Fang-Yu Hong, Shi-Jie Xiong
Dispersive interactions of matter qubits with bright squeezed light in a high-Q cavity is studied. Numerical simulation shows that higher fidelity of operations to obtain a certain phase shift of the pulse through the dispersive light-matter interaction may be reached using bright squeezed light than that using bright coherent light.
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Pure kinetic inductance coupling for cQED with flux qubits
本研究提出一种基于纯动电感耦合的通量量子比特cQED读取架构，可独立设计色散频移，验证了多量子比特区域特性，实现了与传统架构相当的高保真低泄漏读取性能。
Dispersive readout of cavity-coupled solid-state sensor with near-unity readout fidelity
本文提出一种色散腔量子电动力学读出技术，克服氮空位中心系综传感器的自旋饱和与功率展宽限制，实现接近单位逆读出保真度和飞特斯拉级灵敏度，并展示随自旋数N的1/N灵敏度标度。
We describe a system for long-distance distribution of quantum entanglement, in which coherent light with large average photon number interacts dispersively with single, far-detuned atoms or semiconductor impurities in optical cavities. Entanglement is heralded by homodyne detection using a second bright light pulse for phase reference. The use of bright pulses leads to a high success probability for the generation of entanglement, at the cost of a lower initial fidelity. This fidelity may be boosted by entanglement purification techniques, implemented with the same physical resources. The nee…
We propose the implementation of a deterministic Hadamard gate for logical photonic qubits encoded in superpositions of coherent states of a harmonic oscillator. The proposed scheme builds on a recently introduced set of conditional operations in the strong dispersive regime of circuit QED [Z. Leghtas et al. Phys. Rev. A. {\bf 87} (2013)]. We further propose an architecture for coupling two such logical qubits and provide a universal set of deterministic quantum gates. Based on parameter values taken from the current state of the art, we give estimates for the achievable gate fidelities accoun…
We investigate the output generation of squeezed radiation of a cavity photon mode coupled to another off-resonant bosonic excitation. By modulating in time their linear interaction, we predict high degree of output squeezing when the dispersive ultrastrong coupling regime is achieved, i.e., when the interaction rate becomes comparable to the frequency of the lowest energy mode. Our work paves the way to squeezed light generation in frequency domains where the ultrastrong coupling is obtained, e.g., solid-state resonators in the GHz, THz and mid-IR spectral range.
Full-counting statistics and quantum information of dispersive readout with a squeezed environment
本文提出了一种基于全计数统计的散射读出框架，用于高效计算光子分布的累积累积量，并展示了其在非线性系统中的鲁棒性和高精度测量能力。
Exponentially Improved Dispersive Qubit Readout with Squeezed Light
本文提出将注入外部压缩与腔内压缩结合用于色散量子比特读出，可在任意测量时间内使信噪比指数级提升，并在短时测量中以两倍压缩参数进一步指数改善，为量子纠错和容错量子计算提供快速高保真读出方案。
We show that a collection of two-level atoms in an optical cavity beyond the rotating wave approximation and in the dispersive and strong-coupling regime constitutes a nonlinear medium and is capable of generating squeezed state of light. It is found that squeezing produced in the strong-coupling regime is significantly higher compared to that produced in the dispersive limit. On the other hand, we also show that it could be possible to observe the Dicke superradiant quantum phase transition in the dispersive regime where the vector potential term is negligible. Such a system can be an essenti…
Raising the Cavity Frequency in cQED
本研究首次实现高频谐振腔的cQED系统，通过提升腔频率至21 GHz，同时保持量子比特在5 GHz，实现了高量子效率和长相干时间，为高频谐振腔的应用提供了新可能。
We propose a method for tailoring the frequency spectrum of bright squeezed vacuum by generating it in a nonlinear interferometer, consisting of two down-converting nonlinear crystals separated by a dispersive medium. Due to a faster dispersive spreading of higher-order Schmidt modes, the spectral width of the radiation at the output is reduced as the length of the dispersive medium is increased. Preliminary results show 30\% spectral narrowing.
