We propose a theoretical scheme to realize the coherent coupling of multiple atoms in a quantum network which is composed of a two-dimensional (2D) array of coupled cavities. In the scheme, the pairing off-resonant Raman transitions of different atoms, induced by the cavity modes and external fields, can lead to selective coupling between arbitrary atoms trapped in separated cavities. Based on this physical mechanism, quantum gates between any pair of qubits and parallel two-qubit operations can be performed in the 2D system. The scheme provides a new perspective for coherent manipulation of quantum systems in 2D quantum networks.
In this paper, we propose a scheme to implement two-qubit logic gates with a controllable and selective interaction in a scalable superconducting circuit of charge qubits. A nanomechanical resonator is used as a data bus to connect qubits. It is indicated that a controllable interaction between qubits can be obtained by making use of the data bus. It is shown that a selective interaction between qubits can be realized when many qubits are involved in the system under our consideration.
We propose a scheme to realize heralded quantum entanglement between two distant matter qubits using two $Λ$ atom systems. Our proposal does not need any photon interference. We also present a general theory of outcome state of non-monochromatic incident light and finite interaction time.
We propose a scheme which implements a controllable change of the state of the target spin qubit in such a way that both the control and the target spin qubits remain in their ground states. The interaction between the two spins is mediated by an auxiliary spin, which can transfer to its excited state. Our scheme suggests a possible relationship between the gate and adiabatic quantum computation.
We show that the coherent coupling of atomic qubits at distant nodes of a quantum network, composed of several cavities linked by optical fibers, can be arbitrarily controlled via the selective pairing of Raman transitions. The adiabatic elimination of the atomic excited states and photonic states leads to selective qubit-qubit interactions, which would have important applications in quantum information processing. Quantum gates between any pair of distant qubits and parallel two-qubit operations on selected qubit pairs can be implemented through suitable choices of the parameters of the exter…
We propose a protocol for Alice to implement a multiqubit quantum operation from the restricted sets on distant qubits possessed by Bob, and then we investigate the communication complexity of the task in different communication scenarios. By comparing with the previous work, our protocol works without prior sharing of entanglement, and requires less communication resources than the previous protocol in the qubit-transmission scenario. Furthermore, we generalize our protocol to $d$-dimensional operations.
The mechanism for entanglement of two flux qubits each interacting with a single mode electromagnetic field is discussed. By performing a Bell state measurements (BSM) on photons we find the two qubits in an entangled state depending on the system parameters. We discuss the results for two initial states and take into consideration the influence of decoherence.
Conditional quantum oscillations are investigated for quantum gate operations in superconducting flux qubits. We present an effective Hamiltonian which describes a conditional quantum oscillation in two-qubit systems. Rabi-type quantum oscillations are discussed in implementing conditional quantum oscillations to quantum gate operations. Two conditional quantum oscillations depending on the states of control qubit can be synchronized to perform controlled-gate operations by varying system parameters. It is shown that the conditional quantum oscillations with their frequency synchronization mak…
We propose a new fast scalable method for achieving a two-qubit entangling gate between arbitrary distant qubits in a network by exploiting dispersionless propagation in uniform chains. This is achieved dynamically by switching on a strong interaction between the qubits and a bus formed by a non-engineered chain of interacting qubits. The quality of the gate scales very efficiently with qubit separations. Surprisingly, a sudden switching of the coupling is not necessary and our gate mechanism is not altered by a possibly gradual switching. The bus is also naturally reset to its initial state m…
The big challenge in quantum computing is to realize scalable multi-qubit systems with cross-talk free addressability and efficient coupling of arbitrarily selected qubits. Quantum networks promise a solution by integrating smaller qubit modules to a larger computing cluster. Such a distributed architecture, however, requires the capability to execute quantum-logic gates between distant qubits. Here we experimentally realize such a gate over a distance of 60m. We employ an ancillary photon that we successively reflect from two remote qubit modules, followed by a heralding photon detection whic…
