Applications In Quantum Information Processing Research Articles

Dynamical Collective Excitations and Entanglement of Two Strongly Correlated Rydberg Superatoms

Based on the dipole blockade effect and with the aid of the superatom (SA) model, we propose a scheme to investigate the correlated evolution of two Rydberg sub-superatoms (SSAs), formed by two spatially separated atomic Rydberg sub-ensembles but in the same blockade region. Starting from the pure separable states, we investigate the in-phase or anti-phase correlated dynamics and explore how two Rydberg SSAs entangle with each other mediated by a single Rydberg excitation. Starting from the entangled states, we discuss the robustness of the system against decoherence induced by the dephasing rate. Our results show that both the correlated evolution of two Rydberg SSAs and their collective-state entanglement are usually sensitive to the number of each Rydberg SSA. This allows us to coherently manipulate the Rydberg ensemble over long distances from the single-quantum level to the mesoscopic level by changing the number of atoms. Furthermore, the method for dividing an SA into two SSAs and obtaining their spin operators without any approximation can be readily generalized to the case of many SSAs. It may have potential promising applications in quantum information processing and provide an attractive platform to study the quantum-classical correspondence, many-body physics and so on.

Quantum information processing with integrated silicon carbide photonics

Color centers in wide bandgap semiconductors are prominent candidates for solid-state quantum technologies due to their attractive properties including optical interfacing, long coherence times, and spin–photon and spin–spin entanglement, as well as the potential for scalability. Silicon carbide color centers integrated into photonic devices span a wide range of applications in quantum information processing in a material platform with quantum-grade wafer availability and advanced processing capabilities. Recent progress in emitter generation and characterization, nanofabrication, device design, and quantum optical studies has amplified the scientific interest in this platform. We provide a conceptual and quantitative analysis of the role of silicon carbide integrated photonics in three key application areas: quantum networking, simulation, and computing.

Multi-Way Noiseless Signal Amplification in a Symmetrical Cascaded Four-Wave Mixing Process

According to the fundamental laws of quantum optics, vacuum noise is inevitably added to the signal when one tries to amplify a signal. However, it has been recently shown that noiseless signal amplification can be realized when a phase-sensitive process is involved. Two phase-sensitive schemes, a correlation injection scheme and a two-beam phase-sensitive amplifier scheme, are both proposed to realize multi-way noiseless signal amplification in a symmetrical cascaded four-wave mixing process. We theoretically study the possibility of the realization of four-way noiseless signal amplification by using these two schemes. The results show that the correlation injection scheme can only realize one-way noiseless signal amplification, but that the two-beam phase-sensitive amplifier scheme can lead to four-way noise figure values below 1. Our results here may find potential applications in quantum information processing, e.g., the realization of quantum information tap and quantum non-demolition measurement, etc.

Heralded Multiplexed High-Efficiency Cascaded Source of Dual-Rail Entangled Photon Pairs Using Spontaneous Parametric Down-Conversion

Deterministic sources of high-fidelity entangled qubit pairs encoded in the dual-rail photonic basis, i.e., presence of a single photon in one of two orthogonal modes, are a key enabling technology of many applications of quantum information processing, including high-rate high-fidelity quantum communications over long distances. The most popular and mature sources of such photonic entanglement, e.g., those that leverage spontaneous parametric down-conversion (SPDC) or spontaneous four-wave mixing (sFWM), generate an entangled (so-called, continuous-variable) quantum state that contains contributions from high-order photon terms that lie outside the span of the dual-rail basis, which is detrimental to most applications. One often uses low pump power to mitigate the effects of those high-order terms. However that reduces the pair generation rate, and the source becomes inherently probabilistic. We investigate a cascaded source that performs a linear-optical entanglement swap between two SPDC sources, to generate a heralded photonic entangled state that has a higher fidelity (to the ideal Bell state) compared to a free-running SPDC source. Further, with the Bell swap providing a heralding trigger, we show how to build a multiplexed source, which despite reasonable switching losses and detector loss and noise, yields a Fidelity versus Success Probability trade-off of a high-efficiency source of high-fidelity dual-rail photonic entanglement. We find however that there is a threshold of $1.5$ dB of loss per switch, beyond which multiplexing hurts the Fidelity versus Success Probability trade-off.