We demonstrate vacuum squeezing at the D1 line of atomic rubidium (795 nm) with a tunable, doubly-resonant, monolithic sub-threshold optical parametric oscillator in periodically-poled Rb-doped potassium titanyl phosphate. The squeezing appears to be undiminished by a strong dispersive optical nonlinearity recently observed in this material.
We show how to use two-mode squeezed light to exponentially enhance cavity-based dispersive qubit measurement. Our scheme enables true Heisenberg-limited scaling of the measurement, and crucially, is not restricted to small dispersive couplings or unrealistically long measurement times. It involves coupling a qubit dispersively to two cavities, and making use of a symmetry in the dynamics of joint cavity quadratures (a so-called quantum-mechanics-free subsystem). We discuss the basic scaling of the scheme and its robustness against imperfections, as well as a realistic implementation in circui…
Sub-microsecond high-fidelity dispersive readout of a spin qubit with squeezed photons
针对电子自旋量子比特快速读取难题，提出用位移压缩真空态光子进行色散读取，在低强度下实现亚微秒级、99%以上保真度的非破坏性测量，助力量子纠错与通用量子计算。
We present a rigorous and quantum-consistent description of dispersive non-Hermitian optical bilayers in the framework of the canonical quantization scheme. Then we investigate the propagation of a normally incident squeezed coherent state of light through such media, particularly at a frequency for which the bilayers become parity-time (PT) symmetric. Furthermore, to check the realization of PT-symmetry in quantum optics, we reveal how dispersion and loss/gain-induced noises and thermal effects in such bilayers can affect quantum features of the incident light, such as squeezing and sub-Poiss…
We present in this work an analysis of the damage imposed by the atom on the field state inside a lossy superconducting cavity. To access such effects, we propose two procedures to preserve a qubit of the decay effects of an imperfect quantum memory: the first by means of an quasi-instantaneous phase kick applied in the atom, and the second by means of controlled resonant and dispersive interactions. We immediately demonstrate that, in both procedures, the dwell time of the qubit in the cavity increases significantly, being expressively higher for the second. A relation between the inaccuracy …
We study universal quantum computation in the cavity quantum electrodynamics (CQED) framework exploiting two orthonormal two-photon generalized binomial states as qubit and dispersive interactions of Rydberg atoms with high-$Q$ cavities. We show that an arbitrary qubit state may be generated and that controlled-NOT and 1-qubit rotation gates can be realized via standard atom-cavity interactions.
Bright squeezed vacuum, a promising tool for quantum information, can be generated by high-gain parametric down-conversion. However, its frequency and angular spectra are typically quite broad, which is undesirable for applications requiring single-mode radiation. We tailor the frequency spectrum of high-gain parametric down-conversion using an SU(1,1) interferometer consisting of two nonlinear crystals with a dispersive medium separating them. The dispersive medium allows us to select a narrow band of the frequency spectrum to be exponentially amplified by high-gain parametric amplification. …
Using circuit QED, we consider the measurement of a superconducting transmon qubit via a coupled microwave resonator. For ideally dispersive coupling, ringing up the resonator produces coherent states with frequencies matched to transmon energy states. Realistic coupling is not ideally dispersive, however, so transmon-resonator energy levels hybridize into joint eigenstate ladders of the Jaynes-Cummings type. Previous work has shown that ringing up the resonator approximately respects this ladder structure to produce a coherent state in the eigenbasis (a dressed coherent state). We numerically…
Dispersive Qubit Readout of Temperature
本文研究利用注入外部压缩与腔内压缩提高色散量子比特读出中的温度测量精度，证明在特定低温、短时或弱输入条件下可获得指数级提升，并在多比特与热浴耦合时通过大腔损耗和弱耦合实现海森堡标度。
