Trapped ion quantum bits can be linked through
a photonic quantum channel, creating long-distance entanglement
between quantum memories, quantum networks, and distributed quantum
computers.
We are investigating the probabalistic entanglement of
individual remotely-located ions through the interference of photons emitted from the ions. Unlike other sources of probabalistic entanglement (eg.,
two-photon downconversion), after verification this type of
entanglement can be used as a resource for efficient scaling to even
larger entangled states over many nodes.
This approach significantly relaxes conventional approaches to
ion-ion entanglement, as the trapped ions need not be
well-localized, and motional coherence is not relevant.
This type of
ion-ion coupling does not rely on the control of the motional state
of the ions, and therefore does not require advanced cooling and
small, tightly-confining traps. The linear traps used in this
project are large (0.5-1.0 mm characteristic size)
The picture above and to the left is an example of the type of linear trap used in these experiments. The image just to the right of the trap is of four trapped ions.
In order to entangle two remote ions, each ion is first entangled with an emitted photon. The ion represents a nearly ideal quantum memory. Ions have been shown to exhibit extremely long coherence times and near-perfect state read-out, which means you can put information there, and still be able to reliably access that information at a later time. The photon, on the other hand, is a nearly ideal quantum communication channel. Traveling at the speed of light, photons traverse great distances very quickly, often without significant loss of coherence. Direct entanglement between stationary (ion) and "flying" (photon) qubits was first observed in our lab in 2004 (Nature428, 153 (2004)).
After each ion is entangled with its emitted photon, the photons are collected and directed to a 50:50 beamsplitter. Due to the quantum interference of the photons (Nature Physics3, 538 (2007)), only an antisymmetric photonic state will result in a simultaneous detection at both output ports of the beamsplitter. It is this "double-click" of the detectors that heralds the entanglement of the two ions. Thus, even though the photons have been measured (and thus any entanglement between the photons is no longer useful for future operations), the ions are now entangled (Nature449, 68 (2007)). The entanglement between the ions can be used for more advanced quantum information applications.
Quantum
Networking with Photons and Trapped Atoms, D. L. Moehring, M.
J. Madsen, K. C. Younge, R. N. Kohn, Jr., P. Maunz, L.-M. Duan, C.
Monroe, and B. B. Blinov, J. Opt. Soc. Am. B 24, 300
(2007).
