7,883 research outputs found
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
Deterministic and cascadable conditional phase gate for photonic qubits
Previous analyses of conditional \phi-phase gates for photonic qubits that
treat cross-phase modulation (XPM) in a causal, multimode, quantum field
setting suggest that a large (~\pi rad) nonlinear phase shift is always
accompanied by fidelity-degrading noise [J. H. Shapiro, Phys. Rev. A 73, 062305
(2006); J. Gea-Banacloche, Phys. Rev. A 81, 043823 (2010)]. Using an atomic
V-system to model an XPM medium, we present a conditional phase gate that, for
sufficiently small nonzero \phi, has high fidelity. The gate is made cascadable
by using using a special measurement, principal mode projection, to exploit the
quantum Zeno effect and preclude the accumulation of fidelity-degrading
departures from the principal-mode Hilbert space when both control and target
photons illuminate the gate
From Quantum Optics to Quantum Technologies
Quantum optics is the study of the intrinsically quantum properties of light.
During the second part of the 20th century experimental and theoretical
progress developed together; nowadays quantum optics provides a testbed of many
fundamental aspects of quantum mechanics such as coherence and quantum
entanglement. Quantum optics helped trigger, both directly and indirectly, the
birth of quantum technologies, whose aim is to harness non-classical quantum
effects in applications from quantum key distribution to quantum computing.
Quantum light remains at the heart of many of the most promising and
potentially transformative quantum technologies. In this review, we celebrate
the work of Sir Peter Knight and present an overview of the development of
quantum optics and its impact on quantum technologies research. We describe the
core theoretical tools developed to express and study the quantum properties of
light, the key experimental approaches used to control, manipulate and measure
such properties and their application in quantum simulation, and quantum
computing.Comment: 20 pages, 3 figures, Accepted, Prog. Quant. Ele
Measurement of conditional phase shifts for quantum logic
Measurements of the birefringence of a single atom strongly coupled to a
high-finesse optical resonator are reported, with nonlinear phase shifts
observed for intracavity photon number much less than one. A proposal to
utilize the measured conditional phase shifts for implementing quantum logic
via a quantum-phase gate (QPG) is considered. Within the context of a simple
model for the field transformation, the parameters of the "truth table" for the
QPG are determined.Comment: 4 pages in Postscript format, including 4 figures (attached as
uuencoded version of a gzip-file
Non-Local Quantum Gates: a Cavity-Quantum-Electro-Dynamics implementation
The problems related to the management of large quantum registers could be
handled in the context of distributed quantum computation: unitary non-local
transformations among spatially separated local processors are realized
performing local unitary transformations and exchanging classical
communication. In this paper, we propose a scheme for the implementation of
universal non-local quantum gates such as a controlled-\gate{NOT} (\cnot)
and a controlled-quantum phase gate (\gate{CQPG}). The system we have chosen
for their physical implementation is a Cavity-Quantum-Electro-Dynamics (CQED)
system formed by two spatially separated microwave cavities and two trapped
Rydberg atoms. We describe the procedures to follow for the realization of each
step necessary to perform a specific non-local operation.Comment: 12 pages, 5 figures, RevTeX; extensively revised versio
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