arXiv subjects

Ian A. Walmsley

Publications and source records attributed to Ian A. Walmsley.

At least 91 records · Page 5Linked to original sources

arXiv2012

Multi-photon state engineering by heralded interference between single photons and coherent states

We develop a technique for generating multi-photon nonclassical states via interference between coherent and Fock states using quantum catalysis. By modulating the coherent field strength, the number of catalyst photons and the ratio of the beam splitter upon which they interfere, a wide range of nonclassical phenomena can be created, including squeezing of up to 1.25 dB, anti- and super-bunched photon statistics and states exhibiting over 90% fidelity to displaced coherent superposition states. We perform quantum catalysis experimentally, showing tunability into the nonclassical regime. Our protocol is not limited by weak nonlinearities that underlie most known strategies of preparing multi-photon nonclassical states. Successive iterations of this protocol can lead to direct control over the weights of higher-order terms in the Fock basis, paving the way towards conditional preparation of "designer" multi-photon states for applications in quantum computation, communication and metrology.

quant-ph↗

arXiv2012

Multi-photon quantum interference in a multi-port integrated photonic device

Increasing the complexity of quantum photonic devices is essential for many optical information processing applications to reach a regime beyond what can be classically simulated, and integrated photonics has emerged as a leading platform for achieving this. Here, we demonstrate three-photon quantum operation of an integrated device containing three coupled interferometers, eight spatial modes and many classical and nonclassical interferences. This represents a critical advance over previous complexities and the first on-chip nonclassical interference with more than two photonic inputs. We introduce a new scheme to verify quantum behaviour, using classically characterised device elements and hierarchies of photon correlation functions. We accurately predict the device's quantum behaviour and show operation inconsistent with both classical and bi-separable quantum models. Such methods for verifying multiphoton quantum behaviour are vital for achieving increased circuit complexity. Our experiment paves the way for the next generation of integrated photonic quantum simulation and computing devices.

quant-ph↗

arXiv2012

Mapping coherence in measurement via full quantum tomography of a hybrid optical detector

Quantum states and measurements exhibit wave-like --- continuous, or particle-like --- discrete, character. Hybrid discrete-continuous photonic systems are key to investigating fundamental quantum phenomena, generating superpositions of macroscopic states, and form essential resources for quantum-enhanced applications, e.g. entanglement distillation and quantum computation, as well as highly efficient optical telecommunications. Realizing the full potential of these hybrid systems requires quantum-optical measurements sensitive to complementary observables such as field quadrature amplitude and photon number. However, a thorough understanding of the practical performance of an optical detector interpolating between these two regions is absent. Here, we report the implementation of full quantum detector tomography, enabling the characterization of the simultaneous wave and photon-number sensitivities of quantum-optical detectors. This yields the largest parametrization to-date in quantum tomography experiments, requiring the development of novel theoretical tools. Our results reveal the role of coherence in quantum measurements and demonstrate the tunability of hybrid quantum-optical detectors.

quant-ph↗

arXiv2012

High-performance single-photon generation with commercial-grade optical fiber

High-quality quantum sources are of paramount importance for the implementation of quantum technologies. We present here a heralded single-photon source based on commercial-grade polarization-maintaining optical fiber. The heralded photons exhibit a purity of at least 0.84 and an unprecedented heralding efficiency into single-mode fiber of 85%. The birefringent phase-matching condition of the underlying four-wave mixing process can be controlled mechanically to optimize the wavelength tuning needed for interfacing multiple sources, as are required for large-scale entanglement generation.

quant-ph↗

arXiv2011

Shaping speckles: spatio-temporal focussing of an ultrafast pulse through a multiply scattering medium

The multiple scattering of coherent light is a problem of both fundamental and applied importance. In optics, phase conjugation allows spatial focussing and imaging through a multiply scattering medium; however, temporal control is nonetheless elusive, and multiple scattering remains a challenge for femtosecond science. Here, we report on the spatially and temporally resolved measurement of a speckle field produced by the propagation of an ultrafast optical pulse through a thick strongly scattering medium. Using spectral pulse shaping, we demonstrate the spatially localized temporal recompression of the output speckle to the Fourier-limit duration, offering an optical analogue to time-reversal experiments in the acoustic regime. This approach shows that a multiply scattering medium can be put to profit for light manipulation at the femtosecond scale, and has a diverse range of potential applications that includes quantum control, biological imaging and photonics.

physics.optics↗

arXiv2011

On-chip, photon-number-resolving, telecom-band detectors for scalable photonic information processing

