Experimental Investigation Of Non-hermitian Quantum Control On Solid-state Spin

Hermiticity of a Hamiltonian is among the most fundamental premises in standard quantum mechanics.It guarantees the real spectrum of a quantum system and ensures the conservation of probability in the dynamics of the system.Recently,researches have revealed that a class of non-Hermitian Hamiltonians with parity-time(PT)symmetry can also possess entire real spectrum when their PT symmetry are conserved.This astonishing discovery has made non-Hermitian physics a remarkable forefront of scientific research.Although non-Hermitian physics was introduced trying to extend the standard quantum mechanics,it has rapidly gained great success in classical systems and induced many applications.By delicately manipulating the gain and loss,researchers have explored rich non-Hermitian related physics in open classical systems,such as photonics,mechanics,acoustics and electrical circuits.Many exciting properties of non-Hermitian system have been discovered and experimentally verified;nano devices based on non-Hermiticity like gain and loss were also invented.However,the researches of non-Hermitian physics in the quantum realm are stumbling.This is because the Hamiltonian of a closed quantum system is always Hermitian,making it difficult to construct a non-Hermitian one,despite that studies of non-Hermitian physics in a real quantum setup are in urgent need.Recently,we developed a dilation method to embed the nonunitary dynamics governed by non-Hermitian Hamiltonians with arbitrary form into a subspace of the enlarged Hilbert space with only one ancilla qubit.Our method paves the way of comprehensively investigating non-Hermitian related physics in extensive quantum systems.Nitrogen-vacancy center(NV center)is a kind of defect in diamond and has been studied intensively in the last two decades.NV center has found applications in the field of quantum computation,quantum communication,quantum simulation and quantum metrology due to its excellent spin and optical properties.Utilizing our dilation method,we carried out a serious of experimental studies about non-Hermitian physics on NV center solid state spin quantum system,including:1.Observation of PT symmetry breaking in a single quantum system.We dilated the non-Hermitian Hamiltonians in both PT symmetric phase and PT symmetry broken phase and realized nonunitary dynamics governed by these Hamiltonians via quantum control method.The PT phase transition was observed by monitoring theses non-Hermitian evolutions in a single NV center.2.Experimental realization of dynamically encircling an exceptional point(EP)in a real quantum system and observing the mode conversion phenomenon.EP is the degeneracy of non-Hermitian Hamiltonian.In the eigen spectrum diagram,a characteristic topology exists in the vicinity of EP and can lead to a kind of spectral mode conversion phenomena when one dynamically encircling the EP in the parameter space.We realized the time-dependent non-Hermitian Hamiltonian associated with the dynamically encircling EP process via non-Hermitian quantum control method and experimentally observed these mode switch phenomena.We also demonstrated that these mode switches were roubust against noises which may be utilized in high fidelity quantum control.Our works established a universal dilation theory to realize arbitrary non-Hermitian Hamiltonians,which pave the way to comprehensively explore the exotic properties of non-Hermitian Hamiltonians in extensive quantum systems.We experimentally demonstrated some unique properties of non-Hermitian system that differs with traditional Hermitian system,this may facilitate the application of non-Hermitian quantum control in the field of quantum information processing.