This poor legendary creature is half dead, half alive. It was brought into being at the beginning of the 20th century by Erwin Schrödinger, one of quantum theory’s founding fathers, and it has been exploited to showcase the strangeness of the theory ever since.

The German mastermind conceived the cat in a thought experiment. It was to illustrate the fact that objects can exists in a so called superposition of states, a core phenomenon of the microworld. Superposition was one of the most striking outcomes of the new science of quanta.

Schrödinger put his imaginary cat in a black box with a lethal gas. The gas is released when a radioactive particle decays. The problem is, that - according to a traditional interpretation of quantum mechanics - the particle is decayed and it isn't at the same time. This makes the cat dead and alive simultaneously. And it is only the act of the measurement that fixes its ontological status. Finding a satisfactory interpretation of this issue has become a major challenge of quantum theory.



Niels Bohr, the father figure of the new science, the Danish giant of physics, shares the blame that reality had to perish. He accepted quantum theory on its own terms, saying something like this: Do not waste your time in futile discussions about the meaning of quantum physics phenomena. Abstract wave functions and their collapses, probabilities, etcetera - that is the domain of science. Don't look for meaning. Who cares whether Schrödinger's cat is dead or alive?! Predictions are what we are after. The rest is just speculation. He used to say: "Shut up and calculate! We don't make philosophy here, it's physics." That became a mantra for several generations of physicists. Bohr overshadowed truly independent scientific thought for many years to come.



Although Albert Einstein gave an initial boost to quantum mechanics, he didn’t like the way it was pursued and developed, particularly by Niels Bohr. He detested the fact that it’s inherently random, that outcomes of experiments depend on how they’re performed. He thought quantum physics is faulty or, in the best case, unfinished. Reality must be independent from us, he said. "I like to think that the Moon is there if I am not looking at it", he teased his younger friends. That is why, together with Nathan Rosen and Boris Podolsky, Einstein invented the following thought experiment.

There are two specially prepared particles, such as photons or electrons. They are quantum, so we cannot know exactly their states, but they're bound in a very subtle way. Einstein, Podolsky and Rosen pointed out that, according to quantum theory, you can separate these entangled particles, send them to opposite ends of the Universe, and they’ll still act like one. They will keep this pan-time, pan-space entanglement, just like ghosts.

Einstein called it “spooky action at a distance”. He hated the idea of quantum entanglement. He was sure it was fiction. He thought the pure fact that one could predict such an outlandish outcome from quantum physics proved that the science was plain wrong, that we were missing something crucial. But it was Einstein who was wrong: quantum entanglement is something that’s easily created in every quantum lab in the world.



David Bohm was quite a complicated figure: independent, conflicted, and chased out of his job in the United States by McCarthyism. He revived the invention of Louis de Broglie who, in the ‘20s, suggested that one can remove the mysterious, causeless randomness of the quantum realm by introducing a wave that accompanies every single particle in the Universe. And Bohm went even further. If the conscious observer plays such a key role in measurement, then take him aboard the new theory, Bohm said. The Universe and the mind are one, in a way, according to this rebel. He became really fringy. Unfortunately Bohm's theory was Baroquely convoluted. Any particle's trajectory, any photon's or electron's fate depends on the configuration of all other particles in the entire Universe. In trying to rescue reality from non-existence, Bohm envisioned it being dreadfully complex.



John Bell was a real Dr Jekyll and Mr Hyde of physics. He was a talented particle physicist in the noble CERN, the European Organization for Nuclear Research in Geneva,by day, and a quantum rebel by night. He insisted on looking for a better, more complete quantum theory. He didn't share the prevailing, dominant attitude of "shut up and calculate” imposed by Niels Bohr. He followed Einstein's and Bohm's footsteps, coming up with a new test of reality.

His famous inequality, which changed the course of quantum mechanics, can be explained with an intuitive example. Suppose there are two kids playing a game in the park. Wherever they are, they both look around. If the first person one of them sees is a man, kid one raises his right arm; if it's a woman, he raises his left arm. Kid two does the same. You have two persons acting identically at different locations, but there is nothing mysterious because they agreed their behaviour in advance. In such a case, Bell's inequality is satisfied. Now let’s change the rule: if both kids see a woman, only one of them has to raise the left arm, the other must now raise their right. In this new game, one of the kids is in trouble. What one must do depends on what the other is seeing, and the kids can’t know that unless they communicate. Their answers cannot be pre-determined. In this case, Bell's inequality does not hold. We say that it is “violated". The question is, do quantum particles behave in a predetermined way? Bell assumed that reality exists independently of us

, that the proverbial Einstein's Moon simply is there, whether we look at it or we don't. From this assumption he derived his inequality. He thought nature would obey his inequalities, therefore showing that the mainstream quantum mechanics he hated so much was wrong. But soon it turned out that he was wrong! Experimentalists like John Clauser and Alain Aspect proved that quantum entanglement, the spooky action at a distance, defies his inequality.



Up until the 70s, cryptography followed a simple formula: to cipher and decipher any message required a single key. The tricky part was handling the key, which is supposed to stay secret. The revolution came when a new, cutting edge technique called public key distribution was developed. The name sounds somewhat self-contradictory, but it works. One key, the public one, is used to cipher a message, the second, the private one, to decipher it. Some very special mathematical functions that are easy to run one way and hard to reverse make the whole procedure extremely secure. Unfortunately, this security is the case only with classical computers. 

Quantum computing machines, when they’re constructed, will make cracking classical ciphers a breeze. But quantum theory also makes available one new cryptographic option - quantum cryptography, which distributes a key without exchanging any actual information. It’s an idea so compelling it was invented twice. In Charles Bennett’s and Gilles Brassard’s protocol, inspired by an idea of Stephen Wiesner, the sender encodes the key into the states of single photons. An eavesdropper is helpless when faced with this method. However he (or she) tries to listen, whatever he does to conceal his presence, he fails. Any act of eavesdropping disturbs the delicate, quantum states of the photons - and gives away an intruder.

Artur Ekert’s protocol was invented when he dusted off the forgotten paper by Einstein, Podolsky and Rosen. Ekert thought: what if we take those entangled pairs of photons and use them for sending a cryptographic key? Photons are sent to the parties who would like to communicate secretly, and their measurements on those photons create the random ones and zeros of the secret key. The parties can detect if anyone was eavesdropping by comparing some portion of their key. As long as nobody was tinkering with the photons, they will find strong correlations in their measurement results. Those correlations are absolutely unachievable in the classical case. That's the magic of quantum entanglement. If the correlations fall below some level, clearly defined by John Bell, it means the photons’ entanglement was broken by an interloper and the line is not secure. Every spying attempt becomes evident. The laws of nature guarantee it.