An on-demand source of bright entangled photon pairs is desirable for quantum key distribution (QKD) and quantum repeaters. The leading candidate to generate entangled photon pairs is based on spontaneous parametric down-conversion (SPDC) in a non-linear crystal. However, a fundamental trade-off exists between entanglement fidelity and efficiency in SPDC sources due to multiphoton emission at high brightness, which limits the pair extraction efficiency to 0.1% when operating at near-unity fidelity. Quantum dots in photonic nanostructures can in principle overcome this trade-off; however, the quantum dots that have achieved an en- entanglement fidelity on par with an SPDC source (99%) have poor pair extraction efficiency of 0.01%. Here, we show a measured peak concurrence of 95.3% ± 0.5% and pair extraction efficiency of 0.65% from an InAsP quantum dot in an InP photonic nanowire waveguide. Additionally, we show that an oscillating two-photon Bell state generated by a semiconductor quantum dot can establish a secure key for peer-to-peer QKD while using all generated photon pairs. Using our time-resolved QKD scheme alleviates the need to remove the exciton fine structure splitting.
Quantum key distribution (QKD) allows for communication with security guaranteed by quantum theory. The main theoretical problem in QKD is to calculate the secret key rate for a given protocol. Analytical formulas are known for protocols with a high degree of symmetry, since symmetry simplifies the analysis. However, experimental imperfections break symmetries, hence the effect of imperfections on key rates is difficult to estimate. Furthermore, it is an interesting question whether (intentionally) asymmetric protocols could outperform symmetric ones. In this work, we develop a robust numerical approach for calculating the key rate for arbitrary discrete-variable QKD protocols. This will allow researchers to study “unstructured” protocols, i.e., those that lack symmetry. Our approach relies on transforming the key rate calculation to the dual optimization problem, which dramatically reduces the number of parameters and hence the calculation time. We illustrate our method, first, by reproducing known literature results for some famous QKD protocols and, second, by investigating some unstructured protocols for which the key rate was previously unknown. Ultimately our vision is to develop user-friendly software that will allow researchers to assess the performance of any QKD protocol simply by running a MATLAB script on their laptop computer. We have taken a step towards that goal by making the key rate calculation more time-efficient. Further details about our work can be found in the following preprint: http://arxiv.org/abs/1510.01294.
We encode mutually unbiased bases (MUBs) using the higher-dimensional orbital angular momentum (OAM) degree of freedom and illustrate how these states are encoded on a phase-only spatial light modulator (SLM). We perform (d - 1)- mutual unbiased measurements in both a classical prepare and measure scheme and on entangled photon pairs for dimensions ranging from d = 2 to 5. The calculated average error rate, mutual information and secret key rate show an increase in information capacity as well as higher generation rates as the dimension increases.
Quantum repeaters enable us to distribute entanglement between remote parties by relying on a network of quantum
memory units that exhibit efficient coupling to light, scalability, and long coherence times. Entanglement
is initially distributed between nearest neighbors and then extended to the far-end nodes using entanglement
swapping techniques. For real-time applications, such as quantum key distribution, the above tasks must be
repeated successively, according to a proper protocol, to generate entangled states at a certain rate. This paper
studies a number of such protocols and the interplay between the rate of entanglement generation, the number
of employed memories, and the coherence time of memory units.
KEYWORDS: Quantum key distribution, Entangled states, Information security, Quantum information, Quantum communications, Data communications, Signal detection, Receivers, Signal processing, Tomography
Practical quantum key distribution can be understood as a two-step
procedure: in a first step two parties exchange quantum mechanical
signals and perform measurements on them, in a second step
auxiliary classical communication protocols are performed over an
authenticated public channel to transform the data of the first step into an information-theoretic secure key. In this article we address the question of necessary conditions on the correlated (classical) data of the first step such that there can be a successful second step at all. As it turns out, a necessary condition is that these data, together with the knowledge about the physical set-up of sender and receiver, allow to establish a proof of effective entanglement between the two parties. We then demonstrate methods to systematically search for such a proof in basic settings, involving the 2-, 4-, and 6-state protocols.
Conference Committee Involvement (10)
Quantum Technologies and Quantum Information Science V
10 September 2019 | Strasbourg, France
Quantum Technologies and Quantum Information Science
10 September 2018 | Berlin, Germany
Quantum Technologies and Quantum Information Science
13 September 2017 | Warsaw, Poland
Quantum Information Science and Technology
26 September 2016 | Edinburgh, United Kingdom
Quantum Information Science and Technology
22 September 2015 | Toulouse, France
Quantum-Physics-Based Information Security
22 September 2014 | Amsterdam, Netherlands
Quantum-Physics-Based Information Security
25 September 2012 | Edinburgh, United Kingdom
Quantum-Physics-Based Information Security
19 September 2011 | Prague, Czech Republic
Quantum Communications Realized II
28 January 2009 | San Jose, California, United States
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