Orbital excitation blockade and algorithmic cooling in quantum gases

Interaction blockade occurs when strong interactions in a confined, few-body system prevent a particle from occupying an otherwise accessible quantum state. Blockade phenomena reveal the underlying granular nature of quantum systems and allow for the detection and manipulation of the constituent particles, be they electrons, spins, atoms or photons. Applications include single-electron transistors based on electronic Coulomb blockade and quantum logic gates in Rydberg atoms. Here we report a form of interaction blockade that occurs when transferring ultracold atoms between orbitals in an optical lattice. We call this orbital excitation blockade (OEB). In this system, atoms at the same lattice site undergo coherent collisions described by a contact interaction whose strength depends strongly on the orbital wavefunctions of the atoms. We induce coherent orbital excitations by modulating the lattice depth, and observe staircase-like excitation behaviour as we cross the interaction-split resonances by tuning the modulation frequency. As an application of OEB, we demonstrate algorithmic cooling of quantum gases: a sequence of reversible OEB-based quantum operations isolates the entropy in one part of the system and then an irreversible step removes the entropy from the gas. This technique may make it possible to cool quantum gases to have the ultralow entropies required for quantum simulation of strongly correlated electron systems. In addition, the close analogy between OEB and dipole blockade in Rydberg atoms provides a plan for the implementation of two-quantum-bit gates in a quantum computing architecture with natural scalability.     

Acceleration of quantum decay processes by frequent observations

In theory, the decay of any unstable quantum state can be inhibited by sufficiently frequent measurements--the quantum Zeno effect. Although this prediction has been tested only for transitions between two coupled, essentially stable states, the quantum Zeno effect is thought to be a general feature of quantum mechanics, applicable to radioactive or radiative decay processes. This generality arises from the assumption that, in principle, successive observations can be made at time intervals too short for the system to change appreciably. Here we show not only that the quantum Zeno effect is fundamentally unattainable in radiative or radioactive decay (because the required measurement rates would cause the system to disintegrate), but also that these processes may be accelerated by frequent measurements. We find that the modification of the decay process is determined by the energy spread incurred by the measurements (as a result of the time-energy uncertainty relation), and the distribution of states to which the decaying state is coupled. Whereas the inhibitory quantum Zeno effect may be feasible in a limited class of systems, the opposite effect--accelerated decay--appears to be much more ubiquitous.     

Laser cooling of a nanomechanical oscillator into its quantum ground state

The simple mechanical oscillator, canonically consisting of a coupled mass-spring system, is used in a wide variety of sensitive measurements, including the detection of weak forces and small masses. On the one hand, a classical oscillator has a well-defined amplitude of motion; a quantum oscillator, on the other hand, has a lowest-energy state, or ground state, with a finite-amplitude uncertainty corresponding to zero-point motion. On the macroscopic scale of our everyday experience, owing to interactions with its highly fluctuating thermal environment a mechanical oscillator is filled with many energy quanta and its quantum nature is all but hidden. Recently, in experiments performed at temperatures of a few hundredths of a kelvin, engineered nanomechanical resonators coupled to electrical circuits have been measured to be oscillating in their quantum ground state. These experiments, in addition to providing a glimpse into the underlying quantum behaviour of mesoscopic systems consisting of billions of atoms, represent the initial steps towards the use of mechanical devices as tools for quantum metrology or as a means of coupling hybrid quantum systems. Here we report the development of a coupled, nanoscale optical and mechanical resonator formed in a silicon microchip, in which radiation pressure from a laser is used to cool the mechanical motion down to its quantum ground state (reaching an average phonon occupancy number of 0.85 ± 0.08). This cooling is realized at an environmental temperature of 20?K, roughly one thousand times larger than in previous experiments and paves the way for optical control of mesoscale mechanical oscillators in the quantum regime.     