A simple scheme to prepare an entanglement between two separated qubits from a given mixed state is proposed. A single qubit (entanglement mediator) is repeatedly made to interact locally and consecutively with the two qubits through rotating-wave couplings and is then measured. It is shown that we need to repeat this kind of process only three times to establish a maximally entangled state directly from an arbitrary initial mixed state, with no need to prepare the state of the qubits in advance or to rearrange the setup step by step. Furthermore, the maximum yield realizable with this scheme …
By using an ancilla qubit as a mediator, two distant qubits can undergo a non-local entangling unitary operation. This is desirable for when attempting to scale up or distribute quantum computation by combining fixed static local sets of qubits with ballistic mediators. Using a model driven by measurements on the ancilla, it is possible to generate a maximally entangling CZ gate while only having access to a less entangling gate between the pair qubits and the ancilla. However this results in a stochastic process of generating control phase rotation gates where the expected time for success do…
Arrays of qubits encoded in the ground-state manifold of neutral atoms trapped in optical (or magnetic) lattices appear to be a promising platform for the realization of a scalable quantum computer. Two-qubit conditional gates between nearest-neighbor qubits in the array can be implemented by exploiting the Rydberg blockade mechanism, as was shown by D. Jaksch et al. [Phys. Rev. Lett. 85, 2208 (2000)]. However, the energy shift due to dipole-dipole interactions causing the blockade falls off rapidly with the interatomic distance and protocols based on direct Rydberg blockade typically fail to …
The dynamics of entanglement and fidelity for a subsystem of two separate spin-1/2 qubits prepared in Bell states is investigated. One of the subsystem qubit labelled $A$ is under the influence of a Heisenberg XY spin-bath, while another one labelled $B$ is uncoupled with that. We discuss two cases: (i) the number of bath spins $N\to\infty$; (ii) $N$ is finite: N=40. In both cases, the bath is initially prepared in a thermal equilibrium state. It is shown that the time dependence of the concurrence and the fidelity of the two subsystem qubits can be controlled by tuning the parameters of the s…
A hybrid quantum system consisting of spatially separated two-level atoms is studied. Two atoms do not interact directly, but they are coupled via an intermediate system which is consisting of a superconducting flux qubit interacting with a mechanical and an electrical resonator which are coupled to one of the atoms. Moreover, the superconducting flux qubit is driven by a classical microwave field. Applying the adiabatic elimination an effective Hamiltonian for the atomic subsystem is obtained. Our results demonstrate that the entanglement degradation decay as well as the fidelity decay in the…
We present a mathematical proof of the algorithm allowing to generate all - symmetric and non-symmetric - total angular momentum eigenstates in remote matter qubits by projective measurements, proposed in Maser et al. [Phys. Rev. A 79, 033833 (2009)]. By deriving a recursion formula for the algorithm we show that the generated states are equal to the total angular momentum eigenstates obtained via the usual quantum mechanical coupling of angular momenta. In this way we demonstrate that the algorithm is able to simulate the coupling of N spin-1/2 systems, and to implement the required Clebsch-G…
We study a tripartite quantum system consisting of a coplanar-waveguide (CPW) resonator and a nanomechanical resonator (NAMR) connected by a flux qubit, where the flux qubit has a large detuning from both resonators. By a unitray transformation and a second-order approximation, we obtain a strong and controllable (i.e., magnetic-field-dependent) effective coupling between the NAMR and the CPW resonator. Due to the strong coupling, vacuum Rabi splitting can be observed from the voltage-fluctuation spectrum of the CPW resonator. We further study the properties of single photon transport as infer…
We study controllable exchange coupling between two singlet-triplet qubits. We start from the original second quantized Hamiltonian of a quadruple quantum dot system and obtain the effective spin-spin interaction between the two qubits using the projection operator method. Under a strong uniform external magnetic field and an inhomogeneous local micro-magnetic field, the effective interqubit coupling is of the Ising type, and the coupling strength can be expressed in terms of quantum dot parameters. Finally, we discuss how to generate various two-qubit operations using this controllable coupli…