Integration is currently the only feasible route towards scalable photonic quantum processing devices that are sufficiently complex to be genuinely useful in computing, metrology, and simulation. Embedded on-chip detection will be critical to such devices. We demonstrate an integrated photon-number resolving detector, operating in the telecom band at 1550 nm, employing an evanescently coupled design that allows it to be placed at arbitrary locations within a planar circuit. Up to 5 photons are resolved in the guided optical mode via absorption from the evanescent field into a tungsten transition-edge sensor. The detection efficiency is 7.2 \pm 0.5 %. The polarization sensitivity of the detector is also demonstrated. Detailed modeling of device designs shows a clear and feasible route to reaching high detection efficiencies.

quant-ph↗

arXiv2011

A compact entanglement distillery

Large-scale quantum-correlated networks could transform technologies ranging from communications and cryptography to computation, metrology, and simulation of novel materials. Critical to achieving such quantum enhancements is distributing high-quality entanglement between distant nodes. This is made possible in the unavoidable presence of decoherence by entanglement distillation. However, current versions of this protocol are prohibitively costly in terms of resources. We introduce a new scheme for continuous-variable entanglement distillation that requires only linear temporal and constant physical or spatial resources, both of which are exponential improvements over existing protocols. Our scheme uses a fixed module - an entanglement distillery - comprising only four quantum memories of at most 50 % storage efficiency and allowing a feasible experimental implementation. Tangible quantum advantages are obtained by using non-ideal quantum memories outside their conventional role of storage. By creating, storing and processing information in the same physical space, the scheme establishes a potentially valuable technique for designing stable, scalable protocols across different quantum technologies.

quant-ph↗

arXiv2011

Quantum metrology with imperfect states and detectors

Quantum enhancements of precision in metrology can be compromised by system imperfections. These may be mitigated by appropriate optimization of the input state to render it robust, at the expense of making the state difficult to prepare. In this paper, we identify the major sources of imperfection an optical sensor: input state preparation inefficiency, sensor losses, and detector inefficiency. The second of these has received much attention; we show that it is the least damaging to surpassing the standard quantum limit in a optical interferometric sensor. Further, we show that photonic states that can be prepared in the laboratory using feasible resources allow a measurement strategy using photon-number-resolving detectors that not only attains the Heisenberg limit for phase estimation in the absence of losses, but also deliver close to the maximum possible precision in realistic scenarios including losses and inefficiencies. In particular, we give bounds for the trade off between the three sources of imperfection that will allow true quantum-enhanced optical metrology.

quant-ph↗

arXiv2011

Vectorial Phase Retrieval for Linear Characterization of Attosecond Pulses

The waveforms of attosecond pulses produced by high-harmonic generation carry information on the electronic structure and dynamics in atomic and molecular systems. Current methods for the temporal characterization of such pulses have limited sensitivity and impose significant experimental complexity. We propose a new linear and all-optical method inspired by widely-used multi-dimensional phase retrieval algorithms. Our new scheme is based on the spectral measurement of two attosecond sources and their interference. As an example, we focus on the case of spectral polarization measurements of attosecond pulses, relying on their most fundamental property -- being well confined in time. We demonstrate this method numerically reconstructing the temporal profiles of attosecond pulses generated from aligned $CO_2$ molecules.

physics.optics↗

arXiv2011

Continuous phase stabilization and active interferometer control using two modes

We present a computer-based active interferometer stabilization method that can be set to an arbitrary phase difference and does not rely on modulation of the interfering beams. The scheme utilizes two orthogonal modes propagating through the interferometer with a constant phase difference between them to extract a common phase and generate a linear feedback signal. Switching times of 50ms over a range of 0 to 6 pi radians at 632.8nm are experimentally demonstrated. The phase can be stabilized up to several days to within 3 degrees.

physics.optics↗

arXiv2010

Space-time coupling of shaped ultrafast ultraviolet pulses from an acousto-optic programmable dispersive filter

A comprehensive experimental analysis of spatio-temporal coupling effects inherent to the acousto-optic programmable dispersive filter (AOPDF) is presented. Phase and amplitude measurements of the AOPDF transfer function are performed using spatially and spectrally resolved interferometry. Spatio-temporal and spatio-spectral coupling effects are presented for a range of shaped pulses that are commonly used in quantum control experiments. These effects are shown to be attributable to a single mechanism: a group-delay--dependent displacement of the shaped pulse. The physical mechanism is explained and excellent quantitative agreement between the measured and calculated coupling speed is obtained. The implications for quantum control experiments are discussed.

physics.optics↗

arXiv2010

Extending electron orbital precession to the molecular case: Can orbital alignment be used to observe wavepacket dynamics?

