135 research outputs found
Controllable quantum point junction on the surface of an antiferromagnetic topological insulator
Engineering and manipulation of unidirectional channels has been achieved in quantum Hall systems, leading to the construction of electron interferometers and proposals for low-power electronics and quantum information science applications. However, to fully control the mixing and interference of edge-state wave functions, one needs stable and tunable junctions. Encouraged by recent material candidates, here we propose to achieve this using an antiferromagnetic topological insulator that supports two distinct types of gapless unidirectional channels, one from antiferromagnetic domain walls and the other from single-height steps. Their distinct geometric nature allows them to intersect robustly to form quantum point junctions, which then enables their control by magnetic and electrostatic local probes. We show how the existence of stable and tunable junctions, the intrinsic magnetism and the potential for higher-temperature performance make antiferromagnetic topological insulators a promising platform for electron quantum optics and microelectronic applications
Teleportation-based realization of an optical quantum two-qubit entangling gate
In recent years, there has been heightened interest in quantum teleportation,
which allows for the transfer of unknown quantum states over arbitrary
distances. Quantum teleportation not only serves as an essential ingredient in
long-distance quantum communication, but also provides enabling technologies
for practical quantum computation. Of particular interest is the scheme
proposed by Gottesman and Chuang [Nature \textbf{402}, 390 (1999)], showing
that quantum gates can be implemented by teleporting qubits with the help of
some special entangled states. Therefore, the construction of a quantum
computer can be simply based on some multi-particle entangled states, Bell
state measurements and single-qubit operations. The feasibility of this scheme
relaxes experimental constraints on realizing universal quantum computation.
Using two different methods we demonstrate the smallest non-trivial module in
such a scheme---a teleportation-based quantum entangling gate for two different
photonic qubits. One uses a high-fidelity six-photon interferometer to realize
controlled-NOT gates and the other uses four-photon hyper-entanglement to
realize controlled-Phase gates. The results clearly demonstrate the working
principles and the entangling capability of the gates. Our experiment
represents an important step towards the realization of practical quantum
computers and could lead to many further applications in linear optics quantum
information processing.Comment: 10 pages, 6 figure
Quantum Optical Systems for the Implementation of Quantum Information Processing
We review the field of Quantum Optical Information from elementary
considerations through to quantum computation schemes. We illustrate our
discussion with descriptions of experimental demonstrations of key
communication and processing tasks from the last decade and also look forward
to the key results likely in the next decade. We examine both discrete (single
photon) type processing as well as those which employ continuous variable
manipulations. The mathematical formalism is kept to the minimum needed to
understand the key theoretical and experimental results
Topological fault-tolerance in cluster state quantum computation
We describe a fault-tolerant version of the one-way quantum computer using a
cluster state in three spatial dimensions. Topologically protected quantum
gates are realized by choosing appropriate boundary conditions on the cluster.
We provide equivalence transformations for these boundary conditions that can
be used to simplify fault-tolerant circuits and to derive circuit identities in
a topological manner. The spatial dimensionality of the scheme can be reduced
to two by converting one spatial axis of the cluster into time. The error
threshold is 0.75% for each source in an error model with preparation, gate,
storage and measurement errors. The operational overhead is poly-logarithmic in
the circuit size.Comment: 20 pages, 12 figure
Brokered Graph State Quantum Computing
We describe a procedure for graph state quantum computing that is tailored to
fully exploit the physics of optically active multi-level systems. Leveraging
ideas from the literature on distributed computation together with the recent
work on probabilistic cluster state synthesis, our model assigns to each
physical system two logical qubits: the broker and the client. Groups of
brokers negotiate new graph state fragments via a probabilistic optical
protocol. Completed fragments are mapped from broker to clients via a simple
state transition and measurement. The clients, whose role is to store the
nascent graph state long term, remain entirely insulated from failures during
the brokerage. We describe an implementation in terms of NV-centres in diamond,
where brokers and clients are very naturally embodied as electron and nuclear
spins.Comment: 5 pages, 3 figure
Quantum Transduction of Telecommunications-band Single Photons from a Quantum Dot by Frequency Upconversion
The ability to transduce non-classical states of light from one wavelength to
another is a requirement for integrating disparate quantum systems that take
advantage of telecommunications-band photons for optical fiber transmission of
quantum information and near-visible, stationary systems for manipulation and
storage. In addition, transducing a single-photon source at 1.3 {\mu}m to
visible wavelengths for detection would be integral to linear optical quantum
computation due to the challenges of detection in the near-infrared. Recently,
