45 research outputs found
Independent high-purity photons created in domain-engineered crystals
Advanced photonic quantum technology relies on multi-photon interference
which requires bright sources of high-purity single photons. Here, we implement
a novel domain-engineering technique for tailoring the nonlinearity of a
parametric down-conversion crystal. We create pairs of independently-heralded
telecom-wavelength photons and achieve high heralding, brightness and spectral
purities without filtering.Comment: 8 pages, 5 figures Imprecise comparison with the experimental results
in [28] has been remove
Tailored quantum light for photonic quantum technologies
Photonic quantum technologies rely on the deterministic preparation of qubits encoded in quantum states of light: advances in this field are therefore contingent
with the development of reliable photon sources. In this Thesis, I address this challenge presenting a novel and versatile approach to single-photon generation based
on nonlinearity engineering in parametric down-conversion. By tailoring the effective nonlinearity of a crystal, this scheme enables access to the spectral degree
of freedom of photonic qubits with unprecedented precision, and translates into a
number of different applications based on the manipulation of the biphoton spectral/temporal properties. A thorough theoretical and numerical description of such
approach is provided and paired with experimental benchmarks conducted in three
main experiments. The first experiment tackles the single-photon spectral purity
problem in down-conversion sources: pure photons are in fact required for achieving perfect two-photon interference, a keystone of most quantum protocols. The
second experiment demonstrates the feasibility of nonlinearity engineering to produce tailored entanglement encoded in the spectrum of biphoton states. Finally, the
third experiment certifies the compatibility of this technique with different degrees
of freedom, demonstrating hyperentanglement of spatially and spectrally structured
quantum light. In conclusion, this Thesis stands as a cookbook for designing simple
yet flexible and highly-efficient single-photon sources based on tailored parametric
down-conversion processes
Experimental investigation on the geometry of GHz states
Nonclassical correlations arising in complex quantum networks are attracting growing interest, both from afundamental perspective and for potential applications in information processing. In particular, in an entanglementswapping scenario a new kind of correlations arise, the so-called nonbilocal correlations that are incompatible withlocal realism augmented with the assumption that the sources of states used in the experiment are independent.In practice, however, bilocality tests impose strict constraints on the experimental setup and in particular to thepresence of shared reference frames between the parties. Here, we experimentally address this point showing thatfalse positive nonbilocal quantum correlations can be observed even though the sources of states are independent.To overcome this problem, we propose and demonstrate a scheme for the violation of bilocality that does notrequire shared reference frames and thus constitutes an important building block for future investigations ofquantum correlations in complex network
Pure down-conversion photons through sub-coherence length domain engineering
Photonic quantum technology relies on efficient sources of coherent single
photons, the ideal carriers of quantum information. Heralded single photons
from parametric down-conversion can approximate on-demand single photons to a
desired degree, with high spectral purities achieved through group-velocity
matching and tailored crystal nonlinearities.
Here we propose crystal nonlinearity engineering techniques with
sub-coherence-length domains. We first introduce a combination of two existing
methods: a deterministic approach with coherence-length domains and
probabilistic domain-width annealing. We then show how the same deterministic
domain-flip approach can be implemented with sub-coherence length domains. Both
of these complementary techniques create highly pure photons, outperforming
previous methods, in particular for short nonlinear crystals matched to
femtosecond lasers.Comment: 12 pages, 4 figures. Minor update to Fig.
Design Considerations for High-purity Heralded Single Photon Sources
When building a parametric downconversion photon-pair source with spectrally
separable photons, e.g. for making high-purity heralded single photons, two
practical issues must be accounted for: the design of the experiment, and its
characterization. To address experiment design, we study the impact on spectral
separability of realistic (sech shaped and chirped) pump fields, realistic
nonlinear crystals with fabrication imperfections, and undesirable PDC
generation far from the central PMF peak coming from nonlinearity shaping
methods. To address experiment characterization, we study the effect of
discretization and spectral range of the measured bi-photon joint spectrum, the
difference between inferring separability from the joint spectral amplitude vs.
the joint spectral intensity, and advantages of interference experiments for
purity characterization over methods based on the joint spectral intensity.
This study will be of practical interest to researchers building the next
generation of nonlinear sources of separable photon pairs.Comment: 13 pages, 14 figure
Direct Generation of Tailored Pulse-Mode Entanglement
Photonic quantum technology increasingly uses frequency encoding to enable
higher quantum information density and noise resilience. Pulsed time-frequency
modes (TFM) represent a unique class of spectrally encoded quantum states of
light that enable a complete framework for quantum information processing.
Here, we demonstrate a technique for direct generation of entangled TFM-encoded
states in single-pass, tailored downconversion processes. We achieve
unprecedented quality in state generation---high rates, heralding efficiency
and state fidelity---as characterised via highly resolved time-of-flight fibre
spectroscopy and two-photon interference. We employ this technique in a
four-photon entanglement swapping scheme as a primitive for TFM-encoded quantum
protocols.Comment: 5 pages, 4 figures, 3 pages supplemental materia
Enhanced Multi-Qubit Phase Estimation in Noisy Environments by Local Encoding
The first generation of multi-qubit quantum technologies will consist of
noisy, intermediate-scale devices for which active error correction remains out
of reach. To exploit such devices, it is thus imperative to use passive error
protection that meets a careful trade-off between noise protection and resource
overhead. Here, we experimentally demonstrate that single-qubit encoding can
significantly enhance the robustness of entanglement and coherence of
four-qubit graph states against local noise with a preferred direction. In
particular, we explicitly show that local encoding provides a significant
practical advantage for phase estimation in noisy environments. This
demonstrates the efficacy of local unitary encoding under realistic conditions,
with potential applications in multi-qubit quantum technologies for metrology,
multi-partite secrecy and error correction.Comment: 7 figure