In this work,We investigate the problem of secretly broadcasting of three-qubit entangled state between two distant partners. The interesting feature of this problem is that starting from two particle entangled state shared between two distant partners we find that the action of local cloner on the qubits and the measurement on the machine state vector generates three-qubit entanglement between them. The broadcasting of entanglement is made secret by sending the measurement result secretly using cryptographic scheme based on orthogonal states. Further we show that this idea can be extended to …
Preparing many body entangled states efficiently using available interactions is a challenging task. One solution may be to couple a system collectively with a probe that leaves residual entanglement in the system. We investigate the entanglement produced between two possibly distant qubits 1 and 2 that interact locally with a third qubit 3 under unitary evolution generated by pairwise Hamiltonians. For the case where the Hamiltonians commute, relevant to certain quantum nondemolition measurements, the entanglement between qubits 1 and 2 is calculated explicitly for several classes of initial …
Control of entanglement between qubits at distant quantum processors using a two-qubit gate is an essential function of a scalable, modular implementation of quantum computation. Among the many qubit platforms, spin qubits in silicon quantum dots are promising for large-scale integration along with their nanofabrication capability. However, linking distant silicon quantum processors is challenging as two-qubit gates in spin qubits typically utilize short-range exchange coupling, which is only effective between nearest-neighbor quantum dots. Here we demonstrate a two-qubit gate between spin qub…
Quantum entanglement between distant qubits is an important feature of quantum networks. Distribution of entanglement over long distances can be enabled through coherently interfacing qubit pairs via photonic channels. Here, we report the realization of optically generated quantum entanglement between electron spin qubits confined in two distant semiconductor quantum dots. The protocol relies on spin-photon entanglement in the trionic $Λ$-system and quantum erasure of the Raman-photon path. The measurement of a single Raman photon is used to project the spin qubits into a joint quantum state w…
We present a flexible scheme to realize non-artificial non-Markovian dynamics of an electronic spin qubit, using a nitrogen-vacancy center in diamond where the inherent nitrogen spin serves as a regulator of the dynamics. By changing the population of the nitrogen spin, we show that we can smoothly tune the non-Markovianity of the electron spin's dynamic. Furthermore, we examine the decoherence dynamics induced by the spin bath to exclude other sources of non-Markovianity. The amount of collected measurement data is kept at a minimum by employing Bayesian data analysis. This allows for a preci…
Logical entanglement distribution between distant 2D array qubits
Graduate School of Engineering Science, Osaka University（大阪大学工学研究科）；NTT Computer and Data Science Laboratories, NTT Corporation（日本电信电话公司计算机与数据科学实验室）；JST, PRESTO（日本科学技术振兴机构PRESTO项目）；Center for Quantum Computing, RIKEN（理化学研究所量子计算中心）；Center for Quantum Information and Quantum Biology, Osaka University（大阪大学量子信息与量子生物学中心）；Faculty of Information Science and Technology, Hokkaido University（北海道大学信息科学与工程学院）
We evaluate the exact concurrence between any two spins in a cyclic XX chain of n spins placed in a uniform transverse magnetic field, both at zero and finite temperature, by means of the Jordan-Wigner transformation plus a number parity projected statistics. It is shown that while at T=0 there is always entanglement between any two spins in a narrow field interval before the transition to the aligned state, at low but non-zero temperatures the entanglement remains non-zero for arbitrarily high fields, for any pair separation L, although its magnitude decreases exponentially with the field. It…
Quantum teleportation is the faithful transfer of quantum states between systems, relying on the prior establishment of entanglement and using only classical communication during the transmission. We report teleportation of quantum information between atomic quantum memories separated by about 1 meter. A quantum bit stored in a single trapped ytterbium ion (Yb+) is teleported to a second Yb+ atom with an average fidelity of 90% over a replete set of states. The teleportation protocol is based on the heralded entanglement of the atoms through interference and detection of photons emitted from e…
Controllable non-Hermitian topology in a dynamically protected cat qubit
