Summary Our topic is a basic discussion of what it is to regard a bit of mathematics as referring to something physical, and the different options one has for explaining -and hence rendering non-miraculous- the empirical success of a mathematical formalism. In order to strip away as much extraneous distraction as possible, we will leave the particular case of the wavefunction in quantum theory aside and tell a simpler tale. A fairy tale, in fact. The aim is to foreground some possible relations between a mathematical formalism and the physical system it is used to represent.

Resumen Nuestro tema es una discusión básica de lo que es considerar que una porción de matemáticas se refiera a algo físico y las diferentes opciones que uno tiene para explicar -y por lo tanto hacer que no sea milagroso- el éxito empírico de un formalismo matemático. Para eliminar tanta distracción externa como sea posible, dejaremos a un lado el caso de la función de onda en la teoría cuántica y contaremos una historia más simple. Un cuento de hadas, de hecho. El objetivo es poner en primer plano algunas posibles relaciones entre un formalismo matemático y el sistema físico al que representamos con el formalismo.

Abstract In this paper a general study of nitrogen-vacancy center internal state in diamond is presented. This study is based on experimental and theoretical findings found in literature. First, a Hamiltonian model for center nitrogen-vacancy internal state is proposed in terms of a complete set of commuting observables (CSCO), which consist of angular momentum L ^, spin momentum S ^, total angular momentum J ^ = L ^ + S ^, and spin momentum on z-direction S ^ z. The second quantization formalism -in steady-state (conservative model)- is used. The creation and annihilation operators are used to described the steady spin-levels structure and in dynamic-state (non-conservative model) and can also describe the system dynamic between different spin-levels transitions. Finally, a discussion is presented about the application of this study in the photochromism phenomenon and solid-state quantum bit (qubit).

Abstract A Fake Calibration Attack process for a Continuous Variable-Quantum Key Distribution system using a Beam Sampler is presented. The Fake Calibration Attack allows a calibration that balances the Standard Quantum Limit for all the optical path in the experiment (differential Standard Quantum Limit is ≈ 0.39 dB), allowing Eve to acquire ≈ 0.0671 for a particular information quadrature which establishes a Quantum Bit Error Rate ≈5.8%. As a final result, the balancing of the Standard Quantum Limit for both states of polarization signals allows maintaining the overall Quantum Bit Error Rate at a particular value ≈ 3%, which implies an important basis for detecting a potential spy considering the minimum Quantum Bit Error Rate.

Abstract: The classical optical communication systems in free space have been extensively studied, in contrast to quantum systems which until relatively recently have acquired great interest for satellite applications and quantum key distribution. Proper design of the elements that collect light in the receiver is critical for good performance. So, the main objective of this paper is to present an optical antennas design methodology used in quantum communications. This article shows feasibility and utility of paraxial theory use for this design which allows the use of optical components such as astronomical telescopes used in conventional classical schemes. An optical antenna for 1550 nm was designed with the help of simulation tools and a commercial Cassegrain type telescope. Also, a proof of concept in the laboratory was performed where three photons per bit were captured, validating as well the suggested methodology.

Resumen: Los sistemas de comunicación óptica clásicos en espacio libre han sido ampliamente estudiados en contraste con los sistemas cuánticos, los cuales hasta fechas relativamente recientes han ido adquiriendo gran interés para aplicaciones satelitales y distribución de llave cuántica. El adecuado diseño de los elementos que recolectan la luz en el receptor es crítico para un buen desempeño. Así, el objetivo principal del presente trabajo es presentar una metodología de diseño de antenas ópticas empleadas en comunicación cuántica. Se muestra la factibilidad y utilidad de la teoría paraxial para este diseño, permitiendo el empleo de componentes ópticos, tales como telescopios astronómicos usados en esquemas clásicos convencionales. Con la ayuda de herramientas de simulación, diseñamos una antena óptica para 1550 nm, empleando un telescopio tipo Cassegrain, y realizamos una prueba de concepto en laboratorio siendo capaces de captar tres fotones por bit validando la metodología sugerida.

