TIQC Research

Cold Neutrals and Cold Ions

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.

This project is supported by the NSF FOCUS Center.