ZT:Quantum engineering moves on



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送交者: xinku 于 2006-6-12, 19:34:09:

回答: 什么是量子系统工程 由 HunHunSheng 于 2006-6-12, 17:31:49:

Quantum engineering moves on
Physics in Action: January 1999

One of the most intriguing features of quantum mechanics is the existence of entangled states. These states result from the superposition of individual quantum states in composite systems, such as a collection of particles, and cannot be expressed as a product of the individual states. The generation and manipulation of entangled states is fundamental to studies of the most basic aspects of quantum mechanics, and provides the basis of applications such as quantum computing, quantum communications and high-precision spectroscopy.


One of the central questions is how to manipulate or "engineer" these entangled states in real physical systems. Indeed, very few physical systems satisfy the stringent requirements that make it possible to control the quantum mechanical Hamilton operator - which determines the time evolution of the system - while at the same time ensuring that the fragile quantum superpositions are not destroyed by interactions with the environment. Suitable systems include trapped ions that are laser cooled to just a few microkelvin and atoms stored in extremely small cavities.

The last few years have seen significant progress towards quantum-state engineering in these systems. In the latest advance researchers at the National Institute of Standards and Technology (NIST) in Boulder, Colorado, have succeeded in creating entangled states of two trapped ions in a controlled way. This differs from earlier experiments in which entangled states were generated as a result of random processes. Such an ability to control and manipulate the entanglement is an important step towards the building of an ion-trap quantum computer (Q A Turchette et al . 1998 Phys. Rev. Lett . 81 3631). This is directly related to the pioneering work of Serge Haroche, Jean-Michelle Raimond and colleagues at the Ecole Normale Supérieure in Paris, who created entangled states by sending two atoms through small rf cavities. In these experiments photons are confined in the cavity for a relatively long time, making it possible to study atom-photon interactions in detail (E Hagley et al. 1997 Phys. Rev. Lett . 79 1).

A key goal for quantum optics is to control quantum systems and the couplings between the different quantum degrees of freedom. In the early days, the dynamics and behaviour of lasers and other quantum optical systems were dominated by dissipative effects and uncontrollable fluctuations of important parameters, such as the number of atoms in the system. However, about 15 years ago it became possible to create single quantum systems by trapping single ions in a potential, or by storing single atoms in a cavity. These single quantum systems can be prepared microscopically and observed under conditions close to idealized and fundamental theoretical models.

These model systems also led to the idea of "engineering" quantum states. Both types of single quantum system comprise a single spin-½ atom with two possible states - spin up or spin down - that is strongly coupled to a harmonic oscillator (the time-dependent trapping potential for the trapped ions, or the photon field in cavity-based experiments). For a trapped ion, a pure quantum state can be created by laser cooling the ion to the ground state of the trapping potential. The ion has internal (electronic) quantum states and a limited number of external (motional) quantum states. Changing the internal state of the ion - for example by using a laser to excite the ion to a higher electronic state - can therefore influence its external motional state.

Researchers at NIST and elsewhere have exploited these techniques to generate superpositions of the ion motion that are analogous to Schrödinger cat states - in other words the same ion can be in two different locations at the same time (see Physics World March 1997 pp37-42). These techniques have also made it possible to create a two-bit quantum gate by creating an entangled state between an internal atomic state, such as the spin, and the quantized centre-of-mass vibration of a single ion. Haroche and colleagues have also used atoms flying through cavities to produce Schrödinger cat states in the lab, and to observe their decay due to coupling with the environment (see Physics World January 1997 pp24-25).

The next challenge for quantum optics is to extend quantum control to systems of several particles and, eventually, to mesoscopic systems. Key elements of this work will be to study the entanglement of particles - in particular the controlled generation of entanglement - as well as decoherence and the quantum measurement process. An important example of such entangled states is the Bell (or EPR) states of two spin-½ particles, which are the starting point for discussions about the violation of Bell's inequalities, teleportation and quantum cryptography.

This new research in quantum optics ties in with the current interest in building a quantum computer (see the special issue on quantum information Physics World March 1998). This task requires a physical realization of quantum gates that can act on a set of quantum bits or "qubits" - a qubit is a two-state quantum system, such as a spin-½ atom. Any operation on the system can be decomposed into rotations of a single qubit and a universal two-bit gate that performs entanglement operations between two qubits.

A string of ions stored in a trap provides a model system of a quantum computer. In the language of quantum computing the quantum bit is stored in (long-lived) internal atomic states, while the string of ions represents a quantum register. Operations on single bits are achieved by directing different laser beams onto each ion, and a two-bit gate operation (i.e. entanglement) is implemented by selectively exciting the collective quantized motion of the ions with a laser. In this case the exchange of phonons acts as a data bus that transfers quantum information between the qubits. The state of the register can be read efficiently with the "quantum jump" technique.

In the latest experiment the NIST group stored two beryllium ions in a tight radiofrequency Paul trap. In this case the qubit is represented by two hyperfine energy levels of the ions' electronic ground state. The beryllium ions have two forms of collective oscillation: the centre-of-mass mode, where the ions oscillate in phase, and the stretch mode, where the ions oscillate out of phase. Transitions connecting the two different vibrational states can be driven by so-called stimulated Raman pulses from pairs of laser beams with frequencies tuned to the internal atomic transitions of the ions.

The states of the individual ions can be altered by using lasers to control their "micromotion". Ions in a Paul trap exhibit a combination of a (slow) secular motion and a (fast) micromotion at the frequency of the applied radiofrequency field. The amplitude of the micromotion, and thus the effective coupling of the laser to the ion, can be selected by using a static field to push the ions from the centre of the trap. By applying an appropriate sequence of laser pulses, the NIST researchers have produced entangled Bell-like states on demand, and with a fidelity of better than 0.7.

The importance of this result is that the system can be scaled to a large number of qubits - at least in principle - and so provides the first step towards the practical realization of an ion-trap quantum computer. This is in contrast to earlier experiments in which entangled states have typically been produced through random processes - either to create the entanglement, as is the case of photon cascades, or by selecting appropriate states from a larger sample of trials. Recent results from quantum systems based on nuclear magnetic resonance in bulk samples have shown entanglement of particle spins, but these correspond to the selection of pseudo-pure states from a thermal ensemble. The signal therefore decreases exponentially with increasing number of spins.

Given the experimental progress at NIST and elsewhere, it seems likely that ion-trap quantum computers containing up to 10 qubits will be built in the next few years. These systems will provide a playground for a new generation of fundamental tests for quantum mechanics, and will allow the demonstration of the basic elements needed for quantum computing, such as error correction. Furthermore, it should be possible to network several small ion-trap quantum computers together via optical cavities connected by fibres. This suggests a scenario of distributed quantum computing on a network.

Although large-scale quantum computing is in the far distant future, these small quantum computers will provide all the necessary hardware for quantum communication. For example, ion traps containing a few qubits could operate as quantum repeaters for long-distance communications.





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