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> Each photon carries one bit of information. More than a bit. Between its wavelength, its presence (or absence), polarization, and timing, a photon can carry

> Each photon carries one bit of information. More than a bit. Between its wavelength, its presence (or absence), polarization, and timing, a photon can carry a great deal more than one bit.

pdonis 6y ago
You can't measure all of these things simultaneously (the operators don't commute), so even if all these observables exist for a photon, they don't all carry independent information. You can only "read out" what you can get from one observable. In most quantum information experiments, that observable is polarization (more precisely, polarization in some particular direction), which is an exact one-bit measurement result. In radio telescope observations, it's typically amplitude and wavelength that are measured. I don't know that "the information content of a single photon" is really meaningful for such observations since they don't observe single photons, they observe the overall intensity at different wavelengths.
- Strilanc 6y ago
  Even accounting for the uncertainty principle, the amount of information you can transfer in principle with a single photon is far more than one bit. For example, you can distinguish the single photon hitting a screen at more than two different times, or at more than two different places, or at more than two different energies.
  - pdonis 6y ago
    > you can distinguish the single photon hitting a screen at more than two different times, or at more than two different places, or at more than two different energies. You can't transfer information by any of these means because you can't control the time or the location or the energy of the photon hitting the screen. The reason polarization is used to carry information in quantum computers is that you can control it, so you can use it to transfer information. But, as I said, a single photon can only be used to read out one bit in this way.
- cryptonector 6y ago
  Wavelength certainly has more than one bit.
  - pdonis 6y ago
    Measurements of wavelength don't measure single photons. They measure overall intensity, which amounts to making a single measurement that involves a huge number of photons.

> Each photon carries one bit of information. More than a bit. Between its wavelength, its presence (or absence), polarization, and timing, a photon can carry a great deal more than one bit.

pdonis 6y ago

You can't measure all of these things simultaneously (the operators don't commute), so even if all these observables exist for a photon, they don't all carry independent information. You can only "read out" what you can get from one observable. In most quantum information experiments, that observable is polarization (more precisely, polarization in some particular direction), which is an exact one-bit measurement result. In radio telescope observations, it's typically amplitude and wavelength that are measured. I don't know that "the information content of a single photon" is really meaningful for such observations since they don't observe single photons, they observe the overall intensity at different wavelengths.

Strilanc 6y ago
Even accounting for the uncertainty principle, the amount of information you can transfer in principle with a single photon is far more than one bit. For example, you can distinguish the single photon hitting a screen at more than two different times, or at more than two different places, or at more than two different energies.
- pdonis 6y ago
  > you can distinguish the single photon hitting a screen at more than two different times, or at more than two different places, or at more than two different energies. You can't transfer information by any of these means because you can't control the time or the location or the energy of the photon hitting the screen. The reason polarization is used to carry information in quantum computers is that you can control it, so you can use it to transfer information. But, as I said, a single photon can only be used to read out one bit in this way.
cryptonector 6y ago
Wavelength certainly has more than one bit.
- pdonis 6y ago
  Measurements of wavelength don't measure single photons. They measure overall intensity, which amounts to making a single measurement that involves a huge number of photons.