Among environmental effects, dissipation allows the stabilization of quantum resources, a fact that may accomplish key advantages over unitary quantum state manipulation. In this paper, we study an open quantum system composed of N two-mode Bose–Einstein condensates (BECs), namely, BEC qubits, interacting with their environments. Usually, the coupling of a system with its environment results in a dissipation process that washes out the quantum effects, but the environment may also play exactly the opposite role. Here, we show that the coupling of a BEC qubit with its local environments can be exploited to establish dissipative state engineering and dissipative quantum computation. Also, we discuss how to generate entanglement and transfer it using such macroscopic BEC qubits. Generally, the couplings can be manipulated such that the system is driven to a desired steady state. We demonstrate that the steady state of a BEC qubit just depends on the tunneling strength between its two modes and the features of the local environments. By preparing the proper physical conditions and adjusting the parameters involved in the model, it can be shown that the steady state of the system may coincide with its initial state. Also, the interaction between the BEC qubits and local environments can be engineered to provide physically controlled single- and two-qubit operations, which are necessary for performing dissipative universal quantum computation. With the prosperous development of quantum computing, such a system can be employed to implement quantum computers based on non-unitary operations.
Among environmental effects, dissipation allows the stabilization of quantum resources, a fact that may accomplish key advantages over unitary quantum state manipulation. In this paper, we study an open quantum system composed of N two-mode Bose–Einstein condensates (BECs), namely, BEC qubits, interacting with their environments. Usually, the coupling of a system with its environment results in a dissipation process that washes out the quantum effects, but the environment may also play exactly the opposite role. Here, we show that the coupling of a BEC qubit with its local environments can be exploited to establish dissipative state engineering and dissipative quantum computation. Also, we discuss how to generate entanglement and transfer it using such macroscopic BEC qubits. Generally, the couplings can be manipulated such that the system is driven to a desired steady state. We demonstrate that the steady state of a BEC qubit just depends on the tunneling strength between its two modes and the features of the local environments. By preparing the proper physical conditions and adjusting the parameters involved in the model, it can be shown that the steady state of the system may coincide with its initial state. Also, the interaction between the BEC qubits and local environments can be engineered to provide physically controlled single- and two-qubit operations, which are necessary for performing dissipative universal quantum computation. With the prosperous development of quantum computing, such a system can be employed to implement quantum computers based on non-unitary operations.
Superconducting quantum circuits provide a promising avenue for scalable quantum computation and simulation. Their chief advantage is that, unlike physical atoms or electrons, these ``artificial atoms'' can be designed with nearly-arbitrarily large coupling to one another and to their electromagnetic environment. This strong coupling allows for fast quantum bit (qubit) operations, and for efficient readout. However, strong coupling comes at a price: a qubit that is strongly coupled to its environment is also strongly susceptible to losses and dissipation, as coherent information leaks from the quantum system under study into inaccessible ``bath'' modes. Extensive work in the field is dedicated to engineering away these losses to the extent possible, and to using error correction to undo the effects of losses that are unavoidable.This dissertation explores an alternate approach to dissipation: we study avenues by which dissipation itself can be used to generate, rather than destroy, quantum resources. We do so specifically in the context of quantum entanglement, one of the most important and most counter-intuitive aspects of quantum mechanics. Entanglement generation and stabilization is critical to most non-trivial implementations of quantum computing and quantum simulation, as it is the property that distinguishes a multi-qubit quantum system from a string of classical bits. The ability to harness dissipation to generate, purify, and stabilize entanglement is therefore highly desirable.We begin with an overview of quantum dissipation and measurement, followed by an introduction to entanglement and to the superconducting quantum information architecture. We then discuss three sets of experiments that highlight and explore the powerful uses of dissipation in quantum systems. First, we use an entangling measurement to probabilistically generate entanglement between two qubits separated by more than one meter of ordinary cable. This represents the first achievement of remote entanglement in a superconducting qubit system, which will be a critical capability as quantum computers and simulators scale. We then use a nearly-quantum limited amplifier to unravel individual quantum trajectories of the system under that entangling measurement, performing the first systematic exploration of entangled trajectories in any physical implementation. We finally demonstrate deterministic entanglement by engineering a lossy quantum environment to efficiently generate and stabilize entangled states with both frequency and symmetry selectivity. These experiments provide evidence that explicitly building dissipation into an engineered quantum system can enable, rather than hinder, the study of fundamental quantum mechanics and complex many-body Hamiltonians.
