A Practical Approach by Esperanza Cuenca Gómez and Pablo Martín Ramiro
The importance of radars
Before we discuss what a quantum radar is, let's explore how classical radars work. In a few words, a classical radar is a detection system that uses electromagnetic waves to determine the distance and velocity of objects relative to the radar location. This system emits radio waves that can reflect off objects and bounce back to the origin, where they are detected and used to calculate objects' locations and speeds. Radars are broadly used across various industries, especially in the aviation and naval sectors, in applications such as aircraft or ship detection, air traﬃc control, air defense systems, weather monitoring, etc.
Although classical radars have shown a lot of value in these applications, they are limited by their short range and resolution, signal and background noise, and interference with other signals. Therefore, the most valuable information they can provide about an object is its approximate location and speed under certain conditions.
What is a Quantum Radar?
How can we, then, improve the accuracy and range of classical radars? This is where quantum and, more specifically, quantum illumination comes into play. The concept of quantum illumination is central to quantum radars. First proposed by Lloyd in 2008 , quantum illumination exploits the quantum property of entanglement for object detection. In this paradigm, a sender prepares two entangled systems, called signal and idler, using two entangled photons. In the same spirit as conventional radars, the signal is sent to probe the presence of an object in a target region, while the idler is retained at the origin. If there is an object in the target region, the signal would be reflected, and it would eventually be measured back at the origin. The potential reflection of the signal is thus combined with the retained idler system in a joint quantum measurement with two possible outcomes: object present or object absent. In practice, this process would be repeated multiple times with different signal-idler pairs.
The key idea behind this approach is that the signal sent is quantum-correlated with the retained idler system. In this way, in the case it's reflected, the correlation with the idler can be easily identified and distinguished from all the uncorrelated background thermal photons that are also received by the detector. This quantum feature makes quantum illumination very efficient. Indeed, exploiting quantum illumination with N bits of entanglement increases the signal-to-noise detection ratio by a factor of 2^N, an exponential improvement over unentangled illumination. As a result, the main advantage of a quantum radar is that it greatly enhances the probability of success in detecting a target compared to a classical radar, where entanglement is not used.
Challenges and experimental demonstrations
MIT Professor Seth Lloyd and others  have provided solid theoretical formulations that describe how quantum illumination could be used to build quantum radars. However, building functioning experimental prototypes based on this phenomenon is quite challenging. In 2019, researchers at the Institute of Science and Technology Austria created the world's first known prototype of a quantum radar . A few months later, two research groups from the University of Glasgow  presented the first full imaging experimental demonstration of a quantum radar based on quantum illumination, using entangled photon pairs and overcoming the presence of noise and losses. While recent developments have been encouraging, experimental demonstrations are currently far from applicable in industries.
Potential applications of Quantum Radars
The benefits of quantum radars are pretty straightforward: higher accuracy and range enable better navigation and air-traffic control systems in the aviation industry, for example. These examples are valid for the naval industry as well. A crucial aspect here is that we are not only talking about civil aviation and civil naval industry but also military and naval industries.
An important limitation of classical radars is that they cannot distinguish the object they detect. Specifically, they can not distinguish between civil and military aircraft (though this is solved using navigation and air-traffic control protocols, for example, transponder codes). On the other hand, a quantum radar could provide enough detail for radar systems to identify the object based on physical characteristics. An F18 fighter, for example, could be identified by the sweep of its wings, the shape of its nose, and the number of engines .
Due to their energy efficiency, quantum radars are extremely difficult to detect. This is an inherent advantage for the army, which has this technology, because it allows detection of all adversarial objects (airplanes or missiles, for example, even those with stealth capabilities) and, thus, the establishment of the appropriate defensive and offensive measures. Nevertheless, quantum radars are not able to detect hypersonic weapons .
The military applications of quantum radars have profound implications from both geopolitical and ethical perspectives. In the following section, we explore the ethical considerations.
