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arXiv:0808.1666 · Europhysics Letters (EPL)

Perfect excitation of a matter qubit by a single photon in free space

Magdalena Stobińska, G. Alber, Gerd Leuchs

Published Apr 1, 2009• 172 citations
arXivHTMLPDF

Abstract

We propose a scheme for perfect excitation of a single two-level atom by a single photon in free space. The photon state has to match the time reversed photon state originating from spontaneous decay of a two-level system. Here, we discuss its experimental preparation. The state is characterized by a particular asymmetric exponentially shaped temporal profile. Any deviations from this ideal state limit the maximum absorption. Although perfect excitation requires an infinite amount of time, we demonstrate that there is a class of initial one-photon quantum states which can achieve almost perfect absorption even for a finite interaction time. Our results pave the way for realizing perfect coupling between flying and stationary qubits in free space thus opening a possibility for building scalable quantum networks.

DOI
10.1209/0295-5075/86/14007

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We propose a scheme for perfect excitation of a single two-level atom by a single photon in free space. The photon state has to match the time reversed photon state originating from spontaneous decay of a two-level system. Here, we discuss its experimental preparation. The state is characterized by a particular asymmetric exponentially shaped temporal profile. Any deviations from this ideal state limit the maximum absorption. Although perfect excitation requires an infinite amount of time, we demonstrate that there is a class of initial one-photon quantum states which can achieve almost perfect absorption even for a finite interaction time. Our results pave the way for realizing perfect coupling between flying and stationary qubits in free space thus opening a possibility for building scalable quantum networks.
The absorption of a single photon by a single two-level system (TLS) in free space is a fundamental physical process. In this contribution, we present the experimental realization of a model system to simulate the dynamics of such an absorption experiment: Comparing the absorption process of a single photon by a single TLS and the incoupling dynamics of an optical resonator, one finds an analogy between the energy stored inside the resonator and the probability of the photon to be absorbed by the TLS [1]. Both systems, resonators and TLS, respond in an equivalent way to the temporal profile of the incident light pulse. The energy storage in a resonator as well as the absorption of a single photon by a TLS can reach an unit efficiency under idealized conditions. Such an optimized process is achieved by an exponentially rising incident light field with a time constant which matches the lifetime of the system [2]. For such a perfect coupling, the end mirror of the resonator needs to have a reflectivity of unity. This corresponds to the excitation of a TLS from the complete solid angle. For a smaller reflectivity of the end mirror, the cavity simulates the absorption dynamics in the case of excitation from only a part of the solid angle. However, using real mirrors, a good analogy with excitation from a large solid angle can still be obtained by using a strongly asymmetric resonator, i.e. an incoupling mirror which has a significantly lower reflectivity than the end mirror of the cavity.
Rapid advances in quantum technology have made possible the control of quantum states of elementary material quantum systems, such as atoms or molecules, and of the electromagnetic radiation field resulting from spontaneous photon emission of their unstable excited states to such a level of precision that subtle quantum electrodynamical phenomena have become observable experimentally. Recent developments in the area of quantum information processing demonstrate that characteristic quantum electrodynamical effects can even be exploited for practical purposes provided the relevant electromagnetic field modes are controlled by appropriate cavities. A central problem in this context is the realization of an ideal transfer of quantum information between a state of a material quantum system and a quantum state of the electromagnetic radiation field which contains a small number or even a single photon only. Despite much recent work in cavity quantum electrodynamics, which has explored this problem successfully in cases in which the number of accessible electromagnetic field modes is strongly restricted by a cavity, currently still much less is known about ideal quantum state transfer between matter and few-photon quantum states of the electromagnetic field in the extreme opposite case of free space. In this contribution we review recent work which explores characteristic quantum electrodynamical phenomena governing the interaction of a material quantum system with the electromagnetic field in extreme many-mode cases which are close to cavity-free situations. For this purpose the simplest possible quantum electrodynamical situation, namely the interaction of a single material two-level system with the wave packet of a single optical photon, is investigated in free space and in situations in which a large cavity modifies the mode structure of the electromagnetic field modes. It is demonstrated that perfect excitation of an atom by a properly shaped single-photon wave packet is possible even in free space. In particular, it is demonstrated that perfect excitation of an atom can be achieved by a single optical photon which is prepared initially in an appropriate superposition of plane-wave modes, provided this photon is focused subsequently onto the absorbing atom by a parabolic cavity.
We investigate the excitation of a three-level ladder-type atom by a unidirectional field with a pair of indistinguishable photons. Starting from an analytical expression for the two-photon absorption probability, we determine the two-photon state that maximizes the population of the upper atomic state at a chosen time and show that in the limit of an infinitely long pulse, perfect excitation is possible. The optimal state is identified as the time-reversed counterpart of the two-photon state emitted in spontaneous cascade decay. We then compare this ideal excitation strategy with experimentally accessible families of states, including symmetrized Gaussian product states, temporally correlated Gaussian states, and coherent pulses. We analyze how the optimal excitation conditions depend on the ratio of atomic decay rates and on the separation of the atomic transition frequencies. For indistinguishable photons, quantum interference may shift the maxima of the marginal spectral distribution away from the atomic resonances and qualitatively modify the optimal excitation strategy. Our results clarify the role of indistinguishability and correlations in two-photon absorption and provide guidance for designing realistic excitation schemes in quantum-optical light-matter interfaces.
We prepare single photons with a temporal envelope with rising exponential shape, resembling the time-reversed version of photons from the spontaneous decay process using a parametric conversion process in a cold atomic vapor. The mechanism is based on correlated photon pair preparation and heralding of one photon by the other one after engineering the temporal envelope of the herald.1 Such a temporal single photon profile is ideal for absorption by a two level system.2, 3 We demonstrate this in an experiment showcasing the absorption by a single Rubidium atom.4
Scattering of light by matter has been studied extensively in the past. Yet, the most fundamental process, the scattering of a single photon by a single atom, is largely unexplored. One prominent prediction of quantum optics is the deterministic absorption of a travelling photon by a single atom, provided the photon waveform matches spatially and temporally the time-reversed version of a spontaneously emitted photon. Here we experimentally address this prediction and investigate the influence of the photon's temporal profile on the scattering dynamics using a single trapped atom and heralded single photons. In a time-resolved measurement of atomic excitation we find a 56(11)% increase of the peak excitation by photons with an exponentially rising profile compared with a decaying one. However, the overall scattering probability remains unchanged within the experimental uncertainties. Our results demonstrate that envelope tailoring of single photons enables precise control of the photon-atom interaction.
We describe an ideal tuneable input-output open quantum system, realizable as either a single qubit or single resonator system, which can perfectly absorb traveling single photon states of arbitrary temporal profiles. That is, the system has a time-dependent coupling parameter to an external field that can be modulated according to the temporal profile of a single photon. We obtain an explicit analytical form for the modulation function that enables perfect absorption of single photon fields with arbitrary temporal profiles by solving the QSDE describing the system being driven by a traveling single photon field. However, a practical limit on the coupling strength prohibits a faithful physical realization of the absorber, and a numerical example is included to illustrate the absorber's performance under constrained coupling strengths.