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Variational quantum algorithms have been widely demonstrated in both experimental and theoretical contexts to have extensive applications in quantum simulation, optimization, and machine learning. However, the exponential growth in the dimension of the Hilbert space results in the phenomenon of vanishing parameter gradients in the circuit as the number of qubits and circuit depth increase, known as the barren plateau phenomena. In recent years, research in non-equilibrium statistical physics has led to the discovery of the realization of many-body localization. As a type of floquet system, many-body localized floquet system has phase avoiding thermalization with an extensive parameter space coverage and have been experimentally demonstrated can produce time crystals. We applied this circuit to the variational quantum algorithms for the calculation of many-body ground states and studied the variance of gradient for parameter updates under this circuit. We found that this circuit structure can effectively avoid barren plateaus. We also analyzed the entropy growth, information scrambling, and optimizer dynamics of this circuit. Leveraging this characteristic, we designed a new type of variational ansatz, called the 'many-body localization ansatz'. We applied it to solve quantum many-body ground states and examined its circuit properties. Our numerical results show that our ansatz significantly improved the variational quantum algorithm., Comment: We finished the work on June 1 and submitted the work to SCPAM on June 17

Deutsch's algorithm is the first quantum algorithm to show the advantage over the classical algorithm. Here we generalize Deutsch's problem to $n$ functions and propose a new quantum algorithm with indefinite causal order to solve this problem. The new algorithm not only reduces the number of queries to the black-box by half over the classical algorithm, but also significantly reduces the number of required quantum gates over the Deutsch's algorithm. We experimentally demonstrate the algorithm in a stable Sagnac loop interferometer with common path, which overcomes the obstacles of both phase instability and low fidelity of Mach-Zehnder interferometer. The experimental results have shown both an ultra-high and robust success probability $\sim 99.7\%$. Our work opens up a new path towards solving the practical problems with indefinite casual order quantum circuits.

Coherent sources exploiting the stimulated emission of non-equilibrium quantum systems, i.e. gain media, have proven indispensable for advancing fundamental research and engineering. The operating electromagnetic bands of such coherent sources have been continuously enriched for increasing demands.Nevertheless, for a single bench top coherent source, simultaneous generation of radiation in multiple bands, especially when the bands are widely separated, present formidable challenges with a single gain medium. Here, we propose a mechanism of simultaneously realizing the stimulated emission of radiation in the visible and microwave bands, i.e. lasing and masing actions, at ambient conditions by utilizing photoexcited singlet and triplet states of the pentacene molecules that are doped in p-terphenyl. The possibility is validated by the observed amplified spontaneous emission (ASE) at 645 nm with a narrow linewidth around 1 nm from the pentacene-doped p-terphenyl crystal used for masing at 1.45 GHz and consolidated by a 20 fold lower threshold of ASE compared to the reported masing threshold. The overall threshold of the pentacene-based multiband coherent source can be optimized by appropriate alignment of the pump-light polarization with the pentacene's transition dipole moment. Our work not only shows a great promise on immediate realization of multiband coherent sources but also establishes an intriguing solid-state platform for fundamental research of quantum optics in multiple frequency domains., Comment: 30 pages, 4 figures

Boson sampling has been theoretically proposed and experimentally demonstrated to show quantum computational advantages. However, it still lacks the deep understanding of the practical applications of boson sampling. Here we propose that boson sampling can be used to efficiently simulate the work distribution of multiple identical bosons. We link the work distribution to boson sampling and numerically calculate the transition amplitude matrix between the single-boson eigenstates in a one-dimensional quantum piston system, and then map the matrix to a linear optical network of boson sampling. The work distribution can be efficiently simulated by the output probabilities of boson sampling using the method of the grouped probability estimation. The scheme requires at most a polynomial number of the samples and the optical elements. Our work opens up a new path towards the calculation of complex quantum work distribution using only photons and linear optics.

Squeezing light is a critical resource in both fundamental physics and precision measurement. The squeezing light has been generated through optical-parametric amplification inside an optical resonator. However, preparing the squeezing light in an optomechanical system is still a challenge for the thermal noise inevitably coupling to the system. We consider an optically levitated nano-particle in a bichromatic cavity, in which two cavity modes could be excited by the scattering photons of the dual-tweezers respectively. Based on the coherent scattering mechanism, the ultra-strong coupling between the cavity field and torsional motion of nano-particle could be achieved for the current experimental conditions. With the back-action of the optically levtiated nano-particle, the broad single-mode squeezing light can be realized in the bad cavity regime. Even at room temperature, the single-mode light can be squeezed for more than 17 dB, which is far beyond the 3 dB limit. The two-mode squeezing lights can also be generated, if the optical tweezers contain two frequencies, one is on the red sideband of the cavity mode, the other is on the blue sideband. The two-mode squeezing can be maximized near the boundary of the system stable regime, and is sensitive to both the cavity decay rate and the power of the optical tweezers., Comment: 13 pages, 7 figures

