Construction of a qudit using Schrödinger cat states and generation of hybrid entanglement between a discrete-variable qudit and a continuous-variable qudit
We show that a continuous-variable (CV) qudit can be constructed using quasiorthogonal cat states of a bosonic mode, when the phase encoded in each cat state is chosen appropriately. With the constructed CV qudit and the discrete-variable (DV) qudit encoded with Fock states, we propose an approach to generate the hybrid maximally entangled state of a CV qudit and a DV qudit by using two microwave cavities coupled to a superconducting flux qutrit. This proposal relies on the initial preparation of a superposition of Fock states of one cavity and the initial preparation of a cat state of the other cavity. After the initial state of each cavity is prepared, this proposal requires only two basic operations, i.e., the first operation employs the dispersive coupling of both cavities with the qutrit while the second operation uses the dispersive coupling of only one cavity with the qutrit. The entangled state production is deterministic and the operation time decreases as the dimensional size of each qudit increases. In addition, during the entire operation, the coupler qutrit remains in the ground state and thus decoherence from the qutrit is significantly reduced. As an example, we further discuss the experimental feasibility for generating the hybrid maximally entangled state of a DV qutrit and a CV qutrit based on circuit QED. This proposal is universal and can be extended to accomplish the same task, by using two microwave or optical cavities coupled to a natural or artificial three-level atom.
We show that a continuous-variable (CV) qudit can be constructed using quasiorthogonal cat states of a bosonic mode, when the phase encoded in each cat state is chosen appropriately. With the constructed CV qudit and the discrete-variable (DV) qudit encoded with Fock states, we propose an approach to generate the hybrid maximally entangled state of a CV qudit and a DV qudit by using two microwave cavities coupled to a superconducting flux qutrit. This proposal relies on the initial preparation of a superposition of Fock states of one cavity and the initial preparation of a cat state of the other cavity. After the initial state of each cavity is prepared, this proposal requires only two basic operations, i.e., the first operation employs the dispersive coupling of both cavities with the qutrit while the second operation uses the dispersive coupling of only one cavity with the qutrit. The entangled state production is deterministic and the operation time decreases as the dimensional size of each qudit increases. In addition, during the entire operation, the coupler qutrit remains in the ground state and thus decoherence from the qutrit is significantly reduced. As an example, we further discuss the experimental feasibility for generating the hybrid maximally entangled state of a DV qutrit and a CV qutrit based on circuit QED. This proposal is universal and can be extended to accomplish the same task, by using two microwave or optical cavities coupled to a natural or artificial three-level atom.
In recent years, cat-state encoding and high-dimensional entanglement have attracted much attention. However, previous works are limited to generation of entangled states of cat-state qubits (two-dimensional entanglement with cat-state encoding), while how to prepare entangled states of cat-state qutrits or qudits (high-dimensional entanglement with cat-state encoding) has not been investigated. We here propose to generate a maximally-entangled state of multiple cat-state qutrits (three-dimensional entanglement by cat-state encoding) in circuit QED. The entangled state is prepared with multiple microwave cavities coupled to a superconducting flux ququart (a four-level quantum system). This proposal operates essentially by the cavity-qutrit dispersive interaction. The circuit hardware resource is minimized because only a coupler ququart is employed. The higher intermediate level of the ququart is occupied only for a short time, thereby decoherence from this level is greatly suppressed during the state preparation. Remarkably, the state preparation time does not depend on the number of the qutrits, thus it does not increase with the number of the qutrits. As an example, our numerical simulations demonstrate that, with the present circuit QED technology, the high-fidelity preparation is feasible for a maximally-entangled state of two cat-state qutrits. Furthermore, we numerically analyze the effect of the inter-cavity crosstalk on the scalability of this proposal. This proposal is universal and can be extended to accomplish the same task with multiple microwave or optical cavities coupled to a natural or artificial four-level atom.
Compared with a qubit, a qudit (i.e., a d-level or d-state quantum system) provides a larger Hilbert space to store and process information. On the other hand, qudit-based hybrid quantum computing usually requires performing hybrid quantum gates with qudits that are different in their nature or in their encoding format. In this work, we consider the qutrit case, i.e., the case for a qudit with d=3. We propose a simple method to realize a hybrid quantum controlled-sum gate with one superconducting (SC) qutrit and a cat-state qutrit. This gate plus single-qutrit gates form a universal set of ternary logic gates for quantum computing with qutrits. Our proposal is based on circuit QED and operates essentially by employing a SC ququart (a four-level quantum system) dispersively coupled to a microwave cavity. The gate implementation is quite simple because it only requires a single basic operation. Neither classical pulse nor measurement is needed. The auxiliary higher energy level of the SC ququart is virtually excited during the gate operation, thus decoherence from this level is greatly suppressed. As an application of this gate, we discuss the generation of a hybrid maximally entangled state of one SC qutrit and one cat-state qutrit. We further analyze the experimental feasibility of creating such a hybrid entangled state in circuit QED. This proposal is quite general and can be extended to accomplish the same task in a wide range of physical system, such as a four-level natural or artificial atom coupled to an optical or microwave cavity.