Fujian Key Laboratory of Quantum Information and Quantum Optics, Fuzhou University（福建量子信息与量子光学重点实验室，福州大学）；Department of Physics, Fuzhou University（福州大学物理系）；School of Physics and Mechanical and Electrical Engineering, Longyan University（龙岩大学物理与机电工程学院）；Hefei National Laboratory（合肥国家实验室）
We propose a theoretical scheme to generate a controllable and switchable coupling between two double-quantum-dot (DQD) spin qubits by using a transmission line resonator (TLR) as a bus system. We study dynamical behaviors of quantum correlations described by entanglement correlation (EC) and discord correlation (DC) between two DQD spin qubits when the two spin qubits and the TLR are initially prepared in $X$-type quantum states and a coherent state, respectively. We demonstrate that in the EC death regions there exist DC stationary states in which the stable DC amplification or degradation c…
It is known that maximally entangled Bell state and three-qubit W-type states are very useful in various quantum information processing task. Thus the problem of preparation of these type of states is very important in quantum information theory. But the factor which prohibit the generation of the above mentioned pure states shared between two and three distant partners is decoherence. When we send one qubit, from a two qubit state, through decoherence channel like amplitude damping channel, the purity of the qubit is lost and it ends up with a mixed state. Therefore it is very difficult to ke…
We theoretically study the non-equilibrium correlations and entanglement between distant semiconductor qubits in a one-dimensional coupled-mechanical-resonator chain. Each qubit is defined by a double quantum dot (DQD) and embedded in a mechanical resonator. The two qubits can be coupled, correlated and entangled through phonon transfer along the resonator chain. We calculate the non-equilibrium correlations and steady-state entanglement at different phonon-phonon coupling rates, and find a maximal steady entanglement induced by a population inversion. The results suggest that highly tunable c…
The ability to connect distant qubits plays a fundamental role in quantum computing. Therefore, quantum systems candidates for quantum computation must be able to interact all their constituent qubits. Here, we model the quantum dot spin qubits by a spin chain with nearest-neighbors interaction. Within this model, we can perform the interaction of distant qubits by the action of consecutive SWAP gates. The SWAP gate exchange the information of two different qubits and it is obtained by a time-dependent interaction of nearest-neighbors qubits that is switched on and off as the quantum informati…
We present an experimentally implementable method to couple Josephson charge qubits and to generate and detect macroscopic entangled states. A large-junction superconducting quantum interference device is used in the qubit circuit for both coupling qubits and implementing the readout. Also, we explicitly show how to achieve a microwave-assisted macroscopic entanglement in the coupled-qubit system.
Two-level system fluctuators (TLS's) in the tunnel barrier of a Josephson junction have recently been demonstrated to cause novel energy splittings in spectroscopic measurements of superconducting phase qubits. With their strong coupling to the Josephson junction and relatively long decoherence times, TLS's can be considered as potential qubits and demonstrate coherent quantum effects. Here, we study the effective interaction between the TLS qubits that is mediated by a Josephson junction resonator driven by an external microwave source. This effective interaction can enable controlled quantum…
We study state engineering through bilinear interactions between two remote qubits and two-mode Gaussian light fields. The attainable two-qubit states span the entire physically allowed region in the entanglement-versus-global-purity plane. Two-mode Gaussian states with maximal entanglement at fixed global and marginal entropies produce maximally entangled two-qubit states in the corresponding entropic diagram. We show that a small set of parameters characterizing extremally entangled two-mode Gaussian states is sufficient to control the engineering of extremally entangled two-qubit states, wh…
Each Bell state has the property that by performing just local operations on one qubit, the complete Bell basis can be generated. That is, states generated by local operations are totally distinguishable. This remarkable property is due to maximal quantum entanglement between the two particles. We present a set of local unitary transformations that generate out of partially entangled two-qubit state a set of four maximally distinguishable states that are mutually equally distant. We discuss quantum dense coding based on these alphabet states.
