Quantum information science英文原稿:Information science and technology has penetrated into all aspects of society, in which the protagonist -- the development of computer science and technology and application, it is greatly promotes the progress of human civilization.Current computers are based on the classical physical laws, is a classical computer. Over the years, it has been recognized classic computer has some unconquerable limitations. For example, could not produce a true random number sequence, not in a limited time to simulate a conventional quantum mechanics system, not possible in acceptable time factorization of large numbers.From at present the development of microelectronics technology in light of the degree, people have to face such a problem: when the silica surface electric line of small to atomic scales, electronic circuits behavior will no longer obey the law of classical mechanics, replace sb. Is quantum mechanics. That is to say, people have to in the quantum theory under the framework of information science and information system construction.When the science is stillQuantum information (quantum information, QI) science, based on the superposition principle of quantum mechanics, based on studies of information processing a new cutting-edge science, the basic theory of modern physics and information science and technology intersect and produce a full vitality of the discipline. Quantum information science, including quantum computers, quantum state transfer from the material, quantum cryptography communication and quantum non-destructive measurement of other aspects.1980, Feynman [1] and Bennett (C. Bennett) [2] had carried out such as quantum information science theory. They pointed out that the two orthogonal polarization states of photons, atoms or atoms in two spin states, the appropriate level of these two orthogonal quantum states (for example: | 0>, | 1>) can be expressed a bit of quantum information, called quantum bit (qubit). Bit different from the classical, quantum bits in the particles (photons or atoms) not only in the | 0> or state | 1> state, and can at | 0> and | 1> of any kind of superposition state. It is this strange characteristic, so that quantum bits can not be compared with a classic bit of advantageIn the study of quantum information, in addition to quantum algorithms, quantum computers and quantum logic gates in quantum communication quantum state transfer from the material, is that people are most concerned about, the most interesting research topics, has received a preliminary experimental study the results.In addition, to explore methods of quantum information processing done by the process of quantum mechanics experiments, in turn, help people to verify and deepen understanding of the laws of the quantum world, the answer to those still remaining controversial issues. Quantum information science research, not only has important potential applications but also has far-reaching scientific significance.Powerful and efficient computational toolsIn 1985, Oxford University, more than the odd (D. Deutsch) [3] established thetheoretical basis of quantum computers, and promote the development of quantum computers. Similar to the classic computers, quantum computing, but also depends on the realization of the corresponding basic logic components - quantum logic gates (quantum logical gate, QLD). There are four possible experimental scheme of quantum logic gates, which are based on cavity quantum electrodynamics (CQED), ion trap (ion trap), nuclear magnetic resonance (NMR) and quantum dots (quantum dot).(1) cavity quantum electrodynamicsCavity quantum electrodynamics (CQED) The basic idea is that the very small number of atoms placed in a high-quality micro-cavity, the cavity electromagnetic fields (including the vacuum field) can be controlled to change, thus affecting the process of atomic radiation. CQED most successful is to study a small number of particles (photons, atoms) between theThe interaction. The method is possible to make a single photon of the electric field enhancement, so that it can make a single atom response saturation. To achieve this objective, we must achieve single-atom and single photon in the cavity of the strong coupling.As for quantum logic elements CQED quantum information processing, first by Pei Lizha in (T. Pellizzari), and others made. California Polytechnic University, Kimble (J. Kimble) group demonstrated the use of the program initial quantum logic gates. The basic approach is to capture a number of neutral atoms in the high-quality micro-optical cavity, the quantum information stored in the atoms within the state, that is the ground state of neutral atoms and on a metastable state. Contains the quantum state of a qubit is in the atomic ground state | g> and a long-lived metastable state | e> of the linear combination. The state quantum bits can be stored a long time, while the atomic energy in the cavity well with the outside world.CQED quantum logic gate is ideal to achieve one of the options. However, high-quality cavity, the connection between multiple quantum gates still have some technical difficulties.(2) ion trap technologyIon-trap quantum logic gate program first by Cirac (J. Cirac), who suggested that the current in the preliminary experiment has been achieved. In the experiment, each qubit is assigned in the capture in a linear Paul (Paul) trap single ions. Contains a qubit quantum state, is in the ion ground state | g> and a certain long-lived metastable state | e> of the linear combination. Therefore, the same atoms, it also enables qubit storage.The advantage of ion trap, ion Coulomb interaction between the far distance between the ions, so the energy of a single laser pulse tuned to a particular ion of | g> state and | e> state energy difference, we can achieve quantum information to read and change.Ion trap is the largest program in order to establish the ion trap quantum computing speed will be restricted. The reason is time - energy uncertainty relation determine the uncertainty of the laser pulse energy should be higher than the characteristic frequency of the vibration center of mass is small, the duration of each pulse should be longer than the reciprocal of the characteristic frequency; the phonon vibration frequency is generally lower the experiment the characteristic frequency of about 100 kHz, so the slower speed.In CQED, because of the role of the light field and atomic time soon, so there is no ion trap in the problems of slow response.