Schrödinger cat states provide a hardware-efficient platform for bosonic quantum error correction by encoding logical information in protected manifolds of harmonic oscillators. While previous work has demonstrated the dissipative stabilization of multi-mode Schrödinger cat states as robust quantum memories, a framework for universal quantum computation has remained unavailable. Here we work towards this by introducing a universal gate set for dissipatively stabilized multi-mode cat qubits as a necessary resource for computation. Using a chain of Kerr non-linear oscillators coupled through engineered non-local dissipation and an effective low-dimensional description, we show how single-qubit control can be achieved through arbitrary rotation around the X-axis and π/2-rotation around the Z-axis. We further show how coupling two such stabilized arrays through just one oscillator on each respective array enables coherent entangling operations through implementation of the XXπ/2 gate. Numerical simulations demonstrate high-fidelity gate dynamics and entanglement generation under realistic parameters. Finally, we analyze the effects of induced and intrinsic photon loss, disorder, and the validity regime of the effective low-dimensional theory. Our results establish dissipatively stabilized multi-mode Schrödinger cat states as a potential architecture for universal bosonic quantum computation.
Abstract Hybrid quantum systems that integrate complementary advantages of different physical platforms provide a new pathway toward large-scale, fault-tolerant quantum information processing. In this work, we theoretically investigate a hybrid system consisting of a Rydberg atom, a piezoelectric nanomechanical resonator, and a superconducting qubit, aiming to achieve high-fidelity steady-state entanglement. In this scheme, the nanomechanical resonator acts as a quantum bus, resonantly coupling to the superconducting qubit via the piezoelectric effect and to the Rydberg atom via the electric dipole interaction. Under the strong-coupling condition, we introduce a dressed-state transformation to analyze the eigenenergy structure of the system. By properly controlling the laser driving fields, the otherwise detrimental dissipation processes are turned into a crucial resource for entanglement preparation. Numerical simulations show that under optimal parameters the fidelity of the target state can reach above 98 % , and the scheme exhibits strong robustness against fluctuations in the driving field strengths and against spontaneous emission noise. This study provides a feasible theoretical approach and technical reference for cross-platform quantum interfaces, distributed quantum computing, and dissipation-assisted quantum state engineering.
Up to now, the lifetime of experimentally demonstrated entangled states has been limited due to their fragility under decoherence and dissipation. Therefore, they are created under strict isolation conditions. In contrast, new approaches harness the coupling of the system to the environment, which drives the system into the desired state. Following these ideas, we present a robust method for generating steady-state entanglement between two distant atomic ensembles. The proposed scheme relies on the interaction of the two atomic systems with the common vacuum modes of an electromagnetic field which act as an engineered environment. We develop the theoretical framework for two-level systems, including dipole-dipole interactions, and complement it by considering its implementation in multilevel ground states. Based on these results, the realization of entanglement generation by engineered dissipation has been experimentally demonstrated [H. Krauter, C. A. Muschik, K. Jensen, W. Wasilewski, J. M. Petersen, J. I. Cirac, and E. S. Polzik, e-print arXiv:1006.4344].
We investigate the computational power of creating steady-states of quantum dissipative systems whose evolution is governed by time-independent and local couplings to a memoryless environment. We show that such a model allows for efficient universal quantum computation with the result of the computation encoded in the steady state. Due to the purely dissipative nature of the process, this way of doing quantum computation exhibits some inherent robustness and defies some of the DiVincenzo criteria for quantum computation. We show that there is a natural class of problems that can be solved with such a model - the preparation of ground states of frustration free quantum Hamiltonians. This allows for robust and efficient creation of exotic states that exhibit features like topological quantum order and the creation of PEPS and it proves the existence of novel dissipative phase transitions. In particular the latter can in principle be verified experimentally with present day technology such as with optical lattices.
Abstract Entangled state preparation and preservation are the cornerstones of any quantum information platform. However , the strongest adversaries in quantum information science are unwanted environmental effects such as decoherence and dissipation. Here, we address how to control and harness the dissipation that arises from the coupling of a system with its environment, to provide stationary entangled states for quantum machine learning. To do so, we design a dissipative quantum channel, i.e., a two-qubit system interacting with a squeezed vacuum field reservoir, and study the output state of the channel by solving the corresponding master equation, especially, in the small squeezing regime. We show that the time-dependent output state of the channel is the so-called two-qubit X-states that generalize many families of entangled two-qubit states. Also, by considering a general Bell diagonal state as the initial state of the system we reveal that this dissipative channel generates two well-known classes of entangled mixed state and Werner-like states in the steady-state regime. Moreover, this channel provides an efficient way to determine whether a given initial state results in a stationary entangled state or not. Finally, we examine the potential application of the designed two-qubit channel for quantum machine learning. Integrating the non-unitary transformation of the two-qubit channel and parallel-processed neural computing establishes the requirements for a meaningful quantum neural network.