Entangled Photons are Used to Enhance the Sensitivity of Microscope.
Authors: Takafumi Ono, Ryo Okamoto, Shigeki Takeuchi
Research Institute for Electronic Science, Hokkaido University, Japan,
The Institute of Scientific and Industrial Research, Osaka University, Japan.
We demonstrated a microscope whose sensitivity is enhanced by using quantum entanglement -- over the limit set by the conventional (classical) light illumination. This is the first experimental demonstration of the application of entangled photons for microscopy.
Quantum entanglement is a unique feature of quantum particles, like photons, electrons, and so on. Quantum entanglement was first introduced by Schrödinger, and later a famous debate on it occurred between Einstein and Bohr; Einstein called it `spooky action at a distance’. Now, quantum entanglement is attracting attention as the resources for quantum information technologies like quantum cryptography and quantum computation. We demonstrated that quantum entanglement is useful not only for such information technologies, but also in other broader fields, like microscopy.
Figure 1: A schematic image of the entanglement-enhanced microscope.
Some years ago, we reported the experiment of four photon interference with high visibility -- enough to beat the standard quantum limit for the phase sensitivity . In that experiment, we used so called `NOON’ state, a path-entangled state where N-photon state is either in one of the two paths (and 0 photons in the opposite path). We demonstrated the quantum interference fringe using a four-photon NOON state with a high-visibility (91%) that was enough to beat the standard quantum limit of the phase sensitivity.
Perhaps the next natural step is to demonstrate entanglement-enhanced metrology. Among the applications of optical phase measurement, the differential interference contrast microscope (DIM) is widely used for the evaluation of opaque materials or biological tissues. The depth resolution of such measurements is determined by the signal-to-noise ratio (SNR) of the measurement, and the SNR is in principle limited by the standard quantum limit. In the advanced measurements using DIM, the intensity of the probe light is tightly limited for a non-invasive measurement, and the limit of the SNR has become a critical issue.
In our recent work , we proposed and demonstrated an entanglement-enhanced microscope, which is a confocal-type DIM where an entangled photon pair source is used for illumination. An image of a glass plate sample, where a Q shape is carved in relief on the surface with a ultra-thin step of ~17 nm, is obtained with better visibility than with a classical light source. The signal-to-noise ratio is 1.35±0.12 times better than that limited by the standard quantum limit. The success of this research will enable more highly sensitive measurements of living cells and other objects, and it has the potential for application in a wide range of fields, including biology and medicine.
Figure 2: (a) Atomic force microscope (AFM) image of a glass plate sample (BK7) on whose surface a Q shape is carved in relief with an ultra-thin step using optical lithography. (b) The section of the AFM image of the sample, which is the area outlined in red in a. The height of the step is estimated to be 17.3nm from this data. (c) The image of the sample using an entanglement-enhanced microscope where two-photon entangled state is used to illuminate the sample. (d) The image of the sample using single photons (a classical light source).
We believe this experimental demonstration is an important step towards entanglement- enhanced microscopy with ultimate sensitivity, using a higher NOON state or other quantum states of light. There are some other related works harnessing such nonclasical light for metrology[3-5].
 Takafumi Ono, Ryo Okamoto, Shigeki Takeuchi, “An entanglement-enhanced microscope”. Nature Communications, 4, 2426 (2013). Abstract.
 Tomohisa Nagata, Ryo Okamoto, Jeremy L. O'Brien, Keiji Sasaki, Shigeki Takeuchi, “Beating the standard quantum limit with four-entangled photons”. Science, 316, 726–729 (2007). Abstract.
 Andrea Crespi, Mirko Lobino, Jonathan C. F. Matthews, Alberto Politi, Chris R. Neal, Roberta Ramponi, Roberto Osellame, Jeremy L. O’Brien, “Measuring protein concentration with entangled photons”. Applied Physics Letters, 100, 233704 (2012). Abstract.
 Florian Wolfgramm, Chiara Vitelli, Federica A. Beduini, Nicolas Godbout, Morgan W. Mitchell, “Entanglement- enhanced probing of a delicate material system”. Nature Photonics, 7, 28–32 (2013). Abstract.
 Michael A. Taylor, Jiri Janousek, Vincent Daria, Joachim Knittel, Boris Hage, Hans-A. Bachor, Warwick P. Bowen, “Biological measurement beyond the quantum limit”. Nature Photonics, 7, 229–233 (2013). Abstract.