We propose a scheme for generation of SU(2) coherent states for an atomic ensemble and a cavity mode. In the scheme a collection of two-level atoms resonantly interact with a single-mode quantized field. Under certain conditions, the system can evolve from a Fock state to a highly entangled SU(2) coherent state. The operation speed increases as the number of atoms increases, which is important in view of decoherence.

Communications in Theoretical PhysicsMay 1, 20090 citations

Bulletin of the American Physical Society

Generating coherent states of entangled spins

Hongyi Yu, Yu Luo, Wang Yao

Published Mar 1, 2012• 0 citations

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Abstract

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Generating coherent states of entangled spins

Hongyi Yu, Yu Luo, Wang Yao

A coherent state of many spins contains quantum entanglement, which increases with a decrease in the collective spin value. We present a scheme to engineer this class of pure state based on incoherent spin pumping with a few collective raising or lowering operators. In a pumping scenario aimed for maximum entanglement, the steady state of $N$ -pumped spin qubits realizes the ideal resource for the $1\ensuremath{\rightarrow}\frac{N}{2}$ quantum telecloning. We show how the scheme can be implemented in a realistic system of atomic spin qubits in an optical lattice. Error analysis shows that high-fidelity state engineering is possible for $N\ensuremath{\sim}O(100)$ spins in the presence of decoherence. The scheme can also prepare a resource state for the secret sharing protocol and for the construction of the large-scale Affleck-Kennedy-Lieb-Tasaki state.A coherent state of many spins contains quantum entanglement, which increases with a decrease in the collective spin value. We present a scheme to engineer this class of pure state based on incoherent spin pumping with a few collective raising or lowering operators. In a pumping scenario aimed for maximum entanglement, the steady state of $N$-pumped spin qubits realizes the ideal resource for the $1\ensuremath{\rightarrow}\frac{N}{2}$ quantum telecloning. We show how the scheme can be implemented in a realistic system of atomic spin qubits in an optical lattice. Error analysis shows that high-fidelity state engineering is possible for $N\ensuremath{\sim}O(100)$ spins in the presence of decoherence. The scheme can also prepare a resource state for the secret sharing protocol and for the construction of the large-scale Affleck-Kennedy-Lieb-Tasaki state.

arXiv:1103.4355Physical Review ASep 26, 20119 citations

Suppression of collective fluctuations and generation of entanglement in a spin ensemble

