Demonstration of the Interaction between Two Stopped Light Pulses

Abstract

We report the first experimental demonstration that two light pulses were made motionless and interacted with each other via a medium. The interaction time is, in principle, as long as possible and a considerable efficiency can be achieved even below single-photon level. We utilized the optical process of one photon pulse switched by another based on the effect of electromagnetically induced transparency to demonstrate the enhancement of optical nonlinear efficiency. With moving light pulses, the switching is activated at energy per area of 2 photons per atomic absorption cross section in the best situation as discussed in [Phys. Rev. Lett. 82, 4611 (1999)]. With motionless light pulses, we demonstrated that the switching is activated at 0.56 photons per atomic absorption cross section and that the light level can be further reduced by increasing the optical density of the medium. Our work enters a new regime of low light physics. Efficiency of a nonlinear optical process is equal to the product of transition rate and interaction time. Since the transition rate is dependent on the light intensity, a high intensity laser field is usually required in the nonlinear optics in order to achieve sufficient efficiency. On the other hand, if the interaction time can be made as long as possible, high efficiency can also be achieved even at low-light or single-photon level. With the techniques of light storage (LS) [1][2][3][4] and stationary light pulse (SLP) [5,6], we report the first experimental demonstration of enhanced nonlinear efficiency due to long interaction time between two motionless light pulses. In the LS, the storing process converts a light pulse to the ground-state coherence of an ensemble and the retrieving process vice versa. The LS provides a way to transfer quantum states between photons and atoms and can lead to the applications of quantum memory [7][8][9]. The SLP is created by the counter-propagating scheme of four-wave mixing. Unlike the stored light in the LS, an SLP maintains its electromagnetic component which is required for nonlinear optical interactions. Nevertheless, the formation of SLP was observed via the remaining energy released from the medium [5,6]. This work is also the first SLP application and provides the direct evidence of SLPs being electromagnetic fields in the medium. As SLPs significantly increase the interaction time between matter and light, they are very promising for applications in lowlight-level nonlinear optics [10] and studies in quantum many-body physics [11][12][13].As a proof-of-principle experiment, the nonlinear optical process that we considered is the all-optical switching (AOS) proposed by Ref. [14]. A weak probe and a strong coupling fields form the Λ-type configuration of electromagnetically induced transparency (EIT) [15, 16] * Electronic address: yu@phys.nthu.edu.tw and, hence, the absorption of the probe field is suppressed. The presence of a weak switching field enables the probe absorption owing to the third-order susceptibility [17,18]. If one photon ...

Slow, Stored and Stationary Light

“…Stationary light pulses maximize the interaction time and thus can provide a considerable interaction efficiency even at a single-photon level. Interaction of two stationary light pulses through the medium was experimentally demonstrated by the same group 3 years later [21].…”

“…Several ideas have been put forward here to exploit the properties of slow light for implementing strong interactions. For example, the possibility offered by EIT to operate close to atomic resonances without suffering from absorption can be exploited to enhance nonlinear optical processes in atomic media [21,[38][39][40][41][42][43][44]. Another very promising direction is to combine EIT with the so-called Rydberg atoms.…”

“…This is called stationary light. It is formed when two counter-propagating pulses of light are driving the same spin excitations of a properly prepared atomic medium [17][18][19][20][21]. This corresponds to having two types of racing cars, one going from the left to the right, and another one from the right to the left.…”

Electromagnetically induced transparency and nonlinear pulse propagation in a combined tripod and Λ atom-light coupling scheme

“…The EIT has various important applications in quantum and nonlinear optics, such as slow and stored light [7][8][9][10][11][12], stationary light [13,14], multiwave mixing [15][16][17], optical solitons [18][19][20][21][22][23], optical bistability [24,25] and Kerr nonlinearity [26][27][28][29][30]. Using the slow light greatly enhances the light-matter interaction and enables nonlinear optical processes to achieve significant efficiency even at a singlephoton level [26,[31][32][33][34][35][36][37][38].…”

Dynamical observations of self-stabilizing stationary light

“…In fact, true standing wave control fields can cause unwanted coupling between counter-propagating fields and additional decay of the SL pulse [25,26,27,28]. The behaviour of EIT stationary light is well understood [29,30,31,32,33,34,35,36,37], and the interaction between SL and a stored light pulse has been demonstrated [38].…”