The complexity of ultrafast molecular photoionization presents an obstacle to the modelling of pump-probe experiments. Here, a simple optimized model of atomic rubidium is combined with a molecular dynamics model to predict quantitatively the results of a pump-probe experiment in which long range rubidium dimers are first excited, then ionized after a variable delay. The method is illustrated by the outline of two proposed feasible experiments and the calculation of their outcomes. Both of these proposals use Feshbach 87Rb2 molecules. We show that long-range molecular pump-probe experiments should observe spin-orbit precession given a suitable pump-pulse, and that the associated high-frequency beat signal in the ionization probability decays after a few tens of picoseconds. If the molecule was to be excited to only a single fine structure state state, then a low-frequency oscillation in the internuclear separation would be detectable through the timedependent ionization cross section, giving a mechanism that would enable observation of coherent vibrational motion in this molecule.

physics.atom-ph↗

arXiv2009

Quantum states made to measure

Recent progress in manipulating quantum states of light and matter brings quantum-enhanced measurements closer to prospective applications. The current challenge is to make quantum metrologic strategies robust against imperfections.

quant-ph↗

arXiv2009

Demonstrating coherent control in 85Rb2 using ultrafast laser pulses: a theoretical outline of two experiments

Calculations relating to two experiments that demonstrate coherent control of preformed rubidium-85 molecules in a magneto-optical trap using ultrafast laser pulses are presented. In the first experiment, it is shown that pre-associated molecules in an incoherent mixture of states can be made to oscillate coherently using a single ultrafast pulse. A mechanism that can transfer molecular population to more deeply bound vibrational levels is used in the second. Optimal parameters of the control pulse are presented for the application of the mechanism to molecules in a magneto-optical trap. The calculations make use of an experimental determination of the initial state of molecules photoassociated by the trapping lasers in the magneto-optical trap and use shaped pulses consistent with a standard ultrafast laser system.

physics.atom-ph↗

arXiv2009

A pump-probe study of the formation of rubidium molecules by ultrafast photoassociation of ultracold atoms

An experimental pump-probe study of the photoassociative creation of translationally ultracold rubidium molecules is presented together with numerical simulations of the process. The formation of loosely bound excited-state dimers is observed as a first step towards a fully coherent pump-dump approach to the stabilization of Rb$_2$ into its lowest ground vibrational states. The population that contributes to the pump-probe process is characterized and found to be distinct from a background population of pre-associated molecules.

physics.chem-ph↗

arXiv2009

Measuring sub-Planck structural analogues in chronocyclic phase space

The phase space structure of certain quantum states reveals structure on a scale that is small compared to the Planck area. Using an analog between the wavefunction of a single photon and the electric field of a classical ultrashort optical pulse we show that spectral shearing interferometry enables measurement of such structures directly. Thereby extending the idea of Praxmeyer et al. In particular, we use multiple-shear spectral interferometry to fully characterize a pulse consisting of two sub-pulses which are temporally and spectrally disjoint, without a relative-phase ambiguity. This enables us to compute the Wigner distribution of the pulse. This spectrographic representation of the pulse field features fringes that are tilted with respect to both the time- and frequency axes, showing that in general the shortest sub-Planck distances may not be in the directions of the canonical (and easily experimentally accessible) directions. Further, independent of this orientation, evidence of the sub-Planck scale of the structure maybe extracted directly from the measured signal.

physics.optics↗

arXiv2009

Ultrashort pulse characterization by spectral shearing interferometry with spatially chirped ancillae

We report a new version of spectral phase interferometry for direct electric field reconstruction (SPIDER), which enables consistency checking through the simultaneous acquisition of multiple shears and offers a simple and precise calibration method. By mixing the test pulse with two spatially chirped ancilla fields we generate a single-shot interferogram which contains multiple shears, the spectral amplitude of the test pulse, and the reference phase, which is accurate for broadband pulses. All calibration parameters - shear, upconversion-frequency and reference phase position - can be accurately obtained from a single calibration trace.

physics.optics↗

arXiv2009

Tailored photon-pair generation in optical fibers

We experimentally control the spectral structure of photon pairs created via spontaneous four-wave mixing in microstructured fibers. By fabricating fibers with designed dispersion, one can manipulate the photons' wavelengths, joint spectrum, and, thus, entanglement. As an example, we produce photon-pairs with no spectral correlations, allowing direct heralding of single photons in pure-state wave packets without filtering. We achieve an experimental purity of $85.9\pm1.6%$, while theoretical analysis and preliminary tests suggest 94.5% purity is possible with a much longer fiber.

quant-ph↗