5,918 research outputs found
Why Do Shoppers Use Cash? Evidence from Shopping Diary Data
Recent studies find that cash remains a dominant payment choice for small-value transactions despite the prevalence of alternative methods of payment such as debit and credit cards. For policy makers an important question is whether consumers truly prefer using cash or merchants restrict card usage. Using unique shopping diary data, we estimate a payment choice model with individual unobserved heterogeneity (demandside factors) while controlling for merchants’ acceptance of cards (supply-side factors). Based on a policy simulation where we impose universal card acceptance among merchants, we find that overall cash usage would decrease by only 7.7 percentage points, implying that cash usage in small-value transactions is driven mainly by consumers’ preferences
A photon-photon quantum gate based on a single atom in an optical resonator
Two photons in free space pass each other undisturbed. This is ideal for the
faithful transmission of information, but prohibits an interaction between the
photons as required for a plethora of applications in optical quantum
information processing. The long-standing challenge here is to realise a
deterministic photon-photon gate. This requires an interaction so strong that
the two photons can shift each others phase by pi. For polarisation qubits,
this amounts to the conditional flipping of one photon's polarisation to an
orthogonal state. So far, only probabilistic gates based on linear optics and
photon detectors could be realised, as "no known or foreseen material has an
optical nonlinearity strong enough to implement this conditional phase
shift..." [Science 318, 1567]. Meanwhile, tremendous progress in the
development of quantum-nonlinear systems has opened up new possibilities for
single-photon experiments. Platforms range from Rydberg blockade in atomic
ensembles to single-atom cavity quantum electrodynamics. Applications like
single-photon switches and transistors, two-photon gateways, nondestructive
photon detectors, photon routers and nonlinear phase shifters have been
demonstrated, but none of them with the ultimate information carriers, optical
qubits. Here we employ the strong light-matter coupling provided by a single
atom in a high-finesse optical resonator to realise the Duan-Kimble protocol of
a universal controlled phase flip (CPF, pi phase shift) photon-photon quantum
gate. We achieve an average gate fidelity of F=(76.2+/-3.6)% and specifically
demonstrate the capability of conditional polarisation flipping as well as
entanglement generation between independent input photons. Our gate could
readily perform most of the hitherto existing two-photon operations. It also
discloses avenues towards new quantum information processing applications where
photons are essential.Comment: 7 pages, 5 figure
Photon-Mediated Quantum Gate between Two Trapped Neutral Atoms in an Optical Cavity
Quantum logic gates are fundamental building blocks of quantum computers.
Their integration into quantum networks requires strong qubit coupling to
network channels, as can be realized with neutral atoms and optical photons in
cavity quantum electrodynamics. Here we demonstrate that the long-range
interaction mediated by a flying photon performs a gate between two stationary
atoms inside an optical cavity from which the photon is reflected. This single
step executes the gate in . We show an entangling operation
between the two atoms by generating a Bell state with 76(2)% fidelity. The gate
also operates as a CNOT. We demonstrate 74.1(1.6)% overlap between the observed
and the ideal gate output, limited by the state preparation fidelity of
80.2(0.8)%. As the atoms are efficiently connected to a photonic channel, our
gate paves the way towards quantum networking with multiqubit nodes and the
distribution of entanglement in repeater-based long-distance quantum networks.Comment: 10 pages including appendix, 5 figure
Cavity Carving of Atomic Bell States
We demonstrate entanglement generation of two neutral atoms trapped inside an
optical cavity. Entanglement is created from initially separable two-atom
states through carving with weak photon pulses reflected from the cavity. A
polarization rotation of the photons heralds the entanglement. We show the
successful implementation of two different protocols and the generation of all
four Bell states with a maximum fidelity of (90+-2)%. The protocol works for
any distance between cavity-coupled atoms, and no individual addressing is
required. Our result constitutes an important step towards applications in
quantum networks, e.g. for entanglement swapping in a quantum repeater.Comment: 9 pages, 7 figures including Supplemen
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