, generating entanglement of motion, and using interacting particle arrays for optomechanical sensing