Theories of indirect chiral coupling and proposal of Fabry–Perot resonance as a flexible chiral-coupling interface

The chiral coupling of an emitter to waveguide mode, i.e., the propagation direction of the excited waveguide mode is locked to the transverse spin (T-spin) of a circularly polarized emitter, has exhibited unprecedented applications in nanophotonics and quantum information processing. This chiral coupling can be largely enhanced in terms of unidirectivity, efficiency, and spontaneous emission rate by introducing resonant modes as coupling interfaces. However, this indirect chiral coupling still undergoes limitations in flexibility and miniaturization, and the underlying physical mechanisms are to be clarified. Here, we present an intuitive and rigorous approach for analyzing the direct/indirect chiral coupling, and thereout, derive some general relations between the chiral-coupling directionality and the T-spin of the field or emitter. Based on the theories, we propose an indirect chiral-coupling system on the platform of surface plasmon polariton (SPP), with a nanocavity supporting Fabry–Perot (FP) resonance of dual SPP modes serving as a novel coupling interface. The FP resonance provides flexible design freedoms which can modulate the chirality of the T-spin (and the resultant chiral-coupling directionality) to flip or disappear. A unidirectivity up to 99.9% along with a high coupling efficiency and enhancement of spontaneous emission rate is achieved. Two first-principles-based SPP models for the reciprocal and original problems are built up to verify the decisive role of the FP resonance in achieving the chiral coupling. The proposed theories and novel chiral-coupling interface will be beneficial to the design of more compact and flexible chiral-coupling systems for diverse applications.

Quantum squeezing of slow-light dark solitons via electromagnetically induced transparency

We consider the quantum effect of slow light dark soliton (SLDS) in a cold atomic gas with defocuing Kerr nonlinearity via electromagnetically induced transparency (EIT). We calculate the quantum fluctuations of the SLDS by solving the relevant non-Hermitian eigenvalue problem describing the quantum fluctuations, and find that only one zero mode is allowed. This is different from the quantum fluctuations of bright solitons, where two independent zero modes occur. We rigorously prove that the eigenmodes, which consist of continuous modes and the zero mode, are bi-orthogonal and constitute a complete bi-orthonormalized basis, useful for the calculation on the quantum fluctuations of the SLDS. We demonstrate that, due to the large Kerr nonlinearity contributed from the EIT effect, a significant quantum squeezing of the SLDS can be realized; the squeezing efficiency can be manipulated by the Kerr nonlinearity and the soliton's amplitude, which can be much higher than that of bright solitons. Our work contributes to efforts for developing quantum nonlinear optics and non-Hermitian Physics, and for possible applications in quantum information processing and precision measurements.

Observation of anomalous heat transport in a trapped ion chain

Energy transport in low-dimensional systems or systems with long-range interactions shows unusual properties not observed in conventional three-dimensional materials. Trapped linear ion chains have been proposed as an ideal platform to study such phenomena, but many predicted features have not been demonstrated yet and the existing experiments mainly focus on the weak excitation limit where the dynamics can be described by linearized phonon modes. Here, we study the energy transport in a chain of $^{174}\mathrm{Yb}^{+}$ ions using a focused heating laser beam. Nontrivial effects are observed such as a nonmonotonic energy distribution with reflection symmetry, oscillatory heating dynamics, and a change in the heating pattern due to the competition between the external driving and interion coupling. Our work deepens the understanding about transport in low-dimensional systems and also provides guidelines for the sympathetic cooling of long ion chains, with applications in quantum information processing and precision measurement.

Guanine-Specific Chemical Reaction Reveals ssDNA Interactions on Carbon Nanotube Surfaces.

