388 research outputs found
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
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
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
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
Higher-order Process Matrix Tomography of a passively-stable Quantum SWITCH
The field of indefinite causal order (ICO) has seen a recent surge in
interest. Much of this research has focused on the quantum SWITCH, wherein
multiple parties act in a superposition of different orders in a manner
transcending the quantum circuit model. This results in a new resource for
quantum protocols, and is exciting for its relation to issues in foundational
physics. The quantum SWITCH is also an example of a higher-order quantum
operation, in that it not only transforms quantum states, but also other
quantum operations. To date, no higher-order quantum operation has been
completely experimentally characterized. Indeed, past work on the quantum
SWITCH has confirmed its ICO by measuring causal witnesses or demonstrating
resource advantages, but the complete process matrix has only been described
theoretically. Here, we perform higher-order quantum process tomography.
However, doing so requires exponentially many measurements with a scaling worse
than standard process tomography. We overcome this challenge by creating a new
passively-stable fiber-based quantum SWITCH using active optical elements to
deterministically generate and manipulate time-bin encoded qubits. Moreover,
our new architecture for the quantum SWITCH can be readily scaled to multiple
parties. By reconstructing the process matrix, we estimate its fidelity and
tailor different causal witnesses directly for our experiment. To achieve this,
we measure a set of tomographically complete settings, that also spans the
input operation space. Our tomography protocol allows for the characterization
and debugging of higher-order quantum operations with and without an ICO, while
our experimental time-bin techniques could enable the creation of a new realm
of higher-order quantum operations with an ICO.Comment: 20 pages (12 pages, 4 pages appendix + reference list and
introduction), 8 figures; v2 with updated funding informatio
Violation of Heisenberg's Measurement-Disturbance Relationship by Weak Measurements
While there is a rigorously proven relationship about uncertainties intrinsic
to any quantum system, often referred to as "Heisenberg's Uncertainty
Principle," Heisenberg originally formulated his ideas in terms of a
relationship between the precision of a measurement and the disturbance it must
create. Although this latter relationship is not rigorously proven, it is
commonly believed (and taught) as an aspect of the broader uncertainty
principle. Here, we experimentally observe a violation of Heisenberg's
"measurement-disturbance relationship", using weak measurements to characterize
a quantum system before and after it interacts with a measurement apparatus.
Our experiment implements a 2010 proposal of Lund and Wiseman to confirm a
revised measurement-disturbance relationship derived by Ozawa in 2003. Its
results have broad implications for the foundations of quantum mechanics and
for practical issues in quantum mechanics.Comment: 5 pages, 4 figure
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