Quantum entanglement in macroscopic systems is not only essential for practical quantum information processing, but also valuable for the study of the boundary between quantum and classical world. However, it is very challenge to achieve the steady remote entanglement between distant macroscopic systems. We consider two distant nanoparticles, both of which are optically trapped in two cavities. Based on the coherent scattering mechanism, we find that the ultrastrong optomechanical coupling between the cavity modes and the motion of the levitated nanoparticles could achieve. The large and steady entanglement between the filtered output cavity modes and the motion of nanosparticles can be generated, if the trapping laser is under the red sideband. Then through entanglement swapping, the steady motional entanglement between the distant nanoparticles can be realized. We numerically simulate and find that the two nanoparticles with 10 km distance can be entangled for the experimentally feasible parameters, even in room temperature environment., Comment: 6+10 pages, 4+3 figures

We theoretically propose a method to realize optical nonreciprocity in rotating nano-diamond with a nitrogen-vacancy (NV) center. Because of the relative motion of the NV center with respect to the propagating fields, the frequencies of the fields are shifted due to the Doppler effect. When the control and probe fields are incident to the NV center from the same direction, the two-photon resonance still holds as the Doppler shifts of the two fields are the same. Thus, due to the electromagnetically-induced transparency (EIT), the probe light can pass through the NV center nearly without absorption. However, when the two fields propagate in opposite directions, the probe light can not effectively pass through the NV center as a result of the breakdown of two-photon resonance., Comment: 7 pages, 4 figures

Periodically driven systems, or Floquet systems, exhibit many novel dynamics and interesting out-of-equilibrium phases of matter. Those phases arising with the quantum systems' symmetries, such as global $U(1)$ symmetry, can even show dynamical stability with symmetry-protection. Here we experimentally demonstrate a $U(1)$ symmetry-protected prethermal phase, via performing a digital-analog quantum simulation on a superconducting quantum processor. The dynamical stability of this phase is revealed by its robustness against external perturbations. We also find that the spin glass order parameter in this phase is stabilized by the interaction between the spins. Our work reveals a promising prospect in discovering emergent quantum dynamical phases with digital-analog quantum simulators., Comment: 14 pages, 4 figures, and supplementary materials

Ultralow dissipation plays an important role in sensing applications and exploring macroscopic quantum phenomena using micro-and nano-mechanical systems. We report a diamagnetic-levitated micro-mechanical oscillator operating at a low temperature of 3K with measured dissipation as low as 0.59 $\mu$Hz and a quality factor as high as $2 \times 10^7$. To the best of our knowledge the achieved dissipation is the lowest in micro- and nano-mechanical systems to date, orders of magnitude improvement over the reported state-of-the-art systems based on different principles. The cryogenic diamagnetic-levitated oscillator described here is applicable to a wide range of mass, making it a good candidate for measuring both force and acceleration with ultra-high sensitivity. By virtue of the naturally existing strong magnetic gradient, this system has great potential in quantum spin mechanics study., Comment: 6 pages, 3 figures

Time crystal is defined as a phase of matter spontaneously exhibiting a periodicity in time. Previous studies focused on discrete quantum time crystals under periodic drive. Here, we propose a time crystal model based on a levitated charged nanoparticle in a static magnetic field without drive. Both the classical time crystal in thermal equilibrium and the quantum time crystal in the ground state can emerge in the spin rotational mode, under the strong magnetic field or the large charge-to-mass ratio limit. Besides, for the first time, the \emph{time polycrystal} is defined and naturally appears in this model. Our model paves a way for realizing time crystals in thermal equilibrium., Comment: 10 pages, 7 figures

Quantum error correction plays an important role in fault-tolerant quantum information processing. It is usually difficult to experimentally realize quantum error correction, as it requires multiple qubits and quantum gates with high fidelity. Here we propose a simple quantum error-correcting code for the detected amplitude damping channel. The code requires only two qubits. We implement the encoding, the channel, and the recovery on an optical platform, the IBM Q System, and a nuclear magnetic resonance system. For all of these systems, the error correction advantage appears when the damping rate exceeds some threshold. We compare the features of these quantum information processing systems used and demonstrate the advantage of quantum error correction on current quantum computing platforms., Comment: 26 pages, 9 figures, 4 tables

Electron and nuclear spins of diamond nitrogen-vacancy (NV) centers are good candidates for quantum information processing as they have long coherence time and can be initialized and read out optically. However, creating a large number of coherently coupled and individually addressable NV centers for quantum computing has been a big challenge. Here we propose methods to use high-density diamond NV centers coupled by spin-spin interaction with an average separation on the order of 10 nm for quantum computing. We propose to use a strain gradient to encode the position information of each NV center in the energy level of its excited electron orbital state, which causes a shift of its optical transition frequency. With such strain encoding, more than 100 closely-packed NV centers below optical diffraction limit can be read out individually by resonant optical excitation. A magnetic gradient will be used to shift the electron spin resonant (ESR) frequencies of NV centers. Therefore, the spin state of each NV center can be individually manipulated and different NV centers can be selectively coupled. A universal set of quantum operations for two-qubit and three-qubit system is introduced by careful design of external drives. Moreover, entangled states with multiple qubits can be created by this protocol, which is a major step towards quantum information processing with solid-state spins., Comment: 11 pages, 10 figures

