890 research outputs found
Identification of Decoherence-Free Subspaces Without Quantum Process Tomography
Characterizing a quantum process is the critical first step towards applying
such a process in a quantum information protocol. Full process characterization
is known to be extremely resource-intensive, motivating the search for more
efficient ways to extract salient information about the process. An example is
the identification of "decoherence-free subspaces", in which computation or
communications may be carried out, immune to the principal sources of
decoherence in the system. Here we propose and demonstrate a protocol which
enables one to directly identify a DFS without carrying out a full
reconstruction. Our protocol offers an up-to-quadratic speedup over standard
process tomography. In this paper, we experimentally identify the DFS of a
two-qubit process with 32 measurements rather than the usual 256, characterize
the robustness and efficiency of the protocol, and discuss its extension to
higher-dimensional systems.Comment: 6 pages, 5 figure
On the Optimal Choice of Spin-Squeezed States for Detecting and Characterizing a Quantum Process
Quantum metrology uses quantum states with no classical counterpart to
measure a physical quantity with extraordinary sensitivity or precision. Most
metrology schemes measure a single parameter of a dynamical process by probing
it with a specially designed quantum state. The success of such a scheme
usually relies on the process belonging to a particular one-parameter family.
If this assumption is violated, or if the goal is to measure more than one
parameter, a different quantum state may perform better. In the most extreme
case, we know nothing about the process and wish to learn everything. This
requires quantum process tomography, which demands an informationally-complete
set of probe states. It is very convenient if this set is group-covariant --
i.e., each element is generated by applying an element of the quantum system's
natural symmetry group to a single fixed fiducial state. In this paper, we
consider metrology with 2-photon ("biphoton") states, and report experimental
studies of different states' sensitivity to small, unknown collective SU(2)
rotations ("SU(2) jitter"). Maximally entangled N00N states are the most
sensitive detectors of such a rotation, yet they are also among the worst at
fully characterizing an a-priori unknown process. We identify (and confirm
experimentally) the best SU(2)-covariant set for process tomography; these
states are all less entangled than the N00N state, and are characterized by the
fact that they form a 2-design.Comment: 10 pages, 5 figure
Adaptive quantum state tomography improves accuracy quadratically
We introduce a simple protocol for adaptive quantum state tomography, which
reduces the worst-case infidelity between the estimate and the true state from
to . It uses a single adaptation step and just one
extra measurement setting. In a linear optical qubit experiment, we demonstrate
a full order of magnitude reduction in infidelity (from to ) for
a modest number of samples ().Comment: 8 pages, 7 figure
Experimental Entanglement of Temporal Orders
The study of causal relations has recently been applied to the quantum realm,
leading to the discovery that not all quantum processes have a definite causal
structure. While such processes have previously been experimentally
demonstrated, these demonstrations relied on the assumption that quantum theory
can be applied to causal structures and laboratory operations. Here, we present
the first demonstration of entangled temporal orders beyond the quantum
formalism. We do so by proving the incompatibility of our experimental outcomes
with a class of generalized probabilistic theories which satisfy the
assumptions of locality and definite temporal orders. To this end, we derive
physical constraints (in the form of a Bell-like inequality) on experimental
outcomes within such a class of theories. We then experimentally invalidate
these theories by violating the inequality, thus providing an experimental
proof, outside the quantum formalism, that nature is incompatible with the
assumption that the temporal order between events is definite locally.Comment: 20 pages, 8 figures. Thoroughly revised manuscript. Updated
theory-independent proofs including new experimental dat
Scalable Spatial Super-Resolution using Entangled Photons
N00N states -- maximally path-entangled states of N photons -- exhibit
spatial interference patterns sharper than any classical interference pattern.
This is known as super-resolution. However, even with perfectly efficient
number-resolving detectors, the detection efficiency of all previously
demonstrated methods to measure such interference decreases exponentially with
the number of photons in the N00N state, often leading to the conclusion that
N00N states are unsuitable for spatial measurements. Here, we create spatial
super-resolution fringes with two-, three-, and four-photon N00N states, and
demonstrate a scalable implementation of the so-called ``optical centroid
measurement'' which provides an in-principle perfect detection efficiency.
Moreover, we compare the N00N-state interference to the corresponding classical
super-resolution interference. Although both provide the same increase in
spatial frequency, the visibility of the classical fringes decreases
exponentially with the number of detected photons, while the visibility of our
experimentally measured N00N-state super-resolution fringes remains
approximately constant with N. Our implementation of the optical centroid
measurement is a scalable method to measure high photon-number quantum
interference, an essential step forward for quantum-enhanced measurements,
overcoming what was believed to be a fundamental challenge to quantum
metrology
Detection of subclinical keratoconus with a validated alternative method to corneal densitometry
Purpose: To enhance the current standards of subclinical keratoconus screening based on the statistical modeling of the pixel intensity distribution of Scheimpflug images. Methods: Scheimpflug corneal tomographies corresponding to 25 corneal meridians of 60 participants were retrospectively collected and divided into three groups: controls (20 eyes), subclinical keratoconus (20 eyes), and clinical keratoconus (20 eyes). Only right eyes were selected. After corneal segmentation, pixel intensities of the stromal tissue were statistically modeled using a Weibull probability density function from which parameter a (pixel brightness) was derived. Further, data were transformed to polar coordinates, smoothed, and interpolated to build a map of the corneal a parameter. The discriminative power of the method was analyzed using receiver operating characteristic curves. Results: The proposed platform-independent method achieved a higher performance in discriminating subclinical keratoconus from control eyes (90.0% sensitivity, 95.0% specificity, 0.97 area under the curve [AUC]) than the standard method (Belin–Ambrósio enhanced ectasia display), which uses only corneal morphometry (85.0% sensitivity, 85.0% specificity, 0.80 AUC). Conclusions: Analysis of light backscatter at the cornea successfully discriminates subclinical keratoconus from control eyes, upgrading the results previously reported in the literature. Translational Relevance: The proposed methodology has the potential to support clinicians in the detection of keratoconus before showing clinical signs. © 2021 The Authors
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