In recent years there has been a major scientific thrust to harness quantum phenomena to increase the performance of a wide variety of classical devices ranging from computation to sensing, and considerable progress has occurred in the analysis and design of photonic-based quantum sensors (e.g. quantum radar and quantum lidar). These quantum sensors exploit the non-classical correlations embedded in entangled quantum states to improve the extraction of target information in low signal-to-noise ratio environments.
LIDAR (Light Detection and Ranging) is a device that can acquire three dimensional coordinates of both manmade and naturally occurring targets by sending laser pulses to the targets and calculating distances by measuring the received time. They are rapidly gaining maturity as very capable sensors for number of applications such as imaging through clouds, vegetation and camouflage, 3D mapping and terrain visualization, meteorology, navigation and obstacle avoidance for robotics as well as weapon guidance.
For modest-range (1–100 km) terrestrial applications under clear-weather conditions, LADAR (LAser Detection And Ranging) systems offer superior spatial resolution, when compared to microwave radars, owing to their use of much shorter wavelengths. When atmospheric turbulence can be neglected, the spatial resolution of such a system is generally limited by the Rayleigh resolution of its receiving optics (1.22λ/D, where λ is the LADAR wavelength and D is the aperture diameter for an unobscured circular entrance pupil) and the signal-to-noise ratio (SNR). Their phase sensitivity is limited by the shotnoise Δϕ = 1/ sqrt( n) , where n is the mean number of photons that is proportional to the intensity.
Quantum remote sensing can achieve quantum super-resolution beating the classical Rayleigh diffraction limit in resolution by exploiting entangled photons, similarly they can possess quantum super-sensitivity by beating the classical shot noise limit to sensitivity.
In general, there are two types of standoff quantum sensors:
Quantum Illumination: These sensors use two-photon entangled state. Entanglement in quantum mechanics refers to particles whose individual states cannot be written without reference to the state of other particles. Such states are said to be non-separable, and measurements of the state of one entangled particle alter the state of all entangled particles. Entanglement takes place when a part of particles interact physically. For instance, a laser beam fired through a certain type of crystal can cause individual light particles to be split into pairs of entangled photons.
Phase-sensitive ampliﬁcation (PSA) can enhance the signal-to-noise ratio (SNR) of an optical measurement suffering from detection ineﬃciency. This increased SNR improves LADAR-imaging spatial resolution. Optical parametric amplfier (OPA) based receiver has been proposed to realize the target detection advantage of quantum illumination (entangled state interrogation) in high noise, high loss environements.
NOON-State Quantum Interferometry: These sensors are basically made of two mirrors and two 50/50 beam splitters. A far away target can be considered as a large arm in the interferometer. The eﬀect of entanglement is to introduce super-oscillations, faster by a factor N than the non-entangled photons where N represents the number of non-entangled photons used by the sensing device. The use of NOON entangled states helps the detector to beat the Rayleigh limit by a factor of N.
Both types of standoff quantum sensors offer a unique set of advantages and disadvantages. For instance, Quantum Illumination sensors operate better in the presence of strong atmospheric attenuation and background solar radiation. On the other hand, NOON-state quantum interferometers have better performance in the absence of atmospheric attenuation.
Quantum sensor’s advantages survive entanglement breakdown
Members of the Optical and Quantum Communications Group at MIT’s Research Laboratory of Electronics have demonstrated that entanglement can also improve the performance of optical sensors, even when it doesn’t survive light’s interaction with the environment.
In the MIT researchers’ system, two beams of light are entangled, and one of them is stored locally—racing through an optical fiber—while the other is projected into the environment. When light from the projected beam—the “probe”—is reflected back, it carries information about the objects it has encountered. But this light is also corrupted by the environmental influences that engineers call “noise.” Recombining it with the locally stored beam helps suppress the noise, recovering the information.
The local beam is useful for noise suppression because its phase is correlated with that of the probe. If you think of light as a wave, with regular crests and troughs, two beams are in phase if their crests and troughs coincide. If the crests of one are aligned with the troughs of the other, their phases are anti-correlated
In experiments that compared optical systems that used entangled light and classical light, the researchers found that the entangled-light systems increased the signal-to-noise ratio—a measure of how much information can be recaptured from the reflected probe—by 20 percent. That accorded very well with their theoretical predictions.
But the theory also predicts that improvements in the quality of the optical equipment used in the experiment could double or perhaps even quadruple the signal-to-noise ratio. Since detection error declines exponentially with the signal-to-noise ratio that could translate to a million-fold increase in sensitivity.
“This is a breakthrough,” says Stefano Pirandola, an associate professor of computer science at the University of York in England. “One of the main technical challenges was the experimental realization of a practical receiver for quantum illumination. Shapiro and Wong experimentally implemented a quantum receiver, which is not optimal but is still able to prove the quantum illumination advantage. In particular, they were able to overcome the major problem associated with the loss in the optical storage of the idler beam.”
“This research can potentially lead to the development of a quantum LIDAR which is able to spot almost-invisible objects in a very noisy background,” he adds. “The working mechanism of quantum illumination could in fact be exploited at short-distances as well, for instance to develop non-invasive techniques of quantum sensing with potential applications in biomedicine.”
