This project will push experimental limits in the local and nonlocal networking of quantum information. It is generally accepted that future quantum networks will require both stable quantum memories and appropriate quantum channels that can traverse macroscopic distances. We concentrate on the use of hyperfine ground states of trapped ions as quantum memories, and individual photons as the quantum communication channel. Building on previous work from our group in the Yb+ system, we will investigate (a) larger numbers of ions at each network node, (b) enhanced probabilities of heralded gate operations with nearby optical elements, (c) larger distances between trapped ion nodes by exploiting infrared transitions in the Yb+ system. These directions represent promising routes toward a variety of quantum information and quantum communication protocols, including distributed quantum computing and long-distance quantum repeaters circuits. This research may also lead to an attractive method for observing a "loophole-free" Bell inequality test, where the entangled ions are space-like separated with respect to the qubit measurement time. Finally, the ion/photon interfaces discussed here are not necessarily specific to trapped ions memories, and may be applied to other systems such as quantum dots in a semiconductor or doped glass host. This work is at the heart of laboratory quantum information science, and it is expected to attract great interest from students at all levels and backgrounds from a variety of disciplines in physics, computer science, and engineering. The results of this research will be widely disseminated at meetings and workshops, and also through lectures at primarily undergraduate institutions. As part of an internal initiative at U. Maryland to attract the best high school science students in the area to College Park, this work will be aggressively presented at local high schools. Where possible, the presentation of this research to young students and other newcomers to the field will be rooted in a particular approach to the foundations of quantum mechanics that relies more on information theoretic viewpoints such as entropy and measurement and less on wave mechanics, differential equations, and complex mathematics.
We have implemented ultrafast operations on individual atoms that may play a role in the development of quantum computer hardware in the future. We use ultrafast optical pulses (picosecond time scale) to apply forces to atoms that are much faster than any other time scale in the system. This eliminates sensitivity to many conventional noise sources, such as slowly fluctuating background fields and the random motion from finite initial temperature of the atom. We have shown that the internal (qubit) states with a single atom can be controlled at the picosecond time scale, and also shown how momentum kicks can be imparted to the atom in order to create a new type of ultrafast interferometer. By dividing single picosecond pulses from a mode-locked laser into 8 pulses and staggering their arrival on a single atom, we are able to create an interference pattern of the atomic matter wave so that a simple separation of the atom into two position results. This created a simple ultrafast interferometer in position, and we are able to see the interference fringes from the atomic wavepacket. In the future, similar types of operations on multiple atoms can be used for fast quantum logic gates that will be critical to the fabrication of a quantum computer.
