Deterministic teleportation of a quantum gate between two logical qubits (1801.05283v1)

Abstract: A quantum computer has the potential to effciently solve problems that are intractable for classical computers. Constructing a large-scale quantum processor, however, is challenging due to errors and noise inherent in real-world quantum systems. One approach to this challenge is to utilize modularity--a pervasive strategy found throughout nature and engineering--to build complex systems robustly. Such an approach manages complexity and uncertainty by assembling small, specialized components into a larger architecture. These considerations motivate the development of a quantum modular architecture, where separate quantum systems are combined via communication channels into a quantum network. In this architecture, an essential tool for universal quantum computation is the teleportation of an entangling quantum gate, a technique originally proposed in 1999 which, until now, has not been realized deterministically. Here, we experimentally demonstrate a teleported controlled-NOT (CNOT) operation made deterministic by utilizing real-time adaptive control. Additionally, we take a crucial step towards implementing robust, error-correctable modules by enacting the gate between logical qubits, encoding quantum information redundantly in the states of superconducting cavities. Such teleported operations have significant implications for fault-tolerant quantum computation, and when realized within a network can have broad applications in quantum communication, metrology, and simulations. Our results illustrate a compelling approach for implementing multi-qubit operations on logical qubits within an error-protected quantum modular architecture.

• The paper introduces a deterministic protocol for teleporting a CNOT gate between logical qubits via real-time adaptive control.
• It employs superconducting cavities and transmon qubits to generate high-fidelity Bell states, verified through quantum state and process tomography.
• The experimental results, achieving 79% fidelity, underscore its potential to advance scalable, fault-tolerant quantum computing architectures.

Deterministic Teleportation of a Quantum Gate Between Logical Qubits

This paper addresses a significant challenge in the field of quantum computing: the deterministic teleportation of quantum gates between logical qubits. The research presented here focuses on the development and experimental demonstration of a teleported controlled-NOT (CNOT) gate operation between two logical qubits encoded in superconducting cavities. This work is crucial for advancing modular quantum computing architectures, which can manage errors and noise inherent in quantum systems and foster scalability in quantum computing.

Key Contributions and Techniques

The authors utilize a novel approach employing real-time adaptive control to achieve deterministic gate teleportation. This contrasts with prior demonstrations where teleportation was probabilistic and lacked real-time feedback, necessitating postselection that lowers the success probability and scalability of the process. By integrating classical communication and feedback in real-time, the researchers can achieve a deterministic outcome, thus significantly enhancing the practicality of the teleportation protocol.

For the experimental setup, two superconducting microwave cavities serve as logical qubits; these qubits store the quantum information through an encoding scheme that utilizes specific photon number states, representing a fault-tolerant quantum data structure. The qubits are linked via communication qubits formed by transmon qubits, which facilitate entanglement and interaction across modules without direct coupling of the data qubits.

A crucial experimental step involves generating an entangled Bell state within the communication qubits. This state acts as a resource enabling the teleportation of the CNOT gate. The entangling operation, facilitated by bus cavities and optimized single-qubit and two-qubit gates via gradient ascent pulse engineering (GRAPE), is carefully calibrated to minimize error and maintain the fidelity of the quantum process.

Results and Analysis

Experimental results demonstrate the successful teleportation of a CNOT operation, verified through quantum state tomography and process tomography of the logical qubits. The researchers provide a detailed characterization of the generated Bell states and logical operations, including assessments of gate fidelity, which is achieved at approximately 79% for the encoded quantum information. These results are validated against expected theoretical models and highlight potential areas for further optimization in coherence and error correction.

Implications and Future Directions

This work significantly impacts the potential scalability and fault tolerance of quantum computing systems. The deterministic nature of the teleported gate operations suggests practical applications in distributed quantum networks, encompassing quantum communication, sensing, and computation. Future avenues of research could focus on increasing the coherence times of the involved quantum states and further integrating error correction protocols to enhance fidelity beyond current thresholds.

The logical qubit implementation means these states are robust against certain types of quantum errors, marking an advance toward practical, fault-tolerant quantum computing. As quantum networks expand, ensuring such operations can be reliably performed between spatially distant qubits will be crucial, requiring advancements in generating and maintaining remote entanglement.

Finally, the demonstrated techniques open possibilities for implementing a broader range of two-qubit operations within a modular quantum architecture. Ultimately, such developments could pave the way for broader implementations of distributed quantum processors, an essential component of future quantum technologies.