Quantum Space based Space surveillance for Space Security and Missile Defence
Wide-Area Surveillance (WAS) involves the continuous monitoring of an extended region of the surface of the earth, or a volume of space above or beyond the earth, typically to identify elements of a discrete set of speciﬁc targets or events. Example WAS applications from the Strategic Defense Initiative (SDI) era include the Boost Surveillance and Tracking System (BSTS) and the Space Surveillance and Tracking System (SSTS)
The increasing reliance of critical infrastructure on the availability of satellites (e.g., GPS) creates a critical need for sensing capabilities to detect threats posed by natural and man-made debris orbiting the earth as well as small objects (e.g., micro-meteoroids) originating from beyond the earth.
The collision threat from space-based targets is daunting because it involves Variable Size / Signature Targets (VSTs). Objects can be of widely varying absolute and apparant size, e.g., a small benign object near the earth may have the same optical proﬁle as a large object further away, and can have vastly diﬀerent material properties, e.g., one object may be highly visible at some wavelengths while another object of the same size may be nearly invisible in that spectrum. The challenge posed by this problem for classical sensor technologies motivates consideration of non-classical quantum-based sensors.
Marco Lanzagortaa and Jeffrey Uhlmann have proposed a quantum-based approach for wide-area, low-power detection of small objects which can threaten spacecraft and satellites. They described a basic system of orbiting quantum sensors which could be used for a variety of missions such as missile defense, Earth defense against comets and asteroids, and debris tracking for the safe removal and passage of spacecrafts.
The proposed system consists of a set of spaceborne multispectrum quantum sensors operating in the optical and/or X bands to achieve super-resolution (beyond the Rayleigh diffraction limit) and super-sensitivity (beyond the shot noise limit) to provide a quadratic increase in detection sensitivity compared to classical alternatives.
Due to the nature of quantum sensing, these satellite network systems would have reduced cost and higher performance than what can be achieved with today’s technology.
NOON state inferometery based Quantum Satellite network
Even though there have been proposals to circumvent the degrading effects of the environment, NOON state interferometry seems to be better geared to operate in low attenuation environments. Such is the case of outer space.
“Indeed, over the first 150 km, the density of the atmosphere decreases by 9 order of magnitude. And outside Earth’s atmosphere, the density of outer space is of less than one hydrogen atom per cubit meter. This implies a photon mean free path of about 1023 km for signal photons traveling in outer space. As a consequence, one should consider the possibility of deploying quantum sensors based on NOON interferometry in orbit around Earth,” say Marco Lanzagortaa and Jeffrey Uhlmann
Space-Based Quantum Radar for Small NEOS
“In the absence of atmospheric attenuation, quantum sensors are able to have better performance than their classical counterparts. First, it is possible to have super-resolution, as NOON state interferometry can beat the Rayleigh limit by a factor of N. This is an important feature to detect and discriminate small objects. And second, these quantum sensors are able to beat the shot noise limit by a quadratic factor.”
“To take advantage of a quantum sensor with these characteristics, we have investigated the theoretical possibility of having a network of space-borne quantum sensors in orbit around Earth. These satellites would be composed of a power source, a communication system, a quantum sensor, and any required maneuvering boosters.” The sensing system is relatively simple, as it would be reduced to a quantum interferometer and a source of NOON states. If all the raw detection data is broadcasted to a ground station, the satellite itself does not require sophisticated data analysis hardware. In such a case, the principal source of continuous energy consumption is the generation of entangled photons.
Furthermore, these quantum sensors can achieve a O( √ N) improvement in performance, or equivalently, a O( √ N) reduction in number of photons necessary to achieve the same level of performance as traditional classical systems. This means that in principle quantum sensors have much lower power requirements, hence significantly lower expected cost compared with most classical alternatives.
Recent advances in quantum mechanics have enabled many enhanced imaging technologies. Entangled photon-number (N00N) states have allowed Heisenberg-limited phase measurement and led to the development of LIDAR systems with quantum-enhanced resolution. Even without the use of entanglement, the sensitivity of optical ranging and pointing systems has been improved beyond the classical limit by the use of quantum resources.
Mehul Malik, Omar S. Magaña-Loaiza and Robert W. Boyd in Quantum-secured imaging have proposed and demonstrated a quantum enhancement to optical ranging and imaging systems that will make them secure against intercept-resend jamming attacks.
A common concern for active imaging systems today is the threat of jamming, where extraneous or false information is sent to the receiver in order to fool it. More sophisticated methods of jamming are being developed which allow the imaging signal to be intercepted, manipulated, and resent. This allows the object being imaged to bely its actual position or velocity, or even create a false target.
“Using quantum states of light modulated in polarization in an imaging system, we can provide security against such methods of jamming. Quantum-secured sensing based on similar principles has previously been demonstrated for the purpose of sensing intruders using entanglement and interaction-free measurements,” write Mehul Malik, Omar S. Magaña-Loaiza and Robert W. Boyd in Quantum-secured imaging
“Our secure imaging technique is based on a modified version of the BB84 protocol of quantum key distribution (QKD). Instead of an eavesdropper (Eve) located between the sender (Alice) and the receiver (Bob), we now have a jamming object (Jim) at one end and Alice and Bob at the other”.
By virtue of being in the same location, Alice and Bob already share information. Instead, they now use this information to securely query Jim by encoding it in the polarization of a stream of photons. This leaves the position and time degrees of freedom of the photons free for the purpose of obtaining an image of Jim. If Jim were to try to jam this system by intercepting and resending the photons with false position or time information, he will introduce statistical errors in the polarization encoding that will give away his jamming attempt. As in QKD, security is guaranteed due to Jim’s inability to measure a photon simultaneously in two conjugate polarization bases.