A new synchronization scheme based on time division multiplexing and wavelength division multiplexing technology for practical quantum key distribution system

Three clock synchronization schemes for a quantum key distribution system are compared experimentally through the outdoor fibre and the interaction physical model of the the clock signal and the the quantum signal in the quantum key distribution system is analysed to propose a new synchronization scheme based on time division multiplexing and wavelength division multiplexing technology to reduce quantum bits error rates under some transmission rate conditions.The proposed synchronization scheme can not only completely eliminate noise photons from the bright background light of the the clock signal,but also suppress the fibre nonlinear crosstalk.     

Practical security analysis of continuous-variable quantum key distribution with jitter in clock synchronization

How to narrow the gap of security between theory and practice has been a notoriously urgent problem in quantum cryptography. Here, we analyze and provide experimental evidence of the clock jitter effect on the practical continuous-variable quantum key distribution (CV-QKD) system. The clock jitter is a random noise which exists permanently in the clock synchronization in the practical CV-QKD system, it may compromise the system security because of its impact on data sampling and parameters estimation. In particular, the practical security of CV-QKD with different clock jitter against collective attack is analyzed theoretically based on different repetition frequencies, the numerical simulations indicate that the clock jitter has more impact on a high-speed scenario. Furthermore, a simplified experiment is designed to investigate the influence of the clock jitter.     

Realistic clocks, universal decoherence, and the black hole information paradox

Ordinary quantum mechanics is formulated on the basis of the existence of an ideal classical clock external to the system under study. This is clearly an idealization. As emphasized originally by Salecker and Wigner and more recently by others, there exist limits in nature to how "classical" even the best possible clock can be. With realistic clocks, quantum mechanics ceases to be unitary and a fundamental mechanism of decoherence of quantum states arises. We estimate the rate of the universal loss of unitarity using optimal realistic clocks. In particular, we observe that the rate is rapid enough to eliminate the black hole information puzzle: all information is lost through the fundamental decoherence before the black hole can evaporate. This improves on a previous calculation we presented with a suboptimal clock in which only part of the information was lost by the time of evaporation.     

Moving system with speeded-up evolution

In classical (non-quantum) relativity theory, the course of a moving clock is dilated when compared to the course of a clock at rest (the Einstein dilation). Any unstable system may be regarded as a clock. The time evolution (e.g., the decay) of a uniformly moving physical system is considered using relativistic quantum theory. An example of a moving system is given whose evolution turns out to be speeded-up instead of dilated. A discussion of this paradoxical result is presented. The text was submitted by the author in English.     

Quantum state manipulation of single-Cesium-atom qubit in a micro-optical trap

Based on single Cesium atoims trapped in a 1064 nm microscopic optical trap we have exhibited a single qubit encoded in the Cesium ＂clock states＂. The single qubit initialization, detection and the fast state rotation with high efficiencies are demonstrated and this state manipulation is crucial for quantmn information processing. The ground ~ates Rabi flopping rate of 229.0 ± 0.6 kHz is realized hy a two-photon Raman process. A clock states dephasing time of 3.0 ± 0.7 ms is measured, while all irreversible homogeneous dephasing time of 124 ± 17 ms is achieved by using the spin-echo technique. This well-controlled single atom provides an ideal quantmn qubit and quantmn node for quantum information processing.     

Larmor time and proper time

The idea of a Larmor clock is reexamined in the relativistic regime. We propose a concept of proper time for quantum theoretical particles. The Larmor clock can measure, under some relevant conditions, the proper time that passes while the particle stays in a space region. Our approach to Larmor clock is different than those of other researchers in the following two aspects: our concept of Larmor clock does not distinguish whether the particle is transmitted or reflected at the end of its stay, and pointer of our Larmor clock is not the spin but the total angular momentum.     

Entangled quantum clocks for measuring proper-time difference

We report that entangled pairs of quantum clocks (non-degenerate quantum bits) can be used as a specialized detector for precisely measuring difference of proper-times that each constituent quantum clock experiences. We describe why the proposed scheme would be more precise in the measurement of proper-time difference than a scheme of two-separate-quantum-clocks. We consider possibilities that the proposed scheme can be used in precision test of the relativity theory. Received 7 November 2001     

Nondestructive probing of Rabi oscillations on the cesium clock transition near the standard quantum limit

We report on the nondestructive observation of Rabi oscillations on the Cs clock transition. The internal atomic state evolution of a dipole-trapped ensemble of cold atoms is inferred from the phase shift of a probe laser beam as measured using a Mach-Zehnder interferometer. We describe a single color as well as a two-color probing scheme. Using the latter, measurements of the collective pseudospin projection of atoms in a superposition of the clock states are performed and the observed spin fluctuations are shown to be close to the standard quantum limit.     

Recurrence in Quantum Mechanics

We first compare the mathematical structure of quantum and classical mechanics when both are formulated in a C^*-algebraic framework. By using finite von Neumann algebras, a quantum mechanical analogue of Liouville's theorem is then proposed. We proceed to study Poincaré recurrence in C^*-algebras by mimicking the measure theoretic setting. The results are interpreted as recurrence in quantum mechanics, similar to Poincaré recurrence in classical mechanics.     

