Coherent quantum state storage and transfer between two phase qubits via a resonant cavity

Catching the quantum bus Microfabricated superconducting circuit elements can harness the power of quantum behaviour for information processing. Unlike classical information bits, quantum information bits (qubits) can form superpositions or mixture states of ON and OFF, offering a faster, natural form of parallel processing. Previously, direct qubit–qubit coupling has been achieved for up to four qubits, but now two independent groups demonstrate the next crucial step: communication and exchange of quantum information between two superconducting qubits via a quantum bus, in the form of a resonant cavity formed by a superconducting transmission line a few millimetres long. Using this microwave cavity it is possible to store, transfer and exchange quantum information between two quantum bits. It can also perform multiplexed qubit readout. This basic architecture lends itself to expansion, offering the possibility for the coherent interaction of many superconducting qubits. The cover illustrates a zig-zag-shaped resonant cavity or quantum bus linking two superconducting phase qubits. One of two papers that demonstrate the communication of individual quantum states between superconducting qubits via a quantum bus. This quantum bus is a resonant cavity formed by a superconducting transmission line of several millimetres. Quantum information, initially defined in one qubit on one end, can be stored in this quantum bus and at a later time retrieved by a second qubit at the other end. As with classical information processing, a quantum information processor requires bits (qubits) that can be independently addressed and read out, long-term memory elements to store arbitrary quantum states 1 , 2 , and the ability to transfer quantum information through a coherent communication bus accessible to a large number of qubits 3 , 4 . Superconducting qubits made with scalable microfabrication techniques are a promising candidate for the realization of a large-scale quantum information processor 5 , 6 , 7 , 8 , 9 . Although these systems have successfully passed tests of coherent coupling for up to four qubits 10 , 11 , 12 , 13 , communication of individual quantum states between superconducting qubits via a quantum bus has not yet been realized. Here, we perform an experiment demonstrating the ability to coherently transfer quantum states between two superconducting Josephson phase qubits through a quantum bus. This quantum bus is a resonant cavity formed by an open-ended s