High-resolution imaging via quantum remote sensing

The resolution of images obtained using a coherent light source is limited as a result of the classical limit imposed by diffraction. This limit can, however, be overcome by taking advantage of quantum fluctuations in the electromagnetic field of light, also known as quantum noise, to enable an increase in image resolution. One effective method is to make these fluctuations less than the standard quantum limit without violating the principle of uncertainty. When the quantum fluctuations of light are less than the fluctuations of a coherent light source, the light is referred to as squeezed light or, more precisely, quantum optical field squeezed light.^1,2

After 10 years of experimental research, we have developed a method of generating squeezed light using a dual-wavelength laser and an injection pump. The signal is split into two beams, of which one beam passes through a beam splitter into a mode cleaner cavity to act as the local source in balanced homodyne detection. The other beam passes though an optical isolator and then a mode-matching lens. The pumped light passes though an optical isolator and is then mixed with the light from the lens and the signal light in an optical parametric amplifier to generate squeezed light, using balanced homodyne detection for monitoring. A nonlinear crystal is used to tune the optical parameters of the squeezed light.

In our efforts to develop an imaging device based on quantum remote sensing,^{3, 4} we have produced a complete prototype that operates by quantization of the light field. This prototype consists of a control module, a module for generating squeezed light, and an optical beam expander to generate a homogenized beam for transmission to the target. The received signal is processed via an optical imaging system, a detector, and a signal acquisition system for subsequent display on a computer. The prototype uses near-IR radiation, provides a spatial resolution twice as high as is possible with classical imaging, and has a noise compression of 6dB and a continuous working time of more than 60min. It has a length of 1.3m, a width of 0.8m, and a height of 0.3m, and weighs less than 50kg.

Figure 1 shows a comparison of the imaging resolutions obtainable at the same distance with the prototype and a conventional coherent light source of equivalent power. We can see that the definition and edge resolution using quantum imaging are much higher than those with coherent optical imaging, which proves that imaging with quantum optical field squeezed light can overcome the limits of quantum noise and the diffraction limit. As the imaging resolution is higher than with coherent radiation, quantum imaging enables images of remotely sensed objects to be transmitted, received, processed, stored, and displayed while carrying more detailed information about the objects than is possible with conventional methods.^{3, 5,6}

Figure 1. Experimental results show that quantum imaging provides higher resolution than coherent optical imaging at the same distance.

In our next step, we propose using quantum optical field squeezed light in satellite-borne applications for long-distance transmission through the atmosphere, because quantum noise in a vacuum can reduce the energy loss due to compression of light, which affects the image quality. In designing an imaging system for satellite-borne sensing, we aim to use an injection of squeezed light, which is coupled with a laser that enters the optical receiver. By this method, the noise spectrum of the weak signal is modulated by the high-voltage squeezed light, which improves its signal-to-noise ratio. Our proposed system comprises a quantum remote sensing imaging system, a data preprocessing system, a computer and external equipment, and a controller. The quantum remote sensing imaging system includes a transmitter, a receiver, a quantum enhancement module, and a detector. The quantum enhancement module consists of a device for injecting squeezed light and a device for amplifying quantum noise (see Figure 2).

Figure 2. Schematic diagram of proposed imaging system for use in satellite-borne quantum remote sensing.

The quantum noise of the weak echo signal is suppressed by injecting squeezed light via the device in the receiver and using the squeezed light field instead of the vacuum field. The device for amplifying quantum noise solves the problem of low detection efficiency. By using a phase-sensitive amplification method, we expect to obtain an imaging effect that is close to that of an ideal detector, so that imaging with high detection efficiency can be achieved by amplifying quantum noise.