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arXiv:1706.08083 · Physical Review A

Circuit QED with qutrits: Coupling three or more atoms via virtual-photon exchange

Peng Zhao, Xinsheng Tan, Haifeng Yu, Shi-Liang Zhu, Yang Yu

Published Oct 13, 2017• 23 citations
arXivHTMLPDF

Abstract

We present a model to describe a generic circuit QED system which consists of multiple artificial three-level atoms, namely, qutrits, strongly coupled to a cavity mode. When the state transition of the atoms disobeys the selection rules the process that does not conserve the number of excitations can happen determinatively. Therefore, we can realize coherent exchange interaction among three or more atoms mediated by the exchange of virtual photons. In addition, we generalize the one-cavity-mode mediated interactions to the multicavity situation, providing a method to entangle atoms located in different cavities. Using experimentally feasible parameters, we investigate the dynamics of the model including three cyclic-transition three-level atoms, for which the two lowest energy levels can be treated as qubits. Hence, we have found that two qubits can jointly exchange excitation with one qubit in a coherent and reversible way. In the whole process, the population in the third level of atoms is negligible and the cavity photon number is far smaller than 1. Our model provides a feasible scheme to couple multiple distant atoms together, which may find applications in quantum information processing.

DOI
10.1103/physreva.96.043833

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We present a model to describe a generic circuit QED system which consists of multiple artificial three-level atoms, namely, qutrits, strongly coupled to a cavity mode. When the state transition of the atoms disobeys the selection rules the process that does not conserve the number of excitations can happen determinatively. Therefore, we can realize coherent exchange interaction among three or more atoms mediated by the exchange of virtual photons. In addition, we generalize the one-cavity-mode mediated interactions to the multicavity situation, providing a method to entangle atoms located in different cavities. Using experimentally feasible parameters, we investigate the dynamics of the model including three cyclic-transition three-level atoms, for which the two lowest energy levels can be treated as qubits. Hence, we have found that two qubits can jointly exchange excitation with one qubit in a coherent and reversible way. In the whole process, the population in the third level of atoms is negligible and the cavity photon number is far smaller than 1. Our model provides a feasible scheme to couple multiple distant atoms together, which may find applications in quantum information processing.
When an atom is strongly coupled to a cavity, the two systems can exchange a single photon through a coherent Rabi oscillation. This process enables precise quantum-state engineering and manipulation of atoms and photons in a cavity, which play a central role in quantum information and measurement. Recently, a new regime of cavity QED was reached experimentally where the strength of the interaction between light and artificial atoms (qubits) becomes comparable to the atomic transition frequency or the resonance frequency of the cavity mode. Here we show that this regime can strongly modify the concept of vacuum Rabi oscillations, enabling multiphoton exchanges between the qubit and the resonator. We find that experimental state-of-the-art circuit-QED systems can undergo two- and three-photon vacuum Rabi oscillations. These anomalous Rabi oscillations can be exploited for the realization of efficient Fock-state sources of light and complex entangled states of qubits.
We propose a method for transferring quantum entangled states of two photonic cat-state qubits ( cqubits ) from two microwave cavities to the other two microwave cavities. This proposal is realized by using four microwave cavities coupled to a superconducting flux qutrit. Because of using four cavities with different frequencies, the inter-cavity crosstalk is significantly reduced. Since only one coupler qutrit is used, the circuit resource is minimized. The entanglement transfer is completed with a singlestep operation only, thus this proposal is quite simple. The third energy level of the coupler qutrit is not populated during the state transfer, therefore decoherence from the higher energy level is greatly suppressed. Our numerical simulations show that high-fidelity transfer of two- cqubit entangled states from two transmission line resonators to the other two transmission line resonators is feasible with current circuit QED technology. This proposal is universal and can be applied to accomplish the same task in a wide range of physical systems, such as four microwave or optical cavities, which are coupled to a natural or artificial three-level atom.
We propose a simple scheme for quantum state transfer between two qutrits in circuit quantum electrodynamics (QED). Other than the two qutrits, only one coupling cavity is adopted, and there is no requirement for a classical pulse or measurement. In the whole process, the cavity is virtually excited so that the decoherence caused by photon leakage of the cavity can be greatly suppressed. Numerical simulation shows that high fidelities (⁠|∼0.984\sim0.984∼0.984| on average) can be achieved with present circuit QED technology, in which a low quality factor (⁠|Q≈4.1×104\mathcal{Q}\approx4.1\times10^{4}Q≈4.1×104|⁠) for the cavity has been adopted. Due to its simplicity, this scheme is quite general and can be applied to other quantum systems.
Photon-mediated interactions between atoms are of fundamental importance in quantum optics, quantum simulations, and quantum information processing. The exchange of real and virtual photons between atoms gives rise to nontrivial interactions, the strength of which decreases rapidly with distance in three dimensions. Here, we use two superconducting qubits in an open one-dimensional transmission line to study much stronger photon-mediated interactions. Making use of the possibility to tune these qubits by more than a quarter of their transition frequency, we observe both coherent exchange interactions at an effective separation of 3λ/4 and the creation of super- and subradiant states at a separation of one photon wavelength λ. In this system, collective atom-photon interactions and applications in quantum communication may be explored.
The controlled transfer of quantum entangled states is a fascinating topic in quantum physics, and has potential applications in fields such as quantum network communication, distributed quantum information processing, and development of quantum functional devices. This study focuses on a circuit quantum electrodynamics (QED) system, comprising an input port, two output ports, and a quantum switch. The input port or each output port contains n microwave cavities. The switch is a superconducting transmon qutrit (three-level quantum system). The vacuum and single-photon states in each cavity encode a photonic qubit. We demonstrate that the single-excitation symmetric (SES) entangled state of n photonic qubits can be transferred from the n cavities at the input port to the n cavities at either of the two output ports, controlled by the switch. Remarkably, the operational time decreases as the number n increases. The only use of qutrit-cavity resonant interaction enables rapid quantum state transfer while effectively suppressing system decoherence. This proposal can be used to achieve the controlled transfer of both Bell states and W states of photonic qubits via a quantum switch. To validate the experimental feasibility, we numerically simulate two scenarios: (i) transferring the Bell state of two photonic qubits, and (ii) transferring the W state of three photonic qubits. This proposal may be extended to accomplish the same task in other physical systems, where the switch is implemented with a three-level artificial atom of different types while each cavity is a microwave or optical cavity.
We present a way to transfer maximally- or partially-entangled states of n single-photon-state (SPS) qubits onto n coherent-state (CS) qubits, by employing 2 n microwave cavities coupled to a superconducting flux qutrit. The two logic states of a SPS qubit here are represented by the vacuum state and the single-photon state of a cavity, while the two logic states of a CS qubit are encoded with two coherent states of a cavity. Because of using only one superconducting qutrit as the coupler, the circuit architecture is significantly simplified. The operation time for the state transfer does not increase with the increasing of the number of qubits. When the dissipation of the system is negligible, the quantum state can be transferred in a deterministic way since no measurement is required. Furthermore, the higher-energy intermediate level of the coupler qutrit is not excited during the entire operation and thus decoherence from the qutrit is greatly suppressed. As a specific example, we numerically demonstrate that the high-fidelity transfer of a Bell state of two SPS qubits onto two CS qubits is achievable within the present-day circuit QED technology. Finally, it is worthy to note that when the dissipation is negligible, entangled states of n CS qubits can be transferred back onto n SPS qubits by performing reverse operations. This proposal is quite general and can be extended to accomplish the same task, by employing a natural or artificial atom to couple 2 n microwave or optical cavities.