Searching topological states in artificial systems has recently become a rapidly growing field of research. Meanwhile, significant experimental progresses on observing topological phenomena have been made in superconducting circuits. However, topological insulator states have not yet been reported in this system. Here, for the first time, we experimentally realize a tunable dimerized spin chain model and observe the topological magnon insulator states in a superconducting qubit chain. Via parametric modulations of the qubit frequencies, we show that the qubit chain can be flexibly tuned into topologically trivial or nontrivial magnon insulator states. Based on monitoring the quantum dynamics of a single-qubit excitation in the chain, we not only measure the topological winding numbers, but also observe the topological magnon edge and defect states. Our experiment exhibits the great potential of tunable superconducting qubit chain as a versatile platform for exploring non-interacting and interacting symmetry-protected topological states., Comment: main text 6 pages, 4 figures; supplement 9 pages, 8 figures

The spin in a rotating frame has attracted a lot of attentions recently, as it deeply relates to both fundamental physics such as pseudo-magnetic field and geometric phase, and applications such as gyroscopic sensors. However, previous studies only focused on adiabatic limit, where the rotating frequency is much smaller than the spin frequency. Here we propose to use a levitated nano-diamond with a built-in nitrogen-vacancy (NV) center to study the dynamics and the geometric phase of a rotating electron spin without adiabatic approximation. We find that the transition between the spin levels appears when the rotating frequency is comparable to the spin frequency at zero magnetic field. Then we use Floquet theory to numerically solve the spin energy spectrum, study the spin dynamics and calculate the geometric phase under a finite magnetic field, where the rotating frequency to fulfill the resonant transition condition could be greatly reduced., Comment: 6+2 pages, 3+1 figures

We propose a scheme to realize universal quantum gates between nitrogen-vacancy (NV) centers in an optically trapped nanodiamond, through uniform magnetic field induced coupling between the NV centers and the torsional mode of the levitated nanodiamond. The gates are tolerant to the thermal noise of the torsional mode. By combining the scheme with dynamical decoupling technology, it is found that the high fidelity quantum gates are possible for the present experimental conditions. The proposed scheme is useful for NV-center-based quantum network and distributed quantum computation, Comment: 7 pages, 6 figures

We consider an array of optically levitated nanospheres in vacuum and investigate nontrivial phonon transports in this system. The levitated nanospheres are coupled by optical binding. Key parameters of this system, such as the interaction range, trapping frequencies and mechanical dissipation, are all highly tunable. By tuning the spacing between neighboring spheres, the mechanical dissipation and the trapping frequency of each sphere, counter-intuitive phenomena such as prethermalization and nonreciprocal phonon transport can be achieved. Our system provides a great platform to investigate novel phonon transport and thermal energy transfer., Comment: 9 pages, 9 figures

We show that the gravitational acceleration can be measured with the matter-wave Ramsey interferometry, by using a nitrogen-vacancy center coupled to a nano-mechanical resonator. We propose two experimental methods to realize the Hamiltonian, by using either a cantilever resonator or a trapped nanoparticle. The scheme is robust against the thermal noise, and could be realized at the temperature much higher than the quantum regime one. The effects of decoherence on the interferometry fringe visibility is caculated, including the mechanical motional decay and dephasing of the nitrogen-vacancy center. In addition, we demonstrate that under the various sources of random and systematic noises, our gravimeter can be made on-chip and achieving a high measurement precision. Under experimental feasible parameters, the proposed gravimeter could achieve $10^{-10}$ relative precision., Comment: 7 pages, 4 figures

Faithfully transferring the quantum state is essential for quantum information processing. Here we demonstrate a fast (in 84 ns) and high-fidelity (99.2%) transfer of arbitrary quantum states in a chain of four superconducting qubits with nearest-neighbor coupling. This transfer relies on full control of the effective couplings between neighboring qubits, which is realized only by our parametrically modulating the qubits without increasing circuit complexity. Once the couplings between qubits fulfill a specific ratio, perfect quantum state transfer can be achieved in a single step, and is therefore robust to noise and accumulation of experimental errors. This quantum state transfer can be extended to a larger qubit chain and thus adds a desirable tool for future quantum information processing. The demonstrated flexibility of the coupling tunability is suitable for quantum simulation of many-body physics, which requires different configurations of qubit couplings., Comment: Main text has 4 pages and 4 figures. Appendix has 4 pages and 6 figures

We systematically investigate the multi-stability behaviour and cooling of both librational and translational modes of an optically levitated nonspherical nanoparticle. By expanding the trapping potential to the fourth order of both the translational and librational freedom degrees, we deduce the nonlinearity of them and their nonlinear coupling. Through stability analysis, we find that the system presents multi-stability when either the librational or the translational drive is red-detuned. The system will be stabilized if and only if these two drives are both blue-detuned. In the steady state region, we study the synthetic cooling scheme of translational mode by utilising librational mode. We find that matching the driving amplitude of these two modes and appropriate air pressure can optimize synthetic cooling. The synthetic cooling limit can be greatly improved, if we combine the feedback cooling with the synthetic cooling., Comment: 20 pages, 12 figures