We have realized controllable coupling between two three-junction flux qubits by inserting an additional coupler loop between them, containing three Josephson junctions. Two of these are shared with the qubit loops, providing strong qubit--coupler interaction. The third junction gives the coupler a nontrivial current--flux relation; its derivative (i.e., the susceptibility) determines the coupling strength J, which thus is tunable in situ via the coupler's flux bias. In the qubit regime, J was varied from ~45 (antiferromagnetic) to ~ -55 mK (ferromagnetic); in particular, J vanishes for an int…
Realizing robust quantum information transfer between long-lived qubit registers is a key challenge for quantum information science and technology. Here we demonstrate unconditional teleportation of arbitrary quantum states between diamond spin qubits separated by 3 meters. We prepare the teleporter through photon-mediated heralded entanglement between two distant electron spins and subsequently encode the source qubit in a single nuclear spin. By realizing a fully deterministic Bell-state measurement combined with real-time feed-forward we achieve teleportation in each attempt while obtaining…
The depolarizing quantum operation plays an important role in studying the quantum noise effect and implementing general quantum operations. In this work, we report a scheme which implements a fully controllable input-state independent depolarizing quantum operation for a photonic polarization qubit.
We consider the inelastic scattering of two photons from two qubits separated by an arbitrary distance $R$ and coupled to a one-dimensional transmission line. We present an exact, analytical solution to the problem, and use it to explore a particular configuration of qubits which is transparent to single-photon scattering, thus highlighting non-Markovian effects of inelastic two-photon scattering: Strong two-photon interference and momentum dependent photon (anti-)bunching. This latter effect can be seen as an inelastic generalization of the Hong--Ou--Mandel effect.
We demonstrate a hierarchy of various classes of quantum correlations on experimentally prepared two-qubit Werner-like states with controllable white noise. Werner states, which are white-noise-affected Bell states, are prototypal examples for studying such a hierarchy as a function of the amount of white noise. We experimentally generated Werner states and their generalizations, i.e., partially entangled pure states affected by white noise. These states enabled us to study the hierarchy of the following classes of correlations: separability, entanglement, steering in three- and two-measuremen…
The exchange coupling between quantum dot spin qubits is isotropic, which restricts the types of quantum gates that can be formed. Here, we propose a method for controlling anisotropic interactions between spins arranged in a bus geometry. The symmetry is broken by an external magnetic field, resulting in XXZ-type interactions that can efficiently generate Greenberger-Horne-Zeilinger (GHZ) states or universal gate sets for exchange-only quantum computing. Analysis of the bus ground state provides a link between quantum critical phenomena and the communication capacity.
Efficient communication between qubits relies on robust networks which allow for fast and coherent transfer of quantum information. It seems natural to harvest the remarkable properties of systems characterized by topological invariants to perform this task. Here we show that a linear network of coupled bosonic degrees of freedom, characterized by topological bands, can be employed for the efficient exchange of quantum information over large distances. Important features of our setup are that it is robust against quenched disorder, all relevant operations can be performed by global variations …
We describe a scheme that allows for the generation of any desired N-photon state on demand. Under ideal conditions, this requires only N single photon sources, laser pulses and linear optics elements. First, the sources should be initialised with the help of single-qubit rotations and repeat-until-success two-qubit quantum gates [Lim et al., Phys. Rev. Lett. 95, 030305 (2005)]. Afterwards, the state of the sources can be mapped onto the state of N newly generated photons whenever needed.
