**Mario Krenn, Mehul Malik, Thomas Scheidl, Rupert Ursin, Anton Zeilinger**

The secure communication of information plays an ever increasing role in our society today. Classical methods of encryption inherently rely on the difficulty of solving a problem such as finding prime factors of large numbers and can, in principle, be cracked by a fast enough machine. The burgeoning field of quantum communication relies on the fundamental laws of physics to offer unconditional information security. Here we introduce the key concepts of quantum superposition and entanglement as well as the no-cloning theorem that form the basis of this field. Then, we review basic quantum communication schemes with single and entangled photons and discuss recent experimental progress in ground and space-based quantum communication. Finally, we discuss the emerging field of high-dimensional quantum communication, which promises increased data rates and higher levels of security than ever before. We discuss recent experiments that use the orbital angular momentum of photons for sharing large amounts of information in a secure fashion.

Read more at https://arxiv.org/pdf/1701.00989v1.pdf

# Category Archives: QUANTUM PHYSICS

# Quantum papers of 2016

1. The Mathematics of Entanglement

Fernando G.S.L. Brandao, Matthias Christandl, Aram W. Harrow, Michael Walter: https://arxiv.org/pdf/1604.01790.pdf

2. Quantum Shannon Theory

John Preskill: https://arxiv.org/pdf/1604.07450.pdf

# Maxwell’s Demon Meets Nonequilibrium Quantum Thermodynamics

**Experimental rectification of entropy production by a Maxwell’s Demon in a quantum system**

P. A. Camati, J. P. S. Peterson, T. B. Batalhão, K. Micadei, A. M. Souza, R. S. Sarthour, I. S. Oliveira, R. M. Serra

Maxwell’s demon explores the role of information in physical processes. Employing information about microscopic degrees of freedom, this “intelligent observer” is capable of compensating entropy production (or extracting work), apparently challenging the second law of thermodynamics. In a modern standpoint, it is regarded as a feedback control mechanism and the limits of thermodynamics are recast incorporating information-to-energy conversion. We derive a trade-off relation between information-theoretic quantities empowering the design of an efficient Maxwell’s demon in a quantum system. The demon is experimentally implemented as a spin-1/2 quantum memory that acquires information, and employs it to control the dynamics of another spin-1/2 system, through a natural interaction. Noise and imperfections in this protocol are investigated by the assessment of its effectiveness. This realization provides experimental evidence that the irreversibility on a non-equilibrium dynamics can be mitigated by assessing microscopic information and applying a feed-forward strategy at the quantum scale.

Read more at https://arxiv.org/pdf/1605.08821v1.pdf

Read also http://physics.aps.org/articles/v9/136

# Bringing quantum mechanics to life: from Schrödinger’s cat to Schrödinger’s microbe

**Zhang-qi Yin, Tongcang Li**

The question whether quantum mechanics is complete and the nature of the transition between quantum mechanics and classical mechanics have intrigued physicists for decades. There have been many experimental breakthroughs in creating larger and larger quantum superposition and entangled states since Erwin Schr\”{o}dinger proposed his famous thought experiment of putting a cat in a superposition of both alive and dead states in 1935. Remarkably, recent developments in quantum optomechanics and electromechanics may lead to the realization of quantum superposition of living microbes soon. Recent evidences also suggest that quantum coherence may play an important role in several biological processes. In this review, we first give a brief introduction to basic concepts in quantum mechanics and the Schr\”{o}dinger’s cat thought experiment. We then review developments in creating quantum superposition and entangled states and the realization of quantum teleportation. Non-trivial quantum effects in photosynthetic light harvesting and avian magnetoreception are also discussed. At last, we review recent proposals to realize quantum superposition, entanglement and state teleportation of microorganisms, such as viruses and bacteria.

Read more at http://arxiv.org/pdf/1608.05322v1.pdf

# A Schrodinger Cat Living in Two Boxes

**Chen Wang et al**

Quantum superpositions of distinct coherent states in a single-mode harmonic oscillator, known as “cat states”, have been an elegant demonstration of Schrodinger’s famous cat paradox.

Here, we realize a two-mode cat state of electromagnetic fields in two microwave cavities bridged by a superconducting artificial atom, which can also be viewed as an entangled pair of single-cavity cat states. We present full quantum state tomography of this complex cat state over a Hilbert space exceeding 100 dimensions via quantum non-demolition measurements of the joint photon number parity.

The ability to manipulate such multi-cavity quantum states paves the way for logical operations between redundantly encoded qubits for fault-tolerant quantum computation and communication.

Read more at http://arxiv.org/pdf/1601.05505v1.pdf

**What happens if you saw Schrödinger’s cat in half?**

Read also:

**1.** **Schrödinger’s cat alive and dead even after you saw it in half**

**2.** **Schrödinger’s cat lives and dies in two boxes at once**