Ion traps for scalable quantum computation and quantum simulation

Fig. 1: Schematic illustrations of (a) a two-level linear RF Paul trap representative of what is currently used in quantum computation experiments, and (b) a five-electrode planar ion trap. Ions are trapped along the trap axis, shown as a dotted line. An RF potential is applied to the red electrodes to provide radial confinement, and DC potentials are applied to the blue control electrodes to provide axial confinement and to shuttle ions along the trap axis. Typical dimensions for current two-level traps are a slot width s of 200 to 400 microns.

In recent years, the quantum computing community has demonstrated the basic building blocks of a scalable ion trap quantum computer. The design uses the electronic states of ions trapped in a radiofrequency (RF) Paul trap as qubits and accomplishes logic gates and state readout by laser-ion interactions. Ions are shuttled between zones in an array of ion traps to perform two-qubit gates between arbitrary pairs of qubits. Recent experiments have accomplished state preparation and readout, one- and two-qubit gates, and ion shuttling in straight lines and through tees. Scaling this architecture up to the many thousands of qubits necessary for a useful computation, however, will involve significant physics and engineering challenges.

This project focuses on developing scalable ion traps for quantum computation. The traps used for quantum computation work by confining ions to the null axis of a two-dimensional RF quadrupole electric potential. Ions are moved along the RF null axis, or "trap axis", by applying DC potentials to "control" electrodes. Fig. 1(a) shows a trap geometry typical of what is used in current experiments. In contrast, we are working on a new ion trap design in which all of the electrodes lie in a plane, Fig. 1(b). Such planar ion traps can be built using silicon VLSI technology and thus have the capability to scale to arbitrarily large and complex trap arrays.

Planar traps

Planar ion traps are a general tool with many applications in quantum simulation, quantum computation, and mass spectrometry. Accordingly, our current work is going in multiple directions. One avenue is working on two-dimensional lattice traps which could be used for quantum simulations of Bose-Hubbard models, Fig. 9. Another is using the things we have learned by trapping macroscopic charged particles to move towards trapping single atomic ions in planar traps for quantum computation. Here there will be new challenges to face, including laser cooling the ions and developing control voltage sequences for performing movement operations adiabatically.

Fig. 2: Top view of a two-dimensional Paul trap array. Ions can either be confined to points or free to move along lines, depending on the RF and ground connections. The connections shown (right inset) trap ions above the dots. Left inset: Microspheres are trapped in the 2D array, and seven are shown illuminated by the laser beam. The four-rod loading trap is visible above and to the right.

Toward quantum simulations

Feynman, in 1982, pointed out that quantum systems cannot be efficiently simulated by computers operating according to the laws of classical physics alone, but can be using a machine based on quantum physics. However, progress has been limited by the difficulty of creating controllable quantum systems with coherence times that are long enough for meaningful studies. Our quantum simulation project addresses this challenge.

A quantum simulator is inherently a quantum computer, but there are some important distinctions. The level of complexity is much less than required for a useful quantum computer. Specifically, the physical system used for the simulator need only have a number of degrees of freedom (e.g. controllable internal quantum states) comparable to the system being simulated. For example, to simulate an N-spin Ising model it suffices to have a lattice of ~N atoms. Furthermore, the control scheme is intrinsically simpler and under the right circumstances, can be more robust (i.e. less susceptible to errors) than that required for quantum computation.

These simplicities allow for the possibility of realizing a quantum simulator with technology at hand along the following lines. The working medium is a two-dimensional array of ultracold ions whose internal hyperfine states model degrees of freedom of the system being simulated. The vibrational states of the ions are entangled via the Coulomb interaction, and mapped onto internal states of the ions with laser-driven Raman transitions. This general approach has already been applied to linear chains of four to six ions for metrology, clocks, and simple quantum algorithms, at a variety of institutions around the world.

Extending trapped-ion techniques to systems of tens to hundreds of ions presents a formidable challenge, but we believe that it is possible using a two-dimensional radio-frequency ion trap that we have implemented based on ideas advanced at NIST. The special feature of our implementation is that it is entirely planar; it is fabricated from a single layer of metal, deposited on a glass composite substrate, and lithographically patterned to produce segmented electrodes. The advantages of this geometry are that it is readily fabricated and can be extended in size because it is fully compatible with semiconductor fabrication techniques. Also, it has the intrinsic geometry of two-dimensional neutral atom traps already used at CUA, which is important in loading the cold ions. This approach opens the door to achieving the electrostatic control required to manipulate tens to hundreds of ions.

We have carried out preliminary studies of this system and implemented several key components: construction and demonstration of ion crystallization in a linear trap, lithographic fabrication of a planar ion trap, and development of methods to load shallow traps. Our current experiment realizes a two-dimensional lattice of ions, with a spacing larger than several hundred micrometers. This trap size is being significantly reduced in a second generation realization, to about 100 micrometers, a size at which ion-ion interactions should become measurable. Methods for coupling ions through optical cavities are also being investigated.