Dipole–dipole interactions enhance non-Markovianity and protect information against dissipation

Preserving non-Markovianity and quantum entanglement from decoherence effect is of theoretical and practical significance in the quantum information processing technologies.In this context, we study a system S that is initially correlated with an ancilla A, which interacts with the environment E via an amplitude damping channel.We also consider dipole-dipole interactions(DDIs) between the system and ancilla, which are responsible for strong correlations.We investigate the impact of DDIs and detuning on the non-Markovianity and information exchange in different environments.We show that DDIs are not only better than detuning at protecting the information(without destroying the memory effect) but also induce memory by causing a transition from Markovian to non-Markovian dynamics.In contrast, although detuning also protects the information, it causes a transition from non-Markovian to the Markovian dynamics.In addition, we demonstrate that the non-Markovianity grows with increasing DDI strength and diminishes with increasing detuning.We also show that the effects of negative detuning and DDIs can cancel out each other, causing a certain loss of coherence and information.

A Possible Quantum Spin Liquid Phase in the Kitaev–Hubbard Model

The quantum spin liquid(QSL) state of Kitaev-like materials, such as iridium oxides A_(2)IrO_(3) and α-RuCl_(3),has been explored in depth. The half-filled Kitaev–Hubbard model with bond-dependent hopping terms is used to describe the Kitaev-like materials, which is calculated using the state-of-the-art fermionic projected entangled pair states method. We find a QSL phase near the Mott insulator transition, which has a strong first-order transition to the semi-metal phase with the decrease of Hubbard U. We suggest that a promising approach to finding QSL states is by finding iridium oxides that are near the Mott insulator transition.

Linear optical approach to supersymmetric dynamics

The concept of supersymmetry developed in particle physics has been applied to various fields of modern physics.In quantum mechanics,the supersymmetric systems refer to the systems involving two supersymmetric partner Hamiltonians,whose energy levels are degeneracy except one of the systems has an extra ground state possibly,and the eigenstates of the partner systems can be mapped onto each other.Recently,an interferometric scheme has been proposed to show this relationship in ultracold atoms[Phys.Rev.A 96043624(2017)].Here this approach is generalized to linear optics for observing the supersymmetric dynamics with photons.The time evolution operator is simulated approximately via Suzuki–Trotter expansion with considering the realization of the kinetic and potential terms separately.The former is realized through the diffraction nature of light and the later is implemented using a phase plate.Additionally,we propose an interferometric approach which can be implemented perfectly using an amplitude alternator to realize the non-unitary operator.The numerical results show that our scheme is universal and can be realized with current technologies.

A von-Neumann-like photonic processor and its application in studying quantum signature of chaos

Photonic quantum computation plays an important role and offers unique advantages.Two decades after the milestone work of Knill-Laflamme-Milburn,various architectures of photonic processors have been proposed,and quantum advantage over classical computers has also been demonstrated.It is now the opportune time to apply this technology to real-world applications.However,at current technology level,this aim is restricted by either programmability in bulk optics or loss in integrated optics for the existing architectures of processors,for which the resource cost is also a problem.Here we present a von-Neumann-like architecture based on temporal-mode encoding and looped structure on table,which is capable of multimode-universal programmability,resource-efficiency,phasestability and software-scalability.In order to illustrate these merits,we execute two different programs with varying resource requirements on the same processor,to investigate quantum signature of chaos from two aspects:the signature behaviors exhibited in phase space(13 modes),and the Fermi golden rule which has not been experimentally studied in quantitative way before(26 modes).The maximal program contains an optical interferometer network with 1694 freely-adjustable phases.Considering current state-of-the-art,our architecture stands as the most promising candidate for real-world applications.

Measuring a dynamical topological order parameter in quantum walks

Quantum processes of inherent dynamical nature,such as quantum walks,defy a description in terms of an equilibrium statistical physics ensemble.Until now,identifying the general principles behind the underlying unitary quantum dynamics has remained a key challenge.Here,we show and experimentally observe that split-step quantum walks admit a characterization in terms of a dynamical topological order parameter(DTOP).This integer-quantized DTOP measures,at a given time,the winding of the geometric phase accumulated by the wavefunction during a quantum walk.We observe distinct dynamical regimes in our experimentally realized quantum walks,and each regime can be attributed to a qualitatively different temporal behavior of the DTOP.Upon identifying an equivalent manybody problem,we reveal an intriguing connection between the nonanalytic changes of the DTOP in quantum walks and the occurrence of dynamical quantum phase transitions.

Optical demonstration of quantum fault-tolerant threshold

A major challenge in practical quantum computation is the ineludible errors caused by the interaction of quantum systems with their environment. Fault-tolerant schemes, in which logical qubits are encoded by several physical qubits, enable to the output of a higher probability of correct logical qubits under the presence of errors. However, strict requirements to encode qubits and operators render the implementation of a full fault-tolerant computation challenging even for the achievable noisy intermediate-scale quantum technology. Especially the threshold for fault-tolerant computation still lacks experimental verification. Here, based on an all-optical setup, we experimentally demonstrate the existence of the threshold for the fault-tolerant protocol. Four physical qubits are represented as the spatial modes of two entangled photons, which are used to encode two logical qubits. The experimental results clearly show that when the error rate is below the threshold, the probability of correct output in the circuit, formed with fault-tolerant gates, is higher than that in the corresponding non-encoded circuit. In contrast, when the error rate is above the threshold, no advantage is observed in the fault-tolerant implementation. The developed high-accuracy optical system may provide a reliable platform to investigate error propagation in more complex circuits with fault-tolerant gates.

Experimental verification of generalized eigenstate thermalization hypothesis in an integrable system

Identifying the general mechanics behind the equilibration of a complex isolated quantum system towards a state described by only a few parameters has been the focus of attention in non-equilibrium thermodynamics.And several experimentally unproven conjectures are proposed for the statistical description of quantum(non-)integrable models.The plausible eigenstate thermalization hypothesis(ETH),which suggests that each energy eigenstate itself is thermal,plays a crucial role in understanding the quantum thermalization in non-integrable systems;it is commonly believed that it does not exist in integrable systems.Nevertheless,integrable systems can still relax to the generalized Gibbs ensemble.From a microscopic perspective,understanding the origin of this generalized thermalization that occurs in an isolated integrable system is a fundamental open question lacking experimental investigations.Herein,we experimentally investigated the spin subsystem relaxation in an isolated spin-orbit coupling quantum system.By applying the quantum state engineering technique,we initialized the system with various distribution widths in the mutual eigenbasis of the conserved quantities.Then,we compared the steady state of the spin subsystem reached in a long-time coherent dynamics to the prediction of a generalized version of ETH and the underlying mechanism of the generalized thermalization is experimentally verified for the first time.Our results facilitate understanding the origin of quantum statistical mechanics.