Quantum metrology and control
编者按：
量子信息是量子科学与信息科学的交叉学科，主要包括量子计算和量子通信。1980年，费曼从模拟量子力学体系的需求，班尼奥夫从降低计算机热耗的要求出发，分别提出了量子计算机的概念。1984年，量子密钥分发由班内特和布拉萨德提出，利用量子性质可现场发现窃听行为，从而实现安全地的数据传输。1995年，舒尔提出了大数质因子分解的量子算法，格罗夫提出了无需数据库的量子搜索算法，充分显示了量子计算机的强大计算功能，取得了量子计算的重大突破，从此量子计算的研究走上了世界科研的前沿，成为国际上具有重大战略意义的研究领域。而量子通信则能够抵御量子攻击，随着量子计算机研究的发展，量子通信也迅速发展，量子秘密共享、量子安全直接通信等相继提出。如今量子通信已经接近实用。
量子计量是继量子计算和量子通信后又一个量子信息的重要方向，利用量子性质可以大幅度地提高测量的精度，从而实现高精度的频率、时间和长度等的计量，而量子操控是量子计算和量子通信的核心操作。本刊组织的“量子精密计量与操控”专题，从电子自旋共振、核自旋共振操控，电场测量、纳米检测，以及多光子纠缠态制备、光子角动量态的制备与应用、压缩态产生与干涉、噪声下的量子网络等几个方面对近几年的创新性研究进行系统的介绍和综述，以期对相关研究领域的研究人员有所帮助。
2015, 64 (16): 160301.
doi: 10.7498/aps.64.160301
Abstract +
Nowadays, the nonlinear optical process of spontaneous parametric down-conversion is considered as the canonical approach for creating entangled-photon pairs. We consider three pairs of entangled photons emitted by the parametric down-conversion source, and introduce a setup for evolving these photons based on linear optics, which is composed of several polarizing beam splitters, beam splitters, and half wave plates. By using the parametric down-conversion source and the setup, we carefully design an efficient scheme for preparing six-photon hyperentangled states in both the polarization and the spatial degrees of freedom. Because we use almost all possible behaviors of the three pairs of entangled photons, the present scheme is efficient for creating six-photon hyperentangled states. Next, in the regime of weak nonlinearity we design a quantum nondemolition detection to distinguish the two cases of photons in two special spatial modes. It is worth pointing out that our scheme is much easier to realize, since the strength of the nonlinearities in the process of quantum nondemolition detection can be restricted to the scalable orders of magnitude in practicality.
2015, 64 (16): 160303.
doi: 10.7498/aps.64.160303
Abstract +
Photon system is a promising candidate for quantum information processing, and it can be used to achieve some important tasks with the interaction between a photon and an atom (or a artificial atom), such as the transmission of secret information, the storage of quantum states, and parallel quantum computing. Several degrees of freedom (DOFs) of a photon system can be used to carry information in the realization of quantum information processing, such as the polarization, spatial-mode, orbit-angular-momentum, time-bin, and frequency DOFs. A hyperparallel quantum computer can implement the quantum operations on several DOFs of a quantum system simultaneously, which reduces the operation time and the resources consumed in quantum information processing. The hyperparallel quantum operations are more robust against the photonic dissipation noise than the quantum computing in one DOF of a photon system. Hyperentanglement, defined as the entanglement in several DOFs of a quantum system, can improve the channel capacity and the security of long-distance quantum communication, and it can also be conductive to completing some important tasks in quantum communication. Hyperentangled Bell-state analysis is used to completely distinguish the 16 hyperentangled Bell states, which is very useful in high-capacity quantum communication protocols and quantum repeaters. In order to depress the effect of noises in quantum channel, hyperentanglement concentration and hyperentanglement purification are required to improve the entanglement of the quantum systems in long-distance quantum communication, which is also very useful in high-capacity quantum repeaters. Hyperentanglement concentration is used to distill several nonlocal photon systems in a maximally hyperentangled state from those in a partially hyperentangled pure state, and hyperentanglement purification is used to distill several nonlocal photon systems in a high-fidelity hyperentangled state from those in a mixed hyperentangled state with less entanglement. In this reviewing article, we review some new applications of photon systems with multiple DOFs in quantum information processing, including hyperparallel photonic quantum computation, hyperentangled-Bell-state analysis, hyperentanglement concentration, and hyperentanglement purification.
