Entangled photonic microscope goes beyond the quantum limit

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Standard optical microscopes have surrendered their once dominant position at the forefront of scientific research to more advanced tools.

As we delve deeper into the microscopic world, photons just don’t provide enough detail when there are technologies like X-ray laser and electron microscopy available. Now a team of researchers from Hokkaido University in Japan may have found a way to use ordinary light to detect much smaller objects than has ever been possible before. The key to this technique is one of the more bizarre effects in quantum mechanics known as entanglement.

Most types of microscopy are limited by the Rayleigh diffraction limit. This principle simply states that light cannot be used to resolve a structure smaller than its own wavelength. So if you need to see something smaller than a few hundred nanometers (the shortest wavelength of visible light), a different type of electromagnetic radiation would be needed. X-ray is a popular choice as its wavelength is orders of magnitude shorter than visible light. Scientists have suspected for more than a decade that entanglement could allow photons to circumvent the Rayleigh limit, and that’s exactly what the Hokkaido University team has done.

The first step in imaging smaller objects with visible light is to generate entangled photons, which the researchers did with a special nonlinear crystal. This left them with pairs of entangled photons that were in opposite polarization states. When two particles are entangled, changes to one of them will be reflected in the other even if they are separated by incredible distances. Theoretically, these two entangled beams should be able to provide much more information about a surface.

In order to test the ability of entangled photons to increase image resolution, the team focused the entangled pairs at adjacent spots on a glass plate with a letter Q printed on it. The lettering was only 17nm taller than the surrounding plate, which should be virtually impossible to see with ordinary light. However, theentangled photons returned 1.35 times sharper images than the standard quantum limit — the lettering was completely legible.

Written By: Ryan Whitwam
continue to source article at extremetech.com

8 COMMENTS

    • In reply to #2 by RichardofYork:

      Mass media isn’t the place to learn quantum mechanics , they talk about weird and strange , photons are so small there are no gradients in the state or its position

      Good point, I quickly hit my comprehension event horizon with this stuff, but I also think these things do need to be widely disseminated as some of us muggles May be encouraged to look into it at greater depth. I feel you may be giving battle in vain on this one.

  1. This principle simply states that light cannot be used to resolve a structure smaller than its own wavelength. So if you need to see something smaller than a few hundred nanometers (the shortest wavelength of visible light), a different type of electromagnetic radiation would be needed. X-ray is a popular choice as its wavelength is orders of magnitude shorter than visible light.

    Gamma rays have even smaller wavelengths. Are they too dangerously full of energy to be used to examine even smaller scales?

    I’m not sure I fully understand how entangled photons are supposed to pick out finer details than regular photons. If the issue is that photon wavelengths are too large to pick out details, then the solution seems to be simply to go with a photon with a smaller wavelength and use that instead. What does entangling it with another photon for instantaneous feedback add to the procedure? Is the problem that the smaller wavelength photons are still too small for us to see? Is the second photon of a larger wavelength, and so able to magnify the tiny effects visited upon its smaller partner so that we can detect it?

    • In reply to #4 by Zeuglodon:

      What does entangling it with another photon for instantaneous feedback add to the procedure? Is the problem that the smaller wavelength photons are still too small for us to see? Is the second photon of a larger wavelength, and so able to magnify the tiny effects visited upon its smaller partner so that we can detect it?

      It’s actually to do with wave interference. The entangled photons don’t travel exactly the same distance, because of the surface’s height. When the photons arrive at a detector, they have a phase difference$ proportional to the difference in the distances they’ve travelled. That difference is a lot smaller than a full wavelength, but it’s a measurable phase shift. Based on the shift’s size, you know the distances’ difference, so a computer can calculate the shape of whatever’s been scanned.

      $ I’ll simplify the details to avoid complex numbers. You’ve probably seen a “sine wave” depiction of waves before. Let’s call one photon’s wave A sin kx, with x the distance travelled, kx in radians and a wavelength of 2pi/k. The other photon’s wave is A sin (kx+ky), and ky is the phase shift. Well, sin (kx) + sin (kx+ky) = 2 cos ky/2 sin (kx+ky/2). You can therefore infer cos ky/2 from the amplitude of the overall result. The reason lasers are so bright is because for them y=0, so the cosine factor doesn’t reduce the light’s intensity. If you’ve ever heard of “noise cancellation”, that’s based on an attempt to make the cosine as small as possible.

      • In reply to #5 by Jos Gibbons:

        In reply to #4 by Zeuglodon:

        What does entangling it with another photon for instantaneous feedback add to the procedure? Is the problem that the smaller wavelength photons are still too small for us to see? Is the second photon of a larger wavelength, and so able to magnify the tiny effects visite…

        Thank you, got it.

      • In reply to #5 by Jos Gibbons:

        Thank you for your reply. I just want to check I’ve got this, as quantum mechanics isn’t my strong point and I’ll admit I had to check the terms used before replying.

        Without entanglement, a photon with a small wavelength (say, an EM wave in the X-ray part of the EM spectrum, with a wavelength of one tenth of a nanometre) would still not be able to detect a difference at a scale much smaller than its given wavelength. Simply beaming two photons at the structure would do nothing to address this because their independent waves would create random patterns of coherence and incoherence, and thus drown out any discrepancy between them as they pass through the surface they’re scanning.

        By contrast, the entangled photons could have the difference in their wavelengths be as small as one hundredth of the photons’ own wavelengths, and all that matters is the disparity between the wavelengths. Moreover, entangled photons are perfectly synchronized such that any small discrepancies between their waves (the phase shift) can be traced back solely to the differences between their routes through the scanned object. This is because the quantum state of one can’t be described without affecting the other instantaneously and in a specific, corresponding way. If you know the quantum state and the equations needed to understand what they reveal about the photons’ paths, you can use this discrepancy to reconstruct the original material they passed through, similar to how you can discover the composition of the crust by sending waves through it in systematic explosions and recording their properties and timing when they reach you (albeit when reflected off the underground material rather than when they emerge from the other side).

        Thus, the key virtue of the entangled photonic microscope is that it can be used for smaller scales than the wavelength would allow while it also reduces noise in order to expose a meaningful phase shift. I doubt my account does justice to the way the quantum entanglement actually works, but have I at least found a helpful way of conceiving of the principles being used in the microscope?

        • In reply to #8 by Zeuglodon:

          Suppose K differs from k. Then sin kx + sin (Kx+Ky) = 2 cos ((Ky+(K-k)x)/2) sin (Ky+(K+k)x/2), which isn’t very helpful to calculate y. Thus “phase shifts” are only really useful when you look at waves of the same wavelength.

          From my reading of the paper, the trick isn’t to take photons of different wavelengths; it’s to do with photon polarisations. We state by creating entangled pairs of photons, each pair in a superposition of the states where both photons are horizontally polarised and where both are vertically polarised. This involves an even number of photons, say N. When the photons return from the sample, you count the numbers in horizontal and vertical polarisations. Whether these numbers are both even or both odd is a crucial part of interpreting the readings.

          It takes a lot of mathematics to show that, whereas the precision achievable with N unentangled photons is inversely proportional to the square root of N, the precision achievable with N entangled photons is inversely proportional to N. That’s better in principle (e.g. 1/100 is much smaller than 1/10). This experiment only entangled photon pairs, rather than all N photons together, so the improvement isn’t as large as it could be, but it’s a start.

          The photon pairs used in this experiment are the N=2 special case of “NOON states”: http://en.wikipedia.org/wiki/NOON_state#Applications

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