Yu Luo

Spin degrees of freedom have been extensively explored in the context of quantum information processing. Many proposals of quantum computation architectures use spins as carriers of quantum of information. A central problem is to efficiently generate quantum entanglement between spin qubits which proves to be a crucial resource for quantum information tasks. On the other hand, uncontrollable spin degrees of freedom in the environment of spin qubits are the major causes of errors at low temperature, for example, the lattice nuclear spins hyperfine coupled to single electron spin qubit localized in semiconductor nano-structures. An outstanding problem for scalable quantum computation is to suppress the collective fluctuations from such spin baths so that the coherence time of the spin qubit can be improved. With these two motivations, the problems of suppressing collective spin fluctuations and generating entanglement in various spin ensembles are addressed in this thesis.\n\n\n\nIn the first half of the thesis, two approaches are introduced for suppressing the collective fluctuations in the nuclear spin bath so that the quantum coherence time of electron spin qubit in semiconductor quantum dots can be improved. The first approach works for a coupled double dot system. A theory for the interaction with the nuclear spins is developed when the two-electron singlet state is in resonance with one of the triplet state in moderate external magnetic field. At this resonance condition, the nuclear-electron flip-flop process caused by the hyperfine interaction can lead to a feedback mechanism, which can be used to suppress the nuclear hyperfine field. The second approach works for a single dot system. It is shown that strong pumping of the nuclear spins in dynamic nuclear polarization processes can saturate the nuclear spin bath towards the collective “dark states”. In such dark states, the transverse nuclear field fluctuation can be substantially suppressed compared to the value at thermal equilibrium. Two physical schemes are proposed to realize the nuclear dark states for suppression of the nuclear field fluctuations.\n\n\n\nIn the second half of the thesis, schemes are presented for generating large scale quantum entanglement in two types of spin qubit systems. For atomic spin qubits in optical lattices, schemes are proposed on how to prepare pure spin coherent state (SCS) with low collective spin by incoherent pumping with collective spin raising and lowering operations. Such SCS realize networks of mutually entangled spins which can be idea resources for the quantum telecloning algorithm. For donor nuclear spin qubits in silicon architecture, proposals are shown on how to deterministic prepare Dicke states which constitute an important class of multipartite entangled states. Our scheme is capable of preparing both symmetric and asymmetric Dicke states which form a complete basis set of the spin Hilbert space. The required controls are in situ to the prototype Kane’s quantum computer. The preparation is robust because each desired Dicke state is the steady state under designed pumping process. The schemes presented here also make possible the construction of decoherence free subspaces where quantum information is protected from collective noises.Spin degrees of freedom have been extensively explored in the context of quantum information processing. Many proposals of quantum computation architectures use spins as carriers of quantum of information. A central problem is to efficiently generate quantum entanglement between spin qubits which proves to be a crucial resource for quantum information tasks. On the other hand, uncontrollable spin degrees of freedom in the environment of spin qubits are the major causes of errors at low temperature, for example, the lattice nuclear spins hyperfine coupled to single electron spin qubit localized in semiconductor nano-structures. An outstanding problem for scalable quantum computation is to suppress the collective fluctuations from such spin baths so that the coherence time of the spin qubit can be improved. With these two motivations, the problems of suppressing collective spin fluctuations and generating entanglement in various spin ensembles are addressed in this thesis.\n\n\n\nIn the first half of the thesis, two approaches are introduced for suppressing the collective fluctuations in the nuclear spin bath so that the quantum coherence time of electron spin qubit in semiconductor quantum dots can be improved. The first approach works for a coupled double dot system. A theory for the interaction with the nuclear spins is developed when the two-electron singlet state is in resonance with one of the triplet state in moderate external magnetic field. At this resonance condition, the nuclear-electron flip-flop process caused by the hyperfine interaction can lead to a feedback mechanism, which can be used to suppress the nuclear hyperfine field. The second approach works for a single dot system. It is shown that strong pumping of the nuclear spins in dynamic nuclear polarization processes can saturate the nuclear spin bath towards the collective “dark states”. In such dark states, the transverse nuclear field fluctuation can be substantially suppressed compared to the value at thermal equilibrium. Two physical schemes are proposed to realize the nuclear dark states for suppression of the nuclear field fluctuations.\n\n\n\nIn the second half of the thesis, schemes are presented for generating large scale quantum entanglement in two types of spin qubit systems. For atomic spin qubits in optical lattices, schemes are proposed on how to prepare pure spin coherent state (SCS) with low collective spin by incoherent pumping with collective spin raising and lowering operations. Such SCS realize networks of mutually entangled spins which can be idea resources for the quantum telecloning algorithm. For donor nuclear spin qubits in silicon architecture, proposals are shown on how to deterministic prepare Dicke states which constitute an important class of multipartite entangled states. Our scheme is capable of preparing both symmetric and asymmetric Dicke states which form a complete basis set of the spin Hilbert space. The required controls are in situ to the prototype Kane’s quantum computer. The preparation is robust because each desired Dicke state is the steady state under designed pumping process. The schemes presented here also make possible the construction of decoherence free subspaces where quantum information is protected from collective noises.

Jan 1, 20120 citations

Generation of a Maximally Entangled State Using Collective Optical Pumping

Maciej Malinowski, Chi Zhang, Vlad Negnevitsky +7 more

We propose and implement a novel scheme for dissipatively pumping two qubits into a singlet Bell state. The method relies on a process of collective optical pumping to an excited level, to which all states apart from the singlet are coupled. We apply the method to deterministically entangle two trapped ^{40}Ca^{+} ions. Within 16 pumping cycles, an initially separable state is transformed into one with 83(1)% singlet fidelity, and states with initial fidelity of ⪆70% converge onto a fidelity of 93(1)%. We theoretically analyze the performance and error susceptibility of the scheme and find it to be insensitive to a large class of experimentally relevant noise sources.We propose and implement a novel scheme for dissipatively pumping two qubits into a singlet Bell state. The method relies on a process of collective optical pumping to an excited level, to which all states apart from the singlet are coupled. We apply the method to deterministically entangle two trapped ^{40}Ca^{+} ions. Within 16 pumping cycles, an initially separable state is transformed into one with 83(1)% singlet fidelity, and states with initial fidelity of ⪆70% converge onto a fidelity of 93(1)%. We theoretically analyze the performance and error susceptibility of the scheme and find it to be insensitive to a large class of experimentally relevant noise sources.