Combating the detrimental effects of noise remains a major challenge in realizing a scalable quantum computer. To help to address this challenge, we introduce a model realizing a controllable qubit-bath coupling using a sequence of LC resonators. The model establishes a strong coupling to a low-temperature environment which enables us to lower the effective qubit temperature making ground state initialization more efficient. The operating principle is similar to that of a recently proposed coplanar-waveguide cavity (CPW) system, for which our work introduces a complementary and convenient expe…
Remote Adiabatic Controlled-Z Gate between Distant Superconducting Qubits
Beijing Academy of Quantum Information Sciences（北京量子信息科学研究院）；Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所）；University of Chinese Academy of Sciences（中国科学院大学）；Hefei National Laboratory（合肥国家实验室）
We consider a one-dimensional (1D) structure where non-interacting spin-$s$ scattering centers, such as quantum impurities or multi-level atoms, are embedded at given positions. We show that the injection into the structure of unpolarized flying qubits, such as electrons or photons, along with {path} detection suffice to accomplish spin-state teleportation between two centers via a third ancillary one. {No action over the internal quantum state of both the spin-$s$ particles and the flying qubits is required. The protocol enables the transfer of quantum information between well-seperated stati…
We show how to generate and control the correlations in a set of two distant quantum emitters coupled to a one-dimensional dissipative plasmonic waveguide. An external laser field enhances the dimer's steady-state correlations and allows an active control (switching on/off) of nonclassical correlations. The plasmon-assisted dipolar-interacting qubits exhibit persistent correlations, which in turn can be decoupled and made to evolve independently from each other. The setup enables long-distance ($\sim 1\, μ$m) qubit control that works for both resonant and detuned emitters. For suitable emitter…
We propose an approach to coherently transfer populations between selected quantum states in one- and two-qubit systems by using controllable Stark-chirped rapid adiabatic passages (SCRAPs). These {\it evolution-time insensitive} transfers, assisted by easily implementable single-qubit phase-shift operations, could serve as elementary logic gates for quantum computing. Specifically, this proposal could be conveniently demonstrated with existing Josephson phase qubits. Our proposal can find an immediate application in the readout of these qubits. Indeed, the broken parity symmetries of the boun…
We consider two distant spin-$\frac{1}{2}$ particles (or qubits) and a number of interacting objects, all with the same value $S\gg1$ of their respective spin, distributed on a one-dimensional lattice (or large-$S$ spin chain). The quantum states of the chain are constructed by linearly combining tensor products of single-spin coherent states, whose evolution is determined accordingly, i.e., via classical-like equations of motions. We show that the quantum superposition of the above product states resulting from a local interaction between the first qubit and one spin of the chain evolves so t…
We propose an experimentally realizable method to control the coupling between two flux qubits. In our proposal, the bias fluxes are always fixed for these two inductively-coupled qubits. The detuning of these two qubits can be initially chosen to be sufficiently large, so that their initial interbit coupling is almost negligible. When a time-dependent magnetic flux (TDMF) is applied to one of the qubits, a well-chosen frequency of the TDMF can be used to compensate the initial detuning and to couple two qubits. This proposed method avoids fast changes of either qubit frequencies or the amplit…
We investigate a hybrid quantum system consisting of spatially separated resonant exchange qubits, defined in three-electron semiconductor triple quantum dots, that are coupled via a superconducting transmission line resonator. Drawing on methods from circuit quantum electrodynamics and Hartmann-Hahn double resonance techniques, we analyze three specific approaches for implementing resonator-mediated two-qubit entangling gates in both dispersive and resonant regimes of interaction. We calculate entangling gate fidelities as well as the rate of relaxation via phonons for resonant exchange qubit…
We propose an active mechanism for coupling the quantized mode of a nanomechanical resonator to the persistent current in the loop of a superconducting Josephson junction (or phase slip) flux qubit. This coupling is independently controlled by an external coupling magnetic field. The whole system forms a novel solid-state cavity quantum electrodynamics (QED) architecture in the strong coupling limit. This architecture can be used to demonstrate quantum optics phenomena and coherently manipulate the qubit for quantum information processing. The coupling mechanism is applicable for more generali…
All quantum information processes inevitably requires the explicit state preparation of an entangled state. Here we present the construction of a quantum switchboard which can act both as an optimal quantum cloning machine and a quantum demultiplexer based on the preparation of a four-qubit state.