We propose a scheme for long-distance distribution of quantum entanglement in which the entanglement between qubits at intermediate stations of the channel is established by using bright light pulses in squeezed states coupled to the qubits in cavities with a weak dispersive interaction. The fidelity of the entanglement between qubits at the neighbor stations (10 km apart from each other) obtained by postselection through the balanced homodyne detection of 7 dB squeezed pulses can reach F=0.99 without using entanglement purification, at same time, the probability of successful generation of en…
We study the dynamics of a general quartic interaction Hamiltonian under the influence of dissipation and non-classical driving. We show that this scenario could be realised with a cascaded superconducting cavity-qubit system in the strong dispersive regime in a setup similar to recent experiments. In the presence of dissipation, we find that an effective Hartree-type decoupling with a Fokker-Planck equation yields a good approximation. We find that the stationary state is approximately a squeezed vacuum, which is enhanced by the $Q$-factor of the cavity but conserved by the interaction. The q…
The recent demonstration of cooling of a macroscopic silicon nitride membrane based on dissipative coupling makes dissipatively coupled optomechanical systems as promising candidates for squeezing. We theoretically show that such a system in a cavity on resonance can yield good squeezing which is comparable to that produced by dispersive coupling. We also report the squeezing resulting from the combined effects of dispersive and dissipative couplings and thus the device can be operated in one regime or the other. We derive the maximal frequency and quadrature angles to observe squeezing for gi…
Microwave squeezing represents the ultimate sensitivity frontier for superconducting qubit measurement. However, observation of enhancement has remained elusive, in part because integration with conventional dispersive readout pollutes the signal channel with antisqueezed vacuum. Here we induce a stroboscopic light-matter coupling with superior squeezing compatibility, and observe an increase in the room-temperature signal-to-noise ratio of 24%. Squeezing the orthogonal phase controls measurement backaction, slowing dephasing by a factor of 1.8. This protocol enables the practical use of micro…
A cavity QED system is analyzed which duplicates the dynamics of a two-level atom in free space interacting exclusively with broadband squeezed light. We consider atoms in a three or four-level Lambda-configuration coupled to a high-finesse optical cavity which is driven by a squeezed light field. Raman transitions are induced between a pair of stable atomic ground states via the squeezed cavity mode and coherent driving fields. An analysis of the reduced master equation for the atomic ground states shows that a three-level atomic system has insufficient parameter flexibility to act as an effe…
A superconducting qubit in the strong dispersive regime of a circuit quantum electrodynamics system is a powerful probe for microwave photons in a cavity mode. In this regime, a qubit spectrum is split into multiple peaks, with each peak corresponding to an individual photon number in the cavity (discrete ac Stark shift). Here, we measure the qubit spectrum in the cavity that is driven continuously with a squeezed vacuum field generated by a Josephson parametric amplifier. By fitting the qubit spectrum with a model which takes into account the finite qubit excitation power, the photon number d…
Unifying Floquet theory of longitudinal and dispersive readout
本文提出统一纵向与色散读出的Floquet理论，揭示交流斯塔克位移斜率与曲率分别控制纵向耦合和色散位移，并在弱驱动下建立联系，适用于超导与自旋混合电路QED系统。
