860 research outputs found
Experimentally reducing the quantum measurement back-action in work distributions by a collective measurement
In quantum thermodynamics, the standard approach to estimate work
fluctuations in unitary processes is based on two projective measurements, one
performed at the beginning of the process and one at the end. The first
measurement destroys any initial coherence in the energy basis, thus preventing
later interference effects. In order to decrease this back-action, a scheme
based on collective measurements has been proposed in~[PRL 118, 070601 (2017)].
Here, we report its experimental implementation in an optical system. The
experiment consists of a deterministic collective measurement on identically
prepared two qubits, encoded in the polarisation and path degree of a single
photon. The standard two projective measurement approach is also experimentally
realized for comparison. Our results show the potential of collective schemes
to decrease the back-action of projective measurements, and capture subtle
effects arising from quantum coherence.Comment: 9 pages, 4 figure
Deterministic realization of collective measurements via photonic quantum walks
Collective measurements on identically prepared quantum systems can extract
more information than local measurements, thereby enhancing
information-processing efficiency. Although this nonclassical phenomenon has
been known for two decades, it has remained a challenging task to demonstrate
the advantage of collective measurements in experiments. Here we introduce a
general recipe for performing deterministic collective measurements on two
identically prepared qubits based on quantum walks. Using photonic quantum
walks, we realize experimentally an optimized collective measurement with
fidelity 0.9946 without post selection. As an application, we achieve the
highest tomographic efficiency in qubit state tomography to date. Our work
offers an effective recipe for beating the precision limit of local
measurements in quantum state tomography and metrology. In addition, our study
opens an avenue for harvesting the power of collective measurements in quantum
information processing and for exploring the intriguing physics behind this
power.Comment: Close to the published versio
Experimental cyclic inter-conversion between Coherence and Quantum Correlations
Quantum resource theories seek to quantify sources of non-classicality that
bestow quantum technologies their operational advantage. Chief among these are
studies of quantum correlations and quantum coherence. The former to isolate
non-classicality in the correlations between systems, the latter to capture
non-classicality of quantum superpositions within a single physical system.
Here we present a scheme that cyclically inter-converts between these resources
without loss. The first stage converts coherence present in an input system
into correlations with an ancilla. The second stage harnesses these
correlations to restore coherence on the input system by measurement of the
ancilla. We experimentally demonstrate this inter-conversion process using
linear optics. Our experiment highlights the connection between
non-classicality of correlations and non-classicality within local quantum
systems, and provides potential flexibilities in exploiting one resource to
perform tasks normally associated with the other.Comment: 8 pages, 4 figures, comments welcom
Quantum electric-dipole liquid on a triangular lattice
Geometric frustrations and quantum mechanical fluctuations may prohibit the
formation of long-range ordering even at the lowest temperature, and therefore
liquid-like ground states could be expected. A good example is the quantum spin
liquid in frustrated magnets that represents an exotic phase of matter and is
attracting enormous interests. Geometric frustrations and quantum fluctuations
can happen beyond magnetic systems. Here we propose that quantum
electric-dipole liquids, analogs to quantum spin liquids, could emerge in
frustrated dielectrics where antiferroelectrically coupled small electric
dipoles reside on a triangular lattice. The quantum paraelectric hexaferrite
BaFe12O19, in which small electric dipoles originated from the off-center
displacement of Fe3+ in the FeO5 bipyramids constitute a two-dimensional
triangular lattice, represents a promising candidate to generate the
anticipated electric-dipole liquid. We present a series of experimental
evidences, including dielectric permittivity, heat capacity, and thermal
conductivity measured down to 66 mK, to reveal the existence of a nontrivial
ground state in BaFe12O19, characterized by itinerant low-energy excitations
with a small gap, to which we interpret as an exotic liquid-like quantum phase.
