25 research outputs found
Single-shot quantum memory advantage in the simulation of stochastic processes
Stochastic processes underlie a vast range of natural and social phenomena.
Some processes such as atomic decay feature intrinsic randomness, whereas other
complex processes, e.g. traffic congestion, are effectively probabilistic
because we cannot track all relevant variables. To simulate a stochastic
system's future behaviour, information about its past must be stored and thus
memory is a key resource. Quantum information processing promises a memory
advantage for stochastic simulation that has been validated in recent
proof-of-concept experiments. Yet, in all past works, the memory saving would
only become accessible in the limit of a large number of parallel simulations,
because the memory registers of individual quantum simulators had the same
dimensionality as their classical counterparts. Here, we report the first
experimental demonstration that a quantum stochastic simulator can encode the
relevant information in fewer dimensions than any classical simulator, thereby
achieving a quantum memory advantage even for an individual simulator. Our
photonic experiment thus establishes the potential of a new, practical resource
saving in the simulation of complex systems
Heralded quantum steering over a high-loss channel
Entanglement is the key resource for many long-range quantum information
tasks, including secure communication and fundamental tests of quantum physics.
These tasks require robust verification of shared entanglement, but performing
it over long distances is presently technologically intractable because the
loss through an optical fiber or free-space channel opens up a detection
loophole. We design and experimentally demonstrate a scheme that verifies
entanglement in the presence of at least dB of added loss,
equivalent to approximately km of telecommunication fiber. Our protocol
relies on entanglement swapping to herald the presence of a photon after the
lossy channel, enabling event-ready implementation of quantum steering. This
result overcomes the key barrier in device-independent communication under
realistic high-loss scenarios and in the realization of a quantum repeater.Comment: 8 pages, 5 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
Time-resolved double-slit experiment with entangled photons
The double-slit experiment strikingly demonstrates the wave-particle duality
of quantum objects. In this famous experiment, particles pass one-by-one
through a pair of slits and are detected on a distant screen. A distinct
wave-like pattern emerges after many discrete particle impacts as if each
particle is passing through both slits and interfering with itself. While the
direct event-by-event buildup of this interference pattern has been observed
for massive particles such as electrons, neutrons, atoms and molecules, it has
not yet been measured for massless particles like photons. Here we present a
temporally- and spatially-resolved measurement of the double-slit interference
pattern using single photons. We send single photons through a birefringent
double-slit apparatus and use a linear array of single-photon detectors to
observe the developing interference pattern. The analysis of the buildup allows
us to compare quantum mechanics and the corpuscular model, which aims to
explain the mystery of single-particle interference. Finally, we send one
photon from an entangled pair through our double-slit setup and show the
dependence of the resulting interference pattern on the twin photon's measured
state. Our results provide new insight into the dynamics of the buildup process
in the double-slit experiment, and can be used as a valuable resource in
quantum information applications