# How can quantum radar be disturbed?

## Quantum control

We are already making use of quantum effects in many everyday objects. Now physicists are trying to control them in a targeted manner and thus enable completely new applications.

Technology based on quantum mechanics has been around us for a long time. For example, the transistor found on every computer chip is based on an understanding of quantum mechanical processes in semiconductors. Without the discovery of the laser, which is based on the quantum theory of electromagnetic radiation, there would be no blue-ray player and without high-precision atomic clocks there would be no cell phones with GPS. We are currently on the threshold of a second technological revolution: quantum effects are not just used, but actively controlled.

Quantum physics as child's play

But what exactly does quantum control mean? A classic children's game offers an analogy for this. The task is to tilt a wooden maze in such a way that you steer a small metal ball through the maze. In the analogy, the metal ball is supposed to represent a tiny particle, for example a single atom or a small molecule. Unlike the macroscopic sphere, the microscopic particles have both particle and wave properties. You could also imagine the ball as a drop of water, mathematically described by a wave function.

The drop can now spread or even split in the labyrinth and be in several places at the same time - a property known as superposition. Only when a measurement is made does the drop turn into a metal ball again, at a random point. The following applies: the more liquid there is at a certain point in the labyrinth, the more likely it is that the bead will appear there. If several droplets are involved, they can either flow through each other undisturbed - or they can interact with one another and merge into a single drop, which is known as entanglement. If you measure the position of one ball in this case, the position of the other ball is automatically determined.

If the movements and interactions of the drops or their wave functions could now be controlled in a targeted manner, all the beads could appear anywhere in the labyrinth. This would be possible, for example, by replacing the wooden floor with a rubber membrane and guiding the drops along the desired path by skillfully pushing and tugging. Transferred to reality, such quantum control would have far-reaching consequences and would open up new technical possibilities - with the potential to revolutionize our everyday lives.

### Control of molecular reaction dynamics

An example of such an application is the control of molecular reaction dynamics. The chemical properties of molecules depend on the position and movement of the so-called valence electrons. These electrons are located in the outermost orbitals of the atoms that make up the molecule. According to our analogy, electrons correspond classically to metal spheres and quantum mechanically to droplets or a wave function that describes their location. The positively charged atomic nuclei form an electrical potential and thus the rubber membrane with which the wave function of the negatively charged electrons can be manipulated. A laser can now distort this potential and thus bring the wave function into a new shape.

Bond of two atoms

The idea of ​​using laser pulses to control chemical reactions in a targeted manner originated in the mid-1980s. The first experimental implementations were made in the late 1990s. Until recently, however, applications were limited to breaking chemical bonds within a molecule. In 2015, Liat Levin from the Israel Institute of Technology and her colleagues succeeded for the first time in creating a chemical bond between two magnesium atoms in a controlled manner. This makes it possible to control chemical reactions in such a way that the reaction products, i.e. the molecules generated, remain in the desired state.

Quantum information processing is another area where quantum control is fundamental. The computing power of microchips has doubled about every 18 months since the 1970s. In the meantime, the electronic components on a chip are only a few atomic distances apart, so that quantum effects can no longer be neglected. So far, the circuits on chips have operated using macroscopic electrical currents, which - depending on whether they flow or not - represent the ones and zeros. However, as the chips get smaller and smaller, there is no longer any room for the many electrons that make up these currents.

Any further progress will therefore take place at the level of quantum mechanics, with bits now becoming quantum bits, or qubits for short. The operation of a logic circuit corresponds to the development of a quantum state over time, which can be described by the wave function. Scientists use electric or magnetic fields to guide the wave function accordingly - or, in the analogy of children's play, to bend the membrane accordingly.

### Quantum information processing

A quantum computer would not only be faster than a conventional computer. By taking advantage of superposition and entanglement, calculations could be carried out that would not be practicable on a classic computer. One example is searching through a large amount of unsorted data. A classic computer would have to look at each entry individually to determine whether it is the one it is looking for. A quantum computer, on the other hand, could dissolve the wave function across all elements at the same time. By appropriately pushing and distorting the quantum computer, precisely the surges in the quantum fluid can be produced that ensure that the overall wave function of all qubits builds up over the database element sought. After a measurement, the “metal ball” would most likely appear directly on the element being searched for.

