Environment-assisted Manipulation Of Dynamical Evolution Speed In Qubit Systems

As qubit is the basic unit of quantum information processing,how to control the dynamical evolution speed of qubit systems is a fundamental issue and a re-search hotspot.The research of this problem has important research significance and manifold applications in virtually all areas of quantum physics,including quantum computation,quantum communication,quantum optimal control and quantum precision measurement.In general,given a threshold value for the dis-tinction between an initial state and a given final state,the dynamical evolution speed of quantum systems is measured by their evolution time.the shorter time the quantum systems require to arrive this threshold value,the faster dynamical evolution speed is.Instead,the longer evolution time means the slower evolution speed.It is worth noting that,quantum mechanics imposes the maximum speed to the evolution of quantum systems,or sets a lower bound on the evolution time.The quantum speed limit time(QSLT)is defined as the minimal time a quantum system needs to evolve between an initial and a final state separated by a given predetermined distance.It determines the theoretical upper bound on the pro-cessing speed of any quantum device as well as the communication speed through any channel.From another point of view,by first fixing the actual evolution time,QSLT reflects the nature of the dynamical evolution path.That the QSLT is equal to the actual evolution time means the evolution is already along the fastest path and possesses no potential capacity for futher acceleration.But in the case that the QSLT is less than the actual evolution time,the shorter QSLT is,the greater the capacity for potential peedup will be.In this thesis,we mainly investigate the effective control of dynamical evolution speed in qubit systems by designing reasonable,feasible and tunable environments.The specific content of each part is as follows:In chapter 1,we introduce the background materials about the evolution speed control in the qubit systems and its research significance.In chapter 2,we briefly introduce the defination of QSLT in both the closed quantum systems and open quantum systems.In chapter 3,we find two double-quantum-dots spin qubits can be coupled by using a transmission line resonator as a data bus.we have proposed a nwe scheme to control the evolution speed of two double-quantum-dot spin qubits by use of the transmission line resonator.It is shown that both of the actual evolutio time and the QSLT of the two double-quantum-dot spin qubits are controllable through changing the parameters of the transmission line resonator.It is also found that a maximally entangled 0011-type Bell state can evolve to a target state with an arbitrary fidelity at the quantum limit speed.In chapter 4,we study the quantum decoherence speed limit of an impurity qubit in impurity-doped Bose-Einstein-condensate(BEC)system,where the BEC atoms can be used to simulate a phasing damping environment for the doped impurity atom.It is shown that both of the actual decoherence speed and the quantum decoherence speed limit of the impurity are controllable by adjusting the nonlinearity of the BEC reservoir and the spatial form of the trap potential imposed on the impurity-dopped BEC system.The essential physical mechanism behind the evolution speed control is that the effective dimension and the nonlinear interaction in the BEC can change the spectral density from subohmic to ohmic and then to superohmic spectra,while the trap double-well potential of impurity could be used to change the cut-off frequency and the effective coupling strength.In chapter 5,we investigate the possibility to control the capacity of potential speedup of a single dephasing qubit with arbitrary initial states by the use of dynamical decoupling pulses.It is found that the QSLT is determined by quantum coherence of the qubit and non-Markovianity of the system during the evolution when the qubit is subjected to a zero-temperature Ohmic-like dephasing reservoir.We indicate that the control mechanism is that dynamical decoupling pulses can be exploited to engineer quantum coherence and quantum non-Markovianity of the single-qubit open system.We demonstrate that the speedup effect of the qubit in the short-time reime is better than that in the long-time regime when the same number dynamical decoupling pulese are applied on the qubit.In chapter 6,we present a method to control the capacity of potential speedup of multiqubit system by employing dynamical decoupling pulses when the qubis are in the amplitude damping channel.It is found that,when the qubits are initially in W-type or GHZ-type states,the QSLT can be controlled by using the dynamical decoupling pulses in both the weak-and strong-coupling regime.The essential physical mechanism behind the speed control is explained as a result of the joint action of the exicited-state population of qubits and the non-Markovianity of the total system during the evolution.It is shown that both the non-Markovianity and the excited-state population can be controlled by dynamical decoupling pulses.In chapter 7,we present a summary and outlook.