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

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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