Transferring quantum entangled states between multiple single-photon-state qubits and coherent-state qubits in circuit QED

We present a way to transfer maximally- or partially-entangled states of n single-photon-state (SPS) qubits onto ncoherent-state (CS) qubits, by employing 2nmicrowave cavities coupled to a superconducting flux qutrit. The two logic states of a SPS qubit here are represented by the vacuum state and the single-photon state of a cavity, while the two logic states of a CS qubit are encoded with two coherent states of a cavity. Because of using only one superconducting qutrit as the coupler, the circuit architecture is significantly simplified. The operation time for the state transfer does not increase with the increasing of the number of qubits. When the dissipation of the system is negligible, the quantum state can be transferred in a deterministic way since no measurement is required. Furthermore, the higher-energy intermediate level of the coupler qutrit is not excited during the entire operation and thus decoherence from the qutrit is greatly suppressed. As a specific example, we numerically demonstrate that the high-fidelity transfer of a Bell state of two SPS qubits onto two CS qubits is achievable within the present-day circuit QED technology. Finally, it is worthy to note that when the dissipation is negligible, entangled states of n CS qubits can be transferred back onto n SPS qubits by performing reverse operations. This proposal is quite general and can be extended to accomplish the same task, by employing a natural or artificial atom to couple 2nmicrowave or optical cavities.     

Transfer of quantum entangled states between superconducting qubits and microwave field qubits

Transferring entangled states between matter qubits and microwave-field (or optical-field) qubits is of fundamental interest in quantum mechanics and necessary in hybrid quantum information processing and quantum communication. We here propose a way for transferring entangled states between superconducting qubits (matter qubits) and microwave-field qubits. This proposal is realized by a system consisting of multiple superconducting qutrits and microwave cavities. Here, „qutrit” refers to a three-level quantum system with the two lowest levels encoding a qubit while the third level acting as an auxiliary state. In contrast, the microwave-field qubits are encoded with coherent states of microwave cavities. Because the third energy level of each qutrit is not populated during the operation, decoherence from the higher energy levels is greatly suppressed. The entangled states can be deterministically transferred because measurement on the states is not needed. The operation time is independent of the number of superconducting qubits or microwave-field qubits. In addition, the architecture of the circuit system is quite simple because only a coupler qutrit and an auxiliary cavity are required. As an example, our numerical simulations show that high-fidelity transfer of entangled states from two superconducting qubits to two microwave-field qubits is feasible with present circuit QED technology. This proposal is quite general and can be extended to transfer entangled states between other matter qubits (e.g., atoms, quantum dots, and NV centers) and microwave- or optical-field qubits encoded with coherent states.     

Universal and deterministic manipulation of the quantum state of harmonic oscillators: a route to unitary gates for Fock state qubits

We present a simple quantum circuit that allows for the universal and deterministic manipulation of the quantum state of confined harmonic oscillators. The scheme is based on the selective interactions of the referred oscillator with an auxiliary three-level system and a classical external driving source, and enables any unitary operations on Fock states, two by two. One circuit is equivalent to a single qubit unitary logical gate on Fock states qubits. Sequences of similar protocols allow for complete, deterministic, and state-independent manipulation of the harmonic oscillator quantum state.     

All-optical generation of states for "Encoding a qubit in an oscillator"

Most quantum computation schemes propose encoding qubits in two-level systems. Others exploit the use of an infinite-dimensional system. In "Encoding a qubit in an oscillator" [Phys. Rev. A 64, 012310 (2001)], Gottesman, Kitaev, and Preskill (GKP) combined these approaches when they proposed a fault-tolerant quantum computation scheme in which a qubit is encoded in the continuous position and momentum degrees of freedom of an oscillator. One advantage of this scheme is that it can be performed by use of relatively simple linear optical devices, squeezing, and homodyne detection. However, we lack a practical method to prepare the initial GKP states. Here we propose the generation of an approximate GKP state by using superpositions of optical coherent states (sometimes called "Schr?dinger cat states"), squeezing, linear optical devices, and homodyne detection.     