2015, 64 (16): 160304.
doi: 10.7498/aps.64.160304
Abstract +
Reduction of quantum noise in one spin component is a significant tool for enhancing the sensitivities of interferometers and atomic clocks. It has been recently implemented for ultra-cold atomic Bose-Einstein condensate (BEC) interferometer. This type of quantum noise reduction reduces the measurement noise near some predetermined phase. However, if the phase is completely unknown prior to measurement, then it is not known which phase quadrature should be in a squeezed state. We introduce a novel planar squeezing uncertainty relation for spin variance in a plane, and analyze how to obtain such a planar quantum squeezed (PQS) state by using a double-well single component BEC, through the use of local nonlinear S-wave scattering interaction between trapped atoms. Here, we consider the PQS that is generated by using two hyperfine states in a two components BEC system, which is useful for quantum metrology. By comparison with the case of two spatial wells, the Hamiltonian parameters can be controlled in a more efficient way. The spin component can be measured by detecting the occupation number difference between the two internal modes, while one needs to observe a spatial interference pattern in the double well BEC case. This is the major difference between the internal and external cases. Another difference is that one can use the Rabi frequency Ω instead of the Josephson parameters to switch the Hamiltonian parameters through using a diabatic technique. Therefore the coupling could be switched off or on to study the different evolutions. PQS simultaneously reduces the quantum noises of two orthogonal spin projections below the standard quantum limit, while increases the noise in the third dimension. This allows the improvement in phase measurement at any phase-angle. PQS states that reductions of fluctuations everywhere in a plane have potential utility in "one-shot" phase measurement, where iterative or repeated measurement strategies cannot be utilized. The improved interferometric phase measurements and planar uncertainty relations are useful for detecting the entanglement in mesoscopic system between two distinguished modes regardless of the third component.
2015, 64 (16): 160306.
doi: 10.7498/aps.64.160306
Abstract +
The direct communication protocol of quantum network over noisy channel is proposed and investigated in this study. In communication process, all quantum nodes share multiparticle Greenberger-Horne-Zeilinger (GHZ)-states. The sending node takes the GHZ-state particle in the hand as the control qubit and the particle for sending secret information as the target qubit, which carries out the CNOT gate operation for the control and target qubit. Each receiving node takes the GHZ-state particle in the hand as the control qubit and the particle of the received secret information as the target qubit, in which the CNOT gate operation is repeated to obtain the secret information that contains the bit error. Each receiving node uses the extracted part of qubits as the checking qubits, and then corrects the bit-flip errors using parity check matrix together with the rest part of qubits. As a result, all receiving nodes obtain rectified secret information. In addition to the high security analysis, this study also presents the detailed analyses of the throughput efficiency and the communication performance.
2015, 64 (16): 160307.
doi: 10.7498/aps.64.160307
Abstract +
Quantum secure direct communication (QSDC) is one of the most important branches of quantum communication. In contrast to the quantum key distribution (QKD) which distributes a secure key between distant parties, QSDC directly transmits secret message instead of sharing key in advance. To establish a secure QSDC protocol, on the one hand, the security of the quantum channel should be confirmed before the exchange of the secret message. On the other hand, the quantum state should be transmitted in a quantum data block since the security of QSDC is based on the error rate analysis in the theories on statistics. Compared with the deterministic quantum key distribution (DQKD) which can also be used to transmit deterministic information, QSDC schemes do not need extra classical bits to read the secret message except for public discussion. In this article, we introduce the basic principles of QSDC and review the development in this field by introducing typical QSDC protocols chronologically. The first QSDC protocol was proposed by Long and Liu, which can be used to establish a common key between distant parties. In their scheme, the method for transmitting quantum states in a block by block way and in multiple steps was proposed and the information leakage before eavesdropping detection was solved. Subsequently, Deng et al. presented two pioneering QSDC schemes, an entangled-state-based two-step QSDC scheme and a single-photon-state-based quantum one-time pad scheme, in which the basic principle and criteria for QSDC were pointed out. From then on, many interesting QSDC schemes have been proposed, including the high-dimension QSDC scheme based on quantum superdense coding, multi-step QSDC scheme based on Greenberger-Horne-Zeilinger states, QSDC scheme based on quantum encryption with practical non-maximally entangled quantum channel, and so on. We also introduce the anti-noise QSDC schemes which were designed for coping with the collective-dephasing noise and the collective-rotation noise, respectively. In 2011, Wang et al. presented the first QSDC which exploited the hyperentangled state as the information carrier and several QSDC schemes based on the spatial degree of freedom (DOF) of photon, single-photon multi-DOF state and hyperentanglement were proposed subsequently. In addition to the point-to-point QSDC schemes, we also review the QSDC networks. Finally, a perspective of QSDC research is given in the last section.