transduction at single-photon power levels has been accomplished through
frequency upconversion, but it has yet to be demonstrated for a true
single-photon source. Here, we transduce the triggered single-photon emission
of a semiconductor quantum dot at 1.3 {\mu}m to 710 nm with a total detection
(internal conversion) efficiency of 21% (75%). We demonstrate that the 710 nm
signal maintains the quantum character of the 1.3 {\mu}m signal, yielding a
photon anti-bunched second-order intensity correlation, g^(2)(t), that shows
the optical field is composed of single photons with g^(2)(0) = 0.165 < 0.5.Comment: 7 pages, 4 figure
Efficient and long-lived quantum memory with cold atoms inside a ring cavity
Quantum memories are regarded as one of the fundamental building blocks of
linear-optical quantum computation and long-distance quantum communication. A
long standing goal to realize scalable quantum information processing is to
build a long-lived and efficient quantum memory. There have been significant
efforts distributed towards this goal. However, either efficient but
short-lived or long-lived but inefficient quantum memories have been
demonstrated so far. Here we report a high-performance quantum memory in which
long lifetime and high retrieval efficiency meet for the first time. By placing
a ring cavity around an atomic ensemble, employing a pair of clock states,
creating a long-wavelength spin wave, and arranging the setup in the
gravitational direction, we realize a quantum memory with an intrinsic spin
wave to photon conversion efficiency of 73(2)% together with a storage lifetime
of 3.2(1) ms. This realization provides an essential tool towards scalable
linear-optical quantum information processing.Comment: 6 pages, 4 figure
Narrowband Biphotons: Generation, Manipulation, and Applications
In this chapter, we review recent advances in generating narrowband biphotons
with long coherence time using spontaneous parametric interaction in monolithic
cavity with cluster effect as well as in cold atoms with electromagnetically
induced transparency. Engineering and manipulating the temporal waveforms of
these long biphotons provide efficient means for controlling light-matter
quantum interaction at the single-photon level. We also review recent
experiments using temporally long biphotons and single photons.Comment: to appear as a book chapter in a compilation "Engineering the
Atom-Photon Interaction" published by Springer in 2015, edited by A.
Predojevic and M. W. Mitchel
Experimental measurement-based quantum computing beyond the cluster-state model
The paradigm of measurement-based quantum computation opens new experimental
avenues to realize a quantum computer and deepens our understanding of quantum
physics. Measurement-based quantum computation starts from a highly entangled
universal resource state. For years, clusters states have been the only known
universal resources. Surprisingly, a novel framework namely quantum computation
in correlation space has opened new routes to implement measurement-based
quantum computation based on quantum states possessing entanglement properties
different from cluster states. Here we report an experimental demonstration of
every building block of such a model. With a four-qubit and a six-qubit state
as distinct from cluster states, we have realized a universal set of
single-qubit rotations, two-qubit entangling gates and further Deutsch's
algorithm. Besides being of fundamental interest, our experiment proves
in-principle the feasibility of universal measurement-based quantum computation
without using cluster states, which represents a new approach towards the
realization of a quantum computer.Comment: 26 pages, final version, comments welcom
Outcomes of Cardiac Screening in Adolescent Soccer Players.
BACKGROUND: Reports on the incidence and causes of sudden cardiac death among young athletes have relied largely on estimated rates of participation and varied methods of reporting. We sought to investigate the incidence and causes of sudden cardiac death among adolescent soccer players in the United Kingdom. METHODS: From 1996 through 2016, we screened 11,168 adolescent athletes with a mean (±SD) age of 16.4±1.2 years (95% of whom were male) in the English Football Association (FA) cardiac screening program, which consisted of a health questionnaire, physical examination, electrocardiography, and echocardiography. The FA registry was interrogated to identify sudden cardiac deaths, which were confirmed with autopsy reports. RESULTS: During screening, 42 athletes (0.38%) were found to have cardiac disorders that are associated with sudden cardiac death. A further 225 athletes (2%) with congenital or valvular abnormalities were identified. After screening, there were 23 deaths from any cause, of which 8 (35%) were sudden deaths attributed to cardiac disease. Cardiomyopathy accounted for 7 of 8 sudden cardiac deaths (88%). Six athletes (75%) with sudden cardiac death had had normal cardiac screening results. The mean time between screening and sudden cardiac death was 6.8 years. On the basis of a total of 118,351 person-years, the incidence of sudden cardiac death among previously screened adolescent soccer players was 1 per 14,794 person-years (6.8 per 100,000 athletes). CONCLUSIONS: Diseases that are associated with sudden cardiac death were identified in 0.38% of adolescent soccer players in a cohort that underwent cardiovascular screening. The incidence of sudden cardiac death was 1 per 14,794 person-years, or 6.8 per 100,000 athletes; most of these deaths were due to cardiomyopathies that had not been detected on screening. (Funded by the English Football Association and others.)
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