Physicists from the Max Planck Institute of Quantum Optics in Garching have developed a novel quantum gate, an essential component of quantum computers. A future quantum computer would be able to handle certain types of tasks far faster than any classical computer. As a central element of their quantum gate, the Max Planck physicists are using an atom trapped between two mirrors of a resonator. By reflecting the photon off the resonator with the atom, they are able to switch the state of the photon. Moreover, the gate operation can entangle the atom with the photon. When quantum particles are entangled, their properties become interdependent. Entanglement opens up whole new horizons in information processing. The quantum gate recently presented by the Garching-based physicists makes it possible to design quantum networks in which information is transferred between remote quantum processors in the form of photons.
The purpose of the experiments is to explore ways to process data in the form of quantum bits, or qubits for short. Whereas classical bits only exist in the states of “0” or “1”, in qubits superpositions of these two states are possible. When several qubits are combined into a single unit – a phenomenon known as entanglement – it is possible to perform parallel calculations that would simply be inconceivable with conventional computers. “A quantum gate such as the one we have developed is an essential component in the construction of a quantum computer,” says Stephan Ritter, who heads the experiment.
A CNOT gate couples a control bit with a target bit: whether or not the control bit changes the state of the target bit depends on its state. All logic circuits required for quantum calculations can be realized with this logic element and a few other simple operations. Many such logic elements are needed to build a quantum computer. A quantum computer could, within a reasonable period of time perform intricate searches in databases that would take even the fastest computer today months to complete. In addition, a quantum computer could break the encryption commonly used today. To prevent eavesdroppers from gaining access to transmitted data, quantum information technology has a tried-and-tested trick up its sleeve: quantum cryptography, which stops spies from tapping information from a data line undetected.