The ''problem of time" in present physics substantially consists in the fact that a straightforward quantization of the general relativistic evolution equation and constraints generates for the Universe wave function the Wheeler-De Witt equation, which describes a static Universe. Page and Wootters considered the fact that there exist states of a system composed by entangled subsystems that are stationary, but one can interpret the component subsystems as evolving: this leads them to suppose that the global state of the universe can be envisaged as one of this static entangled state, whereas the state of the subsystems can evolve. Here we synthetically present an experiment, based on PDC polarization entangled photons, that shows a practical example where this idea works, i.e. a subsystem of an entangled state works as a 'clock" of another subsystem.The ''problem of time" in present physics substantially consists in the fact that a straightforward quantization of the general relativistic evolution equation and constraints generates for the Universe wave function the Wheeler-De Witt equation, which describes a static Universe. Page and Wootters considered the fact that there exist states of a system composed by entangled subsystems that are stationary, but one can interpret the component subsystems as evolving: this leads them to suppose that the global state of the universe can be envisaged as one of this static entangled state, whereas the state of the subsystems can evolve. Here we synthetically present an experiment, based on PDC polarization entangled photons, that shows a practical example where this idea works, i.e. a subsystem of an entangled state works as a 'clock" of another subsystem.

arXiv:1502.03264Journal of Physics Conference SeriesJul 3, 20154 citations

Globally controlled universal quantum computation with arbitrary subsystem dimension

Gerardo A. Paz-Silva, Gavin K. Brennen, Jason Twamley

We present a scheme to perform universal quantum computation using global control with an arbitrary subsystem dimension (not necessarily finite). The scheme is developed over a one spatial dimension $N$ -element array, using only mirror-symmetric logical encoding, global pulses for single subsystem operations, uniform Ising-type interaction, and an all zero initial pure state. A mechanism using ancillary degrees of freedom for subsystem specific measurement is also presented.We present a scheme to perform universal quantum computation using global control with an arbitrary subsystem dimension (not necessarily finite). The scheme is developed over a one spatial dimension $N$-element array, using only mirror-symmetric logical encoding, global pulses for single subsystem operations, uniform Ising-type interaction, and an all zero initial pure state. A mechanism using ancillary degrees of freedom for subsystem specific measurement is also presented.

arXiv:0811.4245Physical Review ANov 12, 20095 citations

Many-Body Systems

Umberto Lombardo, G. Giuliani, Yifei Niu

UNITEXT for physicsJan 1, 20240 citations

Universal shape-dependence of quantum entanglement in disordered magnets

Natalie Love, István A. Kovács

Abstract Disordered quantum magnets are not only experimentally relevant, but offer efficient computational methodologies to calculate the low energy states as well as various measures of quantum correlations. We present a systematic analysis of quantum entanglement in the paradigmatic random transverse-field Ising model in two dimensions, using an efficient implementation of the asymptotically exact strong disorder renormalization group method. The phase diagram is known to be governed by three distinct infinitely disordered fixed points (IDFPs) that we study here. For square subsystems, it has been recently established that quantum entanglement has a universal logarithmic correction due to the corners of the subsystem at all three IDFPs. This corner contribution has been proposed as an ‘entanglement susceptibility’, a useful tool to locate the phase transition and measure the correlation length critical exponent. Towards a deeper understanding, we quantify how the corner contribution depends on the shape of the subsystem. While the corner contribution remains universal, the shape-dependence is qualitatively different in each universality class, also confirmed by line segment subsystems, a special case of skeletal entanglement. Therefore, unlike in conformally invariant systems, in general different subsystem shapes are versatile probes to unveil new universal information on phase transitions in disordered quantum systems.Abstract Disordered quantum magnets are not only experimentally relevant, but offer efficient computational methodologies to calculate the low energy states as well as various measures of quantum correlations. We present a systematic analysis of quantum entanglement in the paradigmatic random transverse-field Ising model in two dimensions, using an efficient implementation of the asymptotically exact strong disorder renormalization group method. The phase diagram is known to be governed by three distinct infinitely disordered fixed points (IDFPs) that we study here. For square subsystems, it has been recently established that quantum entanglement has a universal logarithmic correction due to the corners of the subsystem at all three IDFPs. This corner contribution has been proposed as an ‘entanglement susceptibility’, a useful tool to locate the phase transition and measure the correlation length critical exponent. Towards a deeper understanding, we quantify how the corner contribution depends on the shape of the subsystem. While the corner contribution remains universal, the shape-dependence is qualitatively different in each universality class, also confirmed by line segment subsystems, a special case of skeletal entanglement. Therefore, unlike in conformally invariant systems, in general different subsystem shapes are versatile probes to unveil new universal information on phase transitions in disordered quantum systems.

arXiv:2507.04557New Journal of PhysicsJan 1, 20260 citations

Many-Body Systems

WORLD SCIENTIFIC eBooksApr 22, 20180 citations

We present a scheme to perform universal quantum computation using global control with an arbitrary subsystem dimension (not necessarily finite). The scheme is developed over a one spatial dimension

N

-element array, using only mirror-symmetric logical encoding, global pulses for single subsystem operations, uniform Ising-type interaction, and an all zero initial pure state. A mechanism using ancillary degrees of freedom for subsystem specific measurement is also presented.