manyChirality is ubiquitous in nature and fundamental in science, from particle physics to metamaterials. The most established technique of chiral discrimination -photoabsorption circular dichroism -relies on the magnetic properties of a chiral medium and yields an extremely weak chiral response. We propose and demonstrate a new, orders of magnitude more sensitive type of circular dichroism in neutral molecules: photoexitation circular dichroism. It does not rely on weak magnetic effects, but takes advantage of the coherent helical motion of bound electrons excited by ultrashort circularly polarized light. It results in an ultrafast chiral response and the efficient excitation of a macroscopic chiral density in an initially isotropic ensemble of randomly oriented chiral molecules. We probe this excitation without 1 arXiv:1612.08764v1 [physics.atm-clus] 27 Dec 2016 Here d 01 , d 02 and d 12 are the dipole transition vectors connecting the ground |0 and the two excited states |1 , |2 (Fig. 1b), ∆E 21 is the energy spacing between the excited states. For more than two states, Eq.(1) will contain the sum over all pairs of excited states n, m, leading to oscillations at all relevant frequencies ∆E nm . As a function of time the induced dipole vector maps out a helix (Fig. 3 1b) and the z-component of the helical current is j P XCD z ∝ σ[ d 01 × d 02 ] d 12 ∆E 21 cos(∆E 21 t). (2) Both d P XCD z and j P XCD z are quintessential chiral observables (see e.g. 19, 20 ). Indeed, both are proportional to the light helicity σ = ±1 and to the triple product of three vectors [ d 01 × d 02 ] d 12 . This product presents a fundamental measure of chirality: it changes sign upon reflection and thus has an opposite sign for left and right enantiomers. For randomly oriented non-chiral molecules d P XCD z = j P XCD z = 0. Eqs.(1,2) lead to the following conclusions. First, the coherent excitation of electronic states leads to a charge displacement in the light propagation direction. Hence, a macroscopic dipole d P XCD z and the corresponding chiral density are created in the excited states, with a chiral current oscillating out of phase for the two enantiomers. Second, PXCD requires no magnetic or quadrupole effects. Hence, it is orders of magnitude stronger than standard photoabsorption CD. While photoabsorption CD exploits the helical pitch of the laser field in space, PXCD takes advantage of the sub-cycle rotation of the light field in time and is inherently ultrafast. Indeed, PXCD arises only if the excitation dipoles d 01 , d 02 are non-collinear: for the angle φ between the two transition dipoles, the PXCD (Eqs. (1,2)) is proportional to σ sin(φ). Since σ = ±1, σ sin(φ) = sin(σφ) = sin(σωτ ), where ω is light frequency and τ = φ/ω is the required time for the light field to rotate by the angle φ. PXCD vanishes if the coherence between excited states |1 and |2 is lost and reflects dynamical symmetry breaking in an isotropic medium.The oscillations of the PXCD signal Eqs.(1,2) appear to suggest that probing it requires the