2015, 64 (16): 160308.
doi: 10.7498/aps.64.160308
Abstract +
The cesium fountain clock as primary frequency standard is widely used in the areas, such as time-keeping system, satellite navigation, fundamental physics research, etc. The principle of operation of cesium fountain clock is introduced. The noise source and frequency shift term are ananlyzed. The major noise source influencing frequency stability are cold atom loading time, microwave phase noise related to Dick effect, and detection laser frequency noise. The major frequency bias influencing frequency uncertainty is blackbody radiation frequency shift,cold atom collision frequency shift,distributed cavity phase frequency shift and microwave leakage frequency shift.The key technique to achieve highperformance cesium fountain clock is sumerized. The application of cesium fountain clock is presented. The status of space cesium clock and future primary frequency standard of optical clock are shown.
2015, 64 (16): 160702.
doi: 10.7498/aps.64.160702
Abstract +
Atom in Rydberg state has large polarizability, large electric dipole and low ionization threshold field. It is very sensitive to electric field, therefore it can be used to measure the amplitude of electric field, especially the microwave electric field. The new developed scheme is based on quantum interference effects (electromagnetically induced transparency and Autler-Townes splitting) in Rydberg atoms. Instead of the direct amplitude measurement, this method tests the Rabi frequency value of the transmission spectrum which is determined by the microwave electric field strength and the corresponding atom nature. The minimum measured strengths of microwave electric fields are far below the standard values obtained by traditional antenna methods. Compared with the traditional methods, this new scheme has several advantages, such as self-calibration, non-perturbation to the measured field and independence of the probe length. Besides, this scheme can also be used to measure the polarization direction of microwave electric field and realize sub-wavelength imaging. Through adjusting the wavelength of coupling laser, a broadband 1-500 GHz microwave electric field measurement can be achieved. This new scheme is benefitial to conducting the continue electric field measurement and the miniaturization of the test equipment. In this paper, the researches about using Rydberg atom to measure electric field with high precision are reviewed. The basic theory and experimental techniques are introduced. Finally, we discuss a promising method of using Rydberg atom interferometer to detect the accumulated phase in the process of interaction between electric field and Rydberg atoms. This method converts amplitude measurement into phase test, which may improve the precision and sensitivity.
2015, 64 (16): 164210.
