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Peter L. Knight

Publications and source records attributed to Peter L. Knight.

35 records · Page 2Linked to original sources
arXiv2004

Post-processing with linear optics for improving the quality of single-photon sources

Triggered single-photon sources produce the vacuum state with non-negligible probability, but produce a much smaller multiphoton component. It is therefore reasonable to approximate the output of these photon sources as a mixture of the vacuum and single-photon states. We show that it is impossible to increase the probability for a single photon using linear optics and photodetection on fewer than four modes. This impossibility is due to the incoherence of the inputs; if the inputs were pure-state superpositions, it would be possible to obtain a perfect single-photon output. In the more general case, a chain of beam splitters can be used to increase the probability for a single photon, but at the expense of adding an additional multiphoton component. This improvement is robust against detector inefficiencies, but is degraded by dark counts or multiphoton components in the input.

quant-ph↗
arXiv2003

Photon-number superselection and the entangled coherent-state representation

We introduce the entangled coherent state representation, which provides a powerful technique for efficiently and elegantly describing and analyzing quantum optics sources and detectors while respecting the photon number superselection rule that is satisfied by all known quantum optics experiments. We apply the entangled coherent state representation to elucidate and resolve the longstanding puzzles of the coherence of a laser output field, interference between two number states, and dichotomous interpretations of quantum teleportation of coherent states.

quant-ph↗
arXiv2003

On improving single photon sources via linear optics and photodetection

In practice, single photons are generated as a mixture of vacuum with a single photon with weights 1-p and p, respectively; here we are concerned with increasing p by directing multiple copies of the single photon-vacuum mixture into a linear optic device and applying photodetection on some outputs to conditionally prepare single photon states with larger p. We prove that it is impossible, under certain conditions, to increase p via linear optics and conditional preparation based on photodetection, and we also establish a class of photodetection events for which p can be improved. In addition we prove that it is not possible to obtain perfect (p=1) single photon states via this method from imperfect (p<1) inputs.

quant-ph↗
arXiv2003

Quantum Computation with a One-Dimensional Optical Lattice

We present an economical dynamical control scheme to perform quantum computation on a one dimensional optical lattice, where each atom encodes one qubit. The model is based on atom tunneling transitions between neighboring sites of the lattice. They can be activated by external laser beams resulting in a two-qubit phase gate or in an exchange interaction. A realization of the Toffoli gate is presented which requires only a single laser pulse and no individual atom addressing.

quant-ph↗
arXiv2003

Quantum walk on the line as an interference phenomenon

We show that the coined quantum walk on a line can be understood as an interference phenomenon, can be classically implemented, and indeed already has been. The walk is essentially two independent walks associated with the different coin sides, coupled only at initiation. There is a simple analogy between the evolution of walker positions and the propagation of light in a dispersive optical fiber.

quant-ph↗
arXiv2003

Measurement-induced Nonlinearity in Linear Optics

We investigate the generation of nonlinear operators with single photon sources, linear optical elements and appropriate measurements of auxiliary modes. We provide a framework for the construction of useful single-mode and two-mode quantum gates necessary for all-optical quantum information processing. We focus our attention generally on using minimal physical resources while providing a transparent and algorithmic way of constructing these operators.

quant-ph↗
arXiv2003

Quantum quincunx in cavity quantum electrodynamics

We introduce the quantum quincunx, which physically demonstrates the quantum walk and is analogous to Galton's quincunx for demonstrating the random walk. In contradistinction to the theoretical studies of quantum walks over orthogonal lattice states, we introduce quantum walks over nonorthogonal lattice states (specifically, coherent states on a circle) to demonstrate that the key features of a quantum walk are observable albeit for strict parameter ranges. A quantum quincunx may be realized with current cavity quantum electrodynamics capabilities, and precise control over decoherence in such experiments allows a remarkable decrease in the position noise, or spread, with increasing decoherence.

quant-ph↗
arXiv2003

Dissipation-assisted quantum computation in atom-cavity systems

The principal obstacle to quantum information processing with many qubits is decoherence. One source of decoherence is spontaneous emission which causes loss of energy and information. Inability to control system parameters with high precision is another possible source of error. Strategies aimed at overcoming one kind of error typically increase sensitivity to others. As a solution we propose quantum computing with dissipation-assisted quantum gates. These can be run relatively fast while achieving fidelities close to one. The success rate of each gate operation can, at least in principle, be arbitrary close to one.

