A new model of quantum computation is considered, in which the connections between gates are programmed by the state of a quantum register. This new model of computation is shown to be more powerful than the usual quantum computation, e.g. in achieving the programmability of permutations of N different unitary channels with 1 use instead of N uses per channel. For this task, a new elemental resource is needed, the quantum switch, which can be programmed to switch the order of two channels with a single use of each one.

A bipartite quantum channel represents the interaction between systems, generally allowing for the exchange of information. A special class of bipartite channels is the no-signaling ones, which do not allow for communication. Piani et al. [Phys. Rev. A 74, 012305 (2006)] conjectured that all no-signaling channels are mixtures of entanglement breaking and localizable channels, which require only local operations and entanglement. Here we provide the general realization scheme, and give a counterexample to the conjecture, achieving no-signaling superquantum correlations while preserving entanglement.

We address the problem of learning an unknown unitary transformation from a finite number of examples. The problem consists in finding the learning machine that optimally emulates the examples, thus reproducing the unknown unitary with maximum fidelity. Learning a unitary is equivalent to storing it in the state of a quantum memory (the memory of the learning machine) and subsequently retrieving it. We prove that, whenever the unknown unitary is drawn from a group, the optimal strategy consists in a parallel call of the available uses followed by a ``measure-and-rotate'' retrieving. Differing from the case of quantum cloning, where the incoherent ``measure-and-prepare'' strategies are typically suboptimal, in the case of learning the ``measure-and-rotate'' strategy is optimal even when the learning machine is asked to reproduce a single copy of the unknown unitary. We finally address the problem of the optimal inversion of an unknown unitary evolution, showing also in this case the optimality of the ``measure-and-rotate'' strategies and applying our result to the optimal approximate realignment of reference frames for quantum communication.

The present short review article illustrates the latest theoretical developments on quantum tomography, regarding general optimization methods for both data processing and setup. The basic theoretical tool is the informationally complete measurement. The optimization theory for the setup is based on the new theoretical approach of quantum combs.

Neurophysiological and neuroimaging studies in the human and the monkey brain indicate that links between action observation and execution are much tighter than previously believed. Indeed, the mere observation of movements performed by other individuals brings about a clear increase in activity in specific fronto-parietal neural networks (mirror system). Here, we report a series of four single-pulse transcranial magnetic stimulation studies of the motor system, which show that observation of index and little finger movements brings about a facilitation of potentials recorded from muscles that would be involved in the actual execution of the observed action. Remarkably, however, a clear representational selectivity was found also during observation of bio-mechanically impossible index or little finger movements. Thus, in movement observation tasks, the human cortico-spinal system reacts similarly to the vision of bio-mechanically possible and impossible movements but it is able to detect which muscle would be involved in the actual execution of the observed movement. Importantly, this system may be more related to coding body part movements than precisely simulating their execution.

The effect of short-term light deprivation on tactile spatial acuity was evaluated by asking 28 adult humans to perform a grating orientation task. The 14 subjects who were kept for 90 minutes in complete dark showed, immediately after deprivation, a reversible improvement of tactile spatial acuity. No acuity change was observed in the 14 nondeprived subjects. Results indicate that even a short-term visual deprivation may disclose highly dynamic plastic interactions between visual and tactile systems.

We evaluated motor evoked potentials (MEPs) and duration of the cortical silent period (CSP) from the right first dorsal interosseous (FDI) muscle to transcranial magnetic stimulation (TMS) of the left motor cortex in ten healthy subjects performing different manual tasks. They abducted the index finger alone, pressed a strain gauge with the thumb and index finger in a pincer grip, and squeezed a 4-cm brass cylinder with all digits in a power grip. The level of FDI EMG activity across tasks was kept constant by providing subjects with acoustic-visual feedback of their muscle activity. The TMS elicited larger amplitude FDI MEPs during pincer and power grip than during the index finger abduction task, and larger amplitude MEPs during pincer gripping than during power gripping. The CSP was shorter during pincer and power grip than during the index finger abduction task and shorter during power gripping than during pincer gripping. These results suggest excitatory and inhibitory task-dependent changes in the motor cortex. Complex manual tasks (pincer and power gripping) elicit greater motor cortical excitation than a simple task (index finger abduction) presumably because they activate multiple synergistic muscles thus facilitating corticomotoneurons. The finger abduction task probably yielded greater motor cortical inhibition than the pincer and power tasks because muscles uninvolved in the task activated the cortical inhibitory circuit. Increased cortical excitatory and inhibitory functions during precision tasks (pincer gripping) probably explain why MEPs have larger amplitudes and CSPs have longer durations during pincer gripping than during power gripping.

Objectives: To examine possible changes of excitability of the human motor system contingent upon immobilisation of two hand fingers. Methods: Two series of 5 transcranial magnetic stimulation (TMS) sessions were carried out on different days (1, 2, 3, 4, and 7). In one series (fingers immobilised, FI), subjects wore for 4 days a device that kept immobilised the left fourth and fifth finger. In the other series (fingers free, FF), no constraining device was used. Focal TMS was applied over the right motor cortex and motor evoked potentials (MEPs) were recorded from left abductor digiti minimi (immobilised) and first dorsal interosseus (non-immobilised) muscles. Intensities of 10, 30, and 50% above the resting motor threshold (rMT), were used. Results: In FI series, rMT for both muscles showed significant increase on days 3, 4, and 7 with respect to day 1. At high stimulation intensity a clear decrease of MEPs amplitude was observed on days 3 and 4 for both muscles. Since no time-related changes of peripheral (M-wave) and spinal (F-wave) excitability were noted, MEPs and rMT changes are likely to have a cortical origin. In FF series, no changes of excitability were detected. Conclusions: Sensorimotor restriction of two fingers induces an early decrease of excitability, possibly at cortical level, which involves not only the immobilised muscle but also muscles with purportedly overlapping neural representations.