We consider a cantilever mechanical oscillator(MO) made of diamond. A nitrogen-vacancy(NV) center lies at the end of the cantilever. Two magnetic tips near the NV center induce a strong second-order magnetic field gradient. Under coherent driving of the MO, we find that the coupling between the MO and the NV center is greatly enhanced. We studied how to generate entanglement between the MO and the NV center and realize quantum state transfer between them. We also propose a scheme to generate two-mode squeezing between different MO modes by coupling them to the same NV center. The decoherence and dissipation effects for both the MO and the NV center are numerically calculated using the present parameter values of the experimental configuration. We have achieved high fidelity for entanglement generation, quantum state transfer, and large twomode squeezing.
Identifying Hamiltonian of a quantum system is of vital importance for quantum information processing.In this article, we realized and benchmarked a quantum Hamiltonian identification algorithm recently proposed(Zhang and Sarovar, 2014). we realized the algorithm on a liquid nuclear magnetic resonance quantum information processor using two types of working media with different forms of Hamiltonian. Our experiment realized the quantum identification algorithm based on free induction decay signals. We also showed how to process data obtained in a practical experiment. We studied the influence of decoherence by numerical simulations. Our experiments and simulations demonstrate that the algorithm is effective and robust.
Cloud-based quantum computing is anticipated to be the most useful and reachable form for public users to experience with the power of quantum. As initial attempts, IBM Q has launched influential cloud services on a superconducting quantum processor in 2016, but no other platforms has followed up yet. Here,we report our new cloud quantum computing service – NMRCloud Q(http://nmrcloudq.com/zh-hans/),where nuclear magnetic resonance, one of the pioneer platforms with mature techniques in experimental quantum computing, plays as the role of implementing computing tasks. Our service provides a comprehensive software environment preconfigured with a list of quantum information processing packages,and aims to be freely accessible to either amateurs that look forward to keeping pace with this quantum era or professionals that are interested in carrying out real quantum computing experiments in person. In our current version, four qubits are already usable with in average 99.10% single-qubit gate fidelity and 97.15% two-qubit fidelity via randomized benchmaking tests. Improved control precisions as well as a new seven-qubit processor are also in preparation and will be available later.
Efficient simulations of many-body quantum systems are generally difficult on classical computers due to the exponential resource growth with the system size, while quantum computer has been proved to complete the same task effectively. In this article, we studied quantum algorithms for digital simulation of the dynamics of interacting quantum systems in real space. As an illustrative example, we concentrated on a digital quantum simulation algorithm for a two-fermion system with Coulomb interaction. We experimentally realized our algorithm on a three-qubit NMR device, and interesting phenomena such as the moving toward or against each other of particles under attractive or repulsive interaction have been clearly observed. This experiment demonstrated the very promising potential of quantum computers, even of small scale, to address the simulation of complex quantum systems.
Correlation functions are often employed to quantify the relationships among interdependent variables or sets of data.Recently,a new class of correlation functions,called FORRELATION,has been introduced by Aaronson and Ambainis for studying the query complexity of quantum devices.It was found that there exists a quantum query algorithm solving 2-fold FORRELATION problems with an exponential quantum speedup over all possible classical means,which represents essentially the largest possible separation between quantum and classical query complexities.Here we report an experimental study probing the2-fold and 3-fold FORRELATIONS encoded in nuclear spins.The major experimental challenge is to control the spin fluctuation to within a threshold value,which is achieved by developing a set of optimized GRAPE pulse sequences.Overall,our small-scale implementation indicates that the quantum query algorithm is capable of determining the values of FORRELATIONS within an acceptable accuracy required for demonstrating quantum supremacy,given the current technology and in the presence of experimental noise.
An entanglement measure,multiple entropy measures(MEMS) was proposed recently by using the geometric mean of partial entropies over all possible i-body combinations of the quantum system.In this work,we study the average subsystem von Neumann entropies of the linear cluster state and investigated the quantum entanglement of linear cluster states in terms of MEMS.Explicit results with specific particle numbers are calculated,and some analytical results are given for systems with arbitrary particle numbers.Compared with other example quantum states such as the GHZ states and W states,the linear cluster states are "more entangled" in terms of MEMS,namely their averaged entropies are larger than the GHZ states and W states.
A two-step quantum secure direct dialogue protocol using Einstein-Podolsky-Rosen(EPR)pair block is proposed.In the protocol,the dialogue messages are encoded on series of qubits and sent through a quantum channel directly.The security of the protocol is assured by its connection to the two-step quantum secure direct communication protocol,which has been proved secure.This protocol has several advantages.It is a direct communication protocol that does not require a separate classical communication for the ciphertext.It has high capacity as two bits of secret messages can be transmitted by an EPR pair.As a dialogue protocol,the two parties can speak to each other either simultaneously or sequentially.
Opto-thermal relaxation is one of the most important properties of nonlinear optical materials.Rapid and high precision measurement of this parameter is vital in both fundamental research and applications.Current measurement uses either complicated structure with poor precision or high power heating source with low efficiency.Here,we propose a pump-probe method(PPM) to optically measure the thermal relaxation using whispering gallery mode(WGM) microcavities.When the pump laser shines on a microcavity,the materials absorb the input power resonantly and heat up.Then the heat dissipates from the cavities to the surroundings.The opto-thermal effect induces a refractive index change reflected in the signal light transmission spectra.By analyzing the curve character of the transmission spectra of the signal response in the spontaneous relaxation process,the thermal relaxation time can be rapidly measured with high precision.Additionally,we systematically verify the PPM using microtoroids under various pump powers and at various locking points of the signal laser mode.The small rate of refractive index changes($10à8) can be discerned with an input pump power as low as 11.816 l W.Hence,the PPM can be used to detect refractive index perturbation,like gas or liquid sensing,temperature fluctuations with ultra-high sensitivity and be applied to optical materials analysis efficiently.
Tao WangXiao-Fei LiuYunqi HuGuoqing QinDong RuanGui-Lu Long