The quantum electric-dipole liquids in frustrated dielectrics open up a fresh
playground for fundamental physics and may find applications in quantum
information and computation as well.Comment: 13 pages, 6 figure
Chiral switching of many-body steady states in a dissipative Rydberg gas
Dissipative Rydberg gases are an outstanding platform for the investigation
of many-body quantum open systems. Despite the wealth of existing studies, the
non-equilibrium dynamics of dissipative Rydberg gases are rarely examined or
harnessed from the perspective of non-Hermitian physics, which is but intrinsic
to open systems. Here we report the experimental observation of a chiral
switching between many-body steady states in a dissipative thermal Rydberg
vapor, where the interplay of many-body effects and non-Hermiticity plays a key
role. Specifically, as the parameters are adiabatically varied around a closed
contour, depending on the chirality of the parameter modulation, the Rydberg
vapor can change between two collective steady states with distinct Rydberg
excitations and optical transmissions. Adopting a mean-field description, we
reveal that both the existence of the bistable steady states and chiral
dynamics derive from an exceptional structure in the parameter space, where
multiple steady states of the many-body Liouvillian superoperator coalesce. We
demonstrate that both the exceptional structure and the resulting
state-switching dynamics are tunable through microwave dressing and temperature
variations, confirming their reliance on the many-body dissipative nature of
the Rydberg vapor
Preserving quantum correlations and coherence with non-Markovianity
Open quantum systems exhibit a rich phenomenology, in comparison to closed
quantum systems that evolve unitarily according to the Schr\"odinger equation.
The dynamics of an open quantum system are typically classified into Markovian
and non-Markovian, depending on whether the dynamics can be decomposed into
valid quantum operations at any time scale. Since Markovian evolutions are
easier to simulate, compared to non-Markovian dynamics, it is reasonable to
assume that non-Markovianity can be employed for useful quantum-technological
applications. Here, we demonstrate the usefulness of non-Markovianity for
preserving correlations and coherence in quantum systems. For this, we consider
a broad class of qubit evolutions, having a decoherence matrix separated from
zero for large times. While any such Markovian evolution leads to an
exponential loss of correlations, non-Markovianity can help to preserve
correlations even in the limit . For covariant qubit
evolutions, we also show that non-Markovianity can be used to preserve quantum
coherence at all times, which is an important resource for quantum metrology.
We explicitly demonstrate this effect experimentally with linear optics, by
implementing the required evolution that is non-Markovian at all times
Physical properties and chemical composition of the cores in the California molecular cloud
We aim to reveal the physical properties and chemical composition of the
cores in the California molecular cloud (CMC), so as to better understand the
initial conditions of star formation. We made a high-resolution column density
map (18.2") with Herschel data, and extracted a complete sample of the cores in
the CMC with the \textsl{fellwalker} algorithm. We performed new
single-pointing observations of molecular lines near 90 GHz with the IRAM 30m
telescope along the main filament of the CMC. In addition, we also performed a
numerical modeling of chemical evolution for the cores under the physical
conditions. We extracted 300 cores, of which 33 are protostellar and 267 are
starless cores. About 51\% (137 of 267) of the starless cores are prestellar
cores. Three cores have the potential to evolve into high-mass stars. The
prestellar core mass function (CMF) can be well fit by a log-normal form. The
high-mass end of the prestellar CMF shows a power-law form with an index
that is shallower than that of the Galactic field stellar
mass function. Combining the mass transformation efficiency ()
from the prestellar core to the star of and the core formation
efficiency (CFE) of 5.5\%, we suggest an overall star formation efficiency of
about 1\% in the CMC. In the single-pointing observations with the IRAM 30m
telescope, we find that 6 cores show blue-skewed profile, while 4 cores show
red-skewed profile. []/[HNC] and []/ in protostellar cores are higher than those in prestellar cores;
this can be used as chemical clocks. The best-fit chemical age of the cores
with line observations is ~years.Comment: Accepted by Astronomy & Astrophysics (A&A
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