Hwang Lee

A quantum Rosetta stone for interferometry

Heisenberg-limited measurement protocols can be used to gain an increase in measurement precision over classical protocols. Such measurements can be implemented using, for example, optical Mach—Zehnder interferometers and Ramsey spectroscopes. We address the formal equivalence between the Mach—Zehnder interferometer, the Ramsey spectroscope and a generic quantum logic circuit. Based on this equivalence we introduce the quantum Rosetta stone, and we describe a projective-measurement scheme for generating the desired correlations between the interferometric input states in order to achieve Heisenberg-limited sensitivity. The Rosetta stone then tells us that the same method should work in atom spectroscopy.

Creation of large-photon-number path entanglement conditioned on photodetection

Large-photon-number path entanglement is an important resource for enhanced precision measurements and quantum imaging. We present a general constructive protocol to create any large-photon-number pathentangled state based on the conditional detection of single photons. The influence of imperfect detectors is considered and an asymptotic scaling law is derived.

Single-photon quantum-nondemolition detectors constructed with linear optics and projective measurements

Optical quantum-nondemolition devices can provide essential tools for quantum information processing. Here, we describe several optical interferometers that signal the presence of a single photon in a particular input state without destroying it. We discuss both entanglement-assisted and nonentanglement-assisted interferometers, with ideal and realistic detectors. We found that the existing detectors with 88% quantum efficiency and single-photon resolution can yield output fidelities of up to 89%, depending on the input state. Furthermore, we construct expanded protocols to perform quantum-nondemolition detections of single photons that leave the polarization invariant. For detectors with 88% efficiency, we found polarization-preserving output fidelities of up to 98.5%.

Linear optics and projective measurements alone suffice to create large-photon-number path entanglement

We propose a method for preparing maximal path entanglement with a definite photon-numberN, larger than two, using projective measurements. In contrast with the previously known schemes, our method uses only linear optics. Specifically, we exhibit a way of generating four-photon, path-entangled states of the form u4,0&1u0,4&, using only four beam splitters and two detectors. These states are of major interest as a resource for quantum interferometric sensors as well as for optical quantum lithography and quantum holography. Quantum entanglement plays a central role in quantum communication and computation. It also provides a significant improvement in frequency standards as well as in the performance of interferometric sensors @1,2#. In this context, it has been shown that the Heisenberg limit for phase sensitivity of a Mach-Zehnder interferometer can be reached by using maximally entangled states with a definite number of photons N, that is, uN,0& A,B1u0, N& A,B . Here, A and B denote the two arms of the interferometer. These states, also called path-entangled photon-number states, allow a phase sensitivity of order 1/N, whereas coherent light yields the shot-noise limit of 1/An , with mean photon-number n

Single photons on demand from 3D photonic band-gap structures

We describe a practical implementation of a (semi-deterministic) photon gun based on stimulated Raman adiabatic passage pumping and the strong enhancement of the photonic density of states in a photonic band-gap material. We show that this device allows deterministic and unidirectional production of single photons with a high repetition rate of the order of 100 kHz. We also discuss specific 3D photonic micro-structure architectures in which our model can be realized and the feasibility of implementing such a device using Er3+ ions that produce single photons at the telecommunication wavelength of 1.55 μm.

Exploiting the quantum Zeno effect to beat photon loss in linear optical quantum information processors

We devise a new technique to enhance transmission of quantum information through linear optical quantum information processors. The idea is based on applying the Quantum Zeno effect to the process of photon absorption. By frequently monitoring the presence of the photon through a QND (quantum non-demolition) measurement the absorption is suppressed. Quantum information is encoded in the polarization degrees of freedom and is therefore not affected by the measurement. Some implementations of the QND measurement are proposed.

Towards Scalable Linear-Optical Quantum Computers

AbstractScalable quantum computation with linear optics was considered to be impossible due to the lack of efficient two-qubit logic gates, despite the ease of implementation of one-qubit gates. Two-qubit gates necessarily need a non-linear interaction between the two photons, and the efficiency of this non-linear interaction is typically very small in bulk materials. However, it has recently been shown that this barrier can be circumvented with effective non-linearities produced by projective measurements, and with this work linear-optical quantum computing becomes a new avenue towards scalable quantum computation. We review several issues concerning the principles and requirements of this scheme.nPACS: 03.67.Lx, 03.67.Pp, 42.50.Dv, 42.65.Lm

From linear optical quantum computing to Heisenberg-limited interferometry

The working principles of linear optical quantum computing are based on photodetection, namely, projective measurements. The use of photodetection can provide efficient nonlinear interactions between photons at the single-photon level, which is technically problematic otherwise. We report an application of such a technique to prepare quantum correlations as an important resource for Heisenberg-limited optical interferometry, where the sensitivity of phase measurements can be improved beyond the usual shot-noise limit. Furthermore, using such nonlinearities, optical quantum non-demolition measurements can now be carried out easily at the single-photon level.

Linear optics with projective measurements for fun and profit

The technique of projective measurements in linear optics can provide apparent, efficient nonlinear interaction between photons, which is technically problematic otherwise. We present an application of such a technique to prepare large photon-number path entanglement. Large photon-number path entanglement is an important resource for Heisenberg-limited optical interferometry, where the sensitivity of phase measurements can be improved beyond the usual shot-noise limit. A similar technique can also be applied to signal the presence of a single photon without destroying it. We further show how to build a quantum repeater for long-distance quantum communication.