摘要:

Keeping in mind the ubiquitous standard optical fiber for long‐distance transmission and the widespread availability of efficient active and passive fiber devices, we have been developing telecom‐band resources for practical quantum communication and cryptography in wave‐division‐multiplexed (WDM) optical networks. In this talk I present our recent results on two fronts: i) telecom‐band in‐fiber entanglement generation, storage, and long‐distance distribution and ii) quantum‐noise protected high‐speed data encryption through an optically‐amplified WDM line. Along the first front, with our in‐fiber entanglement source all four Bell states can be readily produced and we have demonstrated violation of Bell’s inequalities by up to 10 standard deviations of measurement uncertainty. With such a source we have demonstrated storage of entanglement for up to 1/8 of a millisecond. Furthermore, when each photon of the entangled pair is propagated in separate 25km‐long standard fibers, high visibility quantum interference is still observed, demonstrating that this system is capable of long‐distance (> 50 km) entanglement distribution. Along the second front, we have implemented a new quantum cryptographic scheme, based on Yuen’s KCQ protocol, in which the inherent quantum noise of coherent states of light is used to perform the cryptographic service of data encryption. In this scheme a legitimate receiver, with use of a short, shared, secret‐key, executes a simple binary decision rule on every transmitted bit. An eavesdropper, on the other hand, who does not possess the secret‐key, is subjected to an irreducible quantum uncertainty in each measurement, even with the use of ideal detectors. We have implemented this scheme to demonstrate quantum‐noise‐protected data encryption at 650 Mbps through a 200 km, in‐line amplified, WDM line. The line simultaneously carried two 10 Gbps standard data channels, 100 GHz on either side of the encrypted channel, which shows that this scheme is compatible with the widely deployed WDM fiber‐optic infrastructure. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1834371

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

The capacityCfor transmitting classical information is investigated for Bosonic channels with isotropic Gaussian noise. For the pure‐loss channel—in which signal photons may be lost in propagation—the exact value ofCis derived. The Holevo information of this channel is shown to be additive, and a “classical” encoding procedure employing coherent states is shown to achieve capacity. For active channel models—in which noise photons are injected from an external environment or the signal is amplified with unavoidable quantum noise—upper and lower bounds are obtained for the capacity. These bounds are asymptotically tight at low and high noise levels. Exact capacity results—given by the lower bounds—would follow from proving the conjecture that a coherent‐state input minimizes the output entropy from such channels. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1834372

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

Coherent‐state inputs are conjectured to minimize the von Neumann entropies at the outputs of two Bosonic channels with thermal noise. Evidence in support of this conjecture is provided, including the fact that coherent‐state inputs minimize the integer‐order Re´nyi entropy and the Wehrl entropy at the outputs of these channels. A stronger conjecture—that output states resulting from coherent‐state inputs majorize the output states from other inputs—is also discussed. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1834373

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

Two prior results on the use of quantum resources to increase classical information‐transmission capacity are generalized from single‐link to multiple‐access scenarios. The first is the use of superdense coding over qudit channels. The second is the use of joint measurements over successive uses of the pure‐loss Bosonic channel. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1834374

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

The concepts of conditional entropy and information between subsystems of a composite quantum system are generalized to include arbitrary indirect measurements (POVMs). Some properties of those quantities differ from those of their classical counterparts; certain equalities and inequalities of classical information theory may be violated. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1834375

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

We explicitly compute the evolution of the density operator of a two‐mode electromagnetic field when the influence of the thermal fluctuation of the vacuum is common for both modes. From this result, we give an example in which the bundle of quantum noisy channels turns out to be noiseless for the special type of signal states due to the correlation. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1834376

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

Several different definitions of quantum games have been proposed over the past five years. Some of the results claimed for the differences between these quantum games and classical games identified as those to which they correspond have been criticized, however, as not being fair comparisons. In this paper I define classical and quantum versions of games withmediated communication, and propose that these are the classical and quantum games that should be compared. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1834377

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

A photon number resolving detector is a fundamental device for realizing quantum computing, to say nothing of improving quantum key distribution. It is considered possible to establish a multipurpose optical quantum computer by combining of a photon number resolving detector and non‐classical light such as a single photon or squeezed light in a feedback system. We are developing a photon number resolving detector at the telecommunication wavelength of 1.5 &mgr;m, in which the attenuation ratio of optical fiber is minimum. We adopt a InGaAs pin photodiode and a GaAs JFET in a charge integration amplifier, that is cooled to 4.2 K to reduce thermal noise and leak current. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1834378

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

We present two kinds of experimental results. One is a continuous variable dense coding experiment, and the other is a photon number detector with high linearity response, the so called charge integration photon detector (CIPD). They can be combined together to be a potential tool for implementing the cubic phase gate which is an important gate element to synthesize the measurement induced nonlinearity for photonic quantum information processing. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1834379

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

We present a model of quantum teleportation protocol based on a double quantum dot array. The unknown qubit is encoded using a pair of quantum dots, with one excess electron, coupled by tunneling. It is shown how to create a maximally entangled state using an adiabatically increasing Coulomb repulsion between different dot pairs. This entangled state is exploited to perform teleportation again using an adiabatic coupling between itself and the incoming unknown state. Finally, a sudden separation of Bob’s qubit allows a time evolution of Alice’s which amounts to a modified version of standard Bell measurement. A transmission over a long distance could be obtained by considering the entangled state of a chain ofNcoupled double quantum dots. The system is shown to be increasingly robust withNagainst decoherence due to phonons. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1834380

出版商:AIP    

年代:1904

数据来源: AIP