The generation of hybrid entangled states between matter qubits and microwave-field qubits is a fascinating area of quantum science and technology. In this work, we propose an efficient method to deterministically prepare hybrid entangled states between multiple superconducting qubits (matter qubits) and microwave-field qubits (flying qubits), by using a system composed of multiple superconducting qutrits and microwave cavities. Here, each superconducting (SC) qubit is encoded with the two lowest levels of a SC qutrit, while each microwave-field (MF) qubit is encoded using discrete-variable (DV) states (e.g., the vacuum and single-photon states) or continuous-variable (CV) states (e.g., coherent states) of a microwave cavity. Due to the nonpopulation of the qutrits' third energy level during the operation, decoherence from this level is significantly reduced. Moreover, the hybrid entangled states can be generated on a large scale. The operation time is independent of the number of SC qubits and MF qubits. Only two steps of the operation are needed for the preparation of the hybrid Greenberger---Horne---Zeilinger (GHZ) states as long as the initial states of the system are ready. As an example, our numerical results demonstrate that high-fidelity creation of hybrid DV-DV entangled states or CV-DV entangled states of three SC qubits and three MF qubits is feasible with current circuit QED technology. This method is universal and can be extended to generate hybrid entangled states between other matter qubits (such as atoms, ions, quantum dots, NV centers, and magnons) and microwave- or optical-field qubits.
Compared to a qubit, a qutrit (a three-level or three-state quantum system) possesses a larger Hilbert space to process and store quantum information. On the other hand, large-scale qutrit-based hybrid quantum computing usually requires performing hybrid multi-qutrit quantum gates with diverse qutrits, different in their nature or in their encoding format. In this work, we consider two types of qutrits, i.e., superconducting (SC) qutrits and cat-state qutrits. We propose to implement a hybrid controlled-SUM gate with one SC qutrit simultaneously controlling multiple-target cat-state qutrits. The gate is implemented in a circuit-QED system, which is composed of an SC ququart and multiple microwave cavities. The SC ququart here refers to a four-level quantum system, with the three lowest levels forming a qutrit and an auxiliary higher energy level utilized for the coherent state manipulation. The gate implementation does not require applying a classical pulse. Because the auxiliary higher energy level of the SC ququart is only virtually excited during the gate operation, decoherence from this level is greatly suppressed. The gate is deterministic, as it requires no measurement of the cavity or SC ququart states. Moreover, the gate operational time is independent of the number of qutrits, thus it does not increase with the number of qutrits. As an application of this gate, we further discuss the generation of a hybrid maximally entangled state of one SC qutrit and multiple cat-state qutrits. We also numerically analyze the experimental feasibility of creating the hybrid entangled state of one SC qutrit and two cat-state qutrits in a circuit QED system. This proposal may be extended to accomplish the same task in other physical systems, such as a four-level artificial atom (e.g., a quantum dot, an NV center, a magnon, etc.) coupled to multiple optical or microwave cavities.
Microwave-optical interactions and hybrid entangled states are crucial for building hybrid quantum networks. In this work, we propose an approach for realizing a cross-Kerr nonlinear interaction between microwave and optical cavities using an ensemble of nitrogen-vacancy centers (NV ensemble). This cross-Kerr interaction is achieved by an NV ensemble dispersively coupled to a microwave cavity and an optical cavity. As an application, we show that the cross-Kerr interaction can be used to create a hybrid entangled state of a discrete-variable (DV) optical qubit and a continuous-variable (CV) optical qubit. The DV optical qubit here refers to a qubit, with two logic states encoded via the vacuum and single-photon states of the optical cavity. The CV optical qubit means a qubit, whose two logic states are encoded through two quasi-orthogonal coherent states or cat states of the microwave cavity. Our method is quite simple because it only requires a single-step operation. Numerical simulations demonstrate that high-fidelity generation of the proposed hybrid state is feasible within current experimental technology. This proposal is universal and the NV ensemble can be replaced by other quantum transducers, such as magnons, rare-earth-doped crystals, and silicon-vacancy centers.
Quantum phase gates are of fundamental interest in quantum physics and play an important role in quantum communication and quantum computing. Here, we propose a scheme to implement a hybrid discrete-variable–continuous-variable (DV–CV) phase gate on multiple photonic qubits by utilizing multiple control microwave cavities and a single target microwave cavity, all coupled to a common superconducting qutrit (a three-level quantum system) in circuit quantum electrodynamics (QED). In our scheme, two logic states of the target qubit are encoded via the vacuum state and the coherent state (CV state) of the microwave cavity, while two logic states of the control qubits are encoded through the vacuum and single-photon states (DV states) of the microwave cavity. This hybrid DV–CV phase gate enables target qubit to acquire a phase that depends on the total number of control qubits in non-vacuum states, when the target qubit is in the coherent state. Our protocol is extensible and operationally efficient, involving just one step and the measurement of only one target qubit. In addition, as a practical application, this hybrid DV–CV phase gate can be employed as a quantum voting machine. Our numerical simulation demonstrates that the high-fidelity construction of a hybrid three-qubit DV–CV phase gate is feasible with current circuit QED technology.