doi: 10.7498/aps.64.164210
Abstract +
Photons are an ideal candidate for encoding both classical and quantum information. Besides spin angular momentum associated with circular polarization, single photon can also carry other fundamentally new degree of freedom of orbital angular momentum related to the spiral phase structure of light. The key significance of orbital angular momentum lies in its potential in realizing a high-dimensional Hilbert space and in encoding a high-dimensional quantum information. Since Allen et al. [Allen L, Beijersbergen M W, Spreeuw R J C, Woerdman J P 1992 Phys. Rev. A 45 8185] recognized the physical reality of photon orbital angular momentum in 1992, rapidly growing interest has been aroused in orbital angular momentum (OAM) from both classical and quantum points of view. Here we present an overall review on the high-order orbital angular momentum of photon, including its preparation and manipulation based on some specific techniques and also its applications. The spatial light modulator is a commercial device that has been widely employed to generate the OAM beams. We make and identify the optical OAM superposition with very high quantum numbers up to l=360. Recently, the metallic spiral phase mirrors were also developed to produce high-order OAM beams up to l=5050. In addition, the Q-plates made of anisotropic and inhomogeneous liquid crystals were invented to generate high-order OAM beams in a polarization-controllable manner, and the OAM superposition of l=± 50 were achieved. Owing to high rotational symmetry, these high OAM beams have been found to have more and more important applications in the fields of high-sensitivity sensing and high-precision measurements. Two fascinating examples are discussed in detail. The first example is that the research group led by Prof. Zeilinger has prepared and observed the quantum entanglement of high orbital angular momenta up to l=±300 by the technique of polarization-OAM entanglement swapping, and they demonstrated that the angular resolution could be significantly improved by a factor of l. Their result was the first step for entangling and twisting even macroscopic, spatially separated objects in two different directions. The second example is that the research group led by Prof. Padgett has demonstrated an elegant experiment of rotational Doppler effects for visible light with l=±20 OAM superposition. They showed that a spinning object with an optically rough surface might induce a Doppler effect in light reflected from the direction parallel to the rotation axis, and the frequency shift was proportional to both the disk's angular speed and the optical OAM. The potential applications in noncontact measurement of angular speed and in significant improvement of angular resolution for remote sensing will be particularly fascinating.
2015, 64 (16): 164211.
doi: 10.7498/aps.64.164211
Abstract +
Cavity optomechanics originated from the research of interferometric detection of gravitational waves, and later became a fast-growing area of techniques and approaches ranging from the fields of atomic, molecular, and optical physics to nano-science and condensed matter physics as well. Recently, it focused on the exploration of operating mechanical oscillators deep in the quantum regime, with an interest ranging from quantum-classical interface tests to high-precision quantum metrology. In this paper, recent theoretical work of our group in the field of quantum measurement with cavity optomechanical systems is reviewed. We explore the quantum measurement theory and its applications with several unconventional cavity optomechanical schemes working in the quantum regime. This review covers the basics of quantum noises in the cavity optomechanical setups and the resulting standard quantum limit of precision displacement and force measurement. Three novel quantum measurement proposals based on the hybrid optomechanical system are introduced. First, we describe a quantum back-action insulated weak force sensor. It is realized by forming a quantum-mechanics-free subsystem with two optomechanical oscillators of reversed effective mass. Then we introduce a role-reversed atomic optomechanical system which enables the preparation and the quantum tomography of a variety of non-classical states of atoms. In this system, the cavity field acts as a mechanical oscillator driven by the radiation pressure force from an ultracold atomic field. In the end, we recommend a multimode optomechanical transducer that can detect intensities significantly below the single-photon level via adiabatic transfer of the microwave signal to the optical frequency domain. These proposals demonstrate the possible applications of optomechanical devices in understanding of quantum-classical crossover and in achieving quantum measurement limit.
2015, 64 (16): 164212.
