Topology change from Kaluza-Klein dimensions

In this letter we show that in a Kaluza-Klein framework we can have arbitrary topology change between the macroscopic (i.e. noncompactified) spacelike 3-hypersurfaces. This is achieved by using the compactified dimensions as a catalyser for topology change. In the case of odd-dimensional spacetimes (such as the 11-dimensional M-theory) this is always possible. In the even-dimensional case, a sufficient condition is the existence of a closed, odd-dimensional manifold as a factor (such as S^1, S^3) in the Kaluza-Klein sector. Since one of the most common manifolds used for compactification is the torus T^k = S^1 \times ... \times S^1, in this case we can again induce an arbitrary topology change on the 3-hypersurfaces.Comment: 5 pages, LaTeX, no figures, uses eps

Schr\"odinger's Cat: Where Does The Entanglement Come From?

Schr\"odinger's cat is one of the most striking paradoxes of quantum mechanics that reveals the counterintuitive aspects of the microscopic world. Here, I discuss the paradox in the framework of quantum information. Using a quantum networks formalism, I analyse the information flow between the atom and the cat. This reveals that the atom and the cat are connected only through a classical information channel: the detector clicks $\rightarrow$ the poison is released $\rightarrow$ the cat is killed. No amount of local operations and classical communication can entangle the atom and the cat, which are initially in a separable state. This casts a new light on the paradox

Proposal for a quantum delayed-choice experiment

Gedanken experiments are important conceptual tools in the quest to reconcile our classical intuition with quantum mechanics and nowadays are routinely performed in the laboratory. An important open question is the quantum behaviour of the controlling devices in such experiments. We propose a framework to analyse quantum-controlled experiments and illustrate the implications by discussing a quantum version of Wheeler's delayed-choice experiment. The introduction of a quantum-controlled device (i.e., quantum beamsplitter) has several consequences. First, it implies that we can measure complementary phenomena with a single experimental setup, thus pointing to a redefinition of complementarity principle. Second, a quantum control allows us to prove there are no consistent hidden-variable theories in which "particle" and "wave" are realistic properties. Finally, it shows that a photon can have a morphing behaviour between "particle" and "wave"; this further supports the conclusion that "particle" and "wave" are not realistic properties but merely reflect how we 'look' at the photon. The framework developed here can be extended to other experiments, particularly to Bell-inequality tests

Entangling spins by measuring charge: a parity-gate toolbox

The parity gate emerged recently as a promising resource for performing universal quantum computation with fermions using only linear interactions. Here we analyse the parity gate (P-gate) from a theoretical point of view in the context of quantum networks. We present several schemes for entanglement generation with P-gates and show that native networks simplify considerably the resources required for producing multi-qubit entanglement, like n-GHZ states. Other applications include a Bell-state analyser and teleportation. We also show that cluster state fusion can be performed deterministically with parity measurements. We then extend this analysis to hybrid quantum networks containing spin and mode qubits. Starting from an easy-to-prepare resource (spin-mode entanglement of single electrons) we show how to produce a spin n-GHZ state with linear elements (beam-splitters and local spin-flips) and charge-parity detectors; this state can be used as a resource in a spin quantum computer or as a precursor for constructing cluster states. Finally, we construct a novel spin CZ-gate by using the mode degrees of freedom as ancillae.Comment: updated to the published versio

Interferometric mass spectrometry

Accelerator mass spectrometry (AMS) is a widely-used technique with multiple applications, including geology, molecular biology and archeology. Although extremely precise, AMS requires tandem accelerators and bulky magnets which confines it to large laboratories. Here we propose interferometric mass spectrometry (IMS), a novel method of mass separation which uses quantum interference. IMS employs the wave-like properties of the samples, and as such is complementary to AMS, in which samples are particle-like. This complementarity has two significant consequences: (i) in IMS separation is performed according to the absolute mass $m$, and not to the mass-to-charge ratio $m/q$, as in AMS; (ii) in IMS the samples are in the low-velocity regime, in contrast to the high-velocity regime used in AMS. Potential applications of IMS are compact devices for mobile applications, sensitive molecules that break at the acceleration stage and neutral samples which are difficult to ionise

Quantum Computation with Ballistic Electrons

We describe a solid state implementation of a quantum computer using ballistic single electrons as flying qubits in 1D nanowires. We show how to implement all the steps required for universal quantum computation: preparation of the initial state, measurement of the final state and a universal set of quantum gates. An important advantage of this model is the fact that we do not need ultrafast optoelectronics for gate operations. We use cold programming (or pre-programming), i.e., the gates are set before launching the electrons; all programming can be done using static electric fields only.Comment: 5 pages, RevTeX4, 5 figures, uses epsf, latexsym, time