Understanding the conformations of physisorbed single-stranded DNA (ssDNA) oligos on single-wall carbon nanotube (SWCNT) surfaces is important for advancing basic nanoscience and for developing applications in biomedicine and quantum information processing. Here we report evidence that the ssDNA strands are partly desorbed from the nanotube surface under common conditions. SWCNT suspensions were prepared in eight ssDNA oligos, each containing 1 guanine and 30 thymine bases but differing in the position of the guanine within the strand. Singlet oxygen exposure then covalently functionalized the guanine to the SWCNT surface, red-shifting the nanotube fluorescence by an amount reflecting the guanine spatial density at the surface. Spectral shifts were greatest for central guanine positions and smallest for end positions. In conjunction with steered molecular dynamics simulations, the results suggest that steric interference between neighboring ssDNA strands on an individual nanotube causes significant dislocation or desorption of the strand ends while central regions remain better wrapped around the nanotube. This effect decreases with decreasing concentrations of free ssDNA.

Statistical properties of non-Gaussian quantum states generated via thermal state truncation

Quantum scissors devices proposed by Pegg are beneficial to obtain highly nonclassical quantum states and indispensable for meeting the requirements of quantum information and computation. In this paper, via inputting and detecting single photon states, quantum scissor operation is equivalent to a mixed superposition of three pure state projection operators, which means that the output states are always truncated for any input state. We theoretically prepare a class of non-Gaussian quantum states via thermal state truncation and investigate their statistical properties using average photon number, gain intensity and signal to noise ratio. It is shown that the intensity gain and signal to noise ratio greater than one can be achieved by modulating the thermal parameter and the transmissivity, which realizes the signal amplification and enhancement. Besides, quantum scissor operation can generate the highly non-classical quantum state by investigating the negativity volume of Wigner function. These results indicate that the usage of the new non-Gaussian states may have potential applications in certain quantum information processing.

Generation of Bell and Greenberger-Horne-Zeilinger states from a hybrid qubit-photon-magnon system

We propose a level-resolved protocol in a hybrid architecture for connecting a superconducting qubit and a magnon mode contained within a microwave cavity (resonator) to generate the local and global entangled states, enabling a wide range of applications in quantum communication, quantum metrology, and quantum information processing. Exploiting the high-degree of controllability in such a hybrid qubit-photon-magnon system, we derive effective Hamiltonians at the second- or the third-order resonant points by virtue of the strong counter-rotating interactions between the resonator and the qubit and between the resonator and the magnon. Consequently, we can efficiently generate the Bell states of the photon-magnon and the qubit-magnon subsystems and the Greenberger-Horne-Zeilinger state of the whole hybrid system. We also check the robustness of our protocol against the environmental noise by the Lindblad master equation. Our work makes this hybrid platform of high-degree of controllability a high-fidelity candidate for the realization of the maximally-entangled multiple states.

Quantum-Memory-Enhanced Preparation of Nonlocal Graph States.

Graph states are an important class of multipartite entangled states. Previous experimental generation of graph states and in particular the Greenberger-Horne-Zeilinger (GHZ) states in linear optics quantum information schemes is subjected to an exponential decay in efficiency versus the system size, which limits its large-scale applications in quantum networks. Here, we demonstrate an efficient scheme to prepare graph states with only a polynomial overhead using long-lived atomic quantum memories. We generate atom-photon entangled states in two atomic ensembles asynchronously, retrieve the stored atomic excitations only when both sides succeed, and further project them into a four-photon GHZ state. Wemeasure the fidelity of this GHZ state and further demonstrate its applications in the violation of Bell-type inequalities and in quantum cryptography. Our work demonstrates the prospect of efficient generation of multipartite entangled states in large-scale distributed systems with applications in quantum information processing and metrology.

