Here we take our knowledge of adaptive quantum control and apply it to enhance phase measurement when we are given a limited set of entangled quantum states. We show the measurement of a completely unknown phase at precision that is in principle below the limit of standard techniques.
It's known to be impossible to, with 100% accuracy, discriminate between two different quantum states that are not orthogonal. In this paper we look at how accurately you can make this determination when you are given multiple identical copies of one of the two nonorthogonal states. We consider different measurements you can perform, and find that a measurement strategy that performs optimally when the states in question are pure actually performs poorer than a naive “majority vote” scheme when the states have some mixture. We experimentally demonstrate these schemes and derive (and also demonstrate) an adaptive measurement scheme that performs optimally in all conditions, and compare it to the fundamental limit.
This paper presents a thorough theoretical treatment (with some bonus new experimental results) of our recent demonstrations of phase measurement algorithms which variously beat the standard limit and achieve the fundamental limit of precision. We show how to do this without resorting to entangled states, both with and without adaptive measurements.
We were (or more specifically, our theory collaborator Prof. Howard Wiseman was) invited to write a paper for IEEE Journal of Selected Topics in Quantum Electronics. Here we describe several of the experiments recently taking place in our (or more specifically, Prof. Geoff Pryde's) laboratory, of which Howard is an integral part. It discusses the recent work on phase measurement, with some bonus theoretical details, as well as touching briefly on some soon-to-be-published work on adaptive quantum state discrimination.
Following up our previous publication, here we theoretically prove and experimentally demonstrate a quantum control algorithm to measure an optical phase at the fundamental Heisenberg limit of precision without entangled states or adaptive mesurements. We also demonstrate a simplified adaptive protocol with accuracy surpassing standard techniques.
This post marks the day of my first publication in a scientific journal. And it just so happens to be in Nature. For those not in the know, Nature is one of the highest-tier multidisciplinary journals in the world (if not the highest-tier: ongoing competition with Science makes that perennially debatable). As you can imagine, I'm pretty chuffed about that.
In this work, we developed and experimentally demonstrated an algorithm for phase measurement utilizing techniques from quantum control and quantum computation to achieve efficiency at the fundamental limit, better than any classical method, without requiring quantum entanglement. A thousand thanks to my coauthors and colleagues who gave me the opportunity to be a leading part of this project.
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