We report experiments on superconducting flux qubits in a circuit quantum electrodynamics (cQED) setup. Two qubits, independently biased and controlled, are coupled to a coplanar waveguide resonator. Dispersive qubit state readout reaches a maximum contrast of $72\,\%$. We find intrinsic energy relaxation times at the symmetry point of $7\,μ\text{s}$ and $20\,μ\text{s}$ and levels of flux noise of $2.6\,μΦ_0/\sqrt{\text{Hz}}$ and $2.7\,μΦ_0/\sqrt{\text{Hz}}$ at 1 Hz for the two qubits. We discuss the origin of decoherence in the measured devices. These results demonstrate the potential of cQED…
We study an optomechanical system consisting of an optical cavity and movable mirror coupled through dispersive linear optomechanical coupling (LOC) and quadratic optomechanical coupling(QOC). We work in the resolved side band limit with a high quality factor mechanical oscillator in a strong coupling regime. We show that the presence of QOC in the conventional optomechanical system (with LOC alone) modifies the mechanical oscillator's frequency and reduces the back-action effects on mechanical oscillator. As a result of this the fluctuations in mechanical oscillator can be suppressed below st…
Interfacing stationary qubits with propagating photons is a fundamental problem in quantum technology. Cavity quantum electrodynamics (CQED) invokes a mediator degree of freedom in the form of a far-detuned cavity mode, the adaptation of which to superconducting circuits (cQED) proved remarkably fruitful. The cavity both blocks the qubit emission and it enables a dispersive readout of the qubit state. Yet, a more direct (cavityless) interface is possible with atomic clocks, in which an orbital cycling transition can scatter photons depending on the state of a hyperfine or quadrupole qubit tran…
We theoretically propose an efficient way to generate and detect squeezed light by a single qubit in circuit QED. By tuning the qubit energy splitting close to the fundamental frequency of the first harmonic mode (FHM) in a transmission line resonator and placing the qubit at the nodal point of the third harmonic mode, one can generate the resonantly enhanced squeezing of the FHM upon pumping with the second harmonic mode. In order to investigate the photon number splitting for the squeezed FHM, we have numerically calculated the qubit absorption spectrum, which exhibits regularly spaced peaks…
We study theoretically the squeezing spectrum and second-order correlation function of the output light for an optomechanical system in which a mechanical oscillator modulates the cavity linewidth (dissipative coupling). We find strong squeezing coinciding with the normal-mode frequencies of the linearized system. In contrast to dispersive coupling, squeezing is possible in the resolved-sideband limit simultaneously with sideband cooling. The second-order correlation function shows damped oscillations, whose properties are given by the mechanical-like, the optical-like normal mode, or both, an…
The quantromon: A qubit-resonator system with orthogonal qubit and readout modes
提出名为quantromon的双模电路，通过设计内在的交叉Kerr耦合实现正交的量子比特与读出模式，在实验中展现出对失谐的弱依赖性、内在Purcell保护及98.3%的无参量放大器单次读出保真度，有望简化量子处理器测量电路。
Achieving strong coupling between light and matter is usually a challenge in Cavity Quantum Electrodynamics (cQED), especially in solid state systems. For this reason is useful taking advantage of alternative approaches to reach this regime, and then, generate reliable quantum polaritons. In this work we study a system composed of a quantized single mode of a mechanical resonator interacting linearly with both a single mode nanocavity and a two-level single quantum dot. In particular, we focus on the behavior of the indirect light-matter interaction when the phonon mode interfaces both parts. …
Strong long-range coupling between distant spins is crucial for spin-based quantum information processing. However, achieving such a strong spin-spin coupling remains challenging. Here we propose to realize a strong coupling between two distant spins via the Kerr effect of magnons in a yttrium-iron-garnet nanosphere. By applying a microwave field on this nanosphere, the Kerr effect of magnons can induce the magnon squeezing, so that the coupling between the spin and the squeezed magnons can be exponentially enhanced. This in turn allows the spin-magnon distance to increase from nano- to microm…
