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Colloquium: Quantum networks with trapped ions

L.-M. Duan and C. Monroe

Quantum computation remains one of the most challenging problems in science. In recent years there have been many developments, theoretical and experimental, in this active area of research. In this Colloquium the issue of quantum networks in the context of information exchange between trapped ions by photons is discussed. Advances in this area can have huge impacts in communication and computation. ...

Individual trapped ions are known as the most advanced
material qubit, as they can be localized in one
place for extended periods like a solid while interacting
weakly with their environment. But trapped ions are
also attractive candidates for quantum networking applications
because they can be replicated in different nodes
with nearly identical characteristics, which can be an important
practical feature for the implementation of photonic
quantum networks. While the coupling of trapped
ion qubits to photons is typically small, this is not a fundamental
shortcoming as there exist probabilistic protocols
for the scalable photonic networking of trapped ion
qubits. It is possible to generate scalable entangled
states of many ions through the use of exclusively probabilistic
gates, although this will likely require gate success
probabilities to approach p10−3 or higher. On the
other hand, when probabilistic gates are combined with
local deterministic gates, a natural situation for small ion
crystals that can be entangled based on their Coulomb
interaction, scalable quantum information processing
can proceed with gate success probabilities that are even
smaller.

Recent experiments have shown the basic features of
photonic networking between separated single trapped
ions, and future technological gains in ion trap photonics
may dramatically increase the probabilistic gate success
rate. Future ion-photonic interfaces will likely exploit
advances in microfabricated ion trap structure Blatt and
Wineland, 2008 and integrated optical elements. One
example of an ion-photonic network architecture is
shown in Fig. 14, where registers of trapped ion crystals
allow local entanglement to proceed though conventional
Coulomb gates, and entanglement between registers
is accomplished through the interference of synchronously
emitted photons from selected ions through
the use of a reconfigurable NN optical cross-connect
switch. Such a hierarchical architecture is promising for
the scaling to very large numbers of trapped ion qubits.
There are many other possible architectures, taking the
lead from interconnects in classical processors.
We note finally that while trapped ions are among the
best controlled matter qubits today, in the future it
should be possible to apply the optical protocols discussed
in this Colloquium to other optically active qubits. ...
All of the protocols discussed in this Colloquium could ultimately be
applied to the NV-diamond system, as well as other optically
active solid-state qubits such as a large class of
rare-earth ion-doped solid-state systems. Such quantum
registers can also be represented by single semiconductor
quantum dots, where the two local qubits are carried
by the electron- and nuclear-spin states. The electronspin
states can be coupled to photons, and separated
dots can be entangled through the probabilistic entangling
protocols discussed above. The local gates on the
electron spin and the nuclear spin could be achieved
with laser manipulation of the hyperfine interaction.

Rev. Mod. Phys. 82, 1209 (2010) – Published April 28, 2010