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August 10, 2011

Diamond's Quantum Memory paving the way to quantum computer chips

The Quantum Chip: In the center, there is the microwave resonator and the dark diamond.

Two completely different quantum systems were successfully joined at Vienna University of Technology (TU Vienna). This should pave the way to feasible quantum-computer microchips. Microwaves have now been coupled to the quantum states of a diamond.

Physical Review Letters - Cavity QED with Magnetically Coupled Collective Spin States



We report strong coupling between an ensemble of nitrogen-vacancy center electron spins in diamond and a superconducting microwave coplanar waveguide resonator. The characteristic scaling of the collective coupling strength with the square root of the number of emitters is observed directly. Additionally, we measure hyperfine coupling to 13C nuclear spins, which is a first step towards a nuclear ensemble quantum memory. Using the dispersive shift of the cavity resonance frequency, we measure the relaxation time of the NV center at millikelvin temperatures in a nondestructive way.

Different Quantum Technologies in One Chip
For a long time, scientists have been looking for suitable building blocks to construct a quantum computer – but without much success. Several ideas for systems which can store quantum mechanical information have been put forward, but quantum information is usually very fragile and easily destroyed. A component of a computer has to meet different criteria. It should be able to switch its state very rapidly, and it has to conserve its quantum state for a sufficient amount of time, so that calculations can be carried out. “There is no single quantum system which meets all the requirements”, Johannes Majer says. He and his team coupled two completely different kinds of quantum systems, in order to use the advantages of both sides: Microwaves and Diamonds.

Photons and Diamonds

Our usual computers have a processor and a memory. The processor carries out fast calculations, the memory is supposed to remember the results for a long time. The relation between the two different quantum systems unified on one quantum chip at TU Vienna is quite similar: fast manipulations are possible due to a so called microwave resonator. Its quantum state is defined by photons in the microwave regime. This microwave resonator is coupled to a thin layer of diamond, in which quantum states can be stored.

Desirable Flaws

For jewellery, diamonds are supposed to be pure and flawless, but for quantum experiments, the opposite is required. Here, flaws in the diamond are desirable. When nitrogen atoms slip into the regular carbon structure of the diamond, the diamond becomes almost black, but it gains the ability to store quantum states. “We could show that in our quantum chip, quantum states can actually be transferred between the microwaves and the nitrogen-centers in the diamond”, Robert Amsüss (TU Vienna) explains. The more nitrogen atoms take part in this transfer of quantum information, the more stable the diamonds “memory” becomes.
Surprisingly, it turned out that also the angular momentum of the atomic nuclei can store quantum information. “This could be the first step towards a nuclear memory device”, Johannes Majer suggests. But first, the diamond quantum chip in its present form should be optimized. All the necessary parts are now there, creating the oppurtunity for reliable operations.

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