Coupling superconducting qubits via a cavity bus

Abstract

Superconducting circuits are promising candidates for constructing quantum bits (qubits) in a quantum computer; single-qubit operations are now routine 1,2 , and several examples 3,4,5,6,7,8,9 of two qubit interactions and gates having been demonstrated. These experiments show that two nearby qubits can be readily coupled with local interactions. Performing gates between an arbitrary pair of distant qubits is highly desirable for any quantum computer architecture, but has not yet been demonstrated. An efficient way to achieve this goal is to couple the qubits to a quantum bus, which distributes quantum information among the qubits. Here we show the implementation of such a quantum bus, using microwave photons confined in a transmission line cavity, to couple two superconducting qubits on opposite sides of a chip. The interaction is mediated by the exchange of virtual rather than real photons, avoiding cavity induced loss. Using fast control of the qubits to switch the coupling effectively on and off, we demonstrate coherent transfer of quantum states between the qubits. The cavity is also used to perform multiplexed control and measurement of the qubit states. This approach can be expanded to more than two qubits, and is an attractive architecture for quantum information processing on a chip.There are several physical systems in which one could realize a quantum bus. A particular example is trapped ions 10,11 in which a variety of quantum operations and algorithms have been performed using the quantized motion of the ions (phonons) as the bus. Photons are another natural candidate as a carrier of quantum information 12,13 , because they are highly coherent and can mediate interactions between distant objects. To create a photon bus, it is helpful to utilize the increased interaction strength provided by the techniques of cavity quantum electrodynamics, where an atom is coupled to a single cavity mode. In the strong coupling limit 14 the interaction is coherent, permitting the transfer of quantum information between the atom and the photon. Entanglement between atoms has been demonstrated with Rydberg atom cavity QED 15,16,17 . Circuit QED 18 is a realization of the physics of cavity QED with superconducting qubits coupled to a microwave cavity on a chip. Previous circuit QED experiments with single qubits have achieved 19 the strong coupling limit and have demonstrated 20 the transfer of quantum information from qubit to photon. Here we perform a circuit QED experiment with two qubits strongly coupled to a cavity, and demonstrate a coherent, non-local coupling between the qubits via this bus.Operations with multiple superconducting qubits have been performed and are a subject of current research. The first solid-state quantum gate has been demonstrated with charge qubits 3 . For flux qubits, two-qubit coupling 5 and a controllable coupling mechanism have been realized 7,8,9 . Two phase qubits have also been successfully coupled 4 and the entanglement between them has been observed 6 . All of these interactions h...

Entanglement and disentanglement in circuit QED architectures

“…At time t = τ off , the interaction is turned off again. As far as we are only interested in the entanglement dynamics of the cavities, we may trace out the qubit and ancilla degrees of freedom in (14). For τ off − τ on = 2π/g SW the resulting density matrix ρ cav,f takes the form…”

“…This would not only allow for performing quantum operations such as quantum gates, but also enable the creation and distribution of entanglement throughout the network. Indeed, a setup to couple two cavities via an ancilla qubit has been recently proposed in [11,12], while in recent experiments [13,14], two qubits have been entangled via one cavity.…”

Fast geometric gate operation of superconducting charge qubits in circuit QED

“…In particularly, the idea of placing superconducting qubits inside a cavity, i.e., the circuit quantum electrodynamics (QED), has been illustrated [2,3] to have several practical advantages including strong coupling strength, immunity to noises, and suppression of spontaneous emission.…”

“…So, how to suppress this infamous decoherence effects is a main task for scalable quantum computation. To fight against cavity decay, in typical circuit [3] and cavity…”

“…So, how to suppress this infamous decoherence effects is a main task for scalable quantum computation. To fight against cavity decay, in typical circuit [3] and cavity[4] QED systems, convectional wisdom is to resort to the so-called large detuning interaction method.Similarly, in trapped thermal ions system, the famous bichromatic excitation scheme [5] couples ions by virtue excitation of phonon mode, also uses the large detuning interaction. Later investigation shows that the logical operation obtained is of the geometric nature [6] and therefore has high fidelity [7].…”