Photo: Up to 14 quantum bits were entangled in an ion trap.
Innsbruck physicists have set another world record: They have achieved controlled entanglement of 14 quantum bits (qubits) and, thus, realized the largest quantum register that has ever been produced. With this experiment the scientists have not only come closer to the realization of a quantum computer but they also show results for the quantum mechanical phenomenon of entanglement.
Since 2005 the research team of Rainer Blatt has held the record for the number of entangled quantum bits realized experimentally. To date, nobody else has been able to achieve controlled entanglement of eight particles, which represents one quantum byte. Now the Innsbruck scientists have almost doubled this record. They confined 14 calcium atoms in an ion trap, which, similar to a quantum computer, they then manipulated with laser light. The internal states of each atom formed single qubits and a quantum register of 14 qubits was produced. This register represents the core of a future quantum computer. In addition, the physicists of the University of Innsbruck have found out that the decay rate of the atoms is not linear, as usually expected, but is proportional to the square of the number of the qubits. When several particles are entangled, the sensitivity of the system increases significantly. “This process is known as superdecoherence and has rarely been observed in quantum processing,“ explains Thomas Monz. It is not only of importance for building quantum computers but also for the construction of precise atomic clocks or carrying out quantum simulations.
By now the Innsbruck experimental physicists have succeeded in confining up to 64 particles in an ion trap. “We are not able to entangle this high number of ions yet,“ says Thomas Monz. “However, our current findings provide us with a better understanding about the behavior of many entangled particles.“ And this knowledge may soon enable them to entangle even more atoms.
Some weeks ago Rainer Blatt’s research group reported on another important finding in this context in the scientific journal Nature: They showed that ions might be entangled by electromagnetic coupling. This enables the scientists to link many little quantum registers efficiently on a micro chip. All these findings are important steps to make quantum technologies suitable for practical information processing,” Rainer Blatt is convinced.
Being able to efficiently generate entangled quantum states involving 10 and more qubits opens a new range of applications. Our system represents the basic building
block for quantum simulation experiments to investigate complex mechanism such as the magnetic sense of birds , to perform exponentially compressed spinchain simulations, and to better understand cosmology and space-time. It may serve as a very well controlled testbed for fundamental questions in quantum physics such as the investigation of the cross-over from superpositions in quantum systems to defined states in macroscopic systems with GHZ states.
In conclusion, we have analyzed the decay of GHZ states in an ion-trap based quantum computer. We find a dependency that scales quadratically with the number of qubits and thus shows superdecoherence . This mechanism is present in every other experiment that relies on a phase reference for performing quantum information processing with energetically non-degenerate qubits.
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