21 research outputs found
Quantum networking with optimised parametric down-conversion sources
Quantum information processing exploits superposition and entanglement to enable
tasks in computation, communication and sensing that are classically inconceivable.
Photonics is a leading platform for quantum information processing owing to the
relative ease in which the encoding and manipulation of quantum information can be
achieved, but there are a set of characteristics that photons themselves must exhibit
in order to be useful. The ideal photon source for building up multi-qubit states
needs to produce indistinguishable photons with high efficiency. Indistinguishability
is crucial for minimising errors in two-photon interference, central to building larger
states, while high heralding rates will be needed to overcome unfavourable loss
scaling. Domain engineering in parametric down-conversion sources negates the
need for lossy spectral filtering allowing one to satisfy these conditions inherently
within the source design.
Contained in this Thesis are two experimental investigations. Within the first
investigation, we present a telecom-wavelength parametric down-conversion photon
source that operates on the achievable limit of domain engineering. The source is
capable of generating photons from independent sources which achieve two-photon
interference visibilities of up to 98.6 ± 1.1% without narrow-band filtering. As
a consequence, we can reach net heralding efficiencies of 67.5%, corresponding to
collection efficiencies exceeding 90%. These sources enable us to efficiently generate
multi-photon graph states, constituting the second experimental investigation.
Graph states, and their underlying formalism, have been shown to be a valuable
resource in quantum information processing. The generation and distribution of a
6-photon graph state—defining the topology of a quantum network—allows us to
explore prospective issues with networks that invoke protocols beyond end-to-end
primitives, where users only require local operations and projective measurements.
In the case where multiple users wish to establish a common key for conference
communication, our proof-of-principle experiment concludes that employing N-user
key distribution methods over 2-user methods, results in a 2.13 ± 0.06 key rate
advantage
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
Hyper-entanglement between pulse modes and frequency bins
Hyper-entanglement between two or more photonic degrees of freedom (DOF) can
enhance and enable new quantum protocols by allowing each DOF to perform the
task it is optimally suited for. Here we demonstrate the generation of photon
pairs hyper-entangled between pulse modes and frequency bins. The pulse modes
are generated via parametric downconversion in a domain-engineered crystal and
subsequently entangled to two frequency bins via a spectral mapping technique.
The resulting hyper-entangled state is characterized and verified via
measurement of its joint spectral intensity and non-classical two-photon
interference patterns from which we infer its spectral phase. The protocol
combines the robustness to loss, intrinsic high dimensionality and
compatibility with standard fiber-optic networks of the energy-time DOF with
the ability of hyper-entanglement to increase the capacity and efficiency of
the quantum channel, already exploited in recent experimental applications in
both quantum information and quantum computation
Experimental test of local observer-independence
The scientific method relies on facts, established through repeated
measurements and agreed upon universally, independently of who observed them.
In quantum mechanics, the objectivity of observations is not so clear, most
dramatically exposed in Eugene Wigner's eponymous thought experiment where two
observers can experience seemingly different realities. The question whether
these realities can be reconciled in an observer-independent way has long
remained inaccessible to empirical investigation, until recent no-go-theorems
constructed an extended Wigner's friend scenario with four observers that
allows us to put it to the test. In a state-of-the-art 6-photon experiment, we
realise this extended Wigner's friend scenario, experimentally violating the
associated Bell-type inequality by 5 standard deviations. If one holds fast to
the assumptions of locality and free-choice, this result implies that quantum
theory should be interpreted in an observer-dependent way.Comment: 5+5 pages, 6 figure
Entanglement-induced collective many-body interference
Entanglement and interference are both hallmark effects of quantum physics.
Particularly rich dynamics arise when multiple (at least partially)
indistinguishable particles are subjected to either of these phenomena. By
combining both entanglement and many-particle interference, we propose an
interferometric setting through which N-particle interference can be observed,
while any interference of lower orders is strictly suppressed. We
experimentally demonstrate this effect in a four-photon interferometer, where
the interference is nonlocal, in principle, as only pairs of photons interfere
at two separate and independent beam splitters. A joint detection of all four
photons identifies a high-visibility interference pattern varying as a function
of their collective four-particle phase, a genuine four-body property
Quantum communication complexity beyond Bell nonlocality
Efficient distributed computing offers a scalable strategy for solving
resource-demanding tasks such as parallel computation and circuit optimisation.
Crucially, the communication overhead introduced by the allotment process
should be minimised -- a key motivation behind the communication complexity
problem (CCP). Quantum resources are well-suited to this task, offering clear
strategies that can outperform classical counterparts. Furthermore, the
connection between quantum CCPs and nonlocality provides an
information-theoretic insights into fundamental quantum mechanics. Here we
connect quantum CCPs with a generalised nonlocality framework -- beyond the
paradigmatic Bell's theorem -- by incorporating the underlying causal
structure, which governs the distributed task, into a so-called nonlocal hidden
variable model. We prove that a new class of communication complexity tasks can
be associated to Bell-like inequalities, whose violation is both necessary and
sufficient for a quantum gain. We experimentally implement a multipartite CCP
akin to the guess-your-neighbour-input scenario, and demonstrate a quantum
advantage when multipartite Greenberger-Horne-Zeilinger (GHZ) states are shared
among three users.Comment: 21 pages, 5 figure