The AtomChip: Bringing Nanofabrication and Quantum Optics Together

By Ron Folman

One of the scientific revolutions of the 20^th century is Quantum Mechanics. It is a weird theory, extremely different from our day to day experiences, in which, for example, a particle can act as a wave or can be in several places at once.

It is also very different from our deterministic classical laws of physics as it is probabilistic in nature. This strange theory has already enabled marvels of technology such as the atomic clock.

One of the technological revolutions of the 20^th century is the electronic integrated circuit. Such circuits, in which millions of transistors are put into a small semi-conductor chip, have enabled the computer and most technological gadgets that today engulf our lives.

It is not uncommon that joining two seemingly unconnected fields of knowledge brings about dividends that reach far beyond the mere sum of the potential embedded in the two parent disciplines.

The AtomChip is such a story. It brings together the best of both worlds: the relatively mature field of micro- and nano- fabrication and the new set of scientific rules provided by quantum theory. Together they form quantum technology, with the promise of devices such as miniature atomic clocks, magnetic sensors, inertial navigation systems, gravitation field sensors, quantum communication and cryptography, and the quantum computer.

The main idea is to make possible the co-existence of a quantum system within a classical environment, so that the classical environment enables effective control of the quantum system as well as effective information exchange by standard devices such as present day electronics and computers.

The main challenge comes from the fact that while we want to couple to the quantum system for control and information exchange, quantum systems survive in their quantum state only under severe conditions of isolation.

The first generation of AtomChips were designed and operated at the turn of the century [1,2]. These chips were based on micro-fabricated electrodes holding currents and charges, and on a quantum system in the form of ultra-cold neutral atoms cooled by lasers and other simple means without the need for a cumbersome cryogenic apparatus. These simple cooling methods gave rise to Nobel prizes in 1997 and 2001.

Thus, neutral atoms were trapped in vacuum a few microns above the surface of a room temperature chip. If, in a semi-conductor chip, the system of interest moves within the chip, here, the system of interest is trapped and guided within electric, magnetic and electro-magnetic fields, microns above the chip.

Figure 1. An AtomChip for neutral atoms with which a previously unknown phenomenon in electron transport was observed [5] (collaboration with Joerg Schmiedmayer).

Today the chip has evolved into different formats. For example, in addition to neutral atoms, cold isolated ions, molecules and even electrons are trapped above the surface. To increase simplicity, capsules with hot atomic vapor are also used.

In addition, attempts are being made to utilize solid-state lattices which exhibit the unique feature of not destructively coupling to a quantum system embedded within them [e.g. Nitrogen-Vacany (color) centers within a carbon lattice (diamond)].

In numerous laboratories as well as private companies, AtomChip technology is currently being simplified and miniaturized. The vacuum chamber will, for example, be located inside the silicon substrate. Integrated diode lasers and photonics will provide efficient light-matter interaction for delicate information exchange.

We are thus not far from the point in time when AtomChips may be embedded in standard electronic boards alongside 20^th century electronic components. Externally, one will not be able to tell them apart.

For those familiar with fabrication, it is interesting to note that the AtomChip has many completely new figures of merit in terms of quality and yield of chips. It also requires new materials and geometries. For example, while surface or edge roughness of conducting wires are of little concern in the semi-conductor industry (as long as conductivity remains high), quantum optics requires extreme smoothness.

Furthermore, while electrically anisotropic materials are nowhere to be found in the electronics industry, they have been shown to reduce hindering effects for quantum optics by orders of magnitude.

Similarly, while in conventional processes contamination is avoided at all cost, it has been shown that for quantum optics, contamination may be advantageous. The interested reader can learn more about the state-of-the-art in the fabrication of AtomChips in [3].

If one is interested in the specific example of quantum computing, a good overview is given in the special issue of the journal Quantum Information Processing, about to be published [4].

At Ben-Gurion University of the Negev (BGU) we have constructed one of the first, if not the first, fabrication facility designed from the start to address the needs of the AtomChip community.

In the figures, I present two chips fabricated at BGU. The first is an AtomChip for neutral atoms, which featured in the Science magazine in 2008, and which due to the ultra sensitivity to the atom-surface interaction, enabled the observation of a completely unknown phenomenon in electron transport. A tiny cloud of a few thousand neutral atoms trapped microns above the electrodes of an AtomChip is also presented.

The second AtomChip is adapted for charged atoms. Two ions trapped on this chip microns above the surface are shown in the figure. Finally, I present a schematic view of how the future AtomChip will be structured.

Figure 2. Figure 2. A dilute cloud of a few thousand ultra-cold atoms a few micron-meters above the surface of an AtomChip. The electrodes of the chip are visible in the background.Taken from [6].

Figure 3. Fluorescence from two ions trapped micron-meters above the surface of the AtomChip shown below in figure 4 (collaboration with Ferdinand Schmidt-Kaler).

Figure 4. AtomChip for charged atoms (Ion Chip) fabricated at BGU and ready to be put into the chamber in Mainz, Germany.

Figure 5. A schematic view of how a future AtomChip device would be structured. The miniature vacuum chamber will be embedded into the silicon substrate. The chip would integrate all required particle/light sources as well as MEMs valves, photonics, high-Q resonators, and readout via fibers and electronics (courtesy of Tim Freegarde)

The AtomChip is not only a mind-boggling example of synergy between two disciplines. It is also a wonderful example of the integration of many different operational elements in one monolithic device: metallic electrodes for currents and charges side by side with photonics and high-Q resonators, MEMs, lasers, and so on.

Further, the AtomChip platform may enable the integration of several different quantum systems. These so-called hybrid quantum systems may include, for example, logic gates made of superconducting qubits (quantum-bits) and quantum memory in the form of trapped atoms.

As Science is after all about knowledge and not only technology, it is perhaps fitting to end this brief outline with a broader and somewhat philosophical point of view. As the AtomChip enables more and more complex quantum operations, it will also enable deeper and deeper insights into quantum theory. Such understanding of one of the strangest realms of nature may have deep implications concerning our concept of the universe as well as our perception of ourselves. For example, answering the enigma of whether brain functions follow quantum logic or classical logic may have consequences concerning the question of free will.

Whatever the future holds for the AtomChip, it is quite clear that this chip has invited us for quite a ride.

[1] R. Folman, P. Kruger, J. Schmiedmayer, J. Denschlag and C. Henkel, Controlling cold atoms using nanofabricated surfaces: Atom Chips, Adv. At. Mol. Opt. Phys. 48, 263 (2002).

[2] J. Reichel, Microchip traps and Bose–Einstein condensation, Appl. Phys. B 75, 469 (2002).

[3] R. Folman, P. Treutlein, J. Schmiedmayer, Fabrication of Atom Chips, in "Atom Chips" (Book by Wiley-VCH, 2011), Eds. Vladan Vuletic and Jakob Reichel.

[4] Quantum Information processing with neutral particles, Special issue in the journal of Quantum Information Processing (http://www.springer.com/physics/journal/11128), Eds. Ron Folman and Howard Brandt.

[5] S. Aigner, L. Della Pietra, Y. Japha, O. Entin-Wohlman, T. David, R. Salem, R. Folman and J. Schmiedmayer, Science 319, 1226 (2008).

[6] Ramon Szmuk, M.Sc. Thesis, Ben-Gurion University of the Negev (2011)