A controlled evolution generated by nonlinear interactions is required to perform full manipulation of a quantum system, and such control is only coherent when the rate of nonlinearity is large compared to the rate of decoherence. As a result, engineered quantum systems typically rely on a bare nonlinearity much stronger than all decoherence rates, and this hierarchy is usually assumed to be necessary. In this work, we challenge this assumption by demonstrating the universal control of a quantum system where the relevant rate of bare nonlinear interaction is comparable to the fastest rate of d…
Non-perturbative theory of spontaneous parametric down-conversion in open and dispersive optical systems
提出基于格林函数量子化的非微扰理论，以描述任意损耗、色散和高增益条件下开放纳米非线性光学系统中的自发参量下转换，并通过集成未探测光子量子光谱学示例发现新的增益依赖效应。
Dipole coupling of a bilayer graphene quantum dot to a high-impedance microwave resonator
本文在双层石墨烯双量子点与高阻抗超导微波谐振器之间实现了电偶极耦合，利用cQED技术实现快速灵敏的电荷检测，最大耦合强度达49.7 MHz，为范德瓦尔斯材料量子点的相干电荷-光子耦合开辟了道路。
Dynamically enhancing qubit-photon interactions with anti-squeezing
本研究利用简并参量振荡器的压缩态放大场涨落，在超导电路实验中以5.5 dB压缩量实现量子比特-光子色散相互作用两倍增强，验证了量子比特-光子相互作用的原位动态控制，为增强相互作用的实验平台提供新方向。
Quantum Sensing of Antiferromagnetic Magnon Two-Mode Squeezed Vacuum
该研究提出利用自旋量子比特通过直接色散耦合传感反铁磁体的双模压缩真空基态，并借助量子比特光谱与驱动控制确定性生成磁振子对Fock态。
Fast and high-fidelity dispersive readout of a spin qubit via squeezing and resonator nonlinearity
本研究探讨了与非线性微波共振器耦合的半导体双量子点中单个自旋的色散测量，通过利用平移压缩真空态，实现了亚微秒级的高保真读出，在约0.9μs内达到99.99%的保真度。
Quantum sensing composite excitations in an anisotropic ferromagnet via a qubit
本文理论发展量子比特光谱技术，可探测铁磁体的量子压缩磁振子态叠加，分析了多压缩福克态叠加激发态的探测难点及低振幅相干压缩磁振子态的权衡关系，为相关探测提供基础与设计方程。
Boson sampling with self-generation of squeezing via interaction of photons and atoms
本研究提出一种玻色采样多模压缩态生成新方案，用含二能级原子的多模谐振器替代线性干涉仪自发生成压缩复合态，无需外部光子源与大量耦合器，其联合概率分布经典计算机难有效计算，可通过现有技术实现。
While relatively easy to engineer, static transverse coupling between a qubit and a cavity mode satisfies the criteria for a quantum non-demolition (QND) measurement only if the coupling between the qubit and cavity is much less than their mutual detuning. This can put significant limits on the speed of the measurement, requiring trade-offs in the circuit design between coupling, detuning, and decoherence introduced by the cavity mode. Here, we study a circuit in which the qubit-cavity and the cavity-feedline coupling can be turned on and off, which helps to isolate the qubit. We do not rely o…
We analyze the creation of spin squeezed atomic ensembles by simultaneous dispersive interactions with several optical frequencies. A judicious choice of optical parameters enables optimization of an interferometric detection scheme that suppresses inhomogeneous light shifts and keeps the interferometer operating in a balanced mode that minimizes technical noise. We show that when the atoms interact with two-frequency light tuned to cycling transitions the degree of spin squeezing $ξ^2$ scales as $ξ^2\sim 1/d$ where $d$ is the resonant optical depth of the ensemble. In real alkali atoms there …
We introduce and analyze a dispersive qubit readout scheme where two-mode squeezing is generated directly in the measurement cavities. The resulting suppression of noise enables fast, high- fidelity readout of naturally weakly coupled qubits, and the possibility to protect strongly coupled qubits from decoherence by weakening their coupling. Unlike other approaches exploiting squeezing, our setup avoids the difficult task of transporting and injecting with high fidelity an externally-generated squeezed state. Our setup is also surprisingly robust against unwanted non-QND backaction effects, as…
Quantum parametric amplifiation of phonon-mediated magnon-spin interaction
该研究在混合三方系统中通过机械运动的量子参量放大实现磁振子与声子的强耦合，达成纳米磁体与氮空位中心间的相干态转移，为量子器件研发开辟新路径。
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