This
project juxtaposes a cold neutral cadmium gas with single
cadmium ions. The neutrals are trapped with a magneto-optic
trap (MOT), and the ions are held in a linear rf ion trap in the
same regions of space. Motivations:
The
neutral cadmium system has a two-electron singlet ground state and
exhibits a variety of novel features, including direct MOT-beam
photoionization from the excited 1P1 state. This may have direct
applications to the controlled loading and selection of cadmium
ions for existing ion trap quantum information experiments.
The
interaction between cold Cd neutrals and a single Cd+ ion may allow experiments analogous to the transport of a single
impurity “hole” moving through a conductor. Through
charge-exchange, the ion (electron hole) will move through the
gas, and it may be possible to probe properties of this single
hole as it traverses the vapor.
More speculatively, the controlled interaction between cold
trapped cadmium ions and neutrals may allow a coherent
charge-exchange process to “gently” neutralize the trapped cadmium
ion, thereby mapping an ionic hyperfine qubit to a neutral-atom
nuclear-spin qubit for reliable transportation of quantum
information over large distances.
Finally, the long-lived triplet 3P0 and 3P2 states may be of
interest in optical frequency metrology, and the extremely low
cooling limit for laser-cooling on the 3P1 state may offer a quick
route to BEC.
Thousands of neutral 114-cadmium atoms confined in a MOT
operating at 228.8 nm.
Ultraviolet "photoionizing" MOT. The vapor pressure of Cd is similar to that of Na near room
temperature, so we will first load the MOT simply from the
background vapor. With 5-10 mW of ultraviolet power near 229 nm,
we expect 2 mm diameter beams to accumulate of order
100,000 atoms. However, the MOT beams can also directly photoionize the trapped atoms from the excited 1P1 state, leading to
a smaller steady-state number of atoms in the vapor cell MOT. For a
background collision loss rate of 1/sec, we expect that
photoionization will dominate loss processes for intensities greater
than about 30 mW/cm2. However, for intensities much
less, it should be possible to precisely control ion production
rates under 1/sec. The
controlled ion production (rate and isotope) may prove indispensable
for reliably loading ion traps situated in the same region of space
as the MOT.
Hiding Qubits in
Cold Neutral Atoms.
Qubits stored in the hyperfine states of atomic ions such as 111-Cd
are among the most reliable quantum memories known. However, it
is difficult to reliably communicate this quantum information to
other trapped ion memories or other quantum systems such as photons
for quantum communication purposes. If a 111-Cd ion is trapped and
cooled in the presence of a neutral 111-Cd atom, the charge exchange
process discussed above might be exploited to convert the qubit in
the hyperfine levels of a trapped ion into a pure nuclear spin qubit
stored within the neutral 111-Cd atom. Because the neutral would be
in a singlet electronic ground state, the nuclear spin would be
protected from most electronic interactions, and could then be
traverse large distances to communicate quantum information to
another, remotely-located ion. Alternatively, the neutral atom
qubit could be manipulated by driving electronic transitions
to excited hyperfine levels. The figure displays an
example of using this interaction to propagate entanglement for use
in teleportation and other quantum communication protocols.
This
will likely require control of the position and motion of both ion
and neutral that is beyond what has been achieved in the laboratory,
but it may nevertheless be possible to extract some level of
coherence in the transfer. For example, a Ramsey experiment could
be performed with the p/2
pulses acting the hyperfine levels of two separated 111-Cd ions, with a neutral 111-Cd atom allowed to controllably
interact with each ion in turn between the Ramsey zones. Another
possibility is to entangle two 111-Cd ion qubits using conventional methods dealing with their coupled
motion, and then allow a 111-Cd neutral to “carry away” one of the entangled qubits to a remote
location.
