Quantum communication

In this equation, C andT refer to control and target qubits, respectively. The resulting state (output of the qugate) is said to be an entangled state of the two qubits, that is, a state that cannot be written as a product of states for each qubit [30]. The occurrence of such entangled states is another characteristic trait of QC, at the basis of secure quantum communication or cryptography. It also implies that, as opposed to what happens with a classical bit, an arbitrary quantum bit cannot be copied (the COPY classical operation is, in fact, based on the application of a succession of classical CNOT gates) [4]. 

Quantum Communications and Cryptography, edited by Alexander V. Sergienko... 

Besides quantum computations, entanglement has also been at the core of other active research such as quantum teleportation [32, 33], dense coding [34, 35], quantum communication [36], and quantum cryptography [37]. It is believed that the conceptual puzzles posed by entanglement have now become a physical source of novel ideas that might result in applications. 

M. Nielsen and I. Chuang, Quantum Computation and Quantum Communication, Cambridge University Press, Cambridge, 2000. 

Entanglement is the main resource of quantum information processing, without which quantum computation will not be faster than its classical counterpart [8] and quantum communication protocols will not work [113-115]. Moreover, as shown... 

To close the loop, teleportation and indeed many other quantum communication protocols do not need a universal set of gates and do quite well with just those gates covered by the Gottesman-Knill theorem [Bartlett 2002], Although these protocols may be easily simulated, they have capabilities beyond classical communication channels - clearly simulation is not everything. 

However, in quantum communication, entanglement is much better understood and clearly sets apart quantum protocols as a distinct class. 

Entanglement is a vital information resource employed in quantum teleportation, dense coding and quantum computation [Nielsen 2000], The fundamental role played by the entanglement in quantum information science was discussed in part I this part of the book is devoted to the generation and characterization of the entanglement of photons and their usage in quantum communication and computation protocols. 

Greenberger 1989 Greenberger 1990], Distributed entanglement thus allows to establish non-classical correlations between distant parties and can therefore be considered as a quantum analogue of the classical communication channel, a quantum communication channel. 

The scheme we have described allows the output photons to travel freely in space, so that they may further be used in quantum communication protocols, and this is achieved by detecting one and only one photon in modes 63 and 64. The fact that we do not yet have single-photon detectors for this wavelength at our disposal actually forces us to implement a four-fold coincidence detection to confirm that photons actually arrive in the output modes 61 and 62. 

We now turn to a quantitative examination of the feasibility of conditional Fock state generation using our preparation and retrieval technique. For applications in long-distance quantum communication, the quality of the atomic state preparation is the most important quantity. Assuming perfect atom-photon correlations in the write Raman processes, we can find the density matrix p for the number of atomic spin-wave excitations conditioned on the detection of ns Stokes photons. Here we consider only the spin-wave modes correlated with our detection mode. For example, in the absence of losses and background, the conditional atomic density matrix is simply p(ns) = ns)(ns. Loss on the Stokes channel (characterized by transmission coefficient a.s) leads to a statistical mixture of spin-wave excitations,... 

The first prototype of quantum cryptographic apparatus came into existence around 1990 [147]. In the meantime, quantum cryptography has become a well-known technique of communication in a provably secure way, and together with an intensive research in the held of quantum computers it has given rise to a whole new branch of science quantum information theory [148]. Viewed from this perspective, quantum cryptography today is only a subset of a broad held of quantum communications that also include quantum teleportation, quantum dense coding, quantum error-correcting codes, and quantum data compression. 

The quantum communication protocols described above may be used to implement quantum counterparts to the classical solutions of cryptographic tasks mentioned in Section VIII.A. Until now most of the efforts were devoted to a quantum solution of the key-distribution problem, which may readily be applied to secure message exchange or can be used as a building block for different cryptographic schemes. 

Quantum Communication wifii Entangled Photons, Hamid Weinfurter... 

A group of theoretical methods exists where the electronic wavefuntion is computed, and the atomic nuclei are propagated (using classical equations of motion). The Car-Parrinello MD method is one of this type [22-24]. These methods he between the extremes of the classical and ab initio methods, as they include some (quantum) electronic information and some (classical) dynamics information. These methods are called ah initio or first principles MD if you come from the classical community and semi-classical MD if you come firom the quantum community [9], Ah initio MD methods are far more expensive and cannot simulate as many molecules for as long as the classical simulations, but they are more flexible in that structures are not predetermined and information on the electronic structure is retained. Semi-classical MD can be carried out under periodic boundary conditions and thus the local liquid environment, and any extended bonding network, vyill be present. These methods hold a great deal of promise for the future study of ionic liquid systems, the first such calciilations on ionic liquids were reported in 2005 [21,25]. 

The concept of quantum decoherence is often at the forefront of discussions on quantum communication and quantum information since it presents a serious obstacle to the extended use of many of the suggested future techniques. At the same time, this concept is a basic ingredient in our understanding of the quantum measurement problem and for the transition from a quantum to a classical description of the physical world. 

Abstract. Quantum key distribution algorithms use a quantum communication channel with quantum information and a classical communication channel for binary information. The classical channel, in aU algorithms to date, was required to be authenticated. Moreover, Lomo-naco [8] claimed that authentication is not possible using only quantum means. This paper reverses this claim. We design an algorithm for quantum key distribution that does authentication by quantum means only. Although a classical channel is still used, there is no need for the channel to be authenticated. The algorithm relies on two protected pubhc keys to authenticate the communication partner. 

As the state-of-the-art experiments have shown, QKD has been advanced beyond proof-of-principle demonstrations in physics laboratories. In fact, the devices developed for QKD are available as commercial products (Stucki et al., 2002), and a whole QKD setup, including the software to perform the classical calculations for privacy amplification, may soon be available. Does this mean a quantum computer is around the corner Unfortunately, the answer is no, because two requirements for quantum computing are very hard to achieve, although they are not necessary for quantum communication. First, in a quantum computer the quantum states of many qubits, the quantum counterpart of the classical bit, must be stored for roughly as long as the computation runs for QKD, qubits may be measured and destroyed as soon as they have reached the receiver. Second, the many qubits of a quantum computer must interact with each other in a carefully controlled way in QKD, the qubits can be sent separately and never have to interact with each other. 

Efficient on-chip quantum communication will be essential for the development of large-scale solid-state quantum computing. Communication between devices is also important in conventional computers, but the need for quantum error correction necessitates the continuous transfer of redundant qubits throughout the computer. Rapid flow of quantum information will thus be essential for scalable quantum computing. 

The central part of this chapter is devoted to a detailed explanation of the ion trap QC proposed by Cirac and Zoller (Sect. 6.4). Here I will explain how quantum communication between single ions can be established by using a mode of collective motion (phonon). This is done by storing the particles in a linear trap. A group at the National Institiute of Standards and Technology in Boulder/Colorado has already successfully demonstrated the main aspects of the proposal experimentally (Monroe et al. 1995). I will briefly discuss their experiment. 

In Sect. 6.6 a proposal to implementing a QC based on optical cavity quantum electrodynamics is described (Pellizzari et al. 1995). The scheme is similar to the ion trap QC in the sense that the atomic quantum bits are resting in a trap. However, quantum communication is provided by photons instead of phonons. 

---