Stable schrodinger cats

Perpetual state between death and life

"Schrodinger’s cat" is a thought experiment, which should clarify the paradoxes of the quantum world. The cat is both dead and alive in a closed experimental arrangement up to observation. The reason why we do not observe such phenomena in reality is that the quantum state is always rapidly destroyed by interaction with the environment. For a quantum computer, however, the state of Schrodinger’s cat is required. Now, two teams of physicists have succeeded in creating six and two quantum states, respectively. eight ions, respectively. to keep in the "cat state.

In classical computers, information units are stored in the form of bits, i.e. 0 or 1. The interconnection of these bits makes the computational processes possible. Quantum computers, on the other hand, are the dream of the computer future, because they can be used to calculate much more efficiently. The drastically higher performance is achieved by the quantum bits (qubits), superpositions of 0 and 1, so-called superpositions (quantum computers). The superposition of quantum mechanical states is the basis of the quantum computer and the thought experiment "Schrodinger’s cat".

Schrodinger’s cat is dead and alive

The physicist Erwin Schrodinger clarified the exotic qualities of the quantum world with the Schrodinger’s cat paradox in 1935 to illustrate the exotic qualities of the quantum world. It is about the superpositions of different states. In the example this is clarified with the probability of the decay of an atom. Schrodinger formulated it like this:

A cat is locked up in a steel chamber, together with the following Hollen machine (which must be secured against the direct access of the cat): in a Geiger’s number tube there is a tiny amount of radioactive substance, so little that in the course of an hour perhaps one of the atoms decays, but just as probably also none; if it happens, then the number tube responds and actuates via a relay a small hammer, which smashes a colb with hydrocyanic acid. If you have left this whole system to itself for an hour, you will say to yourself that the cat is still alive, if in the meantime not an atom has decayed. The first atomic decay was poisoning them. The Y-function of the whole system was expressed in such a way that in it the living and the dead cat are mixed or smeared in equal parts.

In the everyday world we do not observe Schrodinger cats, because the quanta constantly interact with the environment. The cat is not released from its purgatory only at the moment of the Meng, but already by a reaction with the environment like e.g.B. with air molecules or the daylight. This effect of quantum state decay is called decoherence.

The six beryllium ions in the cat state, animation

For quantum computers superposition and a stable entanglement are necessary. Entanglement is a quantum mechanical state and means that a pair of photons has the same properties, even over coarse distances. The two are like intertwined, just entangled. If one photon of such an entangled pair is changed in its properties, then the second photon also changes automatically and immediately, regardless of how far away it is. Albert Einstein once described this effect as "spooky remote action"; quantum physicist Jeff Kimble paraphrases, "entanglement is when you tickle one particle and the other laughs."

Towards quantum computers

Quantum information processing unites the theory of quantum mechanics with the practice of information technology. The idea is already more than 30 years old, but practical applications are only now slowly coming into view (reading superconducting quantum bits simultaneously). Fundamental to this are quantum systems such as atoms, ions or photons, which can be influenced in such a way that they store or process information. These systems must be very well isolated to prevent interactions with the environment that lead to destabilization (decoherence) of the necessary quantum states. Quantum information processing thus relies on techniques that shield all external influences extremely well.

Science has now made further progress towards quantum computing. In the current ie of the science journal Nature, two teams of researchers publish their results of ion systems in the stable quantum state. Dietrich Leibfried and colleagues at the National Institute of Standards and Technology (NIST) succeeded in entangling six beryllium ions together so that they rotated both clockwise and counterclockwise (corresponding to the cat being both alive and dead). "It is very difficult to control six ions precisely long enough to perform such an experiment," explained Dietrich Leibfried. The ions were in the "cat state" – like the poor animal in the trap. Trapped were the ions in an electromagnetic trap, deeply cooled and controlled by ultraviolet laser light. Laser pulses were used to de-individualize the self-rotation, the electrically charged atoms spinning in unison, so to speak.

One run of the experiment lasted a millisecond, the cat state lasted a twentieth as long: 50 microseconds. To be sure, the team ran the test tens of thousands of times. This is a new record, last year a German-Chinese physics group succeeded in interlocking five photons, the smallest units of light (World record: Heidelberg physicists teleport the quantum state of five photons).

Eight calcium ions caught in a Paul trap.

In the new experiment, the ions are interlocked in a controlled way, which is different from the experiment of the second team of scientists published in Nature. Leibfried explains:

During the process, the ions "talk" to each other simultaneously, as in a conference call. The movement in synchronism can be imagined as the "telephone line". The Austrian experiment is more like a series of individual phone calls to the "boss", or movement.

The team led by Hartmut Haffner of the University of Innsbruck and the Institute for Quantum Optics and Quantum Information (IQOQI) created a convoluted state of up to eight calcium ions. Electrically charged atoms were trapped in ion traps, confined in a vacuum by electromagnetic forces, and manipulated one by one with precisely focused laser pulses. In the process, the particles entered a so-called W state, which remains stable even when individual ions drop out (W state generation and effect of cavity photons on the purification of dot-like single quantum well excitons).

The real difficulty in the experiment was to prove that the particles are in fact entangled with each other. Around 650.000 quantities were performed to describe the eight quantum bits (qubits) by numbers. The measurement process alone took more than ten hours, the calculation of the numbers and the graphical implementation kept a high-performance computer of the university busy for several weeks. This duration already shows the superiority of quantum information processing over conventional computers. "What happens to the eight qubits in about a millisecond can only be calculated and characterized in many hours with a normal computer", explains co-author Rainer Blatt.

The Innsbruck quantum physicists succeeded for the first time in the world in realizing a quantum byte (Qubyte) through the controlled entanglement of the eight ions – an important step towards quantum computing. For years, Austrian quantum physicists have successfully used ion traps as potential building blocks (qubits in a triple pack). Hartmut Haffner explains that the state of encryption created by his team is fundamentally different from that of the Americans:

We were able to fully characterize the quantum state and can therefore give precise information about possible decoherence mechanisms. In addition, the W state we have produced has the interesting property that it retains its quantum nature even when ions are lost. Additionally, we have developed and applied new methods to accurately characterize the encryption properties.

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