Baierein猜测与三维Fermi体系中外场对体系熵的影响

本文讨论并计算了量子情况下磁场中三维电子气体系的熵，发现存在磁场时三维电子气体系的熵有可能大于无磁场时的熵。从而证明了对于在外场下的量子体系Ｂａｉｅｒｌｅｉｎ认为对所有的热力学体系，体系的熵总是随外场或粒子间相互作用的引入而减小的观点剑成立。     

Real-time quantum feedback prepares and stabilizes photon number states

Feedback loops are central to most classical control procedures. A controller compares the signal measured by a sensor (system output) with the target value or set-point. It then adjusts an actuator (system input) to stabilize the signal around the target value. Generalizing this scheme to stabilize a micro-system's quantum state relies on quantum feedback, which must overcome a fundamental difficulty: the sensor measurements cause a random back-action on the system. An optimal compromise uses weak measurements, providing partial information with minimal perturbation. The controller should include the effect of this perturbation in the computation of the actuator's operation, which brings the incrementally perturbed state closer to the target. Although some aspects of this scenario have been experimentally demonstrated for the control of quantum or classical micro-system variables, continuous feedback loop operations that permanently stabilize quantum systems around a target state have not yet been realized. Here we have implemented such a real-time stabilizing quantum feedback scheme following a method inspired by ref. 13. It prepares on demand photon number states (Fock states) of a microwave field in a superconducting cavity, and subsequently reverses the effects of decoherence-induced field quantum jumps. The sensor is a beam of atoms crossing the cavity, which repeatedly performs weak quantum non-demolition measurements of the photon number. The controller is implemented in a real-time computer commanding the actuator, which injects adjusted small classical fields into the cavity between measurements. The microwave field is a quantum oscillator usable as a quantum memory or as a quantum bus swapping information between atoms. Our experiment demonstrates that active control can generate non-classical states of this oscillator and combat their decoherence, and is a significant step towards the implementation of complex quantum information operations.     

多能级系统的量子Zeno效应(英文)

众所周知,当不稳定量子系统被频繁测量时,量子Zeno效应就会发生,此效应在量子信息和量子通信中具有重要意义.然而,以前人们所研究的量子Zeno效应多数是针对二能级系统.可以验证三能级系统里同样存在量子Zeno效应,这将对不同量子系统量子态跃迁的阻止和量子Zeno效应的理解带来帮助,同时,在此基础上拓展到高维系统,也证明此效应的存在.     

Controlling cavity reflectivity with a single quantum dot

Solid-state cavity quantum electrodynamics (QED) systems offer a robust and scalable platform for quantum optics experiments and the development of quantum information processing devices. In particular, systems based on photonic crystal nanocavities and semiconductor quantum dots have seen rapid progress. Recent experiments have allowed the observation of weak and strong coupling regimes of interaction between the photonic crystal cavity and a single quantum dot in photoluminescence. In the weak coupling regime, the quantum dot radiative lifetime is modified; in the strong coupling regime, the coupled quantum dot also modifies the cavity spectrum. Several proposals for scalable quantum information networks and quantum computation rely on direct probing of the cavity-quantum dot coupling, by means of resonant light scattering from strongly or weakly coupled quantum dots. Such experiments have recently been performed in atomic systems and superconducting circuit QED systems, but not in solid-state quantum dot-cavity QED systems. Here we present experimental evidence that this interaction can be probed in solid-state systems, and show that, as expected from theory, the quantum dot strongly modifies the cavity transmission and reflection spectra. We show that when the quantum dot is coupled to the cavity, photons that are resonant with its transition are prohibited from entering the cavity. We observe this effect as the quantum dot is tuned through the cavity and the coupling strength between them changes. At high intensity of the probe beam, we observe rapid saturation of the transmission dip. These measurements provide both a method for probing the cavity-quantum dot system and a step towards the realization of quantum devices based on coherent light scattering and large optical nonlinearities from quantum dots in photonic crystal cavities.     

Measurement of the quantum of thermal conductance

The physics of mesoscopic electronic systems has been explored for more than 15 years. Mesoscopic phenomena in transport processes occur when the wavelength or the coherence length of the carriers becomes comparable to, or larger than, the sample dimensions. One striking result in this domain is the quantization of electrical conduction, observed in a quasi-one-dimensional constriction formed between reservoirs of two-dimensional electron gas. The conductance of this system is determined by the number of participating quantum states or 'channels' within the constriction; in the ideal case, each spin-degenerate channel contributes a quantized unit of 2e(2)/h to the electrical conductance. It has been speculated that similar behaviour should be observable for thermal transport in mesoscopic phonon systems. But experiments attempted in this regime have so far yielded inconclusive results. Here we report the observation of a quantized limiting value for the thermal conductance, Gth, in suspended insulating nanostructures at very low temperatures. The behaviour we observe is consistent with predictions for phonon transport in a ballistic, one-dimensional channel: at low temperatures, Gth approaches a maximum value of g0 = pi2kB2T/3h, the universal quantum of thermal conductance.     

