Quantum and classical resources for unitary design of open-system evolutions

A variety of tasks in quantum control, ranging from purification and cooling to quantum stabilisation and open-system simulation, rely on the ability to implement a target quantum channel over a specified time interval within prescribed accuracy. This can be achieved by engineering a suitable unitary dynamics of the system of interest along with its environment, which, depending on the available level of control, is fully or partly exploited as a coherent quantum controller . After formalising a controllability framework for completely positive trace-preserving quantum dynamics, we provide sufficient conditions on the environment state and dimension that allow for the realisation of relevant classes of quantum channels, including extreme channels, stochastic unitaries or simply any channel. The results hinge on generalisations of Stinespring’s dilation via a subsystem principle . In the process, we show that a conjecture by Lloyd on the minimal dimension of th… …

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Shaking Schrödinger’s cat

Frequent measurement of a quantum system’s state can either speed or delay its collapse, effects called the quantum Zeno and quantum anti-Zeno effect. But so too can “quasimeasurements” that only poke the system and garner no information about its state.

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Interplay of light and matter: A ‘perfect’ attosecond experiment

Novel experiments using ultrafast pulses are currently revolutionizing laser physics research. They deliver unprecedented insights into matter – into the structure and dynamics of electrons in atoms, molecules and in condensed phases. Using what is known as an attosecond experiment, physicists have managed to completely measure and describe the quantum-mechanical wave function of an ionized electron.

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Ring, Ring: ‘Earth? It’s space calling, on the quantum line’

Scientists report the successful transmission of entangled photons between suborbital space and Earth.

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Roberto Peccei and Helen Quinn, Driving Around Stanford in a Clunky Jeep

The two physicists who introduced Peccei-Quinn symmetry came up with their idea on and around Stanford University’s campus 40 years ago. …

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Rainer Weiss, Remembering the Little Room in the Plywood Palace

The physicist who designed the LIGO experiment that detected gravitational waves still holes up in a small basement lab surrounded by electronics and optical instruments. …

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Yitang Zhang’s Santa Barbara Beach Walk

An obscure number theorist who became an overnight sensation with a major proof about the gaps between prime numbers now finds quiet inspiration walking along the Pacific Coast. …

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Coherent control in quantum transport: amplification, filtering and switching at finite bias

We consider coherent feedback control of quantum transport and focus on the application of simple controllers and the effects of a finite bias voltage. We show that simple single-parameter controllers can give rise to a range of useful effects such as amplification of changes in plant transmission, increased resolution of energy filtration, and the detection of differences between otherwise indistinguishable plants. We explore how these effects are impacted by the phase-averaging effects associated with finite bias and identify important voltage scales for the maintenance of the functionalities achieved through feedback control. …

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