Optimum Small Beam Displacement Measurement

Vincent Delaubert

We derive the quantum noise limit for the optical beam displacement of a TEM00 mode. Using a multimodal analysis, we show that the conventional split detection scheme for measuring beam displacement is non-optimal with ~80% efficiency. We propose a new displacement measurement scheme that is optimal for small beam displacement. This scheme utilises a homodyne detection setup that has a TEM10 mode local oscillator. We show that although the quantum noise limit to displacement measurement can be surpassed using squeezed light in appropriate spatial modes for both schemes, the TEM10 homodyning scheme out-performs split detection for all values of squeezing.

Low Frequency Squeezing

Kirk McKenzie

Squeezing at audio Gravitational Wave signal frequencies (10Hz-10kHz) could be used to reduce quantum noise in Gravitational Wave detectors. In early 2004 we have demonstrated squeezing down to 200Hz using a degenerate OPO. Look at our poster from the GR17 conference in 2004 for more information. In collaboration, the Centre for Gravitational Physics, the ANU Quantum Optics group, the Quantum Measurement Group at MIT and the Hannover University are developing techniques and technology that will hopefully yield an unprecedented 10dB at 100 Hz.

Quantum Secret Sharing

Thomas Symul

Quantum state sharing is the quantum equivalent of secret sharing as defined by A. Shamir in 1979. In a (k,n) secret sharing protocol, a sender (or dealer) sends a secret message to n recipients (or players) in such a way that only subgroups of k or more participants (the access structure) can reconstruct the message. In a Quantum state sharing protocol the secret message is replaced by an unknown quantum state. We present here the first experimental realisation of (2,3) quantum state sharing in the continuous variable domain.

Quantum Cryptography

Andrew Lance

Quantum cryptography is the science of sending secret messages via a quantum channel. It uses properties of quantum mechanics to establish a secure key, a process known as quantum key distribution. This key can then be used at a later stage to send encrypted information. We have proposed a new coherent state quantum key distribution protocol that eliminates the need to randomly switch between measurement bases. Conventionally, switching randomly between a pair of non-commuting measurement bases is assumed vital in ensuring the security of the protocol. This protocol provides significantly higher secret key rates, with increased bandwidths, than previous schemes that randomly switch between measurement bases. It also offers the further advantage of simplicity compared to all previous protocols which, to date, have relied on switching.

 

Quantum Random Number Generation

Assad Syed

While random number generation using single photon detections have been around for half a century, we aim at generating random numbers using a bright continuous light field. After overcoming technical noise and detector dark noise, the shot noise of a laser could potentially generate a stream of random numbers at a rate much faster than can be achieved through single photon detections. Our first table top experiment already achieved a bit rate better than 50 MHz

 

Thermal States and Continuous Variable Quantum Key Distribution (CVQKD)

Daniel Alton

We investigate the security of a post-selection based continuous variable Quantum Key Distribution (CVQKD) protocol. Currently, two important factors that limit the performance of such a protocol are the channel loss and excess noise (the noise above the shot noise level), which are present, for example, in optical fibers. We present a theoretical framework for the security of the protocol that takes into account both the channel loss and excess noise. We demonstrate experimentally that the protocol is secure against individual and collective Gaussian attacks in the presence of nonzero excess noise.

 

Optimisation of Squeezing Using New Non-Linear Crystals

Michael Stefszky

Many current research topics in quantum optics and other fields could be improved with the use of light with higher levels of squeezing. As the search for these higher levels of squeezing continues, so does the search into new and improved non-linear crystals which could optimise the non-linear effects, as well as improve the damage thresholds and defect densities. We are currently working on setting up an experiment to test some of these new and improved crystals with the hope of optimising the squeezing we can achieve on a table top experiment.