Geometric phases are noise-resilient, and thus provide a robust way towards high fidelity quantum manipulation. Here we experimentally demonstrate arbitrary non-adiabatic holonomic single-qubit quantum gates for both a superconducting transmon qubit and a microwave cavity in a single-loop way. In both cases, an auxiliary state is utilized, and two resonant microwave drives are simultaneously applied with well-controlled but varying amplitudes and phases for the arbitrariness of the gate. The resulting gates on the transmon qubit achieve a fidelity of 0.996 characterized by randomized benchmarking and the ones on the cavity show an averaged fidelity of 0.978 based on a full quantum process tomography. In principle, a nontrivial two-qubit holonomic gate between the qubit and the cavity can also be realized based on our presented experimental scheme. Our experiment thus paves the way towards practical non-adiabatic holonomic quantum manipulation with both qubits and cavities in a superconducting circuit., Comment: main text 6 pages, 4 figures; supplement 5 pages, 5 figures; Added references and corrected typos

Entanglement is an important evidence that a quantum device can potentially solve problems intractable for classical computers. In this paper, we prepare connected graph states involving 8 to 16 qubits on ibmqx5, a 16-qubit superconducting quantum processor accessible via IBM cloud,using low-depth circuits. We demonstrate that the prepared state is fully entangled, i.e. the state is inseparable with respect to any fixed partition., Comment: 23 pages, 9 figures, 2 tables; The full entanglement is clarified; a new section is added on the localized entanglement with distance 3 and 4, accepted version

Quantum transducers can transfer quantum information between different systems. Microwave-optical photon conversion is important for future quantum networks to interconnect remote superconducting quantum computers with optical fibers. Here we propose a high-speed quantum transducer based on a single-photon emitter in an atomically thin membrane resonator that can couple single microwave photons to single optical photons. The 2D resonator is a freestanding van der Waals heterostructure (may consist of hexagonal boron nitride, graphene, or other 2D materials) that hosts a quantum emitter. The mechanical vibration (phonon) of the 2D resonator interacts with optical photons by shifting the optical transition frequency of the single-photon emitter with strain or the Stark effect. The mechanical vibration couples to microwave photons by shifting the resonant frequency of a LC circuit that includes the membrane. Thanks to the small mass of the 2D resonator, both the single-photon optomechanical coupling strength and the electromechanical coupling strength can reach strong coupling regimes. This provides a way for high-speed quantum state transfer between a microwave photon, a phonon, and an optical photon., Comment: 15 pages, 11 figures

We propose a single phonon source based on nitrogen-vacancy (NV) centers, which are located in a diamond phononic crystal resonator. The strain in the lattice would induce the coupling between the NV centers and the phonon mode. The strong coupling between the excited state of the NV centers and the phonon is realized by adding an optical laser driving. This four level NV centers system exhibits the coherent population trapping (CPT), and yields giant resonantly enhanced acoustic nonlinearities, with zero linear susceptibility. Based on this nonlinearity, the single phonon source can be realized. We numerically calculate $g^{(2)}(0)$ of the single phonon source. We discuss the effects of the thermal noise and the external driving strength., Comment: 5 pages, 3 figures

We study the dynamics of the Lipkin-Meshkov-Glick (LMG) model with finite number of spins. In the thermodynamic limit, the ground state of the LMG model with isotropic Hamiltonian in broken phase breaks to a mean-field ground state with a certain direction. However, when the spins number $N$ is finite, the exact ground state is always unique and is not given by a classical mean-field ground state. Here we prove that for $N$ is large but finite, through a tiny external perturbation, a localized state which is close to a mean-field ground state can be prepared, which mimics a spontaneous symmetry breaking (SSB). Besides, we find the localized in-plane spin polarization oscillates with two different frequencies $\sim O(1/N)$, and the lifetime of the localized state is long enough to exhibit this oscillation. We numerically test the analytical results and find that they agree with each other very well. Finally, we link the phenomena to quantum time crystals and quasicrystals., Comment: Accepted for Phys. Rev. A, 10 pages, 7 figures. Added a section of correlation function in time, and more discussion on time quasicrystal

Displacement detection of a sphere particle in focused laser beams with quadrant photodetector (QPD) provides a fast and high precision way to determine the particle location. In contrast to the traditional Gaussian beams, the sensitivity of displacement detection using various doughnut beams are investigated. The sensitivity improvement for large sphere particles along the longitudinal direction is reported. With appropriate vortex charge $l$ of the doughnut beams, they can outperform the Gaussian beam to get more than one order higher sensitivity and thus have potential applications in various high precision measurement. By using the levitating doughnut beam itself to detect the particle displacement, the result will also facilitate the recent proposal of levitating a particle in doughnut beams to suppress the light absorption., Comment: 6 pages, 6 figures

