Trapped ions provide an ideal physical system to realize qubits. Well-defined qubits with long coherence times have been demonstrated, along with an efficient way to initialize and measure qubit states. Several schemes for realizing universal set of logic gates have also been proposed and demonstrated. These demonstrations provide a solid platform for constructing a scalable quantum information processor (QIP).

A Platform To Integrate Functionalities

The realization of large-scale QIP hinges on the availability of a technology platform to integrate all necessary functionalities to construct a functional circuit, just like the integrated circuits (IC) technology has provided for the classical information processors. For this purpose, the interconnect architecture to transport quantum information from one location in the QIP to another is crucial to system design. Many researchers have suggested ideas over the years, that evolved to a realistic distributed quantum multicomputer architecture.

In this architecture (shown in Figure 1), the overall QIP is divided into elementary logic units (ELUs) consisting of ~10² qubits. Each ELU consists of ion trap chip capable of trapping such density of ions, integrated with control electronics for ion transport within the chip, optical beam distribution network to manipulate the internal states of the ions, and measurement optics to measure the qubit states. The ELU also contains a communication port, where an entangled ion-photon pair is generated and the photon is efficiently captured into a single mode fiber. In the overall computer architecture, a large number (~10³) of such ELUs are connected optically through an optical crossconnect (OXC) switch. On the outputs of the OXC, a set of Bell state detectors provide the entanglement swapping procedure by measuring the interference of photons from different ELUs. This architecture enables generation of entangled ion pairs between any two ELUs in the computer, with a flat communication cost that is independent of the distance between the qubits in the computer. 

 

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Figure 1: Architecture for distributed quantum multicomputer with photonic
interconnect network using a large-scale optical crossconnect switch.

The QIP is in Reach

The technology elements needed to construct such a QIP is within reach. A large scale OXC with more than 1,000 input and output ports have been demonstrated in the context of optical communications. The principles of ion-photon entanglement and ion-ion entanglement using photon exchange have been demonstrated recently. Several groups in the world are working on integrated ion trap chips capable of trapping tens to hundreds of ions on a single chip.

There are, however, critical elements that are yet to be developed:

1. Realization of high density ion trap chips with the capability to arbitrarily transport ions on the chip
2. A more efficient way to determine the internal state of the qubits, which requires a better means to collect the photons scattered from the ions during the measurement process
3. Much more efficient ways to collect the photon from the ion-photon entanglement process at the communication port
4. Efficient ways to deliver the necessary control laser beams to the ions in the ELU

Addressing the Challenges

The first, second and third challenge are addressed in this project. Dr. Slusher's group at Georgia Institute of Technology is working on addressing the first challenge, and we are collaborating with their group to add optical functionality by integrating micro-optical components with their ion trap chip . Multi-functional trap architectures fundamentally integrated with optical elements are crucial in addressing the second and the third challenge listed above. An approach that integrates high numerical aperture reflective surfaces with the ion traps will dramatically improve the collection efficiency of the scattered photons beyond the limits available with refractive optics. Ion traps integrated into an optical micro-cavity will dramatically enhance the ion-photon entanglement generation process. 

The last challenge of distributing control beams requires multiplexed laser beam control from outside the ion trap chip. The first steps to addressing this challenge is explored in our second project, using MEMS-based beam steering technology.

Trapped atomic ions and neutral atoms provide exciting possibilities for realizing scalable quantum information processors (QIPs). The qubit is represented by a pair of internal states of these atoms, and most of the qubit manipulation is performed by using laser beams. In a traditional experiment, the laser beams are aligned to the atomic systems using conventional optics holders on an optical table. Flexible beam shifting capabilities are needed to individually address a large number of these atomic qubits in an array. Traditionally, acousto-optic and electro-optic deflectors are used to provide this capability. In our research we utilize micro-electromechanical (MEMS) technology to provide highly flexible, multiplexed beam steering capability over a wide range of optical wavelengths.

MEMS-Based Beam Steering

MEMS-based beam steering has been widely developed for optical communications in last 1990's and early 2000's. The requirements, however, were very different for those applications. For QIP applications, the speed at which the beams must be shifted is a critical parameter. In our research we target the settling times for the beam steering to be on the order of a few microseconds, consistent with the timescales over which the atomic states remain quantum mechanically coherent.

MEMS-based beam steering system starts with a set of tilting mirrors, where the beam tilt is converted to a lateral beam shift using a Fourier lens located a focal length away from the MEMS mirror. In our approach, the x- and y-tilts are provided by two separate mirrors, each providing the steering function in one direction. The tilt motions in the two directions are optically combined in an innovative optical imaging system.

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Figure 1 shows the schematic of our beam steering system. This system is capable of independently controlling two beams over a lattice of 5x5 sites with beam steering times of better than 10 microseconds. The project is jointly developed with Dr. Felix Lu at Applied Quantum Technologies, Inc.

Collaborative Arrangements

We have collaborative arrangements to where our systems will be put to use in real atomic physics experiments. Dr. Didi Leibfried at NIST in Boulder has a system that will enable a real-time monitoring of their ions as they are transported on a chip, as the laser beam that excites the cycling transition for state detection follows the ion as it is transported. Prof. Mark Saffman's group at University of Wisconsin will apply our system to the 2D atomic lattice of Rb atoms they are working with. In the future, this technology will be applied to constructing a system capable of addressing atoms in a 3D lattice, in collaboration with Prof. David Weiss at Penn State University.