Detection And Control Of Multi-Qubit In Solid-state Spin System

With the development of information science,big data,artificial intelligence and other research in recent years,faster information processing has become more and more important.People realize that classical computer with classical algorithms will eventually reach its limit.In order to solve the problem of insufficient computing resources,people started to search for new algorithms and new computer systems.Quantum computing is a very promising one among many research directions.Quantum algorithms based on quantum mechanics,such as integer factorization and searching algorithm,are very useful in cryptography and optimization problems.The intense investigation of quantum computing also simulates the fast development of quantum simulation,quantum sensing and quantum communication.Detection and control of multi-qubit system is at the core of all the above-mentioned research directions.The strategies of quantum control over a multi-qubit system is highly depended its physical realization.Nitrogen-vacancy center in diamond is a solid-state spin system at room temperature.It has a superb spin coherence time,convenient optical readout and microwave manipulation protocols.It is an excellent carrier of solid state quantum computer at room temperature.Moreover,the ¹³C nuclear spins with natural abundance around a nitrogen vacancy center can also be used as qubits,which provides the scalability of the quantum systems.In this paper,the electron spin of a nitrogen vacancy center and ¹³C nuclear spin around it constitute a multi-qubit system.Since the control speed of the nuclear spin is generally slower in the multi-qubit experimental system,higher requirements for the efficiency of fluorescence collection and microwave manipulation,and longer coherence time of the center electron spin are required.We optimize the sample preparation procedures with these purposes.Together with the improvement of the control method of the ion beam etching system,the fluorescence collection efficiency is greatly improved by accurately positioning the solid immersion lens above some selected nitrogen vacancy centers.In the aspect of microwave control,the efficiency of microwave manipulation is greatly improved by depositing an?-shape coplanar waveguide antenna around the solid immersion lens.Next,we detect the ¹³C nuclear spin distribution in the multi-qubit system with dynamic decoupling techniques and measure the coupling strength between these nuclear spins and the NV electron spin.We find several ¹³C nuclear spins with^kHz coupling strength.This means that we can manipulate a large number of weakly coupled nuclear spins with the ancilla electronic spin.This is an important step towards the direction of scalable quantum computing.Though the electron spin can be read out conveniently through a laser pulse in the multi-qubit system,single-shot readout of a nuclear spin is difficult because of it only present weak coupling to the center electron spin in most cases.With repetitively applied weak measurements implemented by dynamic decoupling sequence,we are able to achieve single-shot readout of a weakly coupled nuclear spin around the electron spin.This method is not only universally applicable to weakly coupled nuclear spins,but also robust to the operation error in the experiments.Finally,with the three-level system of the electron spin of nitrogen vacancy center,we experimentally demonstrate the entropy uncertainty relation.Entropy uncertainty relation is the extension of the uncertain relation in quantum mechanics.By verifying the entropy uncertainty relation of multidimensional measurement,we can get a deeper understanding of the quantum theory.The experimental study of multi-qubit system composed of an electron spin and nuclear spins,including the measurement of the coupling strength between the nuclear spin and the electron spin,the manipulation of the nuclear spin and single-shot readout of the nuclear spin state,will lay a solid foundation for the expansion of quantum computation through the nuclear spin around the nitrogen vacancy center in diamond.