基于关联方向测量的量子关联

通过讨论量子失协的上界,给出可达熵量子关联态的一个必要条件．对于双量子比特系统,利用可获得经典关联的最佳测量定义了关联方向上最小的量子失协,分析得到该度量是量子失协很好的逼近结果．最后,讨论量子关联的动力学过程,发现量子关联在衰减的过程中出现了突然的改变,利用关联方向上最小的量子失协给出了该现象的合理解释．     

Circuit cavity electromechanics in the strong-coupling regime

Demonstrating and exploiting the quantum nature of macroscopic mechanical objects would help us to investigate directly the limitations of quantum-based measurements and quantum information protocols, as well as to test long-standing questions about macroscopic quantum coherence. Central to this effort is the necessity of long-lived mechanical states. Previous efforts have witnessed quantum behaviour, but for a low-quality-factor mechanical system. The field of cavity optomechanics and electromechanics, in which a high-quality-factor mechanical oscillator is parametrically coupled to an electromagnetic cavity resonance, provides a practical architecture for cooling, manipulation and detection of motion at the quantum level. One requirement is strong coupling, in which the interaction between the two systems is faster than the dissipation of energy from either system. Here, by incorporating a free-standing, flexible aluminium membrane into a lumped-element superconducting resonant cavity, we have increased the single-photon coupling strength between these two systems by more than two orders of magnitude, compared to previously obtained coupling strengths. A parametric drive tone at the difference frequency between the mechanical oscillator and the cavity resonance dramatically increases the overall coupling strength, allowing us to completely enter the quantum-enabled, strong-coupling regime. This is evidenced by a maximum normal-mode splitting of nearly six bare cavity linewidths. Spectroscopic measurements of these 'dressed states' are in excellent quantitative agreement with recent theoretical predictions. The basic circuit architecture presented here provides a feasible path to ground-state cooling and subsequent coherent control and measurement of long-lived quantum states of mechanical motion.     

Strong dispersive coupling of a high-finesse cavity to a micromechanical membrane

Macroscopic mechanical objects and electromagnetic degrees of freedom can couple to each other through radiation pressure. Optomechanical systems in which this coupling is sufficiently strong are predicted to show quantum effects and are a topic of considerable interest. Devices in this regime would offer new types of control over the quantum state of both light and matter, and would provide a new arena in which to explore the boundary between quantum and classical physics. Experiments so far have achieved sufficient optomechanical coupling to laser-cool mechanical devices, but have not yet reached the quantum regime. The outstanding technical challenge in this field is integrating sensitive micromechanical elements (which must be small, light and flexible) into high-finesse cavities (which are typically rigid and massive) without compromising the mechanical or optical properties of either. A second, and more fundamental, challenge is to read out the mechanical element's energy eigenstate. Displacement measurements (no matter how sensitive) cannot determine an oscillator's energy eigenstate, and measurements coupling to quantities other than displacement have been difficult to realize in practice. Here we present an optomechanical system that has the potential to resolve both of these challenges. We demonstrate a cavity which is detuned by the motion of a 50-nm-thick dielectric membrane placed between two macroscopic, rigid, high-finesse mirrors. This approach segregates optical and mechanical functionality to physically distinct structures and avoids compromising either. It also allows for direct measurement of the square of the membrane's displacement, and thus in principle the membrane's energy eigenstate. We estimate that it should be practical to use this scheme to observe quantum jumps of a mechanical system, an important goal in the field of quantum measurement.     