Clock frequency estimation under spontaneous emission

We investigate the quantum dynamics of a driven two-level system under spontaneous emission and its application in clock frequency estimation. By using the Lindblad equation to describe the system, we analytically obtain its exact solutions, which show three different regimes: Rabi oscillation, damped oscillation, and overdamped decay. From the analytical solutions, we explore how the spontaneous emission affects the clock frequency estimation. We find that under a moderate spontaneous emission rate, the transition frequency can still be inferred from the Rabi oscillation. Our results enable potential practical applications in frequency measurement and quantum control under decoherence.     

Transmission time of a particle in the reflectionless Sech-squared potential: Quantum clock approach

We investigate the time for a particle to pass through the reflectionless Sech-squared potential. Using the Salecker-Wigner and Peres quantum clock an average transmission time of a Gaussian wave packet representing the particle is explicitly evaluated in terms of average momentum and travel distance. The average transmission time is shown to be shorter than the time of free-particle motion and very close to the classical time for wave packets with well-localized momentum states. Since the clock measures the duration of scattering process the average transmission time can be interpreted as the average dwell time.     

Fractional recurrence in discrete-time quantum walk

Quantum recurrence theorem holds for quantum systems with discrete energy eigenvalues and fails to hold in general for systems with continuous energy. We show that during quantum walk process dominated by interference of amplitude corresponding to different paths fail to satisfy the complete quantum recurrence theorem. Due to the revival of the fractional wave packet, a fractional recurrence characterized using quantum Pólya number can be seen.     

How Does an Electron Tell the Time?

We examine a model of a digital clock to clarify the origin of the spacetime approach in special relativity. Specifically, we consider a two photon clock and assemble a statistical mechanics of such clocks to see how Minkowski space relates to local finite frequency clock behaviour. The result suggests that finite frequency clocks measure spacetime area and it is this feature that provides a simple mechanism behind Minkowski space on large scales. The same feature appears to implicate quantum mechanics on small scales.     

Globally controlled quantum wires for perfect qubit transport, mirroring, and computing

We describe a new design for a q wire with perfect transmission using a uniformly coupled Ising spin chain subject to global pulses. In addition to allowing for the perfect transport of single qubits, the design also yields the perfect "mirroring" of multiply encoded qubits within the wire. We further utilize this global-pulse generated perfect mirror operation as a "clock cycle" to perform universal quantum computation on these multiply encoded qubits where the interior of the q wire serves as the quantum memory while the q-wire ends perform one- and two-qubit gates.     

Quantentheorie der Zeitmessung

As a consequence of its dynamical motion a quantum mechanical system may be considered as a quantum mechanical clock. If one demands that the time be an observable which corresponds to a hypermaximal time operator in Hilbert space, then, for systems having a continuous energy spectrum with a lower limit, in the framework of the nonrelativistic theory to be discussed here there must exist an upper limit of energy, too. Furthermore the time operator is not defined on the whole Hilbert space, but only on state functions satisfying a certain condition. Therefrom it results that a quantum mechanical clock of this kind can be read off only in a sequence of equidistant times separated by a “minimal time”. The beginning of the time measurement being arbitrary the scale of time may be shifted according to the homogenity in time. Especially for a free particle beside the minimal time also a minimal length is obtained. The equidistant scale in space is not absolute either, but permits an arbitrary choice of the point of reference according to homogenity in space. The modificated spreading of the probability distribution of particle position is discussed.     

Entangled two-photon wave packet in a dispersive medium

We report an experimental study of group-velocity dispersion effect on an entangled two-photon wave packet, generated via spontaneous parametric down-conversion and propagating through a dispersive medium. Even in the case of using cw laser beam for pump, the biphoton wave packet and the second-order correlation function spread significantly. The study and understanding of this phenomenon is of great importance for quantum information applications, such as quantum communication and distant clock synchronization.     

Quantum key generation based on coding of polarization states of photons

This paper reports on the results of experimental investigations into the distribution of a quantum key on a setup designed for quantum cryptography with single photons. The quantum key is transmitted with pulsed semiconductor lasers by coding polarization states of photons in two alternative nonorthogonal bases. Silicon avalanche photodiodes (C30902S) serve as single-photon detectors. The rate of key generation is equal to 1.8 kbit/s when the clock frequency of laser pulse repetition is 100 kHz and the mean number of photons per pulse is approximately equal to 0.2. The number of errors in the key does not exceed 1%.     

Quantum mechanics of electromagnetically bounded spin-1/2 particles in an expanding universe: I. Influence of the expansion

The quantum mechanically described electron in an external electromagnetic field, both embedded in an expanding universe with shear, is discussed. This is important for the fundamental question if a quantum mechanically treated atomic clock in curved space-time (based on a hydrogen atom) shows proper or gravitational time. Furthermore, contradictory results reported by other authors seem to imply that quantum mechanics cannot be reconciled with curved space-time. It is shown that this is not the case for expanding Robertson-Walker universes. As basis, in this paper a Hilbert space formulation of the problem with special regard to the Hamiltonian is given. The respective influence of the cosmic expansion and the intrinsic and extrinsic curvatures of the cosmic hypersurfaces on bound quantum mechanical systems is treated in general. For the special case of an expanding 3-flat (=0) Robertson-Walker universe it is shown that the energy levels of a hydrogen atom agree completely with the one in 4-flat space-time, so that in this case the hydrogen atom can be taken as atomic clock showing proper time.