#### Entangled Quantum States are not a Fantasy

This year's Nobel Prize in physics is shared by Alain Aspect, John F. Clauser and Anton Zeilinger for their groundbreaking experiments with entangled photons, which ultimately opened the door to the brand-new research field of quantum information. Entangled quantum states are non-separable multi-particle states that follow quite naturally from the mathematical structure of quantum theory, but their existence was long highly controversial – after all, they would have the consequence that quantum theory is a non-local theory. Only the famous Bell's inequality made it possible in principle to pursue the question of a possible non-locality of quantum physics in an experimental way. And it is precisely in these experiments that this year's Nobel Prize winners played a decisive role. Thanks to their ingenious experiments to verify Bell's inequality, we can now say with almost certainty: quantum physics is non-local, and entangled states are not a fantasy!

Thanks to the pioneering work of researchers such as John Francis Clauser, Alain Aspect and Anton Zeilinger, quantum computers, quantum teleportation or quantum cryptography are no longer mere theory but concrete topics in current fundamental research – with an incredible potential for science and industry. Since individual photons or atoms must be precisely controllable for all applications in these fields, temperatures close to absolute zero and a highly reliable ultrahigh vacuum are required. Accordingly, high-tech vacuum valves are very important. VAT has been working closely with key research teams in the field for many years, and has a corresponding wealth of experience when it comes to developing ultra-fast quantum computers, secure quantum cryptography methods, highly sensitive quantum sensors and other exciting quantum information devices. For example, VAT supplies high-precision vacuum and gas dosing valves for the cooling units needed in such quantum experiments. A special feature of VAT solutions: In so-called multi-valve assemblies, several valve functions are combined in one compact housing. In addition to simplified assembly, this reduces the installation volume and also the risk of possible leakage points.

This year's physics Nobel laureate Anton Zeilinger in his laboratory. Right at his side: a VAT high precision UHV vacuum gate valve (series 10.8).

*Source: VAT archive*

In order to adequately appreciate the fundamental importance of the contributions of this year's Nobel laureates, we invite you to take a short journey into the rather bizarre world of multi-particle quantum systems...

**Tracing the Thoughts of Heisenberg and Schrödinger**Even for a single particle such as a photon, quantum physics is quite a challenge to the human mind. Think, for example, of Heisenberg's uncertainty principle, which states that – completely contrary to "normal" intuition – you can know either the location or the velocity of the quantum particle, but never both quantities simultaneously. Rather, these two quantities – like energy and time, or other comparable pairs of variables – are interwoven via an inequality postulated by Werner Heisenberg in 1927, which requires that the measurement precision of the location multiplied by the measurement precision of the velocity must always be greater than the so-called Planck's action quantum. As the name already suggests, this nature constant has a tremendously small value of about 10-35 Js. This is also the reason why the Heisenberg uncertainty principle has no noticeable consequences on the macroscopic level. If, for example, you park your car in the evening, i.e. you set its speed to 0 km/h, you can also say with absolute certainty where the car is. (Whether you still remember it the next morning is another matter.) But the deeper you venture into the microcosm of molecules, atoms and elementary particles, the louder Heisenberg's inequality demands its right to exist. The laws of classical physics simply prove to be insufficient to adequately describe the phenomena occurring in the quantum world. A typical example is the wave nature of quanta and the resulting possibility that two quantum states can superpose and thereby add or cancel each other depending on their phase relationship (constructive or destructive interference, respectively). This superposition principle is a pure quantum phenomenon: Try to shoot two footballs at each other in such a way that they annihilate each other!

Another peculiarity of the quantum world, which is particularly difficult to digest, is that you usually run into a dead end there with either/or-conceptualizations. Because as long as a quantum particle unfolds freely, it is impossible to come up with a definite state description in the sense of "This particle has the property XY". The only information that can be elicited from the particle is a probability that it is in a certain state. But unless you look concretely – i.e., you make a measurement – you have to settle for a strangely vague probability statement. The famous theoretical physicist Erwin Schrödinger summed up this unintuitive quantum behavior very aptly when he devised a thought experiment in which a cat is ingeniously poisoned and is subsequently both alive and dead until you make a measurement, thus forcing the cat to make a definite statement regarding its state. Fortunately, these probability phenomena average out as you move into the macroscopic world. You would probably sleep pretty badly if you knew that there was only an 80 percent probability that your car was still in your garage and a 20 percent probability that it was already out for a little spin, right? No wonder, then, that the pioneers of quantum physics also had extensive debates about how our common notion that a system is in a certain state (whether we look or not) can be carried over into the quantum world. But in the end, all the debating did not help, because experiments showed time and again that the properties of a quantum particle are actually determined only when someone bothers to measure them! It is only by the measurement that the probability cloud of different state possibilities collapses to a so-called eigenstate and that you can "call the particle by a name".

Quite confusing, isn't it? But if you now look at multi-particle quantum systems, it becomes even more confusing! The symbol [0>●[1> can still be interpreted quite intuitively as a two-particle state whose first particle is in the eigenstate [0> and whose second particle is in the eigenstate [1>. Even with a superposition of the form [0>●[0> + [0>●[1>, you can still make a clear separation between the two particles by factoring out: [0>●[0> + [0>●[1> = [0>●([0> + [1>), in other words, the first particle is in the eigenstate [0> and the second sharing is in the (superposition) state [0> + [1>. But try to decompose the state [0>●[0> + [1>●[1> in a similar way into two clearly separated particle spaces. No chance! From a theoretical point of view, this is a perfectly legitimate state of the two-particle system, but the fact that the single particles seem to mix somehow runs diametrically against our common sense.