最大可能熵原理

定义了最大可能熵(PME)的新概念,并提出最大可能熵原理.仅对孤立的平衡系统而言PME才等于系统的熵(Sm=S),一般情况下PME总是大于系统的真实熵(Sm＞S).当一个非平衡开放系统的PME不管因为甚么原因增大时,它的信息和熵就能同时增加.例如,当一个系统的体积和能量增加时,其PME就增大,从而导致它的信息和熵可同时增加.最大可能熵原理建立在理论和近年提出的"熵界"上.由这个原理不但能推导出科学上已有结论,如黑洞无毛定理,还可用来解释宇宙中某些谜一样的问题,如整个宇宙的熵增大,同时地球上的生物却可以不断进化.     

Experimental realization of a one-atom laser in the regime of strong coupling

Conventional lasers (from table-top systems to microscopic devices) typically operate in the so-called weak-coupling regime, involving large numbers of atoms and photons; individual quanta have a negligible impact on the system dynamics. However, this is no longer the case when the system approaches the regime of strong coupling for which the number of atoms and photons can become quite small. Indeed, the lasing properties of a single atom in a resonant cavity have been extensively investigated theoretically. Here we report the experimental realization of a one-atom laser operated in the regime of strong coupling. We exploit recent advances in cavity quantum electrodynamics that allow one atom to be isolated in an optical cavity in a regime for which one photon is sufficient to saturate the atomic transition. The observed characteristics of the atom-cavity system are qualitatively different from those of the familiar many-atom case. Specifically, our measurements of the intracavity photon number versus pump intensity indicate that there is no threshold for lasing, and we infer that the output flux from the cavity mode exceeds that from atomic fluorescence by more than tenfold. Observations of the second-order intensity correlation function demonstrate that our one-atom laser generates manifestly quantum (nonclassical) light, typified by photon anti-bunching and sub-poissonian photon statistics.     

Electromagnetically induced transparency with resonant nuclei in a cavity

The manipulation of light-matter interactions by quantum control of atomic levels has had a profound impact on optical sciences. Such manipulation has many applications, including nonlinear optics at the few-photon level, slow light, lasing without inversion and optical quantum information processing. The critical underlying technique is electromagnetically induced transparency, in which quantum interference between transitions in multilevel atoms renders an opaque medium transparent near an atomic resonance. With the advent of high-brilliance, accelerator-driven light sources such as storage rings or X-ray lasers, it has become attractive to extend the techniques of optical quantum control to the X-ray regime. Here we demonstrate electromagnetically induced transparency in the regime of hard X-rays, using the 14.4-kiloelectronvolt nuclear resonance of the M?ssbauer isotope iron-57 (a two-level system). We exploit cooperative emission from ensembles of the nuclei, which are embedded in a low-finesse cavity and excited by synchrotron radiation. The spatial modulation of the photonic density of states in a cavity mode leads to the coexistence of superradiant and subradiant states of nuclei, respectively located at an antinode and a node of the cavity field. This scheme causes the nuclei to behave as effective three-level systems, with two degenerate levels in the excited state (one of which can be considered metastable). The radiative coupling of the nuclear ensembles by the cavity field establishes the atomic coherence necessary for the cancellation of resonant absorption. Because this technique does not require atomic systems with a metastable level, electromagnetically induced transparency and its applications can be transferred to the regime of nuclear resonances, establishing the field of nuclear quantum optics.     

Baierein猜测与三维Bose体系中外场对体系熵的影响

讨论并计算了量子情况下磁场中三维带电Bose子体系的熵，发现存在磁场时三维带电Bose子体系的熵有可能大于无磁场时的熵，从而证明了对于在外场下Bose子体系 Baierlein的观点不成立。     

量子态和量子态组分布熵的预测规律及验证

研究了在一定的简化模式下，孤立系趋近平衡的过程中量子态和量子态组分布熵随时间变化的预测规律，揭示用有限大量重复性模拟实验所得的量子态组分布熵有多大的实验偏差，并将原文献中只限于对系统的几十个量子态组分布熵的研究扩大到对系统中数以万计的量子态的分布熵的研究，按重复实验法和本文提出的预测法同时设计两套熵增原理的计算机模拟程序，验证表明，预测规律具有高度的准确性。     

Davey—Stewartson1气体的热力学

用(2 1)维量子DS 1系统的波函数来研究DS1气体的热力学。计算了密度矩阵、配分函数和熵。在Gibbs佯谬消除的条件下,发现DS1气体是一类一维费米气体。还对一种新的超导机制做了推测。     

