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Combining quantum information communication and storage

14 Feb 2013

Aalto University researchers from the O.V. Lounasmaa Laboratory and the Department of Applied Physics have successfully connected a superconducting quantum bit, or qubit, with a micrometre-sized drum head and transferred information from the qubit to the resonator and back again. The group’s results were published in Nature. The O.V. Lounasmaa Laboratory is part of the Academy of Finland’s Centre of Excellence in Low Temperature Quantum Phenomena and Devices.

"This work represents the first step towards creating exotic mechanical quantum states. For example, the transfer makes it possible to create a state in which the resonator simultaneously vibrates and doesn’t vibrate," says Mika Sillanpää, professor at the Department of Applied Physics, who runs the research group.

Quantum_systems_framed.jpg

Picture: Schematics of the measurement setup. The figure on the left shows a superconducting quantum bit, or, qubit (green) coupled to a micromechanical oscillator (spring). The state of the qubit is measured using a microwave cavity (blue). On the right, the state of the qubit has been transferred to mechanical motion such that the oscillator is in two places simultaneously.

A qubit is the quantum-mechanical equivalent of the bits we know from computers. A traditional bit can be in a state of 0 or 1, while a qubit can be in both states at the same time. In theory, this inconceivable situation allows for a quantum calculation in which the operations are performed simultaneously for all possible numbers. In the case of a single qubit, this means zero and one, but as the number of qubits increases, the amount of possible numbers and simultaneous calculations grows exponentially.

The quantum state of a qubit is very fragile and easily disturbed between and during the operations. The key to successful quantum calculation is being able to protect the qubit state from disturbances in the environment.

"In this case, the qubit state can be stored as vibration, thus preserving the state for much longer than the qubit itself. The resonator also functions as a mechanical quantum memory, which is something that an ordinary memory can't do," explains Juha Pirkkalainen, who is doing his dissertation on the topic.

The work combined the achievements of both winners of this year’s Nobel Prize for Physics. The qubit state was measured using a superconducting cavity in the same way that Serge Haroche measured atoms, and the qubit state was also linked to mechanical movement as in David Wineland’s experiments. In contrast to these larger-scale measurement arrangements, the experiment at the O.V. Lounasmaa Laboratory was prepared for a tiny silicon microchip. This made it possible to cool the sample to near absolute zero temperatures and then use microwaves.

hybrid_circuit_cavity_Juha_Juvonen_BIG_crop-1.jpg

Picture: Quantum systems with different types of degrees of freedom can intertwine, forming hybrid entangled quantum states with intriguing properties. Researchers at Aalto University have now merged three quantum systems: they used a superconducting quantum qubit, or, qubit (spheres) interacting with two different resonant cavities. A low frequency phonon cavity (vibrating string) was used as a storage of quantum information from the qubit, whereas an electrical microwave cavity (represented by the mirrors) acted as a means of communicating to the outside world. The idea could be used as a building block in the emerging field of quantum information and communication.

The study published in Nature was performed at Aalto University’s O.V. Lounasmaa Laboratory. Some of the sample used in the measurements was produced at Micronova’s Nanofab cleanroom facilities. The project was financed by the Academy of Finland and the European Research Council ERC.

Link to the article: www.nature.com/nature/journal/v494/n7436/full/nature11821.html

More information

  • Professor Mika Sillanpää, mika.sillanpaa(at)aalto.fi, Department of Applied Physics, Aalto University School of Science
  • Juha Pirkkalainen, Doctoral Student, juha.pirkkalainen(at)aalto.fi, O.V. Lounasmaa Laboratory, Low Temperature Laboratory, Aalto University School of Science
  • O.V. Lounasmaa Laboratory website http://ltl.tkk.fi/wiki/LTL
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