11 research outputs found
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
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
Closed timelike curves via post-selection: theory and experimental demonstration
Closed timelike curves (CTCs) are trajectories in spacetime that effectively
travel backwards in time: a test particle following a CTC can in principle
interact with its former self in the past. CTCs appear in many solutions of
Einstein's field equations and any future quantum version of general relativity
will have to reconcile them with the requirements of quantum mechanics and of
quantum field theory. A widely accepted quantum theory of CTCs was proposed by
Deutsch. Here we explore an alternative quantum formulation of CTCs and show
that it is physically inequivalent to Deutsch's. Because it is based on
combining quantum teleportation with post-selection, the
predictions/retrodictions of our theory are experimentally testable: we report
the results of an experiment demonstrating our theory's resolution of the
well-known `grandfather paradox.Comment: 5 pages, 4 figure
Dynamic Stark Effect in Strongly Coupled Microcavity Exciton-Polaritons
We present experimental observations of a non-resonant dynamic Stark shift in
strongly coupled microcavity quantum well exciton-polaritons - a system which
provides a rich variety of solid-state collective phenomena. The Stark effect
is demonstrated in a GaAs/AlGaAs system at 10K by femtosecond pump-probe
measurements, with the blue shift approaching the meV scale for a pump fluence
of 2 mJcm^-2 and 50 meV red detuning, in good agreement with theory. The energy
level structure of the strongly coupled polariton Rabi-doublet remains
unaffected by the blue shift. The demonstrated effect should allow generation
of ultrafast density-independent potentials and imprinting well-defined phase
profiles on polariton condensates, providing a powerful tool for manipulation
of these condensates, similar to dipole potentials in cold atom systems
Observation of exciton-polariton ultrafast dynamic Stark effect
We demonstrate ultrafast phase control of exciton-polaritons in a GaAs/AlGaAs strongly coupled microcavity exploiting the ac Stark effect. Our approach yields meV-scale shifts without carrier generation, providing a powerful tool towards control of polariton BECs