Quantum switch for coupling highly detuned superconducting qubits

We propose to implement a quantum switch scheme for coupling highly detuned superconducting qubits connected by a gap-tunable bridge qubit. By modulating the frequency of the bridge qubit, it can be used as a coupler to switch on/off and adjust the coupling strength between the initially non-interaction qubits. It is shown that the proposals of quantum information transfer and quantum entangled gate between two highly detuned qubits can be implemented with high fidelity. Moreover, we extend the case of coupling the switch to multiple qubits for the generation of W states. The advantages of our scheme are that it eliminates the need for tuning the gaps of the qubits and the cross-talk interaction is greatly suppressed. The influence of decoherence and parameter variation is also investigated by numerical simulation, which suggests that the present scheme is feasible in current experiment.     

Qubit movement-assisted entanglement swapping

We propose a scheme to generate entanglement between two distant qubits(two-level atom) which are separately trapped in their own(in general) non-Markovian dissipative cavities by utilizing entangling swapping, considering the case in which the qubits can move along their cavity axes rather than a static state of motion. We first examine the role of movement of the qubit by studying the entropy evolution for each subsystem. The average entropy over the initial states of the qubit is calculated. Then by performing a Bell state measurement on the fields leaving the cavities, we swap the entanglement between qubit-field in each cavity into qubit-qubit and field-field subsystems. The entangling power is used to measure the average amount of swapped entanglement over all possible pure initial states. Our results are presented in two weak and strong coupling regimes, illustrating the positive role of movement of the qubits on the swapped entanglement.It is revealed that by considering certain conditions for the initial state of qubits, it is possible to achieve a maximally long-leaving stationary entanglement(Bell state) which is entirely independent of the environmental variables as well as the velocity of qubits. This happens when the two qubits have the same velocities.     

Realization of Three-Qubit Controlled-Phase Gate Operation with Atoms in Cavity QED System

We propose a scheme for realization of three-qubit controlled-phase gate via passing two three-level atoms through a high-Q optical cavity in a cavity QED system. In the presented protocol, the two stable ground states of the atoms act as the two controlling qubits and the zero- and one-photon Fock states of the cavity-field form the target qubit, and no auxiliary state or any measurement is required. The numerical simulation shows that the gate fidelities remain at a high level under the influence of the atomic spontaneous emission, the decay of the cavity mode and deviation of the coupling strength. The experimental feasibility of our proposal is also discussed.     

A quantum computer in the scheme of an atomic quantum transistor with logical encoding of qubits

A scheme of a multiqubit quantum computer on atomic ensembles using a quantum transistor implementing two qubit gates is proposed. We demonstrate how multiatomic ensembles permit one to work with a large number of qubits that are represented in a logical encoding in which each qubit is recorded on a superposition of single-particle states of two atomic ensembles. The access to qubits is implemented by appropriate phasing of quantum states of each of atomic ensembles. An atomic quantum transistor is proposed for use when executing two qubit operations. The quantum transistor effect appears when an excitation quantum is exchanged between two multiatomic ensembles located in two closely positioned QED cavities connected with each other by a gate atom. The dynamics of quantum transfer between atomic ensembles can be different depending on one of two states of the gate atom. Using the possibilities of control for of state of the gate atom, we show the possibility of quantum control for the state of atomic ensembles and, based on this, implementation of basic single and two qubit gates. Possible implementation schemes for a quantum computer on an atomic quantum transistor and their advantages in practical implementation are discussed.     

Quantum Information Processing in a Coupled Cavity Array

We consider a one-dimensional array of L identical coupled cavities, and each cavity is doped with a two-level qubit. Experimentally, it has been developed in several varieties by the newest technology. We find that the one-qubit quantum state can be perfectly transferred through the cavity array, and the entanglement between the first two qubits can also be transferred to the last two qubits. In addition, we successfully realized the entangling gate and swap gate in the coupled cavity array.     

