142 research outputs found
Photonic Entanglement for Fundamental Tests and Quantum Communication
Entanglement is at the heart of fundamental tests of quantum mechanics like
tests of Bell-inequalities and, as discovered lately, of quantum computation
and communication. Their technological advance made entangled photons play an
outstanding role in entanglement physics. We give a generalized concept of
qubit entanglement and review the state of the art of photonic experiments.Comment: 54 pages, 33 figures. Review article submitted to QIC (Rinton
Quantum teleportation and nonlocality: the puzzling predictions of entanglement are coming of age
The academic research into entanglement nicely illustrates the interplay
between fundamental science and applications, and the need to foster both
aspects to advance either one. For instance, the possibility to distribute
entangled photons over tens or even hundreds of kilometers is fascinating
because it confirms the quantum predictions over large distances, while quantum
theory is often presented to apply to the very small (see Figure 1). On the
other hand, entanglement enables quantum key distribution (QKD) [1]. This most
advanced application of quantum information processing allows one to distribute
cryptographic keys in a provably secure manner. For this, one merely has to
measure the two halves of an entangled pair of photons. Surprisingly, and being
of both fundamental and practical interest, the use of entanglement removes
even the necessity for trusting most equipment used for the measurements [5].
Furthermore, entanglement serves as a resource for quantum teleportation (see
Figure 2) [1]. In turn, this provides a tool for extending quantum key
distribution to arbitrarily large distances and building large-scale networks
that connect future quantum computers and atomic clocks [6]. In the following,
we describe the counter-intuitive properties of entangled particles as well as
a few recent experiments that address fundamental and applied aspects of
quantum teleportation. While a lot of work is being done using different
quantum systems, including trapped ions, color centers in diamond, quantum
dots, and superconducting circuits, we will restrict ourselves to experiments
involving photons due to their suitability for building future quantum
networks.Comment: This paper is intended to be published in the 2015 fourth issue of
Europhysics News as a "Feature Article
Quantum Key Distribution
This chapter describes the application of lasers, specifically diode lasers,
in the area of quantum key distribution (QKD). First, we motivate the
distribution of cryptographic keys based on quantum physical properties of
light, give a brief introduction to QKD assuming the reader has no or very
little knowledge about cryptography, and briefly present the state-of-the-art
of QKD. In the second half of the chapter we describe, as an example of a
real-world QKD system, the system deployed between the University of Calgary
and SAIT Polytechnic. We conclude the chapter with a brief discussion of
quantum networks and future steps.Comment: 20 pages, 12 figure
Testing nonlocality over 12.4 km of underground fiber with universal time-bin qubit analyzers
We experimentally demonstrate that the nonlocal nature of time-bin entangled
photonic qubits persists when one or two qubits of the pair are converted to
polarization qubits. This is possible by implementing a novel Universal
Time-Bin Qubit Analyzer (UTBA), which, for the first time, allows analyzing
time-bin qubits in any basis. We reveal the nonlocal nature of the emitted
light by violating the Clauser-Horne-Shimony-Holt inequality with measurement
bases exploring all the dimensions of the Bloch sphere. Moreover, we conducted
experiments where one qubit is transmitted over a 12.4 km underground fiber
link and demonstrate the suitability of our scheme for use in a real-world
setting. The resulting entanglement can also be interpreted as hybrid
entanglement between different types of degrees of freedom of two physical
systems, which could prove useful in large scale, heterogeneous quantum
networks. This work opens new possibilities for testing nonlocality and for
implementing new quantum communication protocols with time-bin entanglement.Comment: 6 pages, 5 figure
Device-dependent and device-independent quantum key distribution without a shared reference frame
Standard quantum key distribution (QKD) protocols typically assume that the
distant parties share a common reference frame. In practice, however,
establishing and maintaining a good alignment between distant observers is
rarely a trivial issue, which may significantly restrain the implementation of
long-distance quantum communication protocols. Here we propose simple QKD
protocols that do not require the parties to share any reference frame, and
study their security and feasibility in both the usual device-dependent
case--in which the two parties use well characterized measurement devices--as
well as in the device-independent case--in which the measurement devices can be
untrusted, and the security relies on the violation of a Bell inequality. To
illustrate the practical relevance of these ideas, we present a
proof-of-principle demonstration of our protocols using polarization entangled
photons distributed over a coiled 10-km-long optical fiber. We consider two
situations, in which either the fiber spool freely drifts, or randomly chosen
polarization transformations are applied. The correlations obtained from
measurements allow, with high probability, to generate positive asymptotic
secret key rates in both the device-dependent and device-independent scenarios
(under the fair-sampling assumption for the latter case).Comment: 12 pages, 11 figure
Quantum states prepared by realistic entanglement swapping
Entanglement swapping between photon pairs is a fundamental building block in
schemes using quantum relays or quantum repeaters to overcome the range limits
of long-distance quantum key distribution. We develop a closed-form solution
for the actual quantum states prepared by realistic entanglement swapping,
which takes into account experimental deficiencies due to inefficient
detectors, detector dark counts, and multiphoton-pair contributions of
parametric down-conversion sources. We investigate how the entanglement present
in the final state of the remaining modes is affected by the real-world
imperfections. To test the predictions of our theory, comparison with
previously published experimental entanglement swapping is provided.Comment: 44 pages, 7 figures, Published with minor changes in Phys. Rev.
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