Physicists have now experimentally observed two-path quantum interference using many different particles, including electrons, neutrons, atoms and simple molecules. It's worth checking out this very nice movie of interference fringes for electrons showing up one at a time.
To date the largest objects observed to engage in this quantum jiggery pokery are molecules with 60 carbon atoms arranged like the pattern on a traditional soccer ball and 48 fluorine atoms coated on the surface of that sphere. This object, with a mass of 2.7 x 10-24 kg, is over a million times more massive than a single electron (10-30 kg). Admittedly it is still very small—less than a ten-billionth of the mass of the smallest cell in the human body (a sperm cell; 10-13 kg).
Perhaps even more amazing is the recent observation of interference of the biological molecule porphyrin, a key ingredient in chlorophyll and some blood cells. A porphyrin molecule in effect has several "wings" that can flop about as if joined to the rest of the molecule by hinges. That makes it very unlike an elementary particle such as an electron or even the carbon-60 molecules, which are quite rigid. See the section titled "The wave nature of biomolecules and fluorofullerenes" for more about the carbon-60 and porphyrin experiments.
No one knows how large of an object can be made to interfere with itself. An experiment with carbon-70 molecules highlighted some of the difficulties. When the molecules were heated so that they were likely to emit infrared photons while they were passing through the slits, the contrast of the interference fringes decreased markedly. We can understand this perfectly using the concepts discussed in the main article: the emitted photons acted as which-path labelers, causing the interference to disappear. That is, in principle one could detect the photons and thereby ascertain which slit each molecule passed through. Remember, it's not necessary that anyone actually do that detecting—to destroy the interference it is enough that the photons simply exist.
More to Explore:
• View the slideshow of quantum erasure in action
• Discuss the experiment in the blog
• What You Will Need For the Experiment
• What Polarizers Do To Photons
• How A Quantum Eraser Works
• Notes on Polarizing Film
• Troubleshooting the Experiment
• More Experiments
• Answer to the 3-Polarizer Puzzle Featured in the Print Edition
• Whither Waves? More About Interference
• Cutting-Edge Experiments: Interfering Soccer Balls
• Delayed-Choice Experiments
• What Do the Quantum Particles Really Do?
• What is Being Erased?