Nathan Walk

Heralded noiseless linear amplification and distillation of entanglement

A noiseless linear amplifier for quantum states of an optical field is demonstrated. The amplifier is also used to enhance entanglement through a technique known as distillation. Such amplification and distillation may be useful for quantum cloning, metrology and communications.

Observation of Genuine One-Way Einstein-Podolsky-Rosen Steering.

Within the hierarchy of inseparable quantum correlations, Einstein-Podolsky-Rosen steering is distinguished from both entanglement and Bell nonlocality by its asymmetry-there exist conditions where the steering phenomenon changes from being observable to not observable, simply by exchanging the role of the two measuring parties. While this one-way steering feature has been previously demonstrated for the restricted class of Gaussian measurements, for the general case of positive-operator-valued measures even its theoretical existence has only recently been settled. Here, we prove, and then experimentally observe, the one-way steerability of an experimentally practical class of entangled states in this general setting. As well as its foundational significance, the demonstration of fundamentally asymmetric nonlocality also has practical implications for the distribution of the trust in quantum communication networks.

Security of continuous-variable quantum cryptography with Gaussian postselection

We extend the security proof for continuous variable quantum key distribution protocols using post selection to account for arbitrary eavesdropping attacks by employing the concept of an equivalent protocol where the post-selection is implemented as a series of quantum operations including a virtual distillation. We introduce a particular ‘Gaussian’ post selection and demonstrate that the security can be calculated using only experimentally accessible quantities. Finally we explicitly evaluate the performance for the case of a noisy Gaussian channel in the limit of unbounded key length and find improvements over all pre-existing continuous variable protocols in realistic regimes.

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.

Quantum Communication with an Accelerated Partner

An unsolved problem in relativistic quantum information research is how to model efficient, directional quantum communication between localized parties in a fully quantum field-theoretical framework. We propose a tractable approach to this problem based on calculating expectation values of localized field observables in the Heisenberg picture. We illustrate our approach by analyzing, and obtaining approximate analytical solutions to, the problem of communicating coherent states between an inertial sender, Alice, and an accelerated receiver, Rob. We use these results to determine the efficiency with which continuous variable quantum key distribution could be carried out over such a communication channel. DOI: 10.1103/PhysRevA.87.012327

Nondeterministic noiseless amplification via non-symplectic phase space transformations

We analyse the action of an ideal noiseless linear amplifier operator, , using the Wigner function phase space representation. In this setting we are able to clarify the gain g for which a physical output is produced when this operator is acted upon inputs other than coherent states. We derive compact closed form expressions for the action of N local amplifiers, with potentially different gains, on arbitrary N-mode Gaussian states and provide several examples of the utility of this formalism for determining important quantities including amplification and the strength and purity of the distilled entanglement, and for optimizing the use of the amplification in quantum information protocols.

Channel purification via continuous-variable quantum teleportation with Gaussian postselection

We present a protocol based on continuous-variable quantum teleportation and Gaussian postselection that can be used to correct errors introduced by a lossy channel. We first show that the global transformation enacted by the protocol is equivalent to an effective system composed of a noiseless amplification (or attenuation), and an effective quantum channel, which can in theory have no loss and an amount of thermal noise arbitrarily small, hence tending to an identity channel. An application of our protocol is the probabilistic purification of quantum non-Gaussian states using only Gaussian operations.

Models of reduced-noise, probabilistic linear amplifiers

We construct an amplifier that interpolates between a nondeterministic, immaculate linear amplifier and a deterministic, ideal linear amplifier and beyond to nonideal linear amplifiers. The construction involves cascading an immaculate linear amplifier that has amplitude gain g(1) with a (possibly) nonideal linear amplifier that has gain g(2). With respect to normally ordered moments, the device has output noise mu(2)(G(2) - 1) where G = g(1)g(2) is the overall amplitude gain and mu(2) is a noise parameter. When mu(2) >= 1, our devices realize ideal (mu(2) = 1) and nonideal (mu(2) > 1) linear amplifiers. When 0 <= mu(2) < 1, these devices work effectively only over a restricted region of phase space and with some subunity success probability p (sic). We investigate the performance of our mu(2) amplifiers in terms of a gain-corrected probability-fidelity product and the ratio of input to output signal-to-noise ratios corrected for success probability.

Continuous-variable QKD with post-selection is secure

Post-selection is a standard part of discrete variable QKD protocols, however attempts to prove security when post-selection is deployed in continuous variable QKD protocols have until now been limited. Here, by using a carefully tailored post-selection protocol, we prove unconditional security and show that significant performance improvements are achieved.

Virtual noiseless amplification

The unavoidable addition of noise during amplification is a well known signature of quantum mechanics. It is at the heart of fundamental results such as the no-cloning theorem, quantum limited metrology, quantum key distribution and the impossibility of increasing entanglement by local operations. Nonetheless one can still avoid the unavoidable by moving to a non-deterministic protocol. This novel concept and a linear optics implementation have been proposed [1] and experimentally realised for the case of amplifying coherent states [2-4], qubits [5,6] and the concentration of phase information [7]. All these were extremely challenging experiments, with only [2] demonstrating entanglement distillation and none directly showing the EPR distillation necessary for application to CV QKD. Furthermore the success probability of these experiments was substantially worse than the theoretical considerations would imply. However as has been noted in [8,9] it is possible to virtually implement noiseless amplification (NLA) and hence entanglement distillation via post-selective measurements, achieving significant distillation with a much improved probability of success.