33 research outputs found
Observation of entanglement negativity transition of pseudo-random mixed states
Multipartite entanglement is a key resource for quantum computation. It is
expected theoretically that entanglement transition may happen for multipartite
random quantum states, however, which is still absent experimentally. Here, we
report the observation of entanglement transition quantified by negativity
using a fully connected 20-qubit superconducting processor. We implement
multi-layer pseudo-random circuits to generate pseudo-random pure states of 7
to 15 qubits. Then, we investigate negativity spectra of reduced density
matrices obtained by quantum state tomography for 6 qubits.Three different
phases can be identified by calculating logarithmic negativities based on the
negativity spectra. We observe the phase transitions by changing the sizes of
environment and subsystems. The randomness of our circuits can be also
characterized by quantifying the distance between the distribution of output
bit-string probabilities and Porter-Thomas distribution. Our simulator provides
a powerful tool to generate random states and understand the entanglement
structure for multipartite quantum systems
Revealing inherent quantum interference and entanglement of a Dirac Fermion
The Dirac equation is critical to understanding the universe, and plays an
important role in technological advancements. Compared to the stationary
solution, the dynamical evolution under the Dirac Hamiltonian is less
understood, exemplified by Zitterbewegung. Although originally predicted in
relativistic quantum mechanics, Zitterbewegung can also appear in some
classical systems, which leads to the important question of whether
Zitterbewegung of Dirac Fermions is underlain by a more fundamental and
universal interference behavior without classical analogs. We here reveal such
an interference pattern in phase space, which underlies but goes beyond
Zitterbewegung, and whose nonclassicality is manifested by the negativity of
the phase-space quasiprobability distribution, and the associated
pseudospin-momentum entanglement. We confirm this discovery by numerical
simulation and an on-chip experiment, where a superconducting qubit and a
quantized microwave field respectively emulate the internal and external
degrees of freedom of a Dirac particle. The measured quasiprobability
negativities well agree with the numerical simulation. Besides being of
fundamental importance, the demonstrated nonclassical effects are useful in
quantum technology.Comment: 18 pages, 15 figure