Journal Article

Autonomously stabilized entanglement between two superconducting quantum bits

Mirrahimi, M., Hatridge, M., Leghtas, Z., Sliwa, K. M., Devoret, M. H., Vool, U., Frunzio, L., Girvin, S. M., Narla, A., Shankar, S.

2013

Overview

An entangled Bell state of two superconducting quantum bits can be stabilized for an arbitrary time using an autonomous feedback scheme, that is, one that does not require a complicated external error-correcting feedback loop. Harnessing dissipation in entangled quantum states Entangled states are a key resource in fundamental quantum physics, quantum cryptography and quantum computation. It has been generally assumed that the creation of such states requires the avoidance of contact with a dissipative environment, and minimization of decoherence. Some studies have shown, however, that dissipative interactions can be used to preserve coherence, and in this issue of Nature two groups demonstrate this principle for continuously driven physical systems. Lin et al . use engineered dissipation to deterministically produce and stabilize entanglement between two trapped-ion qubits, independent of their initial state. Shankar et al . use an autonomous feedback scheme to counteract decoherence and demonstrate the stabilization of an entangled Bell state of a quantum register of two superconducting qubits for an arbitrary time. This approach may be applied to a broad range of experimental systems to achieve desired quantum dynamics or steady states. Quantum error correction codes are designed to protect an arbitrary state of a multi-qubit register from decoherence-induced errors 1 , but their implementation is an outstanding challenge in the development of large-scale quantum computers. The first step is to stabilize a non-equilibrium state of a simple quantum system, such as a quantum bit (qubit) or a cavity mode, in the presence of decoherence. This has recently been accomplished using measurement-based feedback schemes 2 , 3 , 4 , 5 . The next step is to prepare and stabilize a state of a composite system 6 , 7 , 8 . Here we demonstrate the stabilization of an entangled Bell state of a quantum register of two superconducting qubits for an arbitrary time. Our result is achieved using an autonomous feedback scheme that combines continuous drives along with a specifically engineered coupling between the two-qubit register and a dissipative reservoir. Similar autonomous feedback techniques have been used for qubit reset 9 , single-qubit state stabilization 10 , and the creation 11 and stabilization 6 of states of multipartite quantum systems. Unlike conventional, measurement-based schemes, the autonomous approach uses engineered dissipation to counteract decoherence 12 , 13 , 14 , 15 , obviating the need for a complicated external feedback loop to correct errors. Instead, the feedback loop is built into the Hamiltonian such that the steady state of the system in the presence of drives and dissipation is a Bell state, an essential building block for quantum information processing. Such autonomous schemes, which are broadly applicable to a variety of physical systems, as demonstrated by the accompanying paper on trapped ion qubits 16 , will be an essential tool for the implementation of quantum error correction.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 639/766/1130/1064 / 639/766/483/481 / Classical and quantum physics: mechanics and fields / Electrodynamics / Energy dissipation / Error-correcting codes / Exact sciences and technology / Humanities and Social Sciences / letter / Methods / multidisciplinary / Physics / Protocol / Quantum computing / Quantum information / Quantum theory / Science / Tomography