Azimuthal controlling of electromagnetically induced phase grating in five-level quantum systems

In this paper, we suggest a theoretical model for creating a two-dimensional (2D) electromagnetically induced phase grating in a five-level quantum system interacting with a weak probe light, two coupling standing light in the x and y directions and a Laguerre–Gaussian (LG) field. By derivation of the Maxwell’s wave equation, we obtain the dynamic response of the probe light in the quantum system. Then, we perform the analytical solution of the probe susceptibility for obtaining the linear and nonlinear properties of the medium. By numerical calculations, we discuss the amplitude and phase modulations and Fraunhofer diffraction patterns of the probe light in different parametric conditions. We show that by adjusting the orbital angular momentum (OAM) of the LG light a 2D grating is observed. Moreover, we find that the probe energy can be transferred from zero order to high order of direction when we change the OAM number of LG light. Our proposed model may have potential applications in fundamental research and quantum information processing based on OAM light in quantum systems.

Polarization-path-frequency entanglement using interferometry and frequency shifters

Higher dimensional Hilbert space along with ability to control multiple degrees of freedom of photon and entangle them has enabled new quantum protocols for various quantum information processing applications. Here, we propose a scheme to generate and control polarization-path-frequency entanglement using the operative elements required to implement a polarization-controlled quantum walk in the path (position) space and frequency domain. Hyperentangled states manifests in the controlled dynamics using an interferometric setup where half-wave plates, beam-splitters and frequency shifters such as those based on the electro-optic effect are used to manipulate the polarization, path and frequency degrees of freedom respectively. The emphasis is on utilizing the polarization to influence the movement to a specific value in the frequency and position space. Negativity between the subspaces is calculated to demonstrate the controllability of the entanglement between the three degrees of freedom and the effect of noise on the entanglement is modelled using the depolarizing channel. Progress reported with experimental demonstration of realization of quantum walk using quantum states of light makes quantum walks a practical approach to generate hyperentangled states.

Generation and dynamic manipulation of frequency degenerate polarization entangled Bell states by a silicon quantum photonic circuit

A silicon quantum photonic circuit was proposed and realized for the generation and the dynamic manipulation of telecom-band frequency-degenerate polarization entangled Bell states. Frequency degenerate biphoton states were generated in four silicon waveguides by spontaneous four wave mixing. They were transformed to polarization entangled Bell states through on-chip quantum interference and quantum superposition, and then coupled to optical fibers. The property of polarization entanglement in generated photon pairs was demonstrated by two-photon interference under two non-orthogonal polarization bases. The output state could be dynamically switched between two Bell states, which was demonstrated by the simplified Bell state measurement. The experiment results indicated that the manipulation speed supported a modulation rate of several tens kHz, showing its potential on applications of quantum communication and quantum information processing requiring Bell state encoding and dynamic control.

Anti-PT-symmetric qubit: Decoherence and entanglement entropy

We investigate the dynamics of a general two-level based anti-parity-time (anti-$\mathcal{PT}$)-symmetric qubit and study its decoherence as well as entanglement entropy properties. We compare our findings with that of the corresponding parity-time ($\mathcal{PT}$)-symmetric and Hermitian qubits. To begin, we consider the time-dependent Dyson map to find the exact analytical dynamics for a general non-Hermitian qubit system weakly coupled with a thermal bath for pure dephasing, before specializing it to the case of a general anti-$\mathcal{PT}$-symmetric qubit. Basing the comparison under the same coupling strength or increasing the non-Hermiticity, we observe that the decoherence function and entanglement entropy of the anti-$\mathcal{PT}$-symmetric qubit decays and grows more slowly, respectively, compared to the $\mathcal{PT}$-symmetric and Hermitian qubits. Similarly, the corresponding variance and area of Fisher information are much higher compared to the $\mathcal{PT}$-symmetric and Hermitian qubits. These results demonstrate that anti-$\mathcal{PT}$-symmetric qubits may be better suited for quantum computing and quantum information processing applications than conventional Hermitian or even $\mathcal{PT}$-symmetric qubits.

