Trevor A. Wheatley

Quantum-enhanced optical phase tracking

Keeping Track of Photon Phase In optical interferometers or optical communications, information is often stored in terms of the phase of the waveform or light pulse. However, fluctuations and noise can give rise to random jitter in the phase and amplitude of the optical pulses, making it difficult to keep track of the phase. Yonezawa et al. (p. 1514) developed a technique based on quantum mechanical squeezing to determine the phase of randomly varying optical waveforms. The quantum mechanical technique enhanced the precision with which the phase could be determined and, as optical technologies continue to be miniaturized, should be helpful in applications within metrology. A quantum mechanical technique is developed to enhance the phase tracking of photons. Tracking a randomly varying optical phase is a key task in metrology, with applications in optical communication. The best precision for optical-phase tracking has until now been limited by the quantum vacuum fluctuations of coherent light. Here, we surpass this coherent-state limit by using a continuous-wave beam in a phase-squeezed quantum state. Unlike in previous squeezing-enhanced metrology, restricted to phases with very small variation, the best tracking precision (for a fixed light intensity) is achieved for a finite degree of squeezing because of Heisenberg’s uncertainty principle. By optimizing the squeezing, we track the phase with a mean square error 15 ± 4% below the coherent-state limit.

Adaptive optical phase estimation using time-symmetric quantum smoothing.

Quantum parameter estimation has many applications, from gravitational wave detection to quantum key distribution. The most commonly used technique for this type of estimation is quantum filtering, using only past observations. We present the first experimental demonstration of quantum smoothing, a time-symmetric technique that uses past and future observations, for quantum parameter estimation. We consider both adaptive and nonadaptive quantum smoothing, and show that both are better than their filtered counterparts. For the problem of estimating a stochastically varying phase shift on a coherent beam, our theory predicts that adaptive quantum smoothing (the best scheme) gives an estimate with a mean-square error up to 2sqrt[2] times smaller than nonadaptive filtering (the standard quantum limit). The experimentally measured improvement is 2.24+/-0.14.

Ultra-wide frequency response measurement of an optical system with a DC photo-detector

Precise knowledge of an optical devices frequency response is crucial for it to be useful in most applications. Traditional methods for determining the frequency response of an optical system (e.g. optical cavity or waveguide modulator) usually rely on calibrated broadband photo-detectors or complicated RF mixdown operations. As the bandwidths of these devices continue to increase, there is a growing need for a characterization method that does not have bandwidth limitations, or require a previously calibrated device. We demonstrate a new calibration technique on an optical system (consisting of an optical cavity and a high-speed waveguide modulator) that is free from limitations imposed by detector bandwidth, and does not require a calibrated photo-detector or modulator. We use a low-frequency (DC) photo-detector to monitor the cavitys optical response as a function of modulation frequency, which is also used to determine the modulators frequency response. Knowledge of the frequency-dependent modulation depth allows us to more precisely determine the cavitys characteristics (free spectral range and linewidth). The precision and repeatability of our technique is demonstrated by measuring the different resonant frequencies of orthogonal polarization cavity modes caused by the presence of a non-linear crystal. Once the modulator has been characterized using this simple method, the frequency response of any passive optical element can be determined to a fine resolution (e.g. kilohertz) over several gigahertz.

Effect of buffering on the throughput of ALOHA

Interest in ALOHA has waned because its throughput is low in comparison to other more complex MAC protocols. Yet those protocols suffer substantial performance degradation in networks with long relative propagation delay. Abramsons well accepted analysis of ALOHA estimates a maximum throughput of 18.6% assuming an infinite number of nodes and a Poisson distribution of transmitted packets. Previous work extended the analysis to allow for a finite number of nodes. However, in modern use buffered transmission is an obvious implementation feature that is not addressed by published analysis. Here we derive a method to calculate the throughput of ALOHA for both buffered and unbuffered cases with a finite number of nodes. We calculate theoretical values for two different collision cases and thereby determine the relationship between offered load and transmitted load. Ultimately, we show by analysis and simulation that the maximum throughput for a low number of active nodes approaches to 32.22% with buffering and 27.37% for unbuffered ALOHA. Therefore, ALOHA is even more attractive than commonly understood as a MAC for networks with large relative propagation delay, such as underwater acoustic networks.

Spectrum analysis with quantum dynamical systems

Measuring the power spectral density of a stochastic process, such as a stochastic force or magnetic field, is a fundamental task in many sensing applications. Quantum noise is becoming a major limiting factor to such a task in future technology, especially in optomechanics for temperature, stochastic gravitational wave, and decoherence measurements. Motivated by this concern, here we prove a measurement-independent quantum limit to the accuracy of estimating the spectrum parameters of a classical stochastic process coupled to a quantum dynamical system. We demonstrate our results by analyzing the data from a continuous optical phase estimation experiment and showing that the experimental performance with homodyne detection is close to the quantum limit. We further propose a spectral photon counting method that can attain quantum-optimal performance for weak modulation and a coherent-state input, with an error scaling superior to that of homodyne detection at low signal-to-noise ratios.

Stabilization of squeezed vacuum states using weak pump depletion

We present a new phase-locking technique based on weak pump depletion. It allows for the first experimental realization of a pump-phase lock of an optical parametric oscillator by reading out the pre-existing phase information in the pump field. No degradation of the detected squeezed states occurs.

Utilizing weak pump depletion to stabilize squeezed vacuum states

We propose and demonstrate a pump-phase locking technique that makes use of weak pump depletion (WPD) - an unavoidable effect that is usually neglected - in a sub-threshold optical parametric oscillator (OPO). We show that the phase difference between seed and pump beam is imprinted on both light fields by the non-linear interaction in the crystal and can be read out without disturbing the squeezed output. In our experimental setup we observe squeezing levels of 1.96 ± 0.01 dB, with an anti-squeezing level of 3.78 ± 0.02 dB (for a 0.55 mW seed beam at 1064 nm and 67.8 mW of pump light at 532 nm). Our new locking technique allows for the first experimental realization of a pump-phase lock by reading out the pre-existing phase information in the pump field. There is no degradation of the detected squeezed states required to implement this scheme.

Practical implications of the new Australian and New Zealand laser safety standard

This paper provides an in context advance look at the practical implications to the Australian Defence Force and general laser community of the pending update to the premier Australian and New Zealand laser safety standard. It discusses the significant changes to the standard, rationale and implications such as the revision of the maximum permissible exposure (MPE) limits, the changes classification system and unavoidable issues created by the adoption process. These details are of value to laser operators, laser safety officers, policy makers, distributors and regulators alike who will be impacted by this change.

Nuisance laser pointer detection and geo-location

We introduce the new research area of real-time detection and source origin geo-location of nuisance lasers pointers above the horizon. We present initial findings using simple image processing techniques and low-cost sensors.

Squeezing-enhanced adaptive optical phase estimation

We demonstrate squeezing-enhanced adaptive optical phase estimation for a stochastically varying phase. By using a continuous-wave phase squeezed beam, estimation accuracy is improved by a factor of 1.4 compared to a coherent beam case.