Science Daily – Physicists at the Max Planck Institute of Quantum Optics have measured the propagation velocity of quantum signals in a many-body system. A quantum computer based on quantum particles instead of classical bits, can in principle outperform any classical computer. However, it still remains an open question, how fast and how efficient quantum computers really may be able to work. A critical limitation will be given by the velocity with which a quantum signal can spread within a processing unit.

The communication and processing of information in a quantum computer is based on concepts that are inherently different from those used in classical computers. This is due to the fundamental differences between quantum particles and classical objects. Whereas the latter are, for example, either black or white, quantum particles can take on both colours at the same time. It is only at the process of measurement that the particles decide on one of the two possible properties. As a consequence of this peculiar behaviour, two quantum objects can form one entangled state in which their properties are strictly connected, i.e. quantum correlated. At present there is no general model for predicting how fast a quantum correlation can travel after it is generated.

*Propagation of quantum correlations in an optical lattice. Left: artist’s view (Graphic by woogie works animation studio). Right: a) In the initial state, each lattice site is filled by exactly one atom. The height of the barrier between the sites is then abruptly lowered, bringing the system out of equilibrium. b) After the barrier has been lowered, an entangled doublon-holon pair is formed. The correlated doublons and holons move across the system with opposite momenta. (Credit: MPQ)*

Nature – Light-cone-like spreading of correlations in a quantum many-body system

In relativistic quantum field theory, information propagation is bounded by the speed of light. No such limit exists in the non-relativistic case, although in real physical systems, short-range interactions may be expected to restrict the propagation of information to finite velocities. The question of how fast correlations can spread in quantum many-body systems has been long studied. The existence of a maximal velocity, known as the Lieb–Robinson bound, has been shown theoretically to exist in several interacting many-body systems (for example, spins on a lattice)—such systems can be regarded as exhibiting an effective light cone that bounds the propagation speed of correlations. The existence of such a ‘speed of light’ has profound implications for condensed matter physics and quantum information, but has not been observed experimentally. Here we report the time-resolved detection of propagating correlations in an interacting quantum many-body system. By quenching a one-dimensional quantum gas in an optical lattice, we reveal how quasiparticle pairs transport correlations with a finite velocity across the system, resulting in an effective light cone for the quantum dynamics. Our results open perspectives for understanding the relaxation of closed quantum systems far from equilibrium, and for engineering the efficient quantum channels necessary for fast quantum computation

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