Dissipative production of a maximally entangled steady state of two quantum bits

Abstract

Entangled states are a key resource in fundamental quantum physics, quantum cryptography, and quantum computation [1]. To date, controlled unitary interactions applied to a quantum system, so-called "quantum gates", have been the most widely used method to deterministically create entanglement [2]. These processes require high-fidelity state preparation as well as minimizing the decoherence that inevitably arises from coupling between the system and the environment and imperfect control of the system parameters. Here, on the contrary, we combine unitary processes with engineered dissipation to deterministically produce and stabilize an approximate Bell state of two trapped-ion qubits independent of their initial state. While previous works along this line involved the application of sequences of multiple time-dependent gates [3] or generated entanglement of atomic ensembles dissipatively but relied on a measurement record for steady-state entanglement [4], we implement the process in a continuous time-independent fashion, analogous to optical pumping of atomic states. By continuously driving the system towards steady-state, the entanglement is stabilized even in the presence of experimental noise and decoherence. Our demonstration of an entangled steady state of two qubits represents a step towards dissipative state engineering, dissipative quantum computation, and dissipative phase transitions [5][6][7]. Following this approach, engineered coupling to the environment may be applied to a broad range of experimental systems to achieve desired quantum dynamics or steady states. Indeed, concurrently with this work, an entangled steady state of two superconducting qubits was demonstrated using dissipation [8].Trapped ions are one of the leading experimental platforms for quantum information processing. Here, advanced protocols using unitary quantum gates have been demonstrated, see for example Refs. [9, 10]. However, decoherence and dissipation from coupling to the environment remains a challenge. One approach to overcome this relies on active feedback [11][12][13][14][15][16][17]. Such feedback techniques may be extended to quantum error correction, which can stabilize entangled states or realize fault-tolerant quantum computations. This will, however, require high-fidelity quantum gates and large qubit overheads that are beyond the reach of current experiments [2]. Recently, a complementary approach has been proposed to create entangled states or perform quantum computing by engineering the continuous interaction of the system with its environment [5][6][7][18][19][20][21][22][23][24][25][26].In our experiment, we take a step towards harnessing dissipation for quantum information processing by producing an entangled state that is inherently stabilized against decoherence by the applied interactions in a setting fully compatible with quantum computation. With this technique, we realize maximally entangled steady states with a fidelity of F = 0.75(3) by simultaneously applying a combination of time-independent fiel...

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