(3) NMR techniquesNMR-based quantum computing scheme in recent years developed a new method of quantum information processing. In NMR, quantum bits are assigned certain specific molecules on the nuclear spin states. At a constant external magnetic field, each nuclear spin is either up or down. System and spin decoherence in degraded state can be kept for a longer time before, so the qubit can be stored.By a pulsed magnetic field acting on the spin-spin Rabi oscillation state to achieve the selected magnetic pulse can also be appropriate to achieve the transformation of a single magnetic spin states, because only those who are in resonance with the spin state of the external magnetic field will produce the role. Meanwhile, the spin state, there are also dipole-dipole interaction, this effect can be used to implement logic gates.NMR for quantum computation, but not as easily accepted as the first two options. Because the NMR system is "hot" nuclear spin temperature (room temperature) is generally caused by fluctuations in energy than the difference between the upper and lower levels of nuclear spin hundreds of times higher. This means that, from a single molecule in the composition of the nuclear spin quantum computer quantum state in a very large thermal noise into. The noise will drown out the quantum information. Further, the actual process is not handled a single molecule, but includes 1023 "quantum computer" macro samples.Read from this device the signal is actually a large number of molecules of the ensemble average, but the quantum algorithm is probabilistic, it comes from the randomness of quantum computing itself, and people took advantage of this randomness. Ensemble average does not mean a single unit on quantum computing. People had put forward some explanations of these difficulties, that the calculated average will not eliminate many useful quantum information. According to reports, the use of NMR methods have producedMulti-qubit logic gate, and use this to achieve a quantum state transfer from the material.Many scholars believe that the existing NMR system could not produce entanglement; arising from entanglement in quantum information is the key. NMR as a quantum information hardware will encounter many difficulties, from the principle limitations are: coherent signal and background noise ratio will be with the nuclear spin of each molecule increases the number of exponential decay. In a real system, complete with a 10-qubit NMR calculations will face serious challenges. Of course, some scholars hold different views on the above arguments, the NMR quantum logic gates to be optimistic. However, NMR will help people understand some of the nuclear spin of things.(4) quantum dotsRelated to nano-scale quantum-dot semiconductor region. These regions showed a small number of electronic states, the single-electron quantum dot can be changed into electronic state, which may be used for quantum information processing, quantum dots placed CQED they may control the materials in the spontaneous emission, enhanced light matter interaction the role. If the mature semiconductor technology combined with quantum devices, may have a practical quantum information systems. However, how toensure the purity of quantum dot materials remains a challenge.Quantum computing in an attempt to actually start, you need to try a variety of quantum logic gates program, which is a challenging work, it has only just begun. Practical quantum computing, to the number of qubits to the quantum logic gates and have made significant progress as a precondition.Magic magic- Quantum state transfer from the materialMaterial transfer from the state (teleportation) from a science fiction film, from the physical meaning of a "complete" information transfer (disembodied transport).Restrictions due to relativistic effects can not be real in an instant from one place to another place. You can achieve the object from the moment things send? Not exceed the limit in the speed of light under the premise seems to be feasible. Because, in principle, as long as all the information that constitute the object, all the quantum states can be reconstructed in any place. However, quantum mechanics tells us that, it is impossible to make accurate measurements of the quantum state can not be accurately all the information about any object. Therefore, reconstruction of this method can not be achieved, which is the quantum no-cloning theorem [4] are limited. However, another phenomenon of quantum mechanics - entanglement (EPR) of non-locality (non-local) [5] - for the realization of quantum state transfer from the material provides a new way.In 1993, six scientists from different countries, made using a combination of classical and quantum methods to achieve quantum state transfer from the object program. Using EPR (entangled state) of the non-locality, without violating the no-cloning theorem of quantum situations, can be an unknown quantum state from one place to another place. In this scheme, EPR source plays a vital role. Quantum mechanics, nonlocality violation of Bell's inequality has been confirmed by experimental results.Quantum state transfer from objects to people, not only in physics understanding and revealing the mysterious laws of nature are very important, and can be used as an information carrier quantum states, quantum state transfer is completed by a large-capacity information transmission, in principle, can achieve decipher the quantum cryptography communication, ultra-dense coding, quantum computing and quantum communication has therefore become the current rapid development of the core areas of quantum information.The protector of the secretResearch and use of password is a very ancient, wide range of issues, current password in addition to one-time password (Vernam password), but not impossible to decipher, the confidentiality of the algorithm depends on the difficulty of deciphering and calculation time. The use of quantum cryptography can guarantee from the principle Confidentiality of communications. Communication between the parties through the public channel