The tools and techniques which will eventually bring quantum computing within our grasp are undergoing fast and furious development – so much so that it is unusual to hear of a completely new approach.

Just before Christmas, however, came a report of a new line of attack toward making optical quantum computers. The method is so simple (at least in principle) that it is rather amazing it hasn’t been tried before.

Optical quantum computers are essentially very complex interferometers, systems of optics that superimpose two or more light waves in such a way that they interfere. Measuring various characteristics of the interference pattern conveys a great deal of information about the two light waves.

The quantum connection is that when individual photons are passed through an interferometer, a single photon can interfere with itself, leading to changes in the quantized light.

Interferometers acting on entangled photons can implement complex mathematical functions. Generating entangled photons is not a simple process, but systems of optical interferometers have recently produced sets of as many as 8 entangled photons. Unfortunately, accomplishing this task requires precise placement and orientation of perhaps 100 individual optical elements on a large optical bench. To accomplish this in an environment with variable temperature, air currents, and vibrations is an experimental tour de force, but not a practical accomplishment.

Professor W.A. Miller of Florida International University and his collaborators have taken a large step outside this box. In work reported on arXiv, they detail implementing interferometers for optical quantum computing in the form of holograms of actual interferometers!

Holograms are somewhat analogous to sound recordings – they are records of light scattered from a set of objects in such a way that the optical effects of the original objects can be reconstructed without using the original objects. Thus, a hologram of an interferometer, when reconstructed, has the same optical properties as does the original interferometer! (There is an excellent description of how holograms work on Wikipedia - scroll down to the section “How Holography Works.”)

Using holograms instead of physical optics offers many advantages. Once an interferometer has been recorded as a hologram, the hologram is far more robust with respect to environmental degradation of its optical properties. The instrument is frozen in place, without vibrations, thermal expansion, mechanical sagging, or other environmental effects. In addition, a near-perfect hologram of optical apparatus can be created from a computer simulation of the optics even if physical implementation is too difficult. Another rather obvious advantage is that picking up a hologram is much easier than picking up 1,000 pounds of lab equipment!

Although the idea is rather obvious, the devil is in the details. To create an accurate hologram of such complex optical apparatus requires a special type of holographic plate. For a three-dimensional hologram, a thick emulsion is required. The required thickness cannot be supplied using standard photographic emulsions, which are actually suspensions of light-sensitive silver salts in a colloidal gel.

The common alternative is known as a photo-thermo-reactive (PTR) glass, which is usually a silicate glass doped with metallic nanoparticles. Proper exposure of this glass to light allows one to record high resolution three-dimensional refractive index variations permanently into the glass. As such, PTR glasses are ideal for recording high-quality three-dimensional holograms. The particular PTR glass suggested by the authors is a proprietary glass manufactured by OptiGrate of Orlando, Fla. This is one of the few PTR glasses with sufficient thickness and resolution to allow the recording of 3-D interferometers.

Once the desired optical functionality is recorded in the form of a hologram, the hologram is simply placed in the optical path where the physical optical train would normally be. In practice, one often makes measurements at intermediate positions in the optical train to force the desired quantum functionality to appear. Such intermediate measurements cannot be made within a hologram. Fortunately, the rules of quantum computing tell us that the final quantum wavefunction is independent of when the measurements are taken. So in holographic quantum computing the measurements which define the result are performed on the light emerging from the hologram.

Although in theory one could record any conceivable optical apparatus in a hologram, in practice there are limits on the complexity that can be recorded. Simple quantum algorithms can be carried out in a single hologram, but more complex algorithms would require multiple holograms. (Multiple holograms can be stacked to combine their optical functions.) A problem linked with this one is that a holographic apparatus cannot be altered, so a holographic quantum computer cannot be reprogrammed.

The operational relationship of holograms to general-purpose quantum computers will probably be analogous to the relation between electronic logic chips and a general-purpose electronic computer. A logic chip contains the circuitry for a simple logical operation, and such logic chips can be combined into a general-purpose programmable computer. If quantum computing holograms do become part of the practical quantum computers of the 2020s, they will probably fill roles as quantum logic chips in a larger apparatus. Key parts – important parts – but not the whole story. Still, wanting to make a cart and merely inventing the wheel isn’t bad for a day’s work!