# Quantum Enhanced Measurements using Entangled States and Quantum Data Compression

### Monday, 07 Apr. 2014, 11:00 - 12:00

**Presenter:** Lee Rozema

**Host:** P. Walther

**Where:** Ernst-Mach-Hörsaal, 2nd floor, Boltzmanngasse 5

In this talk I will discuss two different experimental investigations in quantum information. The first part of the talk will deal with the use of entangled states for quantum metrology. One of the prototypical quantum metrology proposals was the use of the N00N state, a maximally path-entangled state of N photons, to improve the resolution of a spatial interference-based measurement by a factor of N [1]. However, even given perfectly efficient number-resolving detectors, the detection efficiency of all previous schemes to measure this enhanced interference would decrease exponentially with the number of photons in the N00N state. A technique known as the “optical centroid measurement” was proposed to solve this [2, 3]. I will present the results of our implementation of the optical centroid measurement, using an array of 11 single-photon detectors to measure two-, three-, and four-photon N00N state super-resolution [4]. I will also discuss an experimental study on the use of N00N and less entangled “N00N-like” states for the detection and characterization of decoherence.

The second part of the will deal with our experimental demonstration of quantum data compression. Data compression is a ubiquitous aspect of modern information technology, and the advent of quantum information raises the question of what types of compression are feasible for quantum data, where it is especially relevant given the extreme difficulty involved in creating reliable quantum memories. Importantly, the fundamental differences between quantum and classical mechanics open up the possibility of new kinds of data compression with no classical analog. For example, in quantum mechanics an ensemble of identical quantum systems can provide much more information than a single copy – this is not the case classically (since the information encoded in a single system's state can be accessed repeatedly). I will present a protocol in which an ensemble of N identically prepared qubits can be compressed into a memory of size log (N+1) qubits, and discuss an experiment we performed to demonstrate the protocol, compressing a three-qubit ensemble into the state of two qubits.

[1] A.N. Boto, P. Kok, D.S. Abrams, S.L. Braunstein, C.P. Williams, J.P. Dowling, Phys. Rev. Lett. 85, 2733–2736 (2000).

[2] M. Tsang, Phys. Rev. Lett. 102, 253601–4 (2009).

[3] H. Shin, K.W.C. Chan, H.J. Chang, and R.W. Boyd, Phys. Rev. Lett. 107, 083603 (2011).

[4] L.A. Rozema, J.D. Bateman, D.H. Mahler, R. Okamoto, A. Feizpour, A. Hayat, and A.M. Steinberg. arXiv:1312.2012 (2013).