Future universal quantum computers solving problems of practical relevance
are expected to require at least $10^6$ qubits, which is a massive scale-up
from the present numbers of less than 50 qubits operated together. Out of the
different types of qubits, solid state qubits are considered to be viable
candidates for this scale-up, but interfacing to and controlling such a large
number of qubits is a complex challenge that has not been solved yet. One
possibility to address this challenge is to use qubit control circuits located

In this technical note we propose a theoretically motivated and empirically
validated initialization strategy which can resolve the barren plateau problem
for practical applications. The proposed strategy allows for efficient training
of parametrized quantum circuits. The technique involves randomly selecting
some of the initial parameter values, then choosing the remaining values so
that the final circuit is a sequence of shallow unitary blocks that each

Quantum computation promises applications that are thought to be impossible
with classical computation. To realize practical quantum computation, the
following three properties will be necessary: universality, scalability, and
fault-tolerance. Universality is the ability to execute arbitrary multi-input
quantum algorithms. Scalability means that computational resources such as
logical qubits can be increased without requiring exponential increase in
physical resources. Lastly, fault-tolerance is the ability to perform quantum

Variational quantum algorithms dominate contemporary gate-based quantum
enhanced optimization [1], eigenvalue estimation [2] and machine learning [3].
Here we establish the quantum computational universality of variational quantum
computation by developing two constructions which prepare states with high
2-norm overlap with the outputs of quantum circuits. The fleeting resource is
the number of expected values which must be iteratively minimized using a
classical-to-quantum feedback loop. The first approach is efficient in the

Recent researches suggest that the emergence of spacetime is connected to
entanglement. However, the connection is indirectly through the gauge/gravity
or AdS/CFT correspondence. Motivated by searching for direct connection between
entanglement and the geometry properties of gravity, we developed a generic
formulation to calculate an entanglement measure for a bipartite system where
the two subsystems interact via classical gravity. With numerical calculation,

We present new results on realtime alternating, private alternating, and
quantum alternating automaton models. Firstly, we show that the emptiness
problem for alternating one-counter automata on unary alphabets is undecidable.
Then, we present two equivalent definitions of realtime private alternating
finite automata (PAFAs). We show that the emptiness problem is undecidable for
PAFAs. Furthermore, PAFAs can recognize some nonregular unary languages,
including the unary squares language, which seems to be difficult even for some

We consider ensembles of bipartite states resulting from a random passive
Gaussian unitary applied to a fiducial pure Gaussian state. We show that the
symplectic spectra of the reduced density operators concentrate around that of
a thermal state with the same energy. This implies, in particular,
concentration of the entanglement entropy as well as other entropy measures.
Our work extends earlier results on the typicality of entanglement beyond the
two ensembles and the reduced purity measure considered in [A. Serafini, O. C.

We investigate the variation of holographic complexity for two nearby target
states. Based on Nielsen's geometric approach, we find the variation only
depends on the end point of the optimal trajectory, a result which we designate
the first law of complexity. As an example, we examine the complexity=action
conjecture when the AdS vacuum is perturbed by a scalar field excitation, which
corresponds to a coherent state. Remarkably the gravitational contributions to

We introduce a class of communication tests where the task is to communicate
partial ignorance by means of a physical system. We present a full
characterization of the implementations of these tests in the qubit case and
partial results for qudits. A peculiar observation is that two physical systems
with the same operational dimensions may differ with respect to implementations
of these tasks, as is shown to be the case for the qubit and rebit. Finally, we

Along the development of free-electron laser operating at the wavelength of
X-ray, the importance of investigation on the radiation has increased. A
theoretical simulation is an essential tool for studying existing and proposed
experiments. The available simulations preserve net energy conservation over
the timestep, upholding causality between the electrons' power and the radiated
energy flux. However, according to Wheeler-Feynman time-symmetric theory, the
timestep is too short to ensure this. Therefore, the time evolution of