7 research outputs found
Magnetic Monopole Noise
Magnetic monopoles are hypothetical elementary particles exhibiting quantized
magnetic charge and quantized magnetic flux . A classic proposal for detecting such magnetic charges is to measure the
quantized jump in magnetic flux threading the loop of a superconducting
quantum interference device (SQUID) when a monopole passes through it.
Naturally, with the theoretical discovery that a plasma of emergent magnetic
charges should exist in several lanthanide-pyrochlore magnetic insulators,
including DyTiO, this SQUID technique was proposed for their direct
detection. Experimentally, this has proven extremely challenging because of the
high number density, and the generation-recombination (GR) fluctuations, of the
monopole plasma. Recently, however, theoretical advances have allowed the
spectral density of magnetic-flux noise due to GR
fluctuations of magnetic charge pairs to be determined. These
theories present a sequence of strikingly clear predictions for the
magnetic-flux noise signature of emergent magnetic monopoles. Here we report
development of a high-sensitivity, SQUID based flux-noise spectrometer, and
consequent measurements of the frequency and temperature dependence of
for DyTiO samples. Virtually all the elements
of predicted for a magnetic monopole plasma, including the
existence of intense magnetization noise and its characteristic frequency and
temperature dependence, are detected directly. Moreover, comparisons of
simulated and measured correlation functions of the magnetic-flux
noise imply that the motion of magnetic charges is strongly
correlated because traversal of the same trajectory by two magnetic charges of
same sign is forbidden
Uncovering the spin ordering in magic-angle graphene via edge state equilibration
Determining the symmetry breaking order of correlated quantum phases is
essential for understanding the microscopic interactions in their host systems.
The flat bands in magic angle twisted bilayer graphene (MATBG) provide an
especially rich arena to investigate such interaction-driven ground states, and
while progress has been made in identifying the correlated insulators and their
excitations at commensurate moire filling factors, the spin-valley
polarizations of the topological states that emerge at high magnetic field
remain unknown. Here we introduce a new technique based on twist-decoupled van
der Waals layers that enables measurements of their electronic band structure
and, by studying the backscattering between counter-propagating edge states,
determination of relative spin polarization of the their edge modes. Applying
this method to twist-decoupled MATBG and monolayer graphene, we find that the
broken-symmetry quantum Hall states that extend from the charge neutrality
point in MATBG are spin-unpolarized at even integer filling factors. The
measurements also indicate that the correlated Chern insulator emerging from
half filling of the flat valence band is spin-unpolarized, but suggest that its
conduction band counterpart may be spin-polarized. Our results constrain models
of spin-valley ordering in MATBG and establish a versatile approach to study
the electronic properties of van der Waals systems
Measurement-induced entanglement and teleportation on a noisy quantum processor
Measurement has a special role in quantum theory: by collapsing the
wavefunction it can enable phenomena such as teleportation and thereby alter
the "arrow of time" that constrains unitary evolution. When integrated in
many-body dynamics, measurements can lead to emergent patterns of quantum
information in space-time that go beyond established paradigms for
characterizing phases, either in or out of equilibrium. On present-day NISQ
processors, the experimental realization of this physics is challenging due to
noise, hardware limitations, and the stochastic nature of quantum measurement.
Here we address each of these experimental challenges and investigate
measurement-induced quantum information phases on up to 70 superconducting
qubits. By leveraging the interchangeability of space and time, we use a
duality mapping, to avoid mid-circuit measurement and access different
manifestations of the underlying phases -- from entanglement scaling to
measurement-induced teleportation -- in a unified way. We obtain finite-size
signatures of a phase transition with a decoding protocol that correlates the
experimental measurement record with classical simulation data. The phases
display sharply different sensitivity to noise, which we exploit to turn an
inherent hardware limitation into a useful diagnostic. Our work demonstrates an
approach to realize measurement-induced physics at scales that are at the
limits of current NISQ processors