Three-qubit quantum-gate operation with an SQUID in a cavity

We propose a method of realizing a three-qubit quantum gate with a superconducting quantum interference device(SQUID) in a cavity.In this proposal,the gate operation involves the SQUID ground-states and the Fock states of cavity modes b and c.The two field-modes act as the controlling qubits,and the two SQUID states form the target qubit.Since only the metastable lower levels are involved in the gate operation,the gate is not affected by the SQUID decay rates.     

Fast and high-fidelity generation of photonic Greenberger-Horne-Zeilinger states in circuit QED

A fast scheme to generate Greenberger-Horne-Zeilinger states between different cavities in circuit QED systems is proposed. To implement this scheme, we design a feasible experimental device with three qubits and three cavities. In this device, all the couplings between qubit and qubit, cavity and qubit are tunable and are independent with frequencies, and thus the shortcut to adiabaticity technique can be directly applied in our scheme. It is demonstrated that the GHZ state can be generated rapidly with high fidelity in our scheme.     

飞秒激光直写光量子逻辑门

量子比特在同一时刻可处于所有可能状态上的叠加特性使得量子计算机具有天然的并行计算能力,在处理某些特定问题时具有超越经典计算机的明显优势.飞秒激光直写技术因其具有单步骤高效加工真三维光波导回路的能力,在制备通用型集成光量子计算机的基本单元—量子逻辑门中发挥着越来越重要的作用.本文综述了飞秒激光直写由定向耦合器构成的光量子比特逻辑门的进展.主要包括定向耦合器的功能、构成、直写和性能表征,集成波片、哈达玛门和泡利交换门等单量子比特逻辑门、受控非门和受控相位门等两量子比特逻辑门的直写加工,并对飞秒激光加工三量子比特逻辑门进行了展望.     

Preparation of multi-photon Fock states and quantum entanglement properties in circuit QED

We demonstrate the controllable generation of multi-photon Fock states in circuit quantum electrodynamics （circuit QED）. The external bias flux regulated by a counter can effectively adjust the bias time on each superconducting flux qubit so that each flux qubit can pass in turn through the circuit cavity and thereby avoid the effect of decoherence. We further investigate the quantum correlation dynamics of coupling superconducting qubits in a Fock state. The results reveal that the lower the photon number of the light field in the number state, the stronger the interaction between qubits is, then the more beneficial to maintaining entanglement between qubits it will be.     

Implementing multi-qubit entanglement of two-level systems inside a superconducting phase qubit

The interaction between a superconducting phase qubit and the two-level systems located inside the Josephson tunnel barrier is described by the XY model, which is naturally used to implement the i-SWAP gate. With this gate, we propose a scheme to efficiently generate multi-qubit entangled states of such two-level systems, including multipartite W state and cluster states. In particular, it is found that, with the help of the phase qubit, the entanglement witness can be used to efficiently detect the produced multi-qubit entangled states.     

腔QED中相位协变量子克隆机的实现及纠错

应用N个二能级原子和单模真空腔场相互作用,提出了一个1→2的相位协变量子克隆机的方案.同时,基于这种克隆机,我们也提出了量子纠错方案,考虑了由相位和比特反转错误所产生的消相干影响,通过对后两个比特位进行Bell测量,并沿着合适的轴旋转第一比特,就可以恢复初始态.     

Quantum computer hardware based on rare-earth-ion-doped inorganic crystals

We present a scheme for generating multiple, strongly interacting qubits in rare-earth-ion-doped inorganic crystals at cryogenic temperatures. Two ground state hyperfine levels, with hour long lifetimes and ms decoherence times are chosen as qubit states. Controlled logic between the qubits is accomplished using the change in permanent dipole moment induced by an optical transition between the ground and excited state of these ions. The scheme is based on existing material data and established measurement techniques and should therefore be straightforward to realise in practice. The procedure used for creating the qubits can be generalised also to other solid state systems.     

