63 research outputs found
Scheme for a linear-optical controlled-phase gate with programmable phase shift
We present a linear-optical scheme for a controlled-phase gate with tunable
phase shift programmed by a qubit state. In contrast to all previous tunable
controlled-phase gates, the phase shift is not hard-coded into the optical
setup, but can be tuned to any value from 0 to pi by the state of a so-called
program qubit. Our setup is feasible with current level of technology using
only linear-optical components. We provide an experimental feasibility study to
assess the gate's implementability. We also discuss options for increasing the
success probability up to 1/12 which approaches the success probability of a
optimal non-programmable tunable controlled-phase gate.Comment: 7 pages, 3 figure
Experimental measurement of the collectibility of two-qubit states
We present a proof-of-principle experiment demonstrating measurement of the
collectibility, a nonlinear entanglement witness proposed by Rudnicki et al.
[Phys. Rev. Lett. 107, 150502 (2011)]. This entanglement witness works for both
mixed and pure two-qubit states. In the later case it can be used to measure
entanglement in terms of the negativity. We measured the collectibility for
three distinct classes of photonic polarization-encoded two-qubit states, i.e.,
maximally entangled, separable and maximally mixed states. We demonstrate that
the measurement procedure is feasible and robust against typical experimental
shortcomings such as imperfect two-photon indistinguishability.Comment: 7 pages, 4 figure
State-dependent linear-optical qubit amplifier
We propose a linear-optical setup for heralded qubit amplification with
tunable output qubit fidelity. We study its success probability as a function
of output qubit fidelity showing that at the expense of lower fidelity, the
setup can considerably increase probability of successful operation. These
results are subsequently applied in a proposal for state dependent qubit
amplification. Similarly to state-dependent quantum cloning, the a priori
information about the input state allows to optimize the qubit amplification
procedure to obtain better fidelity versus success probability trade-off.Comment: 8 pages, 7 figure
Resource-efficient linear-optical quantum router
All-linear-optical scheme for fully featured quantum router is presented.
This device directs the signal photonic qubit according to the state of one
control photonic qubit. In the introduction we formulate the list of
requirements imposed on a fully quantum router. Then we describe our proposal
showing the exact principle of operation on a linear-optical scheme.
Subsequently we provide generalization of the scheme in order to optimize the
success probability by means of a tunable controlled-phase gate. At the end, we
show how one can modify the device to route multiple signal qubits using the
same control qubit.Comment: 7 pages, 6 figure
Entangling efficiency of linear-optical quantum gates
We propose a new measure of non-classicality of quantum gates which is
particularly suitable for probabilistic devices. This measure enables to
compare, e.g., deterministic devices which prepare entangled states with low
amount of entanglement with probabilistic devices which generate highly
entangled states but which fail sometimes. We provide examples demonstrating
advantages of this new measure over the so far employed entangling power.Comment: 5 pages, 7 figure
Experimental diagnostics of entanglement swapping by a collective entanglement test
The paper reports on experimental diagnostics of entanglement swapping
protocol by means of collective entanglement witness. Our approach is suitable
to detect disturbances occurring in the preparation of quantum states, quantum
communication channel and imperfect Bell-state projection. More specifically we
demonstrate that our method can distinguish disturbances such as
depolarization, phase-damping, amplitude-damping and imperfect Bell-state
measurement by observing four probabilities and estimating collective
entanglement witness. Since entanglement swapping is a key procedure for
quantum repeaters, quantum relays, device-independent quantum communications or
entanglement assisted error correction, this can aid in faster and practical
resolution of quality-of-transmission related problems as our approach requires
less measurements then other means of diagnostics.Comment: 6 pages, 4 figure
Entanglement-assisted scheme for nondemolition detection of the presence of a single photon
In this paper, we propose a resources-optimal linear-optical scheme for
quantum nondemolition detection of single-photon presence. By measuring the
state of ancillary photons, the presence of a photon in signal mode is revealed
with a success probability of 1/2 without any disturbance to its state. We also
show how to tune the setup to perform quantum nondemolition measurement of the
signal photon state, and we provide tradeoff between the extracted information
and the signal state disturbance. Moreover, the optimality of resources and
methods by which to increase the success probability are discussed.Comment: 7 pages, 5 figure
Experimentally attacking quantum money schemes based on quantum retrieval games
The concept of quantum money (QM) was proposed by Wiesner in the 1970s. Its
main advantage is that every attempt to copy QM unavoidably leads to imperfect
counterfeits. In the Wiesner's protocol, quantum banknotes need to be delivered
to the issuing bank for verification. Thus, QM requires quantum communication
which range is limited by noise and losses. Recently, Bozzio et al. (2018) have
demonstrated experimentally how to replace challenging quantum verification
with a classical channel and a quantum retrieval game (QRG). This brings QM
significantly closer to practical realisation, but still thorough analysis of
the revised scheme QM is required before it can be considered secure. We
address this problem by presenting a proof-of-concept attack on QRG-based QM
schemes, where we show that even imperfect quantum cloning can, under some
circumstances, provide enough information to break a QRG-based QM scheme.Comment: 6 pages, 7 figure
Experimental characterization of photon-number noise in Rarity-Tapster type interferometers
In this paper, we develop a simple model describing inherent photon-number
noise in Rarity-Tapster type interferometers. This noise is caused by
generating photon pairs in the process of spontaneous parametric
down-conversion and adding a third photon by attenuating fundamental laser mode
to single-photon level. We experimentally verify our model and present
resulting signal to noise ratios as well as obtained three-photon generation
rates as functions of various setup parameters. Subsequently we evaluate impact
of this particular source of noise on quantum teleportation which is a key
quantum information protocol using this interferometric configuration.Comment: 7 pages, 5 figure
Interplay between strong and weak measurement: Comparison of three experimental approaches to weak value estimation
Weak values are traditionally obtained using a weak interaction between the
measured system and a pointer state. It has, however, been pointed out that
weak coupling can be replaced by a carefully tailored strong interaction. This
paper provides a direct comparison of two strong interaction-based approaches
(strong interaction accompanied by either a suitably prepared pointer state or
quantum erasure) and the traditional weak interaction-based method. Presented
theoretical derivations explicitly prove analytical equivalence of these
approaches which was subsequently certified by an experiment implemented on the
platform of linear optics. We find that strong-interaction-based measurements
are experimentally less demanding on this platform.Comment: 9 pages, 3 figure
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