Such states are called entangled states. Their essential characteristic is that they can no longer be reduced to separate single systems. In the case of [0>●[0> + [1>●[1>, all you know is that the first particle must take the state [0> if the second particle takes the same state [0>, and that the first particle must take the state [1> if the second particle takes the state [1>. (Otherwise, mixed terms of the form [0>●[1> would have to appear in the state function). This rather mathematical consideration has far-reaching philosophical consequences. For this purpose, imagine that you would create the entangled state described above and then let the two particles fly so far away from each other that they can no longer exchange any information by conventional means. (Mind you that in classical physics, information cannot travel faster than the speed of light, so there must be a distance that ensures such an "information gap"). Now the disturbing thing is that the two entangled particles nevertheless "know" about each other! In fact, as soon as you measure the state of the one particle, instantaneously the state of the second particle is also fixed, no matter how far the particles are away from each other. In other words: If entangled states really exist, then quantum theory is a non-local theory.

**Anton Zeilinger and Colleagues have shown its possible**

The idea of having to sacrifice locality – a sacred cow in classical physics – in the quantum realms met with bitter resistance from numerous physicists at the time. Albert Einstein, for example, long derisively dismissed quantum entanglement as a "spooky action at a distance," and theoretical physicists such as David Bohm developed alternative quantum theories based on hidden variables that could be used to explain the quantum world locally again. But all these heated discussions were for a long time only on philosophical-theoretical ground. Then came the hour of John Stewart Bell, a theoretical physicist from Ireland, who derived an ingenious mathematical (in)equation made it possible to verify by experiment whether the quantum world is non-local, or whether quantum objects have additional local properties in the sense of Bohm and Einstein. With the help of Bell's inequality, the question whether quantum physics is a local theory or not suddenly became unambiguously answerable, at least in principle.

Nevertheless, it took more than a decade before physicists were finally technically able to put Bell's inequality to the experimental test. And it was precisely in this test that this year's Nobel Prize winners played a decisive role. In 1974, for example, John Francis Clauser and his colleagues conducted an experiment that for the first time clearly pointed to a violation of Bell's inequality, i.e., they confirmed the non-locality of quantum physics. Of course, the researchers of that time wanted to be absolutely sure about such a fundamental question, which is why many more experiments were performed around Bell's inequality in order to close all possible loopholes. Some of these experiments go back to the account of the now Nobel laureate Alain Aspect, who in 1982 could again exclude the existence of hidden parameters in quantum physics. Then, in 1999, the third laureate Anton Zeilinger and his team provided an even more comprehensive proof of the non-locality of quantum physics.

Another topic that is closely associated with the name of Anton Zeilinger is quantum teleportation. Beam me up, Scottie! – the idea of beaming objects, or even living beings, from one place in the universe to another in no time at all has always fascinated people. No wonder Anton Zeilinger was the talk of the town around 1998, when his research team succeeded in teleporting a quantum state for the first time. Here, too, entangled states and their non-local properties play a central role. In a quantum teleportation, the particle to be teleported is first entangled with another particle, and then the second particle is brought to the destination of the teleportation. By means of a sophisticated measurement procedure, it is then achieved that the state of the original particle is lost and the second particle assumes precisely this state. Mind you, we speak of single photons and most primitive properties like their polarization state, and from there it is still a rather long way until more complex matter or even living beings can be beamed around in this way. Nevertheless, Anton Zeilinger and his colleagues have shown

*that*and how it could be possible. And that alone is certainly worthy of a noble prize!

Without the pioneering work of Clauser, Aspect and Zeilinger, also quantum computers would still be nothing but theory. In contrast to classical computers, quantum computers calculate on the basis of quantum mechanical states, called qubits. Those qubits are not either zero or one as in conventional computers, but can also be additive superpositions of the two basic states. As a result, you can apply all the computational steps that would otherwise be applied to just one state to an entire bundle of states. This parallelism in data processing opens up the space for completely new applications in information technology – with breathtaking computation speed. And not just in theory, but already in very practical applications, as you can learn about in detail in the article Quantum Computing Relies on High-Tech Vacuum Valves!

**VAT Technology Supports Research Teams in Quantum Physics and Technology Worldwide**

Of course, experiments involving entangled states are an enormously subtle undertaking. After all, the quantum states of the individual photons or atoms must be precisely controllable, since entanglement phenomena depend to a large extent on how stable the phase relationship between the quantum particles are (technical term: coherence). One collision of the photon to be teleported with an air molecule, and the experiment is ruined! Accordingly, the hardware of quantum computers is also extremely prone to interference: Any change in the environment, no matter how small, can drastically falsify the calculation result. In short, without continuous cooling to temperatures close to absolute zero and without a highly reliable ultrahigh vacuum, all the experiments mentioned so far would have been doomed to failure. Looking to the future, it is safe to say that if quantum computers are to find widespread application, they will be imperatively dependent on high-tech vacuum valves. It is a good thing that VAT has been working closely with many of the leading research teams in the field since the birth of quantum information technology. So also in 1998, when VAT valves were prominently featured in Anton Zeilinger's famous entanglement swapping experiment to demonstrate the teleportation of entangled states.