Jianjia Fei

Resonant adiabatic passage with three qubits

We investigate the non-adiabatic implementation of an adiabatic quantum teleportation protocol, finding that perfect fidelity can be achieved through resonance. We clarify the physical mechanisms of teleportation, for three qubits, by mapping their dynamics onto two parallel and mutually-coherent adiabatic passage channels. By transforming into the adiabatic frame, we explain the resonance by analogy with the magnetic resonance of a spin-1/2 particle. Our results establish a fast and robust method for transferring quantum states, and suggest an alternative route toward high precision quantum gates.

Heisenberg spin bus as a robust transmission line for quantum-state transfer

We study quantum-state transfer (QST) through a strongly coupled antiferromagnetic spin chain (acting as a spin bus), between weakly coupled external qubits. By treating the weak coupling as a perturbation, we find that QST is enabled specifically by the second-order terms in the perturbative expansion. We show that QST is robust against disorder in the couplings, either within the bus or to the external qubits. We find that the protocol works when the qubits are attached to any node on an even-size bus or to the antiferromagnetic nodes on an odd-size bus. The optimal time for QST is found to depend nonmonotonically on qubit separation.

Probing quantum phase transitions on a spin chain with a double quantum dot

Quantum phase transitions (QPTs) in qubit systems are known to produce singularities in the entanglement, which could in turn be used to probe the QPT. Current proposals to measure the entanglement are challenging however, because of their nonlocal nature. Here we show that a double quantum dot coupled locally to a spin chain provides an alternative and efficient probe of QPTs. We propose an experiment to observe a QPT in a triple dot, based on the well-known singlet projection technique.

Effect of Randomness on Quantum Data Buses of Heisenberg Spin Chains

Department of Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA(Dated: December 2, 2010)Strongly coupled spin chains can be used for quantum data buses. We study odd-size spin 1/2Heisenberg chains with Gaussian random exchange couplings and Gaussian external magnetic fields.It is found that the Gaussian random exchange couplings have no effect on the Zeeman energy gapbetween the ground doublet states of the spin chain, but make the local magnetic moments fluctuate.On the other hand, it is shown that that even though all of external local magnetic fields are appliedin the same direction, their fluctuation in strength could make the ground state of the chain flipped.Also the random external magnetic fields induce avoided crossings, so the ground doublet states havea non-vanishing Zeeman splitting on average even when the average of random external magneticfields is zero.