Estimation of Output Channel Noise for Continuous Variable Quantum Key Distribution

Estimation of channel parameters is important for extending the range and increasing the key rate of continuous variable quantum key distribution protocols. We propose a new estimator for the channel noise parameter based on the method of moments. The method of moments finds an estimator from the moments of the output distribution of the protocol. This estimator has the advantage of being able to use all of the states shared between Alice and Bob. Other estimators are limited to a smaller publicly revealed subset of the states. The proposed estimator has a lower variance for high loss channel than what has previously been proposed. We show that the method of moments estimator increases the key rate by up to an order of magnitude at the maximum transmission of the protocol.Comment: 5 pages, 3 figure

Protocols and Resources for New Generation Continuous Variable Quantum Key Distribution

Quantum optics has been developing into a promising platform for future generation communications protocols. Much of this promise so far has come from the development of quantum key distribution (QKD). The majority of the development of QKD is done with discrete variables (DV), i.e. qubits with the underlying system of single photons. This is one interpretation of an optical field. Alternatively an optical field can be interpreted as wave with the continuous variable (CV) observables of phase and amplitude. This interpretation comes with the advantage of access to high efficiency detection at room temperature and deterministic sources at the cost of susceptibility to noise in lossy channels. This thesis presents an investigation of protocols and resources for the next generation of CV QKD protocols with two directions, the development of quantum state resources and the development of QKD protocols.This thesis starts with the details on the on going development of a low loss squeezed state resource using OPA for use in future communication and estimation experiments. So far the OPA has produced 11dB of squeezing with 13dB predicted with reasonable improvements to losses and locking. Being able to perform a Bell test with a CV Bell state is also key for future CV QKD protocols. Originally developed for DV systems the Bell test is a fundamental test of quantum mechanics. Here the first experimental demonstration of an optical CV bell test is presented. The experiment violated a CHSH Bell inequality with jBj = 2:31. This violation holds promise for being able to realise new device or source independent CV protocols. The second half of this thesis proposes a channel parameter estimation protocol based on the method of moments and presents the results of a one side device independent CV QKD demonstration based on the family of Gaussian QKD protocols. The proposed channel parameter estimation protocol through the use of the method of moments is able to use information usually disregarded for estimation of an adversaries information. The result does not allow for an increase in range of a fully optimised protocol but can increase the key rate by an order of magnitude with high loss channels. Using a newly found entroptic uncertainty relation for CV tripartite states a new security proof was applied to the family of Gaussian CV QKD protocols. This resulted in the discovery of six new protocols with the special property of being one side device independent. Using the new security proof three of the protocols were demonstrated with a positive key rate

Real-Time Source Independent Quantum Random Number Generator with Squeezed States

Random numbers are a fundamental ingredient for many applications including simulation, modelling and cryptography. Sound random numbers should be independent and uniformly distributed. Moreover, for cryptographic applications they should also be unpredictable. We demonstrate a real-time self-testing source independent quantum random number generator (QRNG) that uses squeezed light as source. We generate secure random numbers by measuring the quadratures of the electromagnetic field without making any assumptions on the source; only the detection device is trusted. We use a homodyne detection to alternatively measure the Q and P conjugate quadratures of our source. Using the entropic uncertainty relation, measurements on P allow us to estimate a bound on the min-entropy of Q conditioned on any classical or quantum side information that a malicious eavesdropper may detain. This bound gives the minimum number of secure bits we can extract from the Q measurement. We discuss the performance of different estimators for this bound. We operate this QRNG with a squeezed state and we compare its performance with a QRNG using thermal states. The real-time bit rate was 8.2 kb/s when using the squeezed source and between 5.2-7.2 kb/s when the thermal state source was used.Comment: 11 pages, 9 figure

Violation of Bells inequality using continuous variable measurements

A Bell inequality is a fundamental test to rule out local hidden variable model descriptions of correlations between two physically separated systems. There have been a number of experiments in which a Bell inequality has been violated using discrete-variable systems. We demonstrate a violation of Bells inequality using continuous variable quadrature measurements. By creating a four-mode entangled state with homodyne detection, we recorded a clear violation with a Bell value of $B = 2.31 \pm 0.02$. This opens new possibilities for using continuous variable states for device independent quantum protocols.Comment: 5 pages, 4 figures, lette

Mapping Guaranteed Positive Secret Key Rates for Continuous Variable Quantum Key Distribution

Continuous variable quantum key distribution (CVQKD) is the sharing of secret keys between different parties using the continuous amplitude and phase quadratures of light. There are many protocols in which different modulation schemes are used to implement CVQKD. However, there has been no tool for comparison between different CVQKD protocols to determine the optimal protocol for varying channels while simultaneously taking into account the effects of different parameters. Here, a comparison tool has been developed to map regions of positive secret key rate (SKR), given a channel's transmittance and excess noise, where a user's modulation can be adjusted to guarantee a positive SKR in an arbitrary environment. The method has been developed for discrete modulated CVQKD (DM-CVQKD) protocols but can be extended to other current and future protocols and security proofs.Comment: 15 pages, 9 figure

Experimental demonstration of Gaussian protocols for one-sided device-independent quantum key distribution

Nonlocal correlations, a longstanding foundational topic in quantum information, have recently found application as a resource for cryptographic tasks where not all devices are trusted, for example in settings with a highly secure central hub, such as a bank or government department, and less secure satellite stations which are inherently more vulnerable to hardware "hacking" attacks. The asymmetric phenomena of Einstein-Podolsky-Rosen steering plays a key role in one-sided device-independent quantum key distribution (1sDI-QKD) protocols. In the context of continuous-variable (CV) QKD schemes utilizing Gaussian states and measurements, we identify all protocols that can be 1sDI and their maximum loss tolerance. Surprisingly, this includes a protocol that uses only coherent states. We also establish a direct link between the relevant EPR steering inequality and the secret key rate, further strengthening the relationship between these asymmetric notions of nonlocality and device independence. We experimentally implement both entanglement-based and coherent-state protocols, and measure the correlations necessary for 1sDI key distribution up to an applied loss equivalent to 7.5 km and 3.5 km of optical fiber transmission respectively. We also engage in detailed modelling to understand the limits of our current experiment and the potential for further improvements. The new protocols we uncover apply the cheap and efficient hardware of CVQKD systems in a significantly more secure setting.Comment: Addition of experimental results and (several) new author