quant-ph↗
arXiv2003

On the Preparation of Pure States in Resonant Microcavities

We consider the time evolution of the radiation field (R) and a two-level atom (A) in a resonant microcavity in terms of the Jaynes-Cummings model with an initial general pure quantum state for the radiation field. It is then shown, using the Cauchy-Schwarz inequality and also a Poisson resummation technique, that {\it perfect} coherence of the atom can in general never be achieved. The atom and the radiation field are, however, to a good approximation in a pure state $|ψ>_A\otimes|ψ>_R$ in the middle of what has been traditionally called the ``collapse region'', independent of the initial state of the atoms, provided that the initial pure state of the radiation field has a photon number probability distribution which is sufficiently peaked and phase differences that do not vary significantly around this peak. An approximative analytic expression for the quantity $\Tr[ρ^2_{A}(t)]$, where $ρ_{A}(t)$ is the reduced density matrix for the atom, is derived. We also show that under quite general circumstances an initial entangled pure state will be disentangled to the pure state $|ψ>_{A\otimes R}$.

quant-ph↗
arXiv2001

Quantum Computing in a Macroscopic Dark Period

Decoherence-free subspaces allow for the preparation of coherent and entangled qubits for quantum computing. Decoherence can be dramatically reduced, yet dissipation is an integral part of the scheme in generating stable qubits and manipulating them via one and two bit gate operations. How this works can be understood by comparing the system with a three-level atom exhibiting a macroscopic dark period. In addition, a dynamical explanation is given for a scheme based on atoms inside an optical cavity in the strong coupling regime and we show how spontaneous emission by the atoms can be highly suppressed.

quant-ph↗
arXiv2001

A Bell's Inequality Test with Entangled Atoms

Previous work on Bell's inequality realised in the laboratory has used entangled photons. Here we describe how entangled atoms can violate Bell's inequality, and how these violations can be measured with a very high detection efficiency. We first discuss a simple scheme based on two-level atoms inside a cavity to prepare the entangled state. We then discuss a scheme using four-level atoms, which requires a parameter regime much easier to access experimentally using current technology. As opposed to other schemes, our proposal relies on the presence of finite decay rates and its implementation should therefore be much less demanding.

quant-ph↗
arXiv2001

Verifying Atom Entanglement Schemes by Testing Bell's Inequality

Recent experiments to test Bell's inequality using entangled photons and ions aimed at tests of basic quantum mechanical principles. Interesting results have been obtained and many loopholes could be closed. In this paper we want to point out that tests of Bell's inequality also play an important role in verifying atom entanglement schemes. We describe as an example a scheme to prepare arbitrary entangled states of N two-level atoms using a leaky optical cavity and a scheme to entangle atoms inside a photonic crystal. During the state preparation no photons are emitted and observing a violation of Bell's inequality is the only way to test whether a scheme works with a high precision or not.

quant-ph↗
arXiv2000

Driving Atoms Into Decoherence-Free States

We describe the decoherence-free subspace of N atoms in a cavity, in which decoherence due to the leakage of photons through the cavity mirrors is suppressed. We show how the states of the subspace can be entangled with the help of weak laser pulses, using the high decay rate of the cavity field and strong coupling between the atoms and the resonator mode. The atoms remain decoherence-free with a probability which can, in principle, be arbitrarily close to unity.

quant-ph↗
arXiv2000

Recycling of quantum information: Multiple observations of quantum clocks

How much information about the original state preparation can be extracted from a quantum system which already has been measured? That is, how many independent (non-communicating) observers can measure the quantum system sequentially and give a nontrivial estimation of the original unknown state? We investigate these questions and we show from a simple example that quantum information is not entirely lost as a result of the measurement-induced collapse of the quantum state, and that an infinite number of independent observers who have no prior knowledge about the initial state can gain a partial information about the original preparation of the quantum system.

quant-ph↗
arXiv1998

Copying of entangled states and the degradation of correlations

We investigate the degree to which entanglement survives when a correlated pair of two-state systems are copied using either local or non-local processes. We show how the copying process degrades the entanglement, due to a residual correlation between the copied output and the copying machine (itself made of qubits).

quant-ph↗
arXiv1996

Quantum Interference in Three Photon Down Conversion

We study degenerate three photon down conversion as a potential scheme for generating nonclassical states of light which exhibit clear signatures of phase space interference. The Wigner function representing these states contains an interference pattern manifesting quantum coherence between distinct phase space components, and has substantial areas of negativity. We develop an analytical description of the interference pattern, which demonstrates how the oscillations of the Wigner function are built up by the superposition principle. We analyze the impact of dissipation and pump fluctuations on the visibility of the interference pattern; the results suggest that some signatures of quantum coherence can be observed in the presence of moderate amount of noise.

physics.optics↗