In diamond, the mechanical vibration induced strain can lead to interaction between the mechanical mode and the nitrogen-vecancy (NV) centers. In this work, we propose to utilize the strain induced coupling for the quantum non-demolition (QND) single phonon measurement and memory in diamond. The single phonon in a diamond mechanical resonator can be perfectly absorbed and emitted by the NV centers ensemble (NVE) with adiabatically tuning the microwave driving. An optical laser drives the NVE to the excited states, which have much larger coupling strength to the mechanical mode. By adiabatically eliminating the excited states under large detuning limit, the effective coupling between the mechanical mode and the NVE can be used for QND measurement of the single phonon state. Under realistic experimental conditions, we numerically simulate the scheme. It is found that the fidelity of the absorbing and emitting process can reach a much high value. The overlap between the input and the output phonon shapes can reach $98.57\%$., Comment: 7 pages, 3 figures

We systematically investigate the bistable behavior and squeezing property of the librational mode of a levitated nonspherical nanoparticle trapped by laser beams. By expanding the librational potential to the forth order of the librational angle $\theta$, we find that the nonlinear coefficient of this mode is dependent only on the size and material of nanoparticle, but independent of trapping potential shape. The bistability and hysteresis are displayed when the driving frequency is red-detuned to the librational mode. In the blue-detuned region, we have studied squeezing of the variance of librational mode in detail, which has potential application for measurement of angle and angular momentum., Comment: 16 pages, 6 figures, accepted for Phys. Rev. A

An optically levitated nonspherical nanoparticle can exhibit both librational and translational vibrations due to orientational and translational confinements of the optical tweezer, respectively. Usually, the frequency of its librational mode in a linearly-polarized optical tweezer is much larger than the frequency of its translational mode. Because of the frequency mismatch, the intrinsic coupling between librational and translational modes is very weak in vacuum. Here we propose a scheme to couple its librational and center-of-mass modes with an optical cavity mode. By adiabatically eliminating the cavity mode, the beam splitter Hamiltonian between librational and center-of-mass modes can be realized. We find that high-fidelity quantum state transfer between the librational and translational modes can be achieved with practical parameters. Our work may find applications in sympathetic cooling of multiple modes and quantum information processing., Comment: 6 pages, 3 figures

Hybrid spin-mechanical systems have great potentials in sensing, macroscopic quantum mechanics, and quantum information science. In order to induce strong coupling between an electron spin and the center-of-mass motion of a mechanical oscillator, a large magnetic gradient is usually required, which is difficult to achieve. Here we show that strong coupling between the electron spin of a nitrogen-vacancy (NV) center and the torsional vibration of an optically levitated nanodiamond can be achieved in a uniform magnetic field. Thanks to the uniform magnetic field, multiple spins can strongly couple to the torsional vibration at the same time. We propose to utilize this new coupling mechanism to realize the Lipkin-Meshkov-Glick (LMG) model by an ensemble of NV centers in a levitated nanodiamond. The quantum phase transition in the LMG model and finite number effects can be observed with this system. We also propose to generate torsional superposition states and realize torsional matter-wave interferometry with spin-torsional coupling., Comment: 5 pages, 4 figures

We consider a cantilever mechanical oscillator(MO) made of diamond. There is a nitrogen-vacancy(NV) center at the end of the cantilever. Two magnetic tips induce strong second order magnetic field gradient near the NV center. Under a coherent driving on MO, we find that the coupling between the MO and the NV center can be greatly enhanced. We studied how to realize quantum state transfer between MO and NV center and generate entanglement between them. We also proposed a scheme to generate the two-mode squeezing between different MO modes by coupling them to the same NV center. The decoherence and dissipation effects for both MO and NV center are numerically calculated by taking the present experimental parameters. It is found that high fidelity quantum state transfer, entanglement generation, and large two-mode squeezing could be achieved., Comment: 7 pages, 5 figures

Based on the framework of consistent history theory, the quantum entangled history was proposed in 2015 and experimentally verified through temporal Greenberger-Horne-Zeilinger (GHZ) test with $3$ time nodes in 2016. In this paper, we extend the temporal GHZ test to arbitrary time nodes and even system dimensions. Then, we define a witness to distinguish between the quantum entangled histories and the classical histories. The minimums of the witness for the classical histories are calculated for arbitrary number of time nodes and the system dimensions $2$ and $\infty$. It is found that the minimums of the witness for the classical histories is always larger than the quantum entangled histories minimum $-1$. Only when both the number of time nodes and system dimensions approach to infinity, the minimum of the witness for classical and quantum entangled histories are identical., Comment: 16 pages, 1 figure. GHZ test for entangled histories with dimensions larger than 2 is added. The title is changed accordingly. Accepted for Science Bulletin

