22 research outputs found
Randomness-free Test of Non-classicality: a Proof of Concept
Quantum correlations and non-projective measurements underlie a plethora of
information-theoretic tasks, otherwise impossible in the classical world.
Existing schemes to certify such non-classical resources in a
device-independent manner require seed randomness -- which is often costly and
vulnerable to loopholes -- for choosing the local measurements performed on
different parts of a multipartite quantum system. In this letter, we propose
and experimentally implement a semi-device independent certification technique
for both quantum correlations and non-projective measurements without seed
randomness. Our test is {\it semi-device independent} in the sense that it
requires only prior knowledge of the dimension of the parts. By producing
specific correlated coins from pairs of photons entangled in their transverse
spatial modes we experimentally show a novel quantum advantage in correlated
coin tossing. We establish the advantage by showing that the correlated coin
procured from the entangled photons cannot be obtained from any two 2-level
classical correlated coins. The quantum advantage requires performing qubit
trine positive operator-valued measures (POVMs) on each part of the entangled
pair, thus also certifying such POVMs in a semi-device-independent manner. This
proof of concept firmly establishes a new cost-effective certification
technique for both, generating non-classical shared randomness and implementing
non-classical measurements which will be important for future multi-party
quantum communications
Fundamental Limits to Coherent Photon Generation with Solid-State Atomlike Transitions
Coherent generation of indistinguishable single photons is crucial for many
quantum communication and processing protocols. Solid-state realizations of
two-level atomic transitions or three-level spin- systems offer
significant advantages over their atomic counterparts for this purpose, albeit
decoherence can arise due to environmental couplings. One popular approach to
mitigate dephasing is to operate in the weak excitation limit, where excited
state population is minimal and coherently scattered photons dominate over
incoherent emission. Here we probe the coherence of photons produced using
two-level and spin- solid-state systems. We observe that the coupling
of the atomic-like transitions to the vibronic transitions of the crystal
lattice is independent of driving strength and detuning. We apply a polaron
master equation to capture the non-Markovian dynamics of the ground state
vibrational manifolds. These results provide insight into the fundamental
limitations for photon coherence from solid-state quantum emitters, with the
consequence that deterministic single-shot quantum protocols are impossible and
inherently probabilistic approaches must be embraced.Comment: 16 pages [with supplementary information], 8 figure
Efficient Quantum State Tracking in Noisy Environments
Quantum state tomography, which aims to find the best description of a
quantum state -- the density matrix, is an essential building block in quantum
computation and communication. Standard techniques for state tomography are
incapable of tracking changing states and often perform poorly in the presence
of environmental noise. Although there are different approaches to solve these
problems theoretically, experimental demonstrations have so far been sparse.
Our approach, matrix-exponentiated gradient tomography, is an online tomography
method that allows for state tracking, updates the estimated density matrix
dynamically from the very first measurements, is computationally efficient, and
converges to a good estimate quickly even with noisy data. The algorithm is
controlled via a single parameter, its learning rate, which determines the
performance and can be tailored in simulations to the individual experiment. We
present an experimental implementation of matrix-exponentiated gradient
tomography on a qutrit system encoded in the transverse spatial mode of
photons. We investigate the performance of our method on stationary and
evolving states, as well as significant environmental noise, and find
fidelities of around 95% in all cases.Comment: 10 pages, 3 figure