M. G. Raymer

Continuous-variable optical quantum-state tomography

This review covers latest developments in continuous-variable quantum-state tomography of optical fields and photons, placing a special accent on its practical aspects and applications in quantum information technology. Optical homodyne tomography is reviewed as a method of reconstructing the state of light in a given optical mode. A range of relevant practical topics are discussed, such as state-reconstruction algorithms (with emphasis on the maximum-likelihood technique), the technology of time-domain homodyne detection, mode matching issues, and engineering of complex quantum states of light. The paper also surveys quantum-state tomography for the transverse spatial state (spatial mode) of the field in the special case of fields containing precisely one photon.

Generation and Photonic Guidance of Multi-Octave Optical-Frequency Combs

Ultrabroad coherent comb-like optical spectra spanning several octaves are a chief ingredient in the emerging field of attoscience. We demonstrate generation and guidance of a three-octave spectral comb, spanning wavelengths from 325 to 2300 nanometers, in a hydrogen-filled hollow-core photonic crystal fiber. The waveguidance results not from a photonic band gap but from the inhibited coupling between the core and cladding modes. The spectrum consists of up to 45 high-order Stokes and anti-Stokes lines and is generated by driving the confined gas with a single, moderately powerful (10-kilowatt) infrared laser, producing 12-nanosecond-duration pulses. This represents a reduction by six orders of magnitude in the required laser powers over previous equivalent techniques and opens up a robust and much simplified route to synthesizing attosecond pulses.

Quantum frequency translation of single-photon states in a photonic crystal fiber.

We experimentally demonstrate frequency translation of a nonclassical optical field via four-wave mixing (Bragg-scattering process) in a photonic crystal fiber (PCF). The high nonlinearity and the ability to control dispersion in PCF enable efficient translation between nearby photon channels within the visible to-near-infrared spectral range, useful in quantum networks. Heralded single photons at 683 nm were translated to 659 nm with an efficiency of 28.6±2.2 percent. Second-order correlation measurements on the 683- and 659-nm fields yielded g(683)(2) (0)=0.21±0.02 and g(659)(2) (0)=0.19±0.05, respectively, showing the nonclassical nature of both fields.

Linear optical sampling

We demonstrate the measurement of waveforms and eye diagrams at high bit rates by optical sampling using coherent detection. By simultaneously recording two orthogonal quadratures of the interference between the data stream and a sampling pulse with two balanced detectors, we are able to cancel the phase sensitivity inherent to linear optics. As the device is based on linear optics and square-law low-speed photodetectors, its sensitivity is 1000 times better than nonlinear optical sampling techniques, which makes it attractive for optical signal characterization and monitoring. This new diagnostic tool was used to measure eye diagrams at 10, 40, and 80 Gb/s, and is projected to operate at 160 Gb/s and higher.

Chronocyclic tomography for measuring the amplitude and phase structure of optical pulses

We describe a new method—chronocyclic tomography—for determining the amplitude and phase structure of a short optical pulse. The technique is based on measurements of the energy spectrum of the pulse after it has passed through a time–frequency-domain imaging system. Tomographic inversion of these measured spectra yields the time–frequency Wigner distribution of the pulse, which uniquely determines the amplitude and phase structure.

Optical phase retrieval by phase-space tomography and fractional-order Fourier transforms

Phase-space tomography is experimentally demonstrated for the determination of the spatially varying amplitude and phase of a quasi-monochromatic optical field by measurements of intensity only. Both fully and partially coherent sources are characterized. The method, which makes use of the fractional-order Fourier transform, also yields the Wigner distribution of the field and works in one or two dimensions.

Translation of quantum states by four-wave mixing in fibers.

Optical frequency conversion by four-wave mixing (Bragg scattering) in a fiber is considered. If the frequencies and polarizations of the waves are chosen judiciously, Bragg scattering enables the translation of individual and entangled states, without the noise pollution associated with parametric amplification (modulation instability or phase conjugation), and with reduced noise pollution associated with stimulated Raman scattering.

Quantum noise properties of parametric processes

In this paper the quantum noise properties of phase-insensitive and phase-sensitive parametric processes are studied. Formulas for the field-quadrature and photon-number means and variances are derived, for processes that involve arbitrary numbers of modes. These quantities determine the signal-to-noise ratios associated with the direct and homo-dyne detection of optical signals. The consequences of the aforementioned formulas are described for frequency conversion, amplification, monitoring, and transmission through sequences of attenuators and amplifiers.

Steady-state quantum interference in resonance fluorescence

It is shown that when a monochromatic laser couples a single atomic ground level to two closely spaced excited levels the system can be driven into a state in which quantum interference prevents any fluorescence from the excited levels, regardless of the intensity of the exciting field. This steady-state interference occurs only at a particular excitation frequency which depends on the separation of the excited states and the relative size of the two transition dipole matrix elements. The results are derived from the density matrix equations of motion. It is shown that a correct description of the effect requires the inclusion of generalised Einstein A coefficients which are usually neglected in phenomenological damping theories. A dressed-state analysis is introduced to simplify the generalisation to atoms having more complex manifolds of excited states. Analogous interferences in multiphoton absorption and ionisation are also discussed briefly.

Quantum noise properties of parametric amplifiers driven by two pump waves

In a parametric amplifier (PA) driven by two pump waves the signal sideband is coupled to three idler sidebands, all of which are frequency-converted (FC) images of the signal, and two of which are phaseconjugated (PC) images of the signal. If such a device is to be useful, the signal must be amplified, and the PC and FC idlers must be produced, with minimal noise. In this paper the quantum noise properties of two-sideband (TS) parametric devices are reviewed and the properties of many-sideband devices are determined. These results are applied to the study of two-pump PAs, which are based on the aforementioned four-sideband (FS) interaction. As a general guideline, the more sidebands that interact, the higher are the noise levels. However, if the pump frequencies are tuned to maximize the frequency bandwidth of the FS interaction, the signal and idler noise-figures are only slightly higher than the noise figures associated with the limiting TS interactions.