arXiv:2107.10374Physical Review LettersFeb 22, 202227 citations

A many-body singlet prepared by a central spin qubit

Leon Zaporski, Stijn R. de Wit, Takuya Isogawa +4 more

Controllable quantum many-body systems are platforms for fundamental investigations into the nature of entanglement and promise to deliver computational speed-up for a broad class of algorithms and simulations. In particular, engineering entanglement within a dense spin ensemble can turn it into a robust quantum memory or a computational platform. Recent experimental progress in dense central spin systems motivates the design of algorithms that use a central-spin qubit as a convenient proxy for the ensemble. Here we propose a protocol that uses a central spin to initialize two dense spin ensembles into a pure anti-polarized state and from there creates a many-body entangled state -- a singlet -- from the combined ensemble. We quantify the protocol performance for multiple material platforms and show that it can be implemented even in the presence of realistic levels of decoherence. Our protocol introduces an algorithmic approach to preparation of a known many-body state and to entanglement engineering in a dense spin ensemble, which can be extended towards a broad class of collective quantum states.Controllable quantum many-body systems are platforms for fundamental investigations into the nature of entanglement and promise to deliver computational speed-up for a broad class of algorithms and simulations. In particular, engineering entanglement within a dense spin ensemble can turn it into a robust quantum memory or a computational platform. Recent experimental progress in dense central spin systems motivates the design of algorithms that use a central-spin qubit as a convenient proxy for the ensemble. Here we propose a protocol that uses a central spin to initialize two dense spin ensembles into a pure anti-polarized state and from there creates a many-body entangled state -- a singlet -- from the combined ensemble. We quantify the protocol performance for multiple material platforms and show that it can be implemented even in the presence of realistic levels of decoherence. Our protocol introduces an algorithmic approach to preparation of a known many-body state and to entanglement engineering in a dense spin ensemble, which can be extended towards a broad class of collective quantum states.

arXiv:2301.10258Jan 24, 20230 citations

Many-Body Singlet Prepared by a Central-Spin Qubit

Leon Zaporski, Stijn R. de Wit, Takuya Isogawa +4 more

Controllable quantum many-body systems are platforms for fundamental investigations into the nature of entanglement and promise to deliver computational speed-up for a broad class of algorithms and simulations.In particular, engineering entanglement within a dense spin ensemble can turn it into a robust quantum memory or a computational platform.Recent experimental progress in dense central-spin systems motivates the design of algorithms that use a central-spin qubit as a convenient proxy for the ensemble.Here we propose a protocol that uses a central spin to initialize two dense spin ensembles into a pure antipolarized state and from there creates a many-body entangled state-a singlet-from the combined ensemble.We quantify the protocol performance for multiple material platforms and show that it can be implemented even in the presence of realistic levels of decoherence and diffusion.Our protocol introduces an algorithmic approach to the preparation of a known many-body state and to entanglement engineering in a dense spin ensemble, which can be extended towards a broad class of collective quantum states.Controllable quantum many-body systems are platforms for fundamental investigations into the nature of entanglement and promise to deliver computational speed-up for a broad class of algorithms and simulations.In particular, engineering entanglement within a dense spin ensemble can turn it into a robust quantum memory or a computational platform.Recent experimental progress in dense central-spin systems motivates the design of algorithms that use a central-spin qubit as a convenient proxy for the ensemble.Here we propose a protocol that uses a central spin to initialize two dense spin ensembles into a pure antipolarized state and from there creates a many-body entangled state-a singlet-from the combined ensemble.We quantify the protocol performance for multiple material platforms and show that it can be implemented even in the presence of realistic levels of decoherence and diffusion.Our protocol introduces an algorithmic approach to the preparation of a known many-body state and to entanglement engineering in a dense spin ensemble, which can be extended towards a broad class of collective quantum states.

PRX QuantumDec 14, 20237 citations

Generation of SU(2) Coherent States for a Cavity Mode and a Collective Atomic Mode

Zheng Shi-Biao

Communications in Theoretical PhysicsMay 1, 20090 citations

N

-pumped spin qubits realizes the ideal resource for the $1\ensuremath{\rightarrow}\frac{N}{2}$ quantum telecloning. We show how the scheme can be implemented in a realistic system of atomic spin qubits in an optical lattice. Error analysis shows that high-fidelity state engineering is possible for $N\ensuremath{\sim}O(100)$ spins in the presence of decoherence. The scheme can also prepare a resource state for the secret sharing protocol and for the construction of the large-scale Affleck-Kennedy-Lieb-Tasaki state.