The question whether quantum mechanics is complete and the nature of the transition between quantum mechanics and classical mechanics have intrigued physicists for decades. There have been many experimental breakthroughs in creating larger and larger quantum superposition and entangled states since Erwin Schr\"odinger proposed his famous thought experiment of putting a cat in a superposition of both alive and dead states in 1935. Remarkably, recent developments in quantum optomechanics and electromechanics may lead to the realization of quantum superposition of living microbes soon. Recent evidence also suggests that quantum coherence may play an important role in several biological processes. In this review, we first give a brief introduction to basic concepts in quantum mechanics and the Schr\"odinger's cat thought experiment. We then review developments in creating quantum superposition and entangled states and the realization of quantum teleportation. Non-trivial quantum effects in photosynthetic light harvesting and avian magnetoreception are also discussed. At last, we review recent proposals to realize quantum superposition, entanglement and state teleportation of microorganisms, such as viruses and bacteria., Comment: review article for Contemporary Physics, 28 pages

An optically levitated nanoparticle in vacuum is a paradigm optomechanical system for sensing and studying macroscopic quantum mechanics. While its center-of-mass motion has been investigated intensively, its torsional vibration has only been studied theoretically in limited cases. Here we report the first experimental observation of the torsional vibration of an optically levitated nonspherical nanoparticle in vacuum. We achieve this by utilizing the coupling between the spin angular momentum of photons and the torsional vibration of a nonspherical nanoparticle whose polarizability is a tensor. The torsional vibration frequency can be one order of magnitude higher than its center-of-mass motion frequency, which is promising for ground state cooling. We propose a simple yet novel scheme to achieve ground state cooling of its torsional vibration with a linearly-polarized Gaussian cavity mode. A levitated nonspherical nanoparticle in vacuum will also be an ultrasensitive nanoscale torsion balance with a torque detection sensitivity on the order of $10^{-29} ~\mathrm{N}\cdot \mathrm{m}/\sqrt{\mathrm{ Hz}}$ under realistic conditions.

A quantum network is a promising quantum many-body system because of its tailored geometry and controllable interaction. Here, we propose an external control scheme for the qubit-photon interaction and multiqubit reset in a dissipative quantum network, which comprises superconducting circuit chains with microwave drives and filter-filter couplings. The traditional multiqubit reset of the quantum network requires physically disconnected qubits to prevent their entanglement. However, we use an original effect of dissipation, i.e., consuming the entanglement generated by qubits' interaction, to achieve an external control of the multiqubit reset in an always-connected superconducting circuit. The reset time is independent of the number of qubits in the quantum network. Our proposal can tolerate considerable fluctuations in the system parameters and can be applicable to higher-dimensional quantum networks., Comment: Comments are welcome!

We consider a mechanical resonator made of diamond, which contains a nitrogen-vacancy center (NV center) locating at the end of the oscillator. A second order magnetic gradient is applied and inducing coupling between mechanical modes and the NV center. By applying proper external magnetic field, the energy difference between NV center electron spin levels can be tuned to match the energy difference between two mechanical modes $a$ and $b$. A laser is used for continuously initializing the NV center electron spin. The mode $a$ with lower frequency is driven by a thermal bath. We find that the temperature of the mode $b$ is significantly cooled when the heating bath temperature is increased. We discuss the conditions that quantum regime cooling requires, and confirm the results by numerical simulation. Finally we give the intuitive physical explanation on this unusual effect., Comment: 9 pages, 5 figures; added new figure in Fig. 2; Accepted for Phys. Rev. A

We propose and demonstrate experimentally a scheme to create entangled history states of the Greenberger-Horne-Zeilinger (GHZ) type. In our experiment, the polarization states of a single photon at three different times are prepared as a GHZ entangled history state. We define a GHZ functional which attains a maximum value $1$ on the ideal GHZ entangled history state and is bounded above by $1/16$ for any three-time history state lacking tripartite entanglement. We have measured the GHZ functional on a state we have prepared experimentally, yielding a value of $0.656\pm 0.005$, clearly demonstrating the contribution of entangled histories., Comment: 15 pages, 2 figures v. 2: 16 pages, 2 figures. Fundamental improvements to introduction, added references

We extend the non-Hermitian one-dimensional quantum walk model [Phys. Rev. Lett. 102, 065703 (2009)] by taking the dephasing effect into account. We prove that the feature of topological transition does not change even when dephasing between the sites within units is present. The potential experimental observation of our theoretical results in the circuit QED system consisting of superconducting qubit coupled to a superconducting resonator mode is discussed and numerically simulated. The results clearly show a topological transition in quantum walk and display the robustness of such a system to the decay and dephasing of qubits. We also discuss how to extend this model to higher dimension in the circuit QED system., Comment: 8 pages, 9 figures; published version

We propose a potentially practical scheme for realization of two-mode squeezed state with respect to two distant nitrogen-vacancy center ensembles coupled to two interconnected mechanical modes of diamond nanoresonators. By making use of the tunable phonon-spin interaction and the engineered phonon-phonon tunneling, both the desired excitation transfer process and the optimal two-mode squeezing between spin ensembles can be realized. We investigate the dynamics of the total system under infulences from the mechanical decay using both analytical and numerical methods, where the realistic conditions that leads to optimized squeezing between spin ensembles are analyzed., Comment: 7 pages, 5 pages

