Grover Energy Transfer at Relativistic Speeds

Grover's algorithm for quantum search can also be applied to classical energy transfer. The procedure takes a system in which the total energy is equally distributed among $N$ subsystems and transfers most of the it to one marked subsystem. We show that in a relativistic setting the efficiency of this procedure can be improved. We will consider the transfer of relativistic kinetic energy in a series of elastic collisions. In this case, the number of steps of the energy transfer procedure approaches 1 as the initial velocities of the objects become closer to the speed of light. This is a consequence of introducing non-linearities in the procedure. However, the maximum attainable transfer will depend on the particular combination of speed and number of objects. In the procedure, we will use $N$ elements, like in the classical case, instead of the $log_2(N)$ states of the quantum algorithm

A SWAP gate for qudits

We present a quantum SWAP gate valid for quantum systems of an arbitrary dimension. The gate generalizes the CNOT implementation of the SWAP gate for qubits and keeps its most important properties, like symmetry and simplicity. We only use three copies of the same controlled qudit gate. This gate can be built with two standard higher-dimensional operations, the Quantum Fourier Transform and the d-dimensional version of the CZ gate

Quantum spread spectrum multiple access

We describe a quantum multiple access scheme that can take separate single photon channels and combine them in the same path. We propose an add-drop multiplexer that can insert or extract a single photon into an optical fibre carrying the qubits of all the other users. The system follows the principle of code division multiple access, a spread spectrum technique widely used in cellular networks.Comment: 9 pages, 8 figures. 3 tables. Revised version (v2) with simulation results. Comments welcom

Secure communication in the twin paradox

The channel capacity between two users is affected by relativistic effects. Under the presence of a fixed noise at the receiver, there appears an asymmetry between "slowly aging" and "fast aging" observers which can be used to have private communication transmission. We discuss some models for users inside gravitational wells and in the twin paradox scenario.Comment: Second version. Errata corrected. New figures and extensive appendices with proofs and useful expressions added. Comments welcom

Quantum Multiplexing for Quantum Computer Networks

In communication networks many different channels must share a limited amount of resources. In order to allow for multiple simultaneous communications, multiple access techniques are routinely employed. With quantum communication, it is possible to share a new kind of resource. All of the system channels can be accommodated into a single channel in a larger Hilbert space. In the scheme, a single line combines the information of all the users, and, at the receiver, the original quantum channels are recovered. The given multiplexer/demultiplexer circuit can perform this n qubits to qudit transformation. Connections with superdense coding and classical multiple access schemes are discussed.Comment: 14 pages, 8 figure

Simple quantum password checking

We present a quantum password checking protocol where secrecy is protected by the laws of quantum mechanics. The passwords are encoded in quantum systems that can be compared but have a dimension too small to allow reading the encoded bits. We study the protocol under different replay attacks and show it is robust even for poorly chosen passwords.Comment: 5 pages. Comments welcom

Hidden probe attacks on ultralong fiber laser key distribution systems

In ultralong fiber laser key distribution, two sides use standard optical equipment to create kilometer long fiber lasers in a communication link to establish a secret key. Its security rests on the assumption that any attacker would need much more sophisticated equipment and techniques than those of the legitimate user in order to discover the generated key. We present a challenge to that assumption with a hidden probe attack in which the eavesdropper hides a weak signal in the unavoidable noise floor that appears in the laser during amplification and probes with it the configuration of one or both communication parties. We comment how this attack can compromise different proposals for ultralong laser key distribution and propose possible countermeasures.Comment: 9 pages, 8 figures. Comments welcom

Quantum computer networks with the orbital angular momentum of light

Inside computer networks, different information processing tasks are necessary to deliver the user data efficiently. This processing can also be done in the quantum domain. We present simple optical quantum networks where the orbital angular momentum of a single photon is used as an ancillary degree of freedom which controls decisions at the network level. Linear optical elements are enough to provide important network primitives like multiplexing and routing. First we show how to build a simple multiplexer and demultiplexer which combine photonic qubits and separate them again at the receiver. We also give two different self-routing networks where the OAM of an input photon is enough to make it find its desired destination.Comment: 7 pages, 3 figures, comments welcom

Quantum Interrogation with particles

Interaction-free measurement and quantum interrogation schemes can help in the detection of particles without interacting with them in a classical sense. We present a density matrix study of a quantum interrogation system designed for particles that need not to be perfectly absorptive and compare the results to those of the usual setup.Comment: 7 pages. 7 figures. Comments are welcome. V2: Errata are corrected. Simpler mode

Equivalent Quantum Circuits

Quantum algorithms and protocols are often presented as quantum circuits for a better understanding. We give a list of equivalence rules which can help in the analysis and design of quantum circuits. As example applications we study quantum teleportation and dense coding protocols in terms of a simple XOR swapping circuit and give an intuitive picture of a basic gate teleportation circuit.Comment: 12 pages. Tutorial on quantum circuit analysis. Graphical point of view based on circuit equivalences. Many figure