35 research outputs found
Quasi-Superactivation of Classical Capacity of Zero-Capacity Quantum Channels
One of the most surprising recent results in quantum Shannon theory is the
superactivation of the quantum capacity of a quantum channel. This phenomenon
has its roots in the extreme violation of additivity of the channel capacity
and enables to reliably transmit quantum information over zero-capacity quantum
channels. In this work we demonstrate a similar effect for the classical
capacity of a quantum channel which previously was thought to be impossible. We
show that a nonzero classical capacity can be achieved for all zero-capacity
quantum channels and it only requires the assistance of an elementary
photon-atom interaction process - the stimulated emission.Comment: 52 pages, 6 figures, Journal-ref: Journal of Modern Optics, published
version (minor typo fixed
Long distance quantum teleportation of qubits from photons at 1300 nm to photons at 1550 nm wavelength
Elementary 2-dimensional quantum states (qubits) encoded in 1300 nm
wavelength photons are teleported onto 1550 nm photons. The use of
telecommunication wavelengths enables to take advantage of standard optical
fibre and permits to teleport from one lab to a distant one, 55 m away,
connected by 2 km of fibre. A teleportation fidelity of 81.2 % is reported.
This is large enough to demonstrate the principles of quantum teleportation, in
particular that entanglement is exploited. This experiment constitutes a first
step towards a quantum repeater.Comment: 7 pages, 5 figures, Extended version of Nature lette
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
Measurements in two bases are sufficient for certifying high-dimensional entanglement
High-dimensional encoding of quantum information provides a promising method
of transcending current limitations in quantum communication. One of the
central challenges in the pursuit of such an approach is the certification of
high-dimensional entanglement. In particular, it is desirable to do so without
resorting to inefficient full state tomography. Here, we show how carefully
constructed measurements in two bases (one of which is not orthonormal) can be
used to faithfully and efficiently certify bipartite high-dimensional states
and their entanglement for any physical platform. To showcase the practicality
of this approach under realistic conditions, we put it to the test for photons
entangled in their orbital angular momentum. In our experimental setup, we are
able to verify 9-dimensional entanglement for a pair of photons on a
11-dimensional subspace each, at present the highest amount certified without
any assumptions on the state.Comment: 11+14 pages, 2+7 figure
Quantum Communication
Quantum communication, and indeed quantum information in general, has changed
the way we think about quantum physics. In 1984 and 1991, the first protocol
for quantum cryptography and the first application of quantum non-locality,
respectively, attracted a diverse field of researchers in theoretical and
experimental physics, mathematics and computer science. Since then we have seen
a fundamental shift in how we understand information when it is encoded in
quantum systems. We review the current state of research and future directions
in this new field of science with special emphasis on quantum key distribution
and quantum networks.Comment: Submitted version, 8 pg (2 cols) 5 fig
On-chip generation of high-dimensional entangled quantum states and their coherent control
Optical quantum states based on entangled photons are essential for solving questions in fundamental physics and are at the heart of quantum information science1. Specifically, the realization of high-dimensional states (D-level quantum systems, that is, qudits, with D > 2) and their control are necessary for fundamental investigations of quantum mechanics2, for increasing the sensitivity of quantum imaging schemes3, for improving the robustness and key rate of quantum communication protocols4, for enabling a richer variety of quantum simulations5, and for achieving more efficient and error-tolerant quantum computation6. Integrated photonics has recently become a leading platform for the compact, cost-efficient, and stable generation and processing of non-classical optical states7. However, so far, integrated entangled quantum sources have been limited to qubits (D = 2)8, 9, 10, 11. Here we demonstrate on-chip generation of entangled qudit states, where the photons are created in a coherent superposition of multiple high-purity frequency modes. In particular, we confirm the realization of a quantum system with at least one hundred dimensions, formed by two entangled qudits with D = 10. Furthermore, using state-of-the-art, yet off-the-shelf telecommunications components, we introduce a coherent manipulation platform with which to control frequency-entangled states, capable of performing deterministic high-dimensional gate operations. We validate this platform by measuring Bell inequality violations and performing quantum state tomography. Our work enables the generation and processing of high-dimensional quantum states in a single spatial mode
Entangling Independent Photons by Time Measurement
A quantum system composed of two or more subsystems can be in an entangled
state, i.e. a state in which the properties of the global system are well
defined but the properties of each subsystem are not. Entanglement is at the
heart of quantum physics, both for its conceptual foundations and for
applications in information processing and quantum communication. Remarkably,
entanglement can be "swapped": if one prepares two independent entangled pairs
A1-A2 and B1-B2, a joint measurement on A1 and B1 (called a "Bell-State
Measurement", BSM) has the effect of projecting A2 and B2 onto an entangled
state, although these two particles have never interacted or shared any common
past[1,2]. Experiments using twin photons produced by spontaneous parametric
down-conversion (SPDC) have already demonstrated entanglement swapping[3-6],
but here we present its first realization using continuous wave (CW) sources,
as originally proposed[2]. The challenge was to achieve sufficiently sharp
synchronization of the photons in the BSM. Using narrow-band filters, the
coherence time of the photons that undergo the BSM is significantly increased,
exceeding the temporal resolution of the detectors. Hence pulsed sources can be
replaced by CW sources, which do not require any synchronization[6,7], allowing
for the first time the use of completely autonomous sources. Our experiment
exploits recent progress in the time precision of photon detectors, in the
efficiency of photon pair production by SPDC with waveguides in nonlinear
crystals[8], and in the stability of narrow-band filters. This approach is
independent of the form of entanglement; we employed time-bin entangled
photons[9] at telecom wavelengths. Our setup is robust against thermal or
mechanical fluctuations in optical fibres thanks to cm-long coherence lengths.Comment: 13 pages, 3 figure