Traditional quantum error-correcting codes are designed for the depolarizing channel modeled by generalized Pauli errors occurring with equal probability. Amplitude damping channels model, in general, the decay process of a multilevel atom or energy dissipation of a bosonic system at zero temperature. We discuss quantum error-correcting codes adapted to amplitude damping channels for higher dimensional systems (qudits). For multi-level atoms, we consider a natural kind of decay process, and for bosonic systems,we consider the qudit amplitude damping channel obtained by truncating the Fock basis of the bosonic modes to a certain maximum occupation number. We construct families of single-error-correcting quantum codes that can be used for both cases. Our codes have larger code dimensions than the previously known single-error-correcting codes of the same lengths. Additionally, we present families of multi-error correcting codes for these two channels, as well as generalizations of our construction technique to error-correcting codes for the qutrit $V$ and $\Lambda$ channels., Comment: 22 pages, 3 figures, Part of this work was presented at the 2014 IEEE Symposium on Information Information Theory

Schr\"odinger's thought experiment to prepare a cat in a superposition of both alive and dead states reveals profound consequences of quantum mechanics and has attracted enormous interests. Here we propose a straightforward method to create quantum superposition states of a living microorganism by putting a small cryopreserved bacterium on top of an electromechanical oscillator. Our proposal is based on recent developments that the center-of-mass oscillation of a 15-$\mu$m-diameter aluminium membrane has been cooled to its quantum ground state [Nature 475: 359 (2011)], and entangled with a microwave field [Science 342: 710 (2013)]. A microorganism with a mass much smaller than the mass of the electromechanical membrane will not significantly affect the quality factor of the membrane and can be cooled to the quantum ground state together with the membrane. Quantum superposition and teleportation of its center-of-mass motion state can be realized with the help of superconducting microwave circuits. More importantly, the internal states of a microorganism, such as the electron spin of a glycine radical, can be entangled with its center-of-mass motion and teleported to a remote microorganism. Our proposal can be realized with state-of-art technologies. The proposed setup is a quantum-limited magnetic resonance force microscope. Since internal states of an organism contain information, our proposal also provides a scheme for teleporting information or memories between two remote organisms., Comment: to appear in Science Bulletin

We present a proposal to realize the quantum Zeno effect (QZE) and quantum Zeno-like effect (QZLE) in a proximal $\mathrm{^{13}C}$ nuclear spin by controlling a proximal electron spin of a nitrogen vacancy (NV) center. The measurement is performed by applying a microwave pulse to induce the transition between different electronic spin states. Under the practical experimental conditions, our calculations show that there exist both QZE and QZLE in a $^{13}$C nuclear spin in the vicinity of an NV center., Comment: 9 pages, 2 figures; replot Fig. 2, accepted for Scientific Reports

We present a proposal for heralded entanglement between two quantum dots via Hong--Ou--Mandel effect. Each of the quantum dots, drived off-resonance by two lasers, can be entangled with the coherent cavity mode. The output photons from the two coherent cavity modes interfering by a beamsplitter, we could entangle the two QDs with nearly unit success probability. Our scheme requires neither direct coupling between qubits nor the detection of single photons. Moreover the quantum dots do not need to have the same frequencies and coupling constants.

In this review, we briefly overview recent works on hybrid (nano) opto-mechanical systems that contain both mechanical oscillators and diamond nitrogen-vacancy (NV) centers. We review two different types of mechanical oscillators. The first one is a clamped mechanical oscillator, such as a cantilever, with a fixed frequency. The second one is an optically trapped nano-diamond with a build-in nitrogen-vacancy center. By coupling mechanical resonators with electron spins, we can use the spins to control the motion of mechanical oscillators. For the first setup, we discuss two different coupling mechanisms which are magnetic coupling and strain induced coupling. We summarize their applications such as cooling the mechanical oscillator, generating entanglements between NV centers, squeezing spin ensembles and et al. For the second setup, we discuss how to generate quantum superposition states with magnetic coupling, and realize matter wave interferometer. We will also review its applications as ultra-sensitive mass spectrometer. Finally, we discuss new coupling mechanisms and applications of the field., Comment: 13 pages review, comments are welcome

The past two decades witnessed important developments in the field of non-equilibrium statistical mechanics. Among these developments, the Jarzynski equality, being a milestone following the landmark work of Clausius and Kelvin, stands out. The Jarzynski equality relates the free energy difference between two equilibrium states and the work done on the system through far from equilibrium processes. While experimental tests of the equality have been performed in classical regime, the verification of the quantum Jarzynski equality has not yet been fully demonstrated due to experimental challenges. Here, we report an experimental test of the quantum Jarzynski equality with a single \Yb ion trapped in a harmonic potential. We perform projective measurements to obtain phonon distributions of the initial thermal state. Following that we apply the laser induced force on the projected energy eigenstate, and find transition probabilities to final energy eigenstates after the work is done. By varying the speed of applying the force from equilibrium to far-from equilibrium regime, we verified the quantum Jarzynski equality in an isolated system., Comment: 18 pages, 4 figures, 1 table

