Quantum Information Processing

We explore a way of universal quantum computation with particles which cannot occupy the same position simultaneously and are symmetric under exchange of particle labels. Therefore the associated creation and annihilation operators are neither bosonic nor fermionic. In this work we first show universality of our method and numerically address several examples. We demonstrate dynamics of a Bloch electron system from a viewpoint of adiabatic quantum computation. In addition we provide a novel Majorana fermion system and analyze phase transitions with spin-coherent states and the time average of the out-of-time-order correlator (OTOC). We report that a first-order phase transition is avoided when it evolves in a non-stoquastic manner and the time average of the OTOC diagnoses the phase transitions successfully.

In this paper, we present a collection of results on the observability of quantum mechanical systems, in the case the output is the result of a discrete nonselective measurement. By defining an effective observable, we extend previous results, on the Lie algebraic characterization of observable systems, to general measurements. Further results include the characterization of a ‘best probe’ (i.e. a minimally disturbing probe) in indirect measurement and a study of the relation between disturbance and observability in this case. We also discuss how the observability properties of a quantum system relate to the problem of state reconstruction. Extensions of the formalism to the case of selective measurements are also given.

Quantum correlations are studied for a two-qubit system which indefinitely interacts with its environments via dephasing coupling, where the indefiniteness is created by a control qubit. The time evolution of the whole system is assumed to be modeled in terms of an exactly solvable spin-boson model. The reduced dynamics of the two-qubit system is conditioned on an outcome of measurement which is performed on the controlled qubit. The characteristic feature of the conditional dynamics is that diagonal elements of a density matrix change in time due to the environmental indefiniteness, in spite of the pure dephasing process. Entanglement, non-local correlation and quantum discord of the two qubits as well as their coherence and energy are examined.

This paper proposes an efficiency quantum image steganography protocol based on improved exploiting modification direction algorithm (QIS-IEMD). The new methods of expanding modification range and dynamically sharing between subgroups are introduced in the new protocol. Compared with the quantum image steganography protocol based on EMD algorithm (QIS-EMD), secret information can be embedded on multiple bit planes after expanding pixel’s modification range, and the carrier pixel groups can be dynamically shared to achieve higher embedding efficiency, embedding rate and larger capacity. In addition, in order to reflect the feasibility and practicability of QIS-IEMD, dedicated quantum circuits for embedding and extracting secret information are designed for it. Finally, the experimental results and detailed mathematical analysis prove that, compared with QIS-EMD, QIS-IEMD not only performs well in imperceptibility, but also significantly increases embedding efficiency, embedding rate and capacity.

We propose a scheme to construct a controlled phase gate on two distant nitrogen-vacancy centers (NV-centers) assisted by a quantized nanomechanical cantilevel resonator (NAMR). Unlike the previous work to complete the gate in the dispersive regime to let NV-centers detune with the NAMR largely, our gate is completed by using resonant operations between NV-centers and the NAMR, and single-qubit operations on the NV-center, which let the gate to be achieved within a short time with a high fidelity. To study the performance of the gate for universal quantum computation, we simulate a two-qubit Grover’s search algorithm on the NV-centers with a fidelity of $$98.46\%$$.

The quantum approximate optimization algorithm (QAOA) is considered to be one of the most promising approaches towards using near-term quantum computers for practical application. In its original form, the algorithm applies two different Hamiltonians, called the mixer and the cost Hamiltonian, in alternation with the goal being to approach the ground state of the cost Hamiltonian. Recently, it has been suggested that one might use such a set-up as a parametric quantum circuit with possibly some other goal than reaching ground states. From this perspective, a recent work (Lloyd, arXiv:1812.11075 ) argued that for one-dimensional local cost Hamiltonians, composed of nearest neighbour ZZ terms, this set-up is quantum computationally universal and provides a universal gate set, i.e. all unitaries can be reached up to arbitrary precision. In the present paper, we complement this work by giving a complete proof and the precise conditions under which such a one-dimensional QAOA might produce a universal gate set. We further generalize this type of gate-set universality for certain cost Hamiltonians with ZZ and ZZZ terms arranged according to the adjacency structure of certain graphs and hypergraphs.

The ability to perform quantum error correction is a significant hurdle for scalable quantum information processing. A key requirement for multiple-round quantum error correction is the ability to dynamically extract entropy from ancilla qubits. Heat-bath algorithmic cooling is a method that uses quantum logic operations to move entropy from one subsystem to another and permits cooling of a spin qubit below the closed system (Shannon) bound. Gamma-irradiated, $$^{13}$$ C-labeled malonic acid provides up to five spin qubits: one spin-half electron and four spin-half nuclei. The nuclei are strongly hyperfine-coupled to the electron and can be controlled either by exploiting the anisotropic part of the hyperfine interaction or by using pulsed electron nuclear double resonance techniques. The electron connects the nuclei to a heat-bath with a much colder effective temperature determined by the electron’s thermal spin polarization. By accurately determining the full spin Hamiltonian and performing realistic algorithmic simulations, we show that an experimental demonstration of heat-bath algorithmic cooling beyond the Shannon bound is feasible in both three-qubit and five-qubit variants of this spin system. Similar techniques could be useful for polarizing nuclei in molecular or crystalline systems that allow for non-equilibrium optical polarization of the electron spin.

The novel quantum dialogue (QD) protocol by using the three-qubit GHZ states (Quantum Inf Process (2019) 18: 37) is cryptanalyzed. It is found that there is the information leakage problem in this QD protocol. To be specific, half information of the secret messages exchanged is leaked out unconsciously. Afterwards, a way to improve this protocol is discussed.