One answer to the question of "Why don't we see any aliens?" prompted me to consider:
The computer was a comparatively simple design, what the natives would call “single core” Von Neumann architecture machine. But it made up for its simplicity in raw speed. More than that; its tremendous processing speed required simplicity.
The single central processing unit was made up of patterns of excitement and interference of quasiparticles that were created and herded by a barrel-shaped spool of carefully doped carbon nanotube.
Within the central hollow—inside the barrel—was a core of carefully arranged atoms that stored bits as the orientation of electron spin. In the most general terms, the information storage was using the same principle as the native’s “hard drives”, albeit at a smaller scale. This spin crystal was three dimensional, not flat; and the domains were single electrons. All told, it stored about a petabyte of raw data. In operation, a hologram-like interference pattern played over the entire volume of the core, and the precise pattern allowed it to select individual spin states from the crystal and combine them with simple logic operations, to produce a “bright” or “dark” spot representing the result, on a third location. Intense beams of magnetic flux converged on the result spot in the crystal at the exact moment, allowing the result to be stored as a spin state.
The concept of the three dimensional interference pattern playing over the entire memory naturally allowed only one operation to access it at a time. But the speed of the logic was limited only by the speed of light across the diameter of the barrel, and this severely limited the processing speed to 3 × 1012 logical operations on bits per second. That was not quite enough for the task it was engineered for, and differences between actual verses theoretical maximum performance made the gulf even wider.
So the builders got clever and coaxed more speed out of the system. Processing could be pipelined to some extent, operating in waves that move across the volume, if the data is suitably arranged in space. Many operations operate on words larger than a bit, so this picks up much of the slack and allows, for example, addition of two words to be done in a wave that picks up each bit position in turn, rather than processing one bit at a time.
Another speed boost comes from doing the bulk of the processing over a smaller volume in the center of the volume. Only data at the outer edges would need the full speed-of-light delay across the span. So data was arranged suitably and cached in successive layers like an onion. This computer was a tenth of a millimeter across, and weighed about a microgram. Nearly an order of magnitude more mass was needed to shield and isolate it, so its quantum heart would not feel any whispers from the outside universe.
The craft housing the computer was a hundred times the mass of the shielded computer. The budget for structural members was horded, trying to keep everything as light as possible, yet still be strong and durable. The housing was packed with necessary systems, including sensors, propulsion, power generation, communications, and internal command and control systems. Everything was threaded with tubing for delivery of nanomachines and raw materials for self repair.
The craft, whose total mass was about one milligram, was roughly camouflaged as a member of the local fauna. Any of the locals, sentient or not, who saw it would assume it was some kind of mosquito.
They're here, but we have not noticed them because they are the size of a mosquito.
Story-wise, so far, so good: the alien ("Bill") is an uploaded personality in this tiny craft, studying here in the field. He gets cut off from his peers, and needs to make contact with a human to get help. Local guy discovers Bill after some adventure, they work out a communication mechanism, and strike a deal. Sprinkle with side plots and allegories.
So, my question is, how can Bill get someone's attention, and convince him that he's an intelligent being?
(Block quote above is from a 2011 draft of a short story by myself. It is reproduced here for reference only. Excerpt ©JMD, all rights reserved. I need to label this usage as I'll want to publish it some day.)
I'll discuss the micro vehicle itself in another Q. But, the capabilities of the vehicle are crutial to this question. So some ground rules:
Power is chemical, so energy throughput is about the same as an animal that size. This affects how much it can lift and manipulate.
Senses physically possible in such a small form include those things that real animals can do, but work to the limit of what's physically possible, sometimes an order of magnitude better than what evolved.
Vision is acute in spectra and any properties of a photon received, but naturally limited by the size of the eyes.
Smell is good, collecting and analyzing molecules along the length of the body. Again, acute in data extracted, but limited in quantity of air sampled.
Radio is problematic. Receiving might be possible based on what I've read about a single nanotube receiver, but doesn't have antenna size or power to transmit on anything used by Eartly technology.
Emitting coded light would be the camera in reverse: few photons, precicely tuned. A receiver at some distance needs to know to look for an exact frequency and pulse structure, to distinguish it from background light. It might use properties of light not employed by Earthly technology, like orbital angular momentum or entanglement with reference photons stored in the receiver.
Hearing is limited due to small size and mass. Maybe a lateral line pressure sensor can sense pressure waves in the direction of the body length, so better than a real animal. There is no mechanism to produce sound, but the incedental sound made by the mechanism (flying) is like a mosquito.
Bill doesn't have nanotechnology to mess with outside objects or amplify the actions. His capabilities are those of a mosquito, with human-level intelligence. Any fancy tech will require prior arrangement and can't be used for the initial contact scenario.