Hybrid quantum systems usually consist of two or more subsystems, which may take the advantages of the different systems. Recently, the hybrid system consisting of circuit electromechanical subsystems have attracted great attention due to its advanced fabrication and scalable integrated photonic circuit techniques. Here, we propose a scheme for high fidelity quantum state transfer between a superconducting qubit and a nitrogen-vacancy center in diamond, which are coupled to a superconducting transmission-line resonator with coupling strength $g_1$ and a nanomechanical resonator with coupling strength $g_2$, respectively. Meanwhile, the two resonators are parametrically coupled with coupling strength $J$. The system dynamics, including the decoherence effects, is numerical investigated. It is found that both the small ($J \ll \{g_1, g_2\}$) and large ($J \gg \{g_1, g_2\}$) coupling regimes of this hybrid system can not support high fidelity quantum state transfer before significant technique advances. However, in the intermediate coupling regime ($J \sim g_1 \sim g_2$), in contrast to a conventional wisdom, high fidelity quantum information transfer can be implemented, providing a promising route towards high fidelity quantum state transfer in similar coupled resonators systems.

We propose a scheme to realize quantum networking of superconducting qubits based on the opto-mechanical interface. The superconducting qubits interact with the microwave photons, which then couple to the optical photons through the opto-mechanical interface. The interface generates a quantum link between superconducting qubits and optical flying qubits with tunable pulse shapes and carrier frequencies, enabling transmission of quantum information to other superconducting or atomic qubits. We show that the scheme works under realistic experimental conditions and it also provides a way for fast initialization of the superconducting qubits under 1 K instead of 20 mK operation temperature., Comment: 6.1 pages, 4 figures, accepted for PRA

We demonstrate quantum bath engineering for preparation of any orbital state with controllable phase factor of a superconducting flux qubit assisted by a microwave coplanar waveguide resonator. We investigate the polarization efficiency of the arbitrary direction rotating on the Bloch sphere, and obtain an effective Rabi frequency by using the convergence condition of Markovian master equation. The processes of polarization can be implemented effectively in a dissipative environment created by resonator photon loss when the spectrum of the microwave resonator matches with the specially tailored Rabi and resonant frequencies of the drive. Our calculations indicate that state-preparation fidelities in excess of 99\% and the required time on the order of magnitude of microsecond are in principle possible for experimentally reasonable sample parameters. Furthermore, our proposal could be applied to other systems with spin-based qubits., Comment: 12 pages, 4 figures, accepted by PRA

We propose an ultra-sensitive mass spectrometer based on a coupled quantum-bit-oscillator system. Under dynamical decoupling control of the quantum bit (qubit), the qubit coherence exhibits a comb structure in time domain. The time-comb structure enables high precision measurement of oscillator frequency, which can be used as an ultra-sensitive mass spectrometer. Surprisingly, in ideal case, the sensitivity of the proposed mass spectrometer, which scales with the temperature $T$ as $T^{-1/2}$, has better performance in higher temperature. While taking into account qubit and oscillator decay, we show that the optimal sensitivity is independent on environmental temperature $T$. With present technology on solid state spin qubit and high-quality optomechanical system, our proposal is feasible to realize an ultra-sensitive mass spectrometer in room temperature., Comment: 5 pages, 3 figures

We review recent works on optomechanics of optically trapped microspheres and nanoparticles in vacuum, which provide an ideal system for studying macroscopic quantum mechanics and ultrasensitive force detection. An optically trapped particle in vacuum has an ultrahigh mechanical quality factor as it is well-isolated from the thermal environment. Its oscillation frequency can be tuned in real time by changing the power of the trapping laser. Furthermore, an optically trapped particle in vacuum may rotate freely, a unique property that does not exist in clamped mechanical oscillators. In this review, we will introduce the current status of optical trapping of dielectric particles in air and vacuum, Brownian motion of an optically trapped particle at room temperature, Feedback cooling and cavity cooling of the Brownian motion. We will also discuss about using optically trapped dielectric particles for studying macroscopic quantum mechanics and ultrasensitive force detection. Applications range from creating macroscopic Schr{\"{o}}dinger's cat state, testing objective collapse models of quantum wavefunctions, measuring Casimir force, searching short-range non-Newtonian gravity, to detecting gravitational waves., Comment: review article, 16 pages

We propose a method to generate and detect large quantum superposition states and arbitrary Fock states for the oscillational mode of an optically levitated nanocrystal diamond. The nonlinear interaction required for the generation of non-Gaussian quantum states is enabled through the spin-mechanical coupling with a built-in nitrogen-vacancy center inside the nanodiamond. The proposed method allows the generation of large superpositions of nanoparticles with millions of atoms and the observation of the associated spatial quantum interference under reasonable experimental conditions., Comment: 6 pages, 6 figures, published version