Syed M. Assad

Real time demonstration of high bitrate quantum random number generation with coherent laser light

We present a random number generation scheme that uses broadband measurements of the vacuum field contained in the radio-frequency sidebands of a single-mode laser. Even though the measurements may contain technical noise, we show that suitable algorithms can transform the digitized photocurrents into a string of random numbers that can be made arbitrarily correlated with a subset of the quantum fluctuations (high quantum correlation regime) or arbitrarily immune to environmental fluctuations (high environmental immunity). We demonstrate up to 2 Gbps of real time random number generation that were verified using standard randomness tests.

Observation of Entanglement between Two Light Beams Spanning an Octave in Optical Frequency

We have experimentally demonstrated how two beams of light separated by an octave in frequency can become entangled after their interaction in a chi;{(2)} nonlinear medium. The entangler was a nonlinear optical resonator that was strongly driven by coherent light at the fundamental and second-harmonic wavelengths. An interconversion between the fields created quantum correlations in the amplitude and phase quadratures, which were measured by two independent homodyne detectors. Analysis of the resulting correlation matrix revealed a wave function inseparability of 0.74(1)<1, thereby satisfying the criterion of entanglement.

Experimental demonstration of post-selection-based continuous-variable quantum key distribution in the presence of Gaussian noise

In realistic continuous-variable quantum key distribution protocols, an eavesdropper may exploit the additional Gaussian noise generated during transmission to mask her presence. We present a theoretical framework for a post-selection-based protocol which explicitly takes into account excess Gaussian noise. We derive a quantitative expression of the secret key rates based on the Levitin and Holevo bounds. We experimentally demonstrate that the post-selection-based scheme is still secure against both individual and collective Gaussian attacks in the presence of this excess noise.

Measurement-based noiseless linear amplification for quantum communication

Entanglement distillation is an indispensable ingredient in extended quantum communication networks. Distillation protocols are necessarily non-deterministic and require non-trivial experimental techniques such as noiseless amplification. We show that noiseless amplification could be achieved by performing a post-selective filtering of measurement outcomes. We termed this protocol measurement-based noiseless linear amplification (MBNLA). We apply this protocol to entanglement that suffers transmission loss of up to the equivalent of 100km of optical fibre and show that it is capable of distilling entanglement to a level stronger than that achievable by transmitting a maximally entangled state through the same channel. We also provide a proof-of-principle demonstration of secret key extraction from an otherwise insecure regime via MBNLA. Compared to its physical counterpart, MBNLA not only is easier in term of implementation, but also allows one to achieve near optimal probability of success.

Electromagnetically induced transparency and fourwave mixing in a cold atomic ensemble with large optical depth

We report on the delay of optical pulses using electromagnetically induced transparency (EIT) in an ensemble of cold atoms with an optical depth exceeding 500. To identify the regimes in which four-wave mixing (4WM) impacts on EIT behaviour, we conduct the experiment in both 85Rb and 87Rb. Comparison with theory shows excellent agreement in both isotopes. In 87Rb negligible 4WM was observed and we obtained one pulse-width of delay with 50% efficiency. In 85Rb 4WM contributes to the output. In this regime we achieve a delay-bandwidth product of 3.7 at 50% efficiency, allowing temporally multimode delay, which we demonstrate by compressing two pulses into the memory medium.

Experimental verification of quantum discord in continuous-variable states

We introduce a simple and efficient technique to verify quantum discord in unknown Gaussian states and certain class of non-Gaussian states. We show that any separation in the peaks of the marginal distributions of one subsystem conditioned on two different outcomes of homodyne measurements performed on the other subsystem indicates correlation between the corresponding quadratures and hence nonzero quantum discord. We also demonstrate that under certain measurement constraints, discord between bipartite systems can be consumed to encode information that can only be accessed by coherent quantum interaction.

Maximization of Extractable Randomness in a Quantum Random-Number Generator

The generation of random numbers via quantum processes is an efficient and reliable method to obtain true indeterministic random numbers that are of vital importance to cryptographic communication and large-scale computer modeling. However, in realistic scenarios, the raw output of a quantum random-number generator is inevitably tainted by classical technical noise. The integrity of the device can be compromised if this noise is tampered with, or even controlled by some malicious party. To safeguard against this, we propose and experimentally demonstrate an approach that produces side-information independent randomness that is quantified by min-entropy conditioned on this classical noise. We present a method for maximizing the conditional min-entropy of the number sequence generated from a given quantum-to-classical-noise ratio. The detected photocurrent in our experiment is shown to have a real-time random-number generation rate of 14 (Mbit/s)/MHz. The spectral response of the detection system shows the potential to deliver more than 70 Gbit/s of random numbers in our experimental setup.

Replicating the benefits of Deutschian closed timelike curves without breaking causality

Sending messages back in time can be remarkably powerful, even if no one ever reads them, says an international research team. Peculiarities of general relativity called ‘closed timelike curves’ (CTCs) effectively allow particles to travel backwards in time, and are consistent with current quantum theory. However, CTCs break causality, the fundamental notion that cause must precede effect, and thus their existence remains highly controversial. Mile Gu at Tsinghua University in China and colleagues in Australia, Singapore, the UK and Canada investigated ‘open timelike curves’ (OTCs), which keep all time-travelling particles isolated from the past and thus respect causality. The researchers showed that despite such restrictions, OTCs allow quantum computers to clone quantum states, defy Heisenberg’s uncertainty principle, and efficiently solve previously intractable mathematical problems. This greatly improves prospects for relativistically enhanced quantum computation.

Surpassing the no-cloning limit with a heralded hybrid linear amplifier for coherent states

The no-cloning theorem states that an unknown quantum state cannot be cloned exactly and deterministically. However, this limit can be circumvented by abandoning determinism and using probabilistic methods. Here, we report an experimental demonstration of probabilistic cloning of arbitrary coherent states that clearly surpasses the no-cloning limit. Our scheme is based on a hybrid linear amplifier that combines an ideal deterministic linear amplifier with a heralded measurement-based noiseless amplifier. We demonstrate the production of up to five clones with the fidelity of each clone clearly exceeding the corresponding no-cloning limit. Moreover, since successful cloning events are heralded, our scheme has the potential to be adopted in quantum repeater, teleportation and computing applications.

Overarching framework between Gaussian quantum discord and Gaussian quantum illumination

We are grateful for funding from the National Research Foundation of Singapore (NRF Award No. NRF-NRFF2016- 02), the John Templeton Foundation Grant No. 53914 “Occam’s Quantum Mechanical Razor: Can Quantum theory admit the Simplest Understanding of Reality?”, the Foundational Questions Institute, and the Australian Research Council Centre of Excellence for Quantum Computation and Communication Technology (Project No. CE110001027). This material is based on research supported in part by the National Research Foundation of Singapore under NRFAward No. NRF-NRFF2013-01. S.T. acknowledges support from the AFOSR under Grant No. FA2386-15-1-4082.