One-step implementation of a multi-target-qubit controlled phase gate with cat-state qubits in circuit QED

We propose a single-step implementation of a muti-target-qubit controlled phase gate with one catstate qubit (cqubit) simultaneously controlling n–1 target cqubits. The two logic states of a cqubit are represented by two orthogonal cat states of a single cavity mode. In this proposal, the gate is implemented with n microwave cavities coupled to a superconducting transmon qutrit. Because the qutrit remains in the ground state during the gate operation, decoherence caused due to the qutrit’s energy relaxation and dephasing is greatly suppressed. The gate implementation is quite simple because only a single-step operation is needed and neither classical pulse nor measurement is required. Numerical simulations demonstrate that high-fidelity realization of a controlled phase gate with one cqubit simultaneously controlling two target cqubits is feasible with present circuit QED technology. This proposal can be extended to a wide range of physical systems to realize the proposed gate, such as multiple microwave or optical cavities coupled to a natural or artificial three-level atom.     

Transferring entangled states of photonic cat-state qubits in circuit QED

We propose a method for transferring quantum entangled states of two photonic cat-state qubits(cqubits)from two microwave cavities to the other two microwave cavities.This proposal is realized by using four microwave cavities coupled to a superconducting flux qutrit.Because of using four cavities with different frequencies,the inter-cavity crosstalk is significantly reduced.Since only one coupler qutrit is used,the circuit resource is minimized.The entanglement transfer is completed with a singlestep operation only,thus this proposal is quite simple.The third energy level of the coupler qutrit is not populated during the state transfer,therefore decoherence from the higher energy level is greatly suppressed.Our numerical simulations show that high-fidelity transfer of two-cqubit entangled states from two transmission line resonators to the other two transmission line resonators is feasible with current circuit QED technology.This proposal is universal and can be applied to accomplish the same task in a wide range of physical systems,such as four microwave or optical cavities,which are coupled to a natural or artificial three-level atom.     

Unconventional Geometric Phase-Shift Gates Based on Superconducting Quantum Interference Devices Coupled to a Single-Mode Cavity

We present a scheme to realize geometric phase-shift gate for two superconducting quantum interference device （SQUID） qubits coupled to a single-mode microwave field. The geometric phase-shift gate operation is performed in two lower flux states, and the excited state [2〉 would not participate in the procedure. The SQUIDs undergo no transitions during the gate operation. Thus, the docoherence due to energy spontaneous emission based on the levels of SQUIDs are suppressed. The gate is insensitive to the cavity decay throughout the operation since the cavity mode is displaced along a circle in the phase space, acquiring a phase conditional upon the two lower flux states of the SQUID qubits, and the cavity mode is still in the original vacuum state. Based on the SQUID qubits interacting with the cavity mode, our proposed approach may open promising prospects for quantum iogic in SQUID-system.     

Towards strongly correlated photons in arrays of dissipative nonlinear cavities under a frequency-dependent incoherent pumping

We report a theoretical study of a quantum optical model consisting of an array of strongly nonlinear cavities incoherently pumped by an ensemble of population-inverted two-level atoms. Projective methods are used to eliminate the atomic dynamics and write a generalized master equation for the photonic degrees of freedom only, where the frequency-dependence of gain introduces non-Markovian features. In the simplest single cavity configuration, this pumping scheme gives novel optical bistability effects and allows for the selective generation of Fock states with a well-defined photon number. For many cavities in a weakly non-Markovian limit, the non-equilibrium steady state recovers a Grand-Canonical statistical ensemble at a temperature determined by the effective atomic linewidth. For a two-cavity system in the strongly nonlinear regime, signatures of a Mott state with one photon per cavity are found.