Quantum Memory In Solid State Systems Enabled By Hamiltonian Engineering

Quantum memory plays an important role in quantum information technology.Quan-tum memory is the core part of quantum repeater,which is used to store and emit entangled photons.Long-distance quantum entanglement transmission faces the problem of exponen-tial decay of photon numbers in optical fibers.The use of quantum repeaters overcomes this problem,thus making long-distance quantum communication possible.Quantum memory is also used in quantum computing to store quantum bits,enabling synchronization of different quantum bits in time.Additionally,quantum memory plays a crucial role in quantum key distribution and quantum networks.Quantum dots are one of the many physical systems with the potential to realize quantum computation,where the charge and spin of the central electron can be used to encode quantum bits.However,the coherence time of the central electron is very short,typically only in the nanosecond range,due to the influence of the hyperfine interaction with the surrounding nu-clear spins.The nuclear spins in the quantum dot system are both a source of decoherence and a quantum resource that can be utilized.In 2003,Taylor et al.proposed a protocol that uses the nuclear spins in the quantum dot as a medium to store the quantum states of the central electron,with a storage time in the millisecond range.However,the traditional resonance-based memory protocol is susceptible to magnetic field noise from the nuclear spins,resulting in low quantum state transfer fidelity.In addition,polarizing the nuclear spins and eliminat-ing the nuclear spin noise is very difficult,with the result that the polarization is usually low.In this thesis we propose a pulsed noise-resistant quantum memory(NRQM)protocol that performs a coherent state transfer between the electronic and nuclear spins using Hamilto-nian engineering of the hyperfine interaction.With analytics and numerics,we show that a high-fidelity quantum state transfer between the electron and the nuclear spins is achievable at relatively low nuclear polarizations,due to the strong suppression of nuclear spin noises.For a quantum dot homogeneously coupled with 10~4nuclear spins via hyperfine interaction,a fidelity surpassing 80%is possible at a polarization as low as 30%.Our approach reduces the demand for high nuclear polarization,making experimentally realizing quantum memory in quantum dots more feasible.In addition,this Hamiltonian engineering approach may be help-ful for further investigations in quantum memory and dynamic nuclear polarization(DNP)in other systems such as nitrogen-vacancy(NV)color centers,doped-ion crystals,and atomic ensembles.NV centers and nuclear spins in diamond are promising candidates for future quantum technologies,including quantum computation,communication,and sensing.Nuclear spins are particularly attractive due to their exceptional isolation from the environment and resulting long coherence times.With the NV color center,nuclear spins can be individually addressed and controlled.They can act as quantum memories,enhancing the sensitivity of quantum sensors and quantum repeaters.In this thesis we describe a scheme for selective quantum states transfer between the NV center and the surrounding nuclear spins.By applying a set of pulse sequences on the NV center to modulate the magnetic dipole-dipole interaction between the NV center and the surrounding nuclei,we can selectively address and transfer quantum state to individual nuclear spins.We find that by selecting the appropriate type and spacing of pulses,the ability to distinguish different nuclear spins can be increased and the accuracy of quantum state transfer can be improved.In addition,the use of unequally spaced pulses can continuously change the coupling strength between the NV center and nuclei,providing more flexibility for controlling.Taking advantage of this point can effectively eliminate false signals and improve the accuracy of quantum state transfer.