I describe our recent work in generating exotic entangled states a sample of laser-cooled atomic spins, recently highlighted in Scientific American [1]. We use quantum non-demolition (QND) measurement techniques [2] to prepare entangled spin states. In the simplest case, we measure a single spin component and generate spin-squeezing in a sample of highly polarised atoms - a useful resource for quantum-enhanced atomic magnetometry [3]. More recently, we have developed techniques for squeezing all three spin components of an unpolarised sample of atoms. This generates a highly entangled macroscopic spin singlet (MSS) [4,5], analogous to the ground state of many fundamental spin models in condensed matter physics. The state we generate is SU(2) invariant, so it is directly useful for background-free measurement of magnetic field gradients [6], and may be useful for quantum information tasks such as storing information in a decoherence free subspace. Combining this with quantum feedback control [7] should allow us to deterministically prepare a MSS. In the outlook I discuss prospects for using these techniques in experiments designed to simulate quantum magnetism [8].

[1] Clara Moskowitz, “Quantum Entanglement Creates New State of Matter, Scientific American, 22 Sept. 2014.
[2] Nat. Photon. 7, 517 (2013).
[3] Phys. Rev. Lett. 109, 253605 (2012).
[4] New J. Phys. 12, 053007 (2010).
[5] Phys. Rev. Lett. 113, 093601 (2014).
[6] Phys. Rev. A 88, 013626 (2013).
[7] Phys. Rev. Lett. 111, 103601 (2013).
[8] Phys. Rev. A 87, 021601 (2013).