At the moment we are still very far from a commercial quantum computer. Practical implementations are still limited to a few qubits. In order to be able to exploit quantum mechanical phenomena, the quantum computer must be almost completely isolated from the environment. Physicists also refer to the unwanted loss of quantum properties as decoherence. Sooner or later it will happen in any case - so the scientists not only have to control the temporal development of the qubits in exactly the right way, but also have to do it as quickly as possible. Modern methods of quantum control make it possible to achieve the so-called quantum speed limit: the maximum speed with which a wave function can be moved from one quantum state to another.

D-Wave computer microchip

In the meantime, it is no longer just academic groups who are researching quantum computers; companies like Google have also invested large sums in the implementation of such a computer. Because with it, the growing databases could be searched efficiently. The space agency NASA is already using an early prototype from the Canadian company D-Wave to search for Earth-like exoplanets. And secret services are interested in quantum computers because they could decipher current encryption with almost no effort. On the other hand, quantum effects also enable encryption methods that are in principle unbreakable - so-called quantum cryptography. The security of a quantum communication channel, which can be a simple fiber optic cable, is based on the quantum mechanical entanglement of light quanta that are sent as a signal over the cable.

Such entangled photons also play an important role in the closely related field of quantum sensors: Researchers use quantum effects to measure lengths or electrical and magnetic fields much more precisely than with traditional techniques. In this way, for example, the resolution of radar systems or the accuracy of GPS, i.e. global navigation satellites, could be improved.

If it is possible to fully control a quantum system, this also opens up the possibility of using it as a “quantum simulator” - and thus researching the properties of another, not directly accessible quantum system. This idea came up in the early 1980s and goes back to the physicist Richard Feynman. Their implementation would lead to enormous advances in materials science, for example in the form of photovoltaic systems with significantly higher efficiency. It could also allow the simulation of complex molecules in our body and thus lead to new medical breakthroughs.

### Magnetic resonance

Magnetic resonance tomography (MRT), which safely and reliably delivers detailed images of the human anatomy, has already arrived in everyday medical practice. It is based on the principle of nuclear magnetic resonance and thus also directly on quantum mechanics. Because it takes advantage of the fact that some atomic nuclei have their own angular momentum or spin and, associated with this, a magnetic moment. In a magnetic field, the spins perform a gyroscopic motion that can be measured. The signal can be used, for example, to infer the type of surrounding tissue or the oxygen content in the blood.

Magnetic resonance imaging

One of the most amazing uses of magnetic resonance is in direct imaging of brain functions, known as functional magnetic resonance imaging. The active areas of the brain are measured via tiny changes in the oxygen content in the blood supply. Quantum control could help maximize image contrast for oxygenated and deoxygenated blood in future generations of MRI machines. Marc Lapert from the Technical University of Munich and his colleagues recently determined the physical limits and suggested experimental ways to reach them.

The common thread through all of these technologies is the central challenge of being able to control the wave function of a quantum system in any way. To achieve this goal, scientists use computer simulations in advance to determine how a manipulation - analogous to the distortion of the plastic membrane - affects the dynamics of the system. With the help of mathematical optimization processes, the virtual interventions are iteratively improved until the wave function corresponds to the desired result.

However, the optimization does not necessarily have to be performed by a computer. Jacob Sherson's team at Aarhus University in Denmark maps the computational problem onto a computer game and lets people from all over the world tinker with it: Without knowing it, the players control what are known as optical tweezers that grasp the localized wave function of an atom and leads to another place in order to interact there with another atom. The strategies found by the players are then used directly in Sherson's laboratory to manipulate atoms with lasers at extremely low temperatures.

It has surprisingly been found that the strategies of the players are in many cases better than those of the best current optimization algorithms. This success raises the question of whether player strategies can be analyzed and incorporated into improved algorithms. Perhaps by combining human intuition with the bundled efforts of hundreds of researchers and the best computer programs, it will be possible to develop quantum control to its full potential - and thus initiate a new technological revolution.