Fast quantum state transfer and entanglement for cavity-coupled many qubits via dark pathways

Quantum state transfer (QST) and entangled state generation (ESG) are important building blocks for modern quantum information processing. To achieve these tasks, convention wisdom is to consult the quantum adiabatic evolution, which is time-consuming, and thus is of low fidelity. Here, using the shortcut to adiabaticity technique, we propose a general method to realize high-fidelity fast QST and ESG in a cavity-coupled many qubits system via its dark pathways, which can be further designed for high-fidelity quantum tasks with different optimization purpose. Specifically, with a proper dark pathway, QST and ESG between any two qubits can be achieved without decoupling the others, which simplifies experimental demonstrations. Meanwhile, ESG among all qubits can also be realized in a single step. In addition, our scheme can be implemented in many quantum systems, and we illustrate its implementation on superconducting quantum circuits. Therefore, we propose a powerful strategy for selective quantum manipulation, which is promising in cavity coupled quantum systems and could find many convenient applications in quantum information processing.

Demonstration of Complete Information Trade-Off in Quantum Measurement.

While an information-disturbance trade-off in quantum measurement has been at the core of foundational quantum physics and constitutes a basis of secure quantum information processing, recently verified reversibility of a quantum measurement requires to refine it toward a complete version of information trade-off in quantum measurement. Here we experimentally demonstrate a trade-off relation among all information contents, i.e., information gain, disturbance, and reversibility in quantum measurement. By exploring quantum measurements applied on a photonic qutrit, we observe that the information of a quantum state is split into three distinct parts accounting for the extracted, disturbed, and reversible information. We verify that such different parts of information are in trade-off relations not only pairwise but also triplewise all at once, and find that the triplewise relation is tighter than any of the pairwise relations. Finally, we realize optimal quantum measurements that inherently preserve quantum information without loss of information, which offer wider applications in measurement-based quantum information processing.

Remote asymmetric Einstein-Podolsky-Rosen steering of magnons via a single pathway of Bogoliubov dissipation

We propose a scheme to generate remote asymmetric Einstein-Podolsky-Rosen (EPR) steering of two magnon modes in a hybrid ferromagnet-superconductor system. In our scheme, the indirect coherent coupling between two distant magnons and a $\mathrm{\ensuremath{\Delta}}$-type superconducting atom is established via exchange of virtual photons in two dispersive cavities. As a consequence, the artificial atom acting as a reservoir can produce asymmetrical magnon-magnon steering via a single dissipation channel. Interestingly, we find that the steering directivity is determined by cooling either Bogoliubov mode selectively rather than increasing the extra noise as in conventional schemes. This provides a novel idea to prepare remote EPR steering in solid devices, which may find potential applications in quantum information processing.

Spontaneous emission induced grating in a Rydberg quantum system

In this paper, we study the electromagnetically induced grating via spontaneous emission induced coherence (SIC) in a four-level quantum system consisting of a Rydberg state. The quantum system interacts with a weak probe light and a strong coupling light with standing wave pattern. We find that under presence of SIC, the amplitude and phase modulations of the medium can be controlled when we tune the detuning of the coupling light. Here, we realize that by tuning the amplitude and phase modulations of the medium, the energy of probe light can be transferred from zero order to high order of grating, respectively. In this case, the switching from amplitude grating to phase grating is possible when we adapt the optical parameters of the medium. Our proposed model may have potential applications in quantum information processing.

Two-Phonon Blockade in Quadratically Coupled Optomechanical Systems

We propose a scheme to realize the two-phonon blockade effect in a quadratically coupled optomechanical system. We consider the case that the optical cavity is simultaneously driven by a strong pumping field and a weak driving field. By strongly driving the optical cavity, the nonlinear interaction between the optical mode and the mechanical resonator can be significantly enhanced and an effective second-order nonlinearity between photons and phonons is induced. Based on this effectively strong nonlinearity, the two-phonon blockade effect can be achieved when a weak driving field is applied into the optical cavity. By contrast, we study the case of weakly driving the mechanical resonator. In this case, the single-phonon blockade is generated, while the two-phonon blockade cannot be observed. By numerically calculating the second-order and third-order correlation function, we investigate the statistical characteristics of phonons. In addition, we also study the influence of the thermal noise on the achieved two-phonon blockade effect. Our work provides an alternative approach for implementing multiphonon blockade and has potential applications in quantum information processing.