doi: 10.7498/aps.64.164212
Abstract +
In this review, the recent development of nano-particle detection using Raman lasers in the whispering gallery mode microcavities is presented. The fabrication of the microcavity, the working principles are given and the recent experimental progress is reviewed. Recent years, the demand for nano-particle sensing techniques was increased, since more and more nano-particles of sizes between 1 nm and 100 nm are employed in areas such as biomedical science and homeland security. In these applications, label-free, rapid and real-time sensing requirements are necessary. Whispering gallery mode (WGM) micro-resonators have high-quality factors and small mode volumes, and have achieved significant progress in the nano-particle sensing field. There are various measurement mechanisms for nano-particle sensing using WGM cavities, including resonance mode broadening, resonance frequency shift, and mode splitting changes. The key point to improve sensing limit is to narrow the resonance mode linewidth, which means reducing the optical cavity losses, or equivalently to enhance quality factor. An important approach to narrowing the mode linewidth is to fabricate active resonators that provide gain and produce laser by doping rare earth irons. According to Schawlow-Townes formula, the linewidth of corresponding laser will be narrower than that of the original optical cavity mode. Active resonators have outstanding performances in particle detection. However, doping process requires complex fabrication steps, and rare earth irons laser demands a certain pumping wavelength band. A new approach is to use low threshold Raman laser in an optical micro-resonator. The binding of nano-particles on WGM micro-resonator induces resonance mode splitting. Raman lasers of the two splitting modes irradiate the same photon detector and generate a beat note signal. By monitoring the jumps of the two split mode differential signals, one can easily recognize the nano-particle binding events, thus achieving real time nanoparticle detection. Using Raman laser in WGM cavities in nano-particle sensing is no longer limited by the stringent requirement of a suitable pump light source, which greatly expands the applicability of this method in different environments. It does not need additional fabrication process as compared with the rare earth doping method. It has also better biological compatibility, which makes it a promising technique in biomedical applications. Recently, two groups, i.e., Li et al. (Proc. Natl. Acad. Sci. 111 14657) from Peking University, and zdemir et al. from University of Washington and Tsinghua University, have successfully completed the demonstration experiments. zdemir et al. (Proc. Natl. Acad. Sci. 111 E3836) have achieved a nano-particle sensing limit down to 10 nm without labelling, and Li et al. (Proc. Natl. Acad. Sci. 111 14657) realized real-time detection of single nano-particles with WGM cavity Raman laser in an aqueous environment. Both experiments have shown the great potential of the new approach. The new technique can also be used in other photonic systems, such as the photonic crystal or metal materials.
2015, 64 (16): 167303.
doi: 10.7498/aps.64.167303
Abstract +
Because of the long coherence time and the easy way to achieve the qubit scalability, quantum dot spin qubit has obtained considerable attentions recently. Single spin manipulation is usually achieved using the traditional electron spin resonance technique. This method not only needs a static Zeeman field, but also needs an ac magnetic field which is perpendicular to the static one. However, it is not easy to produce a local ac magnetic field experimentally. Recently, instead of an ac magnetic field, an ac electric field can also be used to manipulate an electron spin, an effect called electric-dipole spin resonance. As is well-known, there is no direct interaction between the spin and the electric field. Thus, the electric-dipole spin resonance must be mediated by some mechanisms. These mediums in the quantum dot can be: the slanting magnetic field, the spin-orbit coupling, and the electron-nucleus hyperfine interaction. This paper summarizes three main mechanisms of the electron-dipole spin resonance in semiconductor quantum dot.
2015, 64 (16): 167601.
doi: 10.7498/aps.64.167601
Abstract +
With the development of quantum information science, the active manipulation of quantum systems is becoming an important research frontier. To build realistic quantum information processors, one of the challenges is to implement arbitrary desired operations with high precision on quantum systems. A large number of quantum control methods and relevant numerical techniques have been put forward in recent years, such as quantum optimal control and quantum feedback control. Nuclear magnetic resonance (NMR) spin systems offer an excellent testbed to develop benchmark tools and techniques for controlling quantum systems. In this review paper, we briefly introduce some of the basic control ideas developed for NMR systems in recent years. We first explain, for the liquid spin systems, the physics of various couplings and the causes of relaxation effects. These mechanisms govern the system dynamics, and thus are crucial for constructing rigorous and efficient control models. We also identify three types of available control means: 1) raido-frequency fields as coherent controls; 2) phase cycling, gradient fields and relaxation effects as non-unitary controls; 3) radiation damping effect as feedback control mechanism. Then, we elucidate some important control tasks, which may arise from the conventional NMR spectroscopy (e.g., pulse design and polarization transfer) or from quantum information science (e.g., algorithmic cooling and pseudo-pure state preparation). In the last part, we review some of the most important control methods that are applicable to NMR control tasks. For systems with a relatively small number of spins, it is possible to use analytic optimal control theory to realize the target unitary operations. However, for larger systems, numerical methods are necessary. The gradient ascent pulse engineering algorithm and pulse compiler techniques are the most successful techniques for implementing complicated quantum networks currently. There are some interesting topics of utilizing radiation damping and relaxation effects to achieve more powerful controls. Finally, we give an outline of the possible future work.
