18 research outputs found
The sudden change phenomenon of quantum discord
Even if the parameters determining a system's state are varied smoothly, the
behavior of quantum correlations alike to quantum discord, and of its classical
counterparts, can be very peculiar, with the appearance of non-analyticities in
its rate of change. Here we review this sudden change phenomenon (SCP)
discussing some important points related to it: Its uncovering,
interpretations, and experimental verifications, its use in the context of the
emergence of the pointer basis in a quantum measurement process, its appearance
and universality under Markovian and non-Markovian dynamics, its theoretical
and experimental investigation in some other physical scenarios, and the
related phenomenon of double sudden change of trace distance discord. Several
open questions are identified, and we envisage that in answering them we will
gain significant further insight about the relation between the SCP and the
symmetry-geometric aspects of the quantum state space.Comment: Lectures on General Quantum Correlations and their Applications, F.
F. Fanchini, D. O. Soares Pinto, and G. Adesso (Eds.), Springer (2017), pp
309-33
Homodyne-based quantum random number generator at 2.9 Gbps secure against quantum side-information
Quantum random number generators promise perfectly unpredictable random numbers. A popular approach to quantum random number generation is homodyne measurements of the vacuum state, the ground state of the electro-magnetic field. Here we experimentally implement such a quantum random number generator, and derive a security proof that considers quantum side-information instead of classical side-information only. Based on the assumptions of Gaussianity and stationarity of noise processes, our security analysis furthermore includes correlations between consecutive measurement outcomes due to finite detection bandwidth, as well as analog-to-digital converter imperfections. We characterize our experimental realization by bounding measured parameters of the stochastic model determining the min-entropy of the systemâs measurement outcomes, and we demonstrate a real-time generation rate of 2.9 Gbit/s. Our generator follows a trusted, device-dependent, approach. By treating side-information quantum mechanically an important restriction on adversaries is removed, which usually was reserved to semi-device-independent and device-independent schemes
Renyi generalizations of quantum information measures
Quantum information measures such as the entropy and the mutual information find applications in physics, e.g., as correlation measures. Generalizing such measures based on the RĂ©nyi entropies is expected to enhance their scope in applications. We prescribe RĂ©nyi generalizations for any quantum information measure which consists of a linear combination of von Neumann entropies with coefficients chosen from the set {â1,0,1}. As examples, we describe RĂ©nyi generalizations of the conditional quantum mutual information, some quantum multipartite information measures, and the topological entanglement entropy. Among these, we discuss the various properties of the RĂ©nyi conditional quantum mutual information and sketch some potential applications. We conjecture that the proposed RĂ©nyi conditional quantum mutual informations are monotone increasing in the RĂ©nyi parameter, and we have proof of this conjecture for some special cases
Hadamard quantum broadcast channels
We consider three different communication tasks for quantum broadcast
channels, and we determine the capacity region of a Hadamard broadcast channel
for these various tasks. We define a Hadamard broadcast channel to be such that
the channel from the sender to one of the receivers is entanglement-breaking
and the channel from the sender to the other receiver is complementary to this
one. As such, this channel is a quantum generalization of a degraded broadcast
channel, which is well known in classical information theory. The first
communication task we consider is classical communication to both receivers,
the second is quantum communication to the stronger receiver and classical
communication to other, and the third is entanglement-assisted classical
communication to the stronger receiver and unassisted classical communication
to the other. The structure of a Hadamard broadcast channel plays a critical
role in our analysis: the channel to the weaker receiver can be simulated by
performing a measurement channel on the stronger receiver's system, followed by
a preparation channel. As such, we can incorporate the classical output of the
measurement channel as an auxiliary variable and solve all three of the above
capacities for Hadamard broadcast channels, in this way avoiding known
difficulties associated with quantum auxiliary variables.Comment: v2: 20 pages, accepted for publication in Quantum Information
Processin