Brian T. Kirby

Entanglement swapping of two arbitrarily degraded entangled states

We consider entanglement swapping, a key component of quantum network operations and entanglement distribution. Pure entangled states, which are the desired input to the swapping protocol, are typically mixed by environmental interactions causing a reduction in their degree of entanglement. Thus an understanding of entanglement swapping with partially mixed states is of importance. Here we present a general analytical solution for entanglement swapping of arbitrary two-qubit states. Our result provides a comprehensive method for analyzing entanglement swapping in quantum networks. First, we show that the concurrence of a partially mixed state is conserved when this state is swapped with a Bell state. Then, we find upper and lower bounds on the concurrence of the state resulting from entanglement swapping for various classes of input states. Finally, we determine a general relationship between the ranks of the initial states and the rank of the final state after swapping.

Entanglement-enabled interferometry using telescopic arrays

We consider quantum enhancement of direct-detection interferometric measurements to increase telescope resolution. We propose a protocol of measuring interferometric visibility function using imperfectly entangled states shared between remote telescopes. We show how errors in visibility measurement, and in turn, errors in intensity distribution of a distant object depend on the degree of entanglement of the shared quantum resource. We determine that these errors are sufficiently small over a wide range of resource states which makes our technique feasible in practical environments.ABSTRACT A protocol of measuring interferometric visibility function using imperfectly entangled states shared between remote telescopes is proposed. We demonstrate how quantum entanglement can be utilized to increase the baseline size of telescopic arrays thereby providing substantial enhancement to the resolution of direct-detection interferometric measurements. We demonstrate, through a comprehensive analysis, how errors in visibility measurements and in the intensity distribution of a distant object show dependence on the entanglement degree of the shared quantum resource. We analyse the feasibility of the protocol using currently available technology and identify the nature of sources that can benefit most from it.

Entanglement swapping with two imperfect states

We present a formal description of entanglement swapping of any two, arbitrarily mixed, density matrices. Application of this result reveals bounds on the rank and concurrence of the final state for several classes of input states.

Quantum-scheme for improving interferometric visibility with imperfect distributed entangled-states

We analyze a quantum-scheme for measuring interferometric visibility for telescopic applications with imperfectly entangled states as a resource. The scheme overcomes photon loss thus permitting measurement with larger baseline leading to finer resolution of angular intensity distribution of remote sources of light.

In-situ calibration of fiber-optics entangled photon distribution system

A source of entangled photons is connected via optical fibers to two single photon detectors. By simultaneously measuring pump power dependencies of the detection probabilities at each detector, as well as the probability of coincidence counts, we reliably extract all relevant system parameters.

Messages per secret bit when authentication and a min-entropy secrecy criterion are required

Suppose Alice and Bob share a secret key, of which Eve is initially oblivious. Clearly, Alice and Bob can use this key to ensure that any particular plain-message sent is both authentic and secure. This paper investigates how many plain-messages can be sent per bit of secret key, while still ensuring both secrecy and authentication. In particular the secrecy tolerance relates to the min-entropy of any individual plain message, given all observed cipher-messages, while the authentication tolerance directly relates to the maximum allowable probability of Bob erroneously accepting a cipher-message from Eve. In this paper we characterize the set of all messages to secret bit ratios. We do this by providing a direct and converse, showing that the maximum ratio is directly determined by the tolerance parameters for authentication and secrecy, and is independent of the ratio of the length of the plain-message to the length of the secret key.

Analysis of modal loss in the successful transduction of an entangled qubit from polarization to OAM

We successfully convert an entangled photonic degree of freedom between polarization and OAM while the photon is inside an optical fiber. Frequency dependent modal loss and temporal drifts are the major impairments to the conversion.

The utility of entanglement swapping in quantum communications

The nonlocal correlations between quantum states in an entangled system are essential to many quantum communications applications. A basic quantum operation, which permits the distribution of entanglement between two initially uncorrelated systems, is entanglement swapping. Here we present a rigorous formulation of entanglement swapping of any two partially mixed two-qubit states without limiting ourselves to any specific type of state or noise. Further, for two important classes of the input states, Bell diagonal and pure states, we describe how the concurrence of the final state is related to the concurrence of the initial states. First, we consider Bell diagonal states, and find bounds on the concurrence of the final state in terms of the concurrences of the initial states. These bounds are important for communications applications because polarization mode dispersion in fibers produces Bell diagonal states up to a local unitary rotation. Second, we show that swapping pure states occasionally results in a state of higher concurrence than either of the initial states. In addition, we find that two pure states are most likely to be capable of swapping to a state of increased concurrence when the two initial states have similar concurrences. Our analysis offers a completely general framework for investigating the behavior of any pair of two-qubit states when used for entanglement swapping.