Advanced quantum technologies最新文献

Nonreciprocal devices, allowing to manipulate one-way signals, are crucial to quantum information processing and quantum networks. Here a nonlinear cavity-magnon system is proposed, consisting of a microwave cavity coupled to one or two yttrium–iron–garnet (YIG) spheres supporting magnons with Kerr nonlinearity, to investigate nonreciprocal unconventional photon blockade. The nonreciprocity originates from the direction-dependent Kerr effect, distinctly different from previous proposals with spinning cavities and dissipative couplings. For a single sphere case, nonreciprocal unconventional photon blockade can be realized by manipulating the nonreciprocal destructive interference between two active paths, via varying the Kerr coefficient from positive to negative, or vice versa. By optimizing the system parameters, the perfect and well-tuned nonreciprocal unconventional photon blockade can be predicted. For the case of two spheres with opposite Kerr effects, only reciprocal unconventional photon blockade can be observed when two cavity-magnon coupling strengths Kerr strengths are symmetric. However, when coupling strengths or Kerr strengths become asymmetric, nonreciprocal unconventional photon blockade appears. This implies that two-sphere nonlinear cavity-magnon systems can be used to switch the transition between reciprocal and nonreciprocal unconventional photon blockades. This study offers a potential platform for investigating the nonreciprocal photon blockade effect in nonlinear cavity magnonics.

Nonlinear mean field dynamics enables quantum information processing operations that are impossible in linear one-particle quantum mechanics. In this approach, a register of bosonic qubits (such as neutral atoms or polaritons) is initialized into a symmetric product state $�{�|�\psi �⟩�}^{��\otimes �n�}$ through condensation, then subsequently controlled by varying the qubit-qubit interaction. An experimental implementation of quantum state discrimination, an important subroutine in quantum computation, with a toroidal Bose–Einstein condensate is proposed. The condensed bosons here are atoms, each in the same superposition of angular momenta 0 and $�\hslash$, encoding a qubit. A nice feature of the protocol is that only a readout of individual quantized circulation states (not superpositions) is required.

Quantum error correction codes based on continuous variables play an important role for the implementation of quantum communication systems. A natural application of such codes occurs within quantum repeater systems which are used to combat severe channel losses and local gate errors. In particular, channel loss drastically reduces the distance of communication between remote users. Here, a cavity-quantum electrodynamics (QED) based repeater scheme is considered to address the losses in the quantum channel. This repeater scheme relies on the transmission of a specific class of rotationally invariant error-correcting codes. Several rotation-symmetric bosonic codes (RSBCs) are compared for encoding the initial states of two remote users connected by a quantum repeater network against the convention of the cat codes and the performance of the system is quantified by using the secret key rate. In particular, the number of stations required to exchange a secret key over a fixed distance is determined and establish the resource overhead. For higher-loss codes, the results show that a secret key rate (SKR) value of 0.01 bit per channel use can be achieved at a distance of 10000 km, with an elementary distance of 1.3 km.

A protocol is proposed for generating three-particle Knill-Laflamme-Milburn (KLM) states in a system composed of two frequency-tunable flux qubits and a coplanar waveguide resonator. With the help of the counter-rotating interaction, the protocol for generating three-particle KLM states can be realized. The numerical results reveal that the protocol is robust against the effects induced by decoherence and frequency-tuning operations. It is hope that the protocol provides an alternative method to generate entangled states.

The entanglement generation of two two-level giant atoms coupled to a photonic waveguide, which is formed by a Su–Schrieffer–Heeger (SSH) type coupled-cavity array is studied. Here, each atom is coupled to the waveguide through two coupling points.The two-atom separate-coupling case is studied, and 16 coupling configurations are considered for the coupling-point distributions between the two atoms and the waveguide. Quantum master equations are derived to govern the evolution of the two atoms and characterize atomic entanglement by calculating the concurrence of the two-atom states. It is found that the two giant-atom entanglement depends on the coupling configurations and the coupling-point distance of the giant atoms. In particular, the entanglement dynamics of the two giant atoms in 14 coupling configurations depend on the dimerization parameter of the SSH waveguide. According to the self-energies of the two giant atoms, it is found that ten of these 16 coupling configurations can be divided into five pairs. It is also showed that the delayed sudden birth of entanglement between the two giant atoms is largely enhanced in these five pairs of coupling configurations. This work will promote the study of quantum effects and coherent manipulation in giant-atom topological-waveguide-QED.