Linear and nonlinear optical spectroscopy of a strongly coupled microdisk-quantum dot system

Cavity quantum electrodynamics, the study of coherent quantum interactions between the electromagnetic field and matter inside a resonator, has received attention as both a test bed for ideas in quantum mechanics and a building block for applications in the field of quantum information processing. The canonical experimental system studied in the optical domain is a single alkali atom coupled to a high-finesse Fabry-Perot cavity. Progress made in this system has recently been complemented by research involving trapped ions, chip-based microtoroid cavities, integrated microcavity-atom-chips, nanocrystalline quantum dots coupled to microsphere cavities, and semiconductor quantum dots embedded in micropillars, photonic crystals and microdisks. The last system has been of particular interest owing to its relative simplicity and scalability. Here we use a fibre taper waveguide to perform direct optical spectroscopy of a system consisting of a quantum dot embedded in a microdisk. In contrast to earlier work with semiconductor systems, which has focused on photoluminescence measurements, we excite the system through the photonic (light) channel rather than the excitonic (matter) channel. Strong coupling, the regime of coherent quantum interactions, is demonstrated through observation of vacuum Rabi splitting in the transmitted and reflected signals from the cavity. The fibre coupling method also allows us to examine the system's steady-state nonlinear properties, where we see a saturation of the cavity-quantum dot response for less than one intracavity photon. The excitation of the cavity-quantum dot system through a fibre optic waveguide is central to applications such as high-efficiency single photon sources, and to more fundamental studies of the quantum character of the system.     

Quantum-coherent coupling of a mechanical oscillator to an optical cavity mode

Optical laser fields have been widely used to achieve quantum control over the motional and internal degrees of freedom of atoms and ions, molecules and atomic gases. A route to controlling the quantum states of macroscopic mechanical oscillators in a similar fashion is to exploit the parametric coupling between optical and mechanical degrees of freedom through radiation pressure in suitably engineered optical cavities. If the optomechanical coupling is 'quantum coherent'--that is, if the coherent coupling rate exceeds both the optical and the mechanical decoherence rate--quantum states are transferred from the optical field to the mechanical oscillator and vice versa. This transfer allows control of the mechanical oscillator state using the wide range of available quantum optical techniques. So far, however, quantum-coherent coupling of micromechanical oscillators has only been achieved using microwave fields at millikelvin temperatures. Optical experiments have not attained this regime owing to the large mechanical decoherence rates and the difficulty of overcoming optical dissipation. Here we achieve quantum-coherent coupling between optical photons and a micromechanical oscillator. Simultaneously, coupling to the cold photon bath cools the mechanical oscillator to an average occupancy of 1.7?±?0.1 motional quanta. Excitation with weak classical light pulses reveals the exchange of energy between the optical light field and the micromechanical oscillator in the time domain at the level of less than one quantum on average. This optomechanical system establishes an efficient quantum interface between mechanical oscillators and optical photons, which can provide decoherence-free transport of quantum states through optical fibres. Our results offer a route towards the use of mechanical oscillators as quantum transducers or in microwave-to-optical quantum links.     

Tunnelling between the edges of two lateral quantum Hall systems

The edge of a two-dimensional electron system in a magnetic field consists of one-dimensional channels that arise from the confining electric field at the edge of the system. The crossed electric and magnetic fields cause electrons to drift parallel to the sample boundary, creating a chiral current that travels along the edge in only one direction. In an ideal two-dimensional electron system in the quantum Hall regime, all the current flows along the edge. Quantization of the Hall resistance arises from occupation of N one-dimensional edge channels, each contributing a conductance of e2/h. Here we report differential conductance measurements, in the integer quantum Hall regime, of tunnelling between the edges of a pair of two-dimensional electron systems that are separated by an atomically precise, high-quality, tunnel barrier. The resultant interaction between the edge states leads to the formation of new energy gaps and an intriguing dispersion relation for electrons travelling along the barrier: for example, we see a persistent conductance peak at zero bias voltage and an absence of tunnelling features due to electron spin. These features are unexpected and are not consistent with a model of weakly interacting edge states. Remnant disorder along the barrier and charge screening may each play a role, although detailed numerical studies will be required to elucidate these effects.