to build their own key.Different from the classical mechanics, quantum mechanics, any time of the measurement system is a function of the system will change the system state (except in the role of operator eigenstates). Quantum cryptography can be used to encode a singlephoton polarization state. Incompatible in the two orthogonal polarization basis to measure a photon's polarization state, the result is completely random, it is impossible to get a measurement in a photon polarized in two different base in the results.Eavesdropper can not know because communication between the parties will be randomly selected each time what kind of polarization-based, so it can not accurately reproduce the signal eavesdropping, communication between the parties as long as the public than some random channel measurement results will know whether the key is eavesdropping, to discover the key insecure, you can re-establish the key until you are satisfied.Extremely accurate rulerBasic principles of quantum mechanics tells us that, due to the quantum uncertainty principle, using the general method, the measurement accuracy will eventually be shot-noise limit restrictions, it is impossible for a quantum system for unlimited precision measurements. Meanwhile, the measurement process will inevitably interfere with and affect the measured quantum state of the system, which often lead to even more accurate measurement results. The use of non-classical light field effects (ie, the unique quantum effects, there is no corresponding classical properties), the use of quantum measurement methods, can be cleverly "avoided" quantum uncertainties, and thus improve the measurement accuracy.(1) non-destructive measurement of quantumQuantum non-destructive measurement (quantum non-demolition detection, QND), 1970's by Braginski (VBBraginsky) [6] and so on, its purpose is to overcome the measurement process on the measured system caused by the interference of quantum state measurement inaccurate results, to be able to repeat the measurement without affecting the system under test is measured.QND measurement is one of the main characteristics repeatable. Measurement process must first choose a conjugate quantity of the good, the measurement process in the amount of interference on one another does not affect the amount of conjugate, and will be measured (signal field) to the probe field.In 1989 scientists from the experimental nonlinear parametric process to achieve this reaction escape, 1993 Grande Audigier (P. Grangier) through the sodium vapor-phase modulation to achieve a QND measurement. Subsequently, the national quantum optics laboratory and the use of different systems to achieve a different type of QND measurement, the transmission efficiency and the quantum state preparation ability are constantly improving. Institute of Optoelectronics, Shanxi University, 1998, the first time, the intensity difference fluctuations class QND measurement [7].(2) exceeded the limit of shot noise measurementsDue to the dominance of quantum mechanics, there is a minimum light field uncertainty, that shot noise limit. Coherent states for general light field, the shot noise limit is the amount of ups and downs two conjugate equal to the product by the uncertainty relation for the determination of limit values. Under normal circumstances, the measurement accuracy is always subject to the limit of shot noise limit, and has nothing to do with measuring instruments.Nonlinear processes of non-classical light field - state light field compression, you cankeep the product of two conjugate quantity under conditions of constant ups and downs make the ups and downs the amount of a conjugate is much smaller than the other. This means that one of the conjugate is less than the amount of ups and downs have been shot noise limit. The use of compressed light field of this feature, you can break through the measurement accuracy limit of shot noise limit, when the compression degree is 100 percent, the measurement accuracy in principle, unlimited increase.In 1987, Shaw (M. Xiao) were used with the Grand Jiyeh light field quadrature squeezed vacuum state, so that shot noise measurement sensitivity limit break. 1997 Years, Shanxi University, Institute for the direct use of optical light field intensity difference squeezing (twin beam on) for weak absorption measurements, the measurement results exceeded the signal light shot noise limit, signal to noise ratio (S / N) than the shot noise limit the signal light increased by 4 dB. In addition, there are many types of compressed light field applied in the measurement reports.Although quantum information processing with speed, capacity, safety, and the great advantages of high accuracy and a very attractive prospect, but also attracted the attention of scientists and government departments, but in addition to quantum cryptography communication may soon enter the practical stage, the quantum computer to be true, kind of away from the material sent, there is still a long way to go.One important reason is because the quantum state is "fragile." Any minor role with the external environment will lead to collapse of quantum states, namely decoherence. It must remain within a certain time quantum state from the outside world, before the collapse in the state to complete the necessary quantum computing. Although theoretically it is possible that the experimental efforts to achieve it need to do. On the other hand, quantum information processing system of storage, isolation and the accuracy of quantum logic gate operation has certain requirements, there is involved in the interaction between single photons and single atoms and other technical issues is no easy task .At present, the theoretical and experimental physicists are also working through a variety of possible ways to try to solve these problems, I believe that in the near future, quantum information science will be a breakthrough.[1] Feynman R. Int J Theor Phys, 1982,21: 4627[2] Bennett C. J Stat Phys, 1980,22: 563[3] Deutsch D. Proc Roy Soc Lond, 1985,A400: 97[4] Wootters W, et al. Nature, 1982,298: 802[5] Einstein A, et al. Phys Rev, 1935,47: 777[6] Braginsky V, et al. Usp Fiz Nauk, 1979,114: 41[7] Wang H, et al. Phys Rev Lett, 1999,82: 1414中文翻译：量子信息学信息科学与技术已经深入到社会的各个方面，其中的主角——计算机科学与技术的发展与应用，更是极大地促进了人类文明的进程。