Entre os primeiros conceitos que alunos de graduação em física encontram nos seus estudos, há os de grandezas e dimensões físicas. Neste artigo pedagógico de revisão, usando a poderosa ferramenta da análise dimensional, vou partir desses conceitos para uma viagem através de vários ramos da física teörica, da mecânica até a gravidade quântica. Entre outras coisas, vou discutir um pouco sobre as constantes fundamentais da Natureza, o chamado “cubo da física” e o sistema de unidades naturais.

Physical quantities and physical dimensions are among the first concepts encountered by students in their undergraduate career. In this pedagogical review, I will start from these concepts and, using the powerful tool of dimensional analysis, I will embark in a journey through various branches of physics, from basic mechanics to quantum gravity. I will also discuss a little bit about the fundamental constants of Nature, the so-called “cube of Physics”, and the natural system of units.

We describe an autonomous, fully functional implementation of the BB84 quantum key distribution protocol using open source hardware microcontrollers for the synchronization, communication, key sifting and real-time key generation diagnostics. The quantum bits are prepared in the polarization of weak optical pulses generated with light emitting diodes, and detected using a sole single-photon counter and a temporally multiplexed scheme. The system generates a shared cryptographic key at a rate of 365 bps, with a raw quantum bit error rate of 2.7%. A detailed description of the peripheral electronics for control, driving and communication between stages is released as supplementary material. The device can be built using simple and reliable hardware and it is presented as an alternative for a practical realization of sophisticated, yet accessible quantum key distribution systems.

We present FPGA-based emulation of a synchronous phase-coded quantum cryptography system. Several of the emulated subsystems are used for implementation in a free space demonstrative QPSK scheme for quantum key distribution with continuous variables (CV-QKD) using a base and optical phase synchronization. The CV-QKD systems are commonly implemented using QPSK modulation with switched or simultaneous detection. In this paper we only make use of one base of the QPSK system in order to get a simpler modulation (BPSK) scheme, used for demonstrative purposes. The reported results from the emulation and the experiment in terms of Quantum Bit Error Rate (QBER) and mutual information for different values of the number of photons per bit are in good agreement.

Se propone un circuito LC con carga eléctrica discreta como sistema alternativo para la realización de un bit cuántico. Considerando la energía de una fuente oscilatoria como perturbación, se da una descripción de la manipulación de un bit cuántico a través de las oscilaciones de Rabi. A diferencia de las oscilaciones de Rabi estándar, se demuestra que la frecuencia de Rabi tiene una dependencia con el parámetro mesoscópico C/L.

We propose an LC circuit with discrete electric charge as an alternative system for the realization of a quantum bit. Considering the energy of an oscillatory source as a perturbative term, we describe the manipulation of such quantum bit by means of Rabi oscillations. Unlike the standard Rabi oscillations, we show that the Rabi frequency depends on the mesoscopic parameter C/L

Usar a teoria da informação de Shannon é uma estratégia comum para medir todo tipo de variabilidade em um sinal ou fenômeno. Alguns métodos foram desenvolvidos para adaptar a medida de entropia informacional a dados de cantos de pássaro, tentando enfatizar seus aspectos de versatilidade. Essa abordagem clássica, usando o conceito de bit, produz resultados interessantes. Agora, a idéia original desenvolvida neste artigo é usar a teoria quântica da informação e o conceito de q-bit, com a finalidade de proporcionar uma visão mais completa dos resultados experimentais.

Using Shannon information theory is a common strategy to measure any kind of variability in a signal or phenomenon. Some methods were developed to adapt information entropy measures to bird song data trying to emphasize its versatility aspect. This classical approach, using the concept of bit, produces interesting results. Now, the original idea developed in this paper is to use the quantum information theory and the quantum bit (q-bit) concept in order to provide a more complete vision of the experimental results.

We study the effects of counter rotating terms in the interaction of quantized light with a two-level atom, by using the method of small rotations. We give an expression for the wave function of the composed system atom plus field and point out one initial wave function that generates a quantum bit of the electromagnetic field with arbitrary amplitudes.

Estudiamos los efectos de los términos contra-rotantes en la interacción entre un campo electromagnético cuantizado y un átomo de dos niveles, usando el método de pequeñas rotaciones. Obtenemos una expresión para la función de onda del sistema compuesto átomo-campo y elegimos un estado inicial de la función de onda que genera un bit cuántico del campo electromagnético.