Program Manager
Dr Elanor Huntington - UNSW@ADFA

Linear Optics Experimental Research Staff
Dr Gregory Milford

Linear Optics Experimental Research Students
Ms Amy Dunlop
Mr James Webb

Collaborating Centre Researchers
A/Prof Timothy Ralph - University of Queensland
Dr Howard Wiseman - Griffith University

Other Collaborating
Dr David Pulford - DSTO Australia
Dr Matthew Sellars - Australian National University
Dr Charles Harb- Australian National University
Mr Craig Robilliard - UNSW@ADFA

PROGRAM DESCRIPTION
The potential of linear optical systems for quantum computation has been clearly illustrated by recent demonstrations of the operation of non-deterministic photonic CNOT gates (T.B.Pittman et al, Physical Review Letters, 88, 257902 (2002) and J.L.O’Brien et al, Nature, 426, 6964 (2003)). The aim of this program is to develop the technologies required for advanced encoding and detection schemes for Linear Optical Quantum Computation (LOQC). Specifically, the aims have been to: develop the technology required to encode an optical qubit in the occupation of one of two different frequency modes; and develop advanced optoelectronic measurement and control schemes.

1. Optical Source for LOQC in the RF-Basis
The main focus of our work in 2005 was to complete the design of a non-classical source of single photons suitable for LOQC in the frequency basis. A schematic of the source is shown in Figure 1. Because of the need for a local oscillator for our detection techniques (see the following section), the output of the laser is first frequency doubled in the second harmonic generation (SHG) cavity and then converted to pairs of photons at 1550nm in the spontaneous parametric down-conversion (SPDC) cavity. By using cavity-enhanced down-conversion, the output photons are emitted in a very narrow frequency band, as required for our encoding scheme. In preparing a detailed quantum optical description of our source, we have shown that the output is a pair of photons in a frequency superposition. Because the downconversion takes place inside an optical cavity, the output photons are emitted in multiple modes of the cavity. At present we are preparing a paper detailing our findings. Our source has characteristics that turn out to be useful for continuous variable quantum optics experiments (A. E. Dunlop et al, Physical Review A, 73, 013817 (2005)). Our source is physically similar to an optical parametric amplifier (OPA), a device that is commonly used to generate squeezed light. Squeezed light displays quadrature fluctuations that are less than the quantum noise limit (QNL) set by the quadrature fluctuations of the vacuum. By recognising that the down-conversion occurs within a cavity, we have shown that squeezing should occur not only at the carrier frequency but also at every resonant frequency of the cavity and limited only by the phase matching bandwidth of the crystal - see Figure 2a. Furthermore, our calculations suggest maximal squeezing at these higher frequencies even in the presence of seed laser noise and cavity fluctuations (Figure 2b). With design and modelling of the source complete, we ordered and, in 2005, took delivery of the major components for the source including a 1W fiber laser (1550nm) and several sets of nonlinear crystals for use in both the frequency doubling and downconversion cavities. Physical construction of the source has commenced and will be the main focus of our work during 2006.

Figure 1 Design of the UNSW@ADFA single photon source for generating optical qubits.	FIGURE 2 Multi-mode OPO (a) without seed and (b) with seed. The free spectral range (FSR) = 10 GHz in both figures.

Figure 3 The laboratory used for LOQC experiments.

2. Ensemble average photon number via homodyne detection
An essential requirement for continued research into the frequency basis encoding of information is the ability to probe the number of photons at optical sideband frequencies. For sufficiently small photon flux the individual photons could be resolved and counted using a number of standard techniques such as avalanche photodiodes (APDs), photomultipliers and superconducting bolometers. In the absence of sharp spectral filtering, however, such detection techniques fail to provide the required frequency selectivity.

Recent work (by J.G. Webb et al, to appear in Physical Review A) has illustrated the use of homodyne detection whereby an optical mode may be probed by first interfering it with a strong reference field – a local oscillator – before detecting its intensity. The intensity detection is performed by linear photodiodes that cannot resolve individual photons but instead allow measurement of the continuous spectrum of the signals’ conjugate amplitude quadratures. In this case, frequency resolution is achieved via electronic (as opposed to optical) filtering, allowing for comparatively superior frequency domain selectivity. We have shown that the mean number of photons at a specific sideband frequency may be inferred from such a measurement.

Whilst the concepts of single photon and homodyne detection are well understood, our work is the first direct comparison between discrete photon and continuous variable measurements of the same field. Aside from providing frequency resolution, the homodyne detection scheme was also observed to have superior speed and dynamic range compared to a discrete photon counting approach. Comparativemeasurements are illustrated in Figure 4 and examples of the data are shown in Figure 5. Implementation of the detector is experimentally straightforward and hence easy to incorporate into subsequent experiments of frequency basis LOQC.

By virtue of simultaneous quadrature and discrete photon measurement, our data also permits a direct comparison of two contemporary optical vacuum noise models. Our results quantitatively support a quantum mechanical description of the vacuum over the semiclassical stochastic electrodynamic model as determined by comparison of the weighted mean square errors between the predicted and observed data. Given the absence of experimental free parameters, the quantum mechanical noise model permits close agreement between inferred and actual values of experimental photon flux as shown in Figure 6.

Figure 4 Comparison between single photon detector and homodyne measurements over four orders of magnitude of photon flux. The green line indicates the ideal 1:1 relationship.

Figure 5 Example data from the homodyne experiments.

Figure 6 Illustrating the strong agreement between the line of best fit (red) for measured discrete and homodyne photon flux using a quantum mechanical noise model and the ideal 1:1 relationship (green).