The phenomenon of nonreciprocity arises from the disruption of time reversal symmetry, enabling the one-way transfer of signals through specific channels. In the framework of cavity optomechanics, this symmetry breaking is attributed to a nonuniform radiation pressure force resulting from the interaction between light and matter. This study investigates a hybrid cavity optomechanical system (COMS) comprising two optical modes, directly coupled to each other via photon hopping interaction and indirectly via a common mechanical excitation in the form of a movable mirror, and an additional parallel metallic plate that induces a dynamical Casimir force (DCF) interaction between the plates. The two optical cavities are driven by two strong laser fields, accompanied by two weak probe classical fields from each port. The primary focus lies in exploring the nonreciprocal behavior of the light field across ports one and two, strongly manipulated by the DCF. The DCF plays a pivotal role in providing extra degrees of flexibility and manipulation in controlling the nonreciprocal signal transmission by modifying the resonance conditions of the fields within the hybrid COMS and is responsible for the amplification and swapping of information between the two ports.

Despite the anticipated speed-up of quantum computing, the achievement of a measurable advantage remains subject to ongoing debate. Adiabatic Quantum Computers (AQCs) are quantum devices designed to solve quadratic uncostrained binary optimization (QUBO) problems, but their intrinsic thermal noise can be leveraged to train computationally demanding machine learning algorithms such as the Boltzmann Machine (BM). Despite an asymptotic advantage is expected only for large networks, a limited quantum speed up can be already achieved on a small $��16�\times �16�$ BM is shown, by exploiting parallel adiabatic computation. This approach exhibits a 8.6-fold improvement in wall time on the $��4�\times �4�$ Bars and Stripes dataset when compared to a parallelized classical Gibbs sampling method, which has never been outperformed before by quantum approaches.

Randomness extraction is a key problem in cryptography and theoretical computer science. With the recent rapid development of quantum cryptography, quantum-proof randomness extraction has also been widely studied, addressing the security issues in the presence of a quantum adversary. In contrast with conventional quantum-proof randomness extractors characterizing the input raw data as min-entropy sources, it is found that the input raw data generated by a large class of trusted-device quantum random number generators can be characterized as the so-called reverse block source. This fact enables us to design improved extractors. Two novel quantum-proof randomness extractors for reverse block sources that realize real-time block-wise extraction are proposed specifically. In comparison with the general min-entropy randomness extractors, the designs achieve a significantly higher extraction speed and a longer output data length with the same seed length. In addition, they enjoy the property of online algorithms, which process the raw data on the fly without waiting for the entire input raw data to be available. These features make the design an adequate choice for the real-time post-processing of practical quantum random number generators. Applying the extractors to the raw data generated by a widely used quantum random number generator, a simulated extraction speed as high as 300 Gbps is achieved.

The long coherence time of donor atom nuclear spin states and of its bounded electron in $��{}^{28}�\mathrm{Si}�$ can be exploited to define a qubit. This work is focused on a type of donor- and quantum dot-based qubit, the flip-flop (FF) qubit, that leverages antiparallel electron-nuclear spin states of a $��{}^{31}�P�$ donor atom controlled by an electric field. It can provide long-range inter-qubit interactions in the order of some hundreds of nanometers, thus relaxing the common constraints and tolerances on inter-qubit distances in donor-based qubits. Simulation results of linear array (LA) and square array (SA) of four FF qubits are presented to study the effect of noise, idle qubits, and simultaneous gating (parallel gating) on gate fidelity. The impact of noise and qubit mutual coupling for both considered types of array are presented and the obtained fidelity results are compared.

The nitrogen−vacancy (NV) center in diamond gains its versatility when negatively charged (NV⁻) but is mediocre when neutrally charged. Particularly, the charge states of NV centers are convertible under optical pumping and during the dark intervals, whose dynamics are mixed with the NV⁻s’ spin polarizations and relaxations, making them difficult to detect. Here, a microwave-pulses-assisted charge state dynamics (CSD) measurement method of NV centers in the dark time (DT) is proposed. The microwave pulses are designed to manipulate the populations of the NV⁻s’ ground state spin triplets (qutrit) to the equilibrium state before the DT. Thus, the longitudinal relaxations of the qutrit are balanced, and pure CSD can be detected. Interestingly, in an annealed bulk diamond, not only the traditional tunneling-induced fast exponential CSD are observed, but also a slow and long-term recharging process, which is probably attributed to the exchanging of the electrons between NV centers and the high-energy-level charge traps such as vacancy clusters. Furthermore, results demonstrate a 40% increase in NV⁻s’ contrast by properly extending the recharging DT. These results are significant for the in-depth study of the NV centers’ CSD and can improve the sensing abilities of the NV⁻ ensemble.