The potential proliferation of quantum radars in the near future could have significant, fatal consequences. Although quantum radars could be employed in the same sectors as their classical counterparts (air, sea, and terrestrial traffic control, navigation systems, etc.), their main potential application could be in warfare systems. In addition, if we imagine a future in which quantum radars are a ubiquitous technology in the defense sector, this would encourage the development of even more deadly warfare armament to surpass the capabilities of quantum detection systems. Therefore, quantum radars represent a hazardous destabilizing effect with potentially catastrophic consequences.
With great power comes great responsibility. Over the last decades, powerful technologies have played a key role in reshaping society and the economy on a global scale. Although most technologies are not inherently good or bad, there are choices over how they are applied to impact society positively. Technology offers a wealth of exciting opportunities if managed responsibly. With this in mind, tech companies, governments, and non-governmental organizations should work together to create a set of minimum standards that must be met in tech governance, ethics, transparency, and safety.
A technology like quantum radars has high strategic importance and significant geopolitical consequences. For this reason, little is known about research in quantum radars in the defense sector. However, it is suspected that the US, China, and possibly Russia are also actively researching in this field. Any technological system with such a significant social impact must be kept accountable to the areas and contexts in which it is applied. Therefore, legal and policy resources should be created to ensure international organizations closely regulate technologies such as quantum radars. Crucially, important sanctions should be established for misusing this and other increasingly powerful technologies.
At the development level, it is crucial to integrate ethical and social impact considerations into the research, development, and deployment of all new technologies. This requires a commitment of the entire tech community to design systems with a human-centric perspective at their core. This is critical to ensure that technological innovations are aligned with a common good, which helps humanity progress in all respects.
 S. Lloyd, Quantum Illumination, Cambridge, 2008.
 S.-H. Tan, B. I. Erkmen, V. Giovannetti, S. Guha, S. Lloyd, L. Maccone, S. Pirandola and J. H. Shapiro, "Quantum Illumination with Gaussian States," Physical Review Letters, vol. 101, no. 25, 2008.
 S. Barzanjeh, S. Pirandola, D. Vitali and J. Fink, "Microwave quantum illumination using a digital receiver," Science Advances, vol. 6, no. 19, 2020.
 T. Gregory, P.-A. Moreau, E. Toninelli and M. Padgett, "Imaging through noise with quantum illumination," Science Advances, vol. 6, no. 6, 2020.
 K. Mizokami, "How Quantum Radar Could Completely Change Warfare," Popular Mechanics, 26 August 2019. [Online]. Available: https://www.popularmechanics.com/military/a28818232/quantum-radar/. [Accessed 9 August 2022].
 A. Gagaridis, "Warfare Evolved: Quantum Radar," Geopolitical Monitor, 2 July 2021. [Online]. Available: https://www.geopoliticalmonitor.com/warfare-evolved-quantum-radar/. [Accessed 9 August 2022].
About the Authors
Esperanza Cuenca Gómez is Head of Strategy and Outreach at Multiverse Computing. She is a digital transformation enthusiast with more than 10 years of experience in consumer finance and banking, and more than 5 years in strategy and operations consulting. Quantum mechanics has always fascinated Esperanza, so she decided to study and research in quantum computing and communications. As an engineer, Esperanza sees applied science and engineering as ways to build new technologies, solve problems, and contribute to society. Esperanza also serves as Head of Change Navigation at the Quantum Strategy Institute.
Pablo Martín Ramiro is a Machine Learning Engineer at Multiverse Computing, where he works with strategic clients to provide tangible value through the intersection of machine learning, artificial intelligence and quantum technologies. Pablo holds a PhD in Physics from the Universidad Autónoma de Madrid, where his research was focused on using machine learning techniques to study fundamental particle physics. Pablo has also been a research affiliate at Berkeley Lab, working on unsupervised methods for anomaly detection. In addition to his PhD, Pablo also holds a MSc in Physics at Durham University and a BSc in Physics at Universidad Complutense de Madrid.