22 research outputs found
Engineering Dynamical Sweet Spots to Protect Qubits from 1/ Noise
Protecting superconducting qubits from low-frequency noise is essential for
advancing superconducting quantum computation. Based on the application of a
periodic drive field, we develop a protocol for engineering dynamical sweet
spots which reduce the susceptibility of a qubit to low-frequency noise. Using
the framework of Floquet theory, we prove rigorously that there are manifolds
of dynamical sweet spots marked by extrema in the quasi-energy differences of
the driven qubit. In particular, for the example of fluxonium biased slightly
away from half a flux quantum, we predict an enhancement of pure-dephasing by
three orders of magnitude. Employing the Floquet eigenstates as the
computational basis, we show that high-fidelity single- and two-qubit gates can
be implemented while maintaining dynamical sweet-spot operation. We further
confirm that qubit readout can be performed by adiabatically mapping the
Floquet states back to the static qubit states, and subsequently applying
standard measurement techniques. Our work provides an intuitive tool to encode
quantum information in robust, time-dependent states, and may be extended to
alternative architectures for quantum information processing
Floquet-engineered enhancement of coherence times in a driven fluxonium qubit
We use the quasienergy structure that emerges when a fluxonium
superconducting circuit is driven periodically to encode quantum information
with dynamically induced flux-insensitive sweet spots. The framework of Floquet
theory provides an intuitive description of these high-coherence working points
located away from the half-flux symmetry point of the undriven qubit. This
approach offers flexibility in choosing the flux bias point and the energy of
the logical qubit states as shown in [\textit{Huang et al., 2020}]. We
characterize the response of the system to noise in the modulation amplitude
and DC flux bias, and experimentally demonstrate an optimal working point which
is simultaneously insensitive against fluctuations in both. We observe a
40-fold enhancement of the qubit coherence times measured with Ramsey-type
interferometry at the dynamical sweet spot compared with static operation at
the same bias point.Comment: 12 pages, 7 figure
Symplectic geometry and circuit quantization
Circuit quantization is an extraordinarily successful theory that describes
the behavior of quantum circuits with high precision. The most widely used
approach of circuit quantization relies on introducing a classical Lagrangian
whose degrees of freedom are either magnetic fluxes or electric charges in the
circuit. By combining nonlinear circuit elements (such as Josephson junctions
or quantum phase slips), it is possible to build circuits where a standard
Lagrangian description (and thus the standard quantization method) does not
exist. Inspired by the mathematics of symplectic geometry and graph theory, we
address this challenge, and present a Hamiltonian formulation of
non-dissipative electrodynamic circuits. The resulting procedure for circuit
quantization is independent of whether circuit elements are linear or
nonlinear, or if the circuit is driven by external biases. We explain how to
re-derive known results from our formalism, and provide an efficient algorithm
for quantizing circuits, including those that cannot be quantized using
existing methods.Comment: 30 pages, 8 figure
Moving beyond the transmon: Noise-protected superconducting quantum circuits
Artificial atoms realized by superconducting circuits offer unique
opportunities to store and process quantum information with high fidelity.
Among them, implementations of circuits that harness intrinsic noise protection
have been rapidly developed in recent years. These noise-protected devices
constitute a new class of qubits in which the computational states are largely
decoupled from local noise channels. The main challenges in engineering such
systems are simultaneously guarding against both bit- and phase-flip errors,
and also ensuring high-fidelity qubit control. Although partial noise
protection is possible in superconducting circuits relying on a single quantum
degree of freedom, the promise of complete protection can only be fulfilled by
implementing multimode or hybrid circuits. This Perspective reviews the
theoretical principles at the heart of these new qubits, describes recent
experiments, and highlights the potential of robust encoding of quantum
information in superconducting qubits
Visualizing Heavy Fermion Confinement and Pauli-Limited Superconductivity in Layered CeCoIn5
Layered material structures play a key role in enhancing electron-electron
interactions to create correlated metallic phases that can transform into
unconventional superconducting states. The quasi-two-dimensional electronic
properties of such compounds are often inferred indirectly through examination
of their bulk properties. Here we use scanning tunneling microscopy and
spectroscopy to directly probe in cross section the quasi-two-dimensional
correlated electronic states of the heavy fermion superconductor CeCoIn5. Our
measurements reveal the strong confined nature of heavy quasi-particles,
anisotropy of tunneling characteristics, and layer-by-layer modulated behavior
of the precursor pseudogap gap phase in this compound. Examining the interlayer
coupled superconducting state at low temperatures, we find that the orientation
of line defects relative to the d-wave order parameter determines whether
in-gap states form due to scattering. Spectroscopic imaging of the anisotropic
magnetic vortex cores directly characterizes the short interlayer
superconducting coherence length and shows an electronic phase separation near
the upper critical in-plane magnetic field, consistent with a Pauli-limited
first-order phase transition into a pseudogap phase
Observation of a Nematic Quantum Hall Liquid on the Surface of Bismuth
Nematic quantum fluids with wavefunctions that break the underlying
crystalline symmetry can form in interacting electronic systems. We examine the
quantum Hall states that arise in high magnetic fields from anisotropic hole
pockets on the Bi(111) surface. Spectroscopy performed with a scanning
tunneling microscope shows that a combination of local strain and many-body
Coulomb interactions lift the six-fold Landau level (LL) degeneracy to form
three valley-polarized quantum Hall states. We image the resulting anisotropic
LL wavefunctions and show that they have a different orientation for each
broken-symmetry state. The wavefunctions correspond precisely to those expected
from pairs of hole valleys and provide a direct spatial signature of a nematic
electronic phase
Visualizing heavy fermions emerging in a quantum critical Kondo lattice
In solids containing elements with f orbitals, the interaction between
f-electron spins and those of itinerant electrons leads to the development of
low-energy fermionic excitations with a heavy effective mass. These excitations
are fundamental to the appearance of unconventional superconductivity and
non-Fermi-liquid behaviour observed in actinide- and lanthanide-based
compounds. Here we use spectroscopic mapping with the scanning tunnelling
microscope to detect the emergence of heavy excitations with lowering of
temperature in a prototypical family of cerium-based heavy-fermion compounds.
We demonstrate the sensitivity of the tunnelling process to the composite
nature of these heavy quasiparticles, which arises from quantum entanglement of
itinerant conduction and f electrons. Scattering and interference of the
composite quasiparticles is used to resolve their energy-momentum structure and
to extract their mass enhancement, which develops with decreasing temperature.
The lifetime of the emergent heavy quasiparticles reveals signatures of
enhanced scattering and their spectral lineshape shows evidence of
energy-temperature scaling. These findings demonstrate that proximity to a
quantum critical point results in critical damping of the emergent heavy
excitation of our Kondo lattice system.Comment: preprint version, 26 pages, 6 figures. Supplementary: 15 pages, 14
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