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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

A single molecule as a high-fidelity photon gun for producing intensity-squeezed light

Xiao‐Liu Chu, Stephan Götzinger, Vahid Sandoghdar

Published Jun 1, 2017• 3 citations
Publisher

Abstract

Summary form only given. One efficient approach for generating single photons is based on using a two-level atom, which inherently cannot emit more than one photon at the time. As early as the 1980s, this quantum feature was identified as the gateway to creating single photon sources (SPS's), where a regular excitation sequence would produce a stream of photons with photon number fluctuations below the shot noise. Such an ideal intensity squeezed SPS would be desirable for a range of applications, including quantum imaging, sensing, enhanced precision measurements and information processing.

DOI
10.1109/cleoe-eqec.2017.8087610

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Summary form only given. One efficient approach for generating single photons is based on using a two-level atom, which inherently cannot emit more than one photon at the time. As early as the 1980s, this quantum feature was identified as the gateway to creating single photon sources (SPS's), where a regular excitation sequence would produce a stream of photons with photon number fluctuations below the shot noise. Such an ideal intensity squeezed SPS would be desirable for a range of applications, including quantum imaging, sensing, enhanced precision measurements and information processing.
The generation of non-classical states of light [1] is an important scientific challenge with many potential applications. For example, squeezed states of light [2] have generated much interest in the past decade, because measurements can be performed with noise levels that are lower than those available with classical light. A particularly novel non-classical source of light is a triggered single-photon source: a source that has the property to emit with a high degree of certainty one (and only one) photon at a user-specified time. Such a deterministic source of single photons can be important for quantum information processing [3], quantum cryptography [4] and certain quantum computation problems [5]. A single quantum emitter such as a molecule or an atom is a good potential source because it can emit only one photon at a time. The emitted light is antibunched [6, 7] and consists of single photons separated by random time intervals, which depend on the excited state lifetime and on the pumping rate. Thus triggered emission of single photons can be obtained with controlled excitation of a single emitter. The first demonstration of a single photons source [8] using a single molecule in a solid is based on a rapid adiabatic passage method [9] to excite the molecule. In the effective spin picture, the Bloch vector follows the driving magnetic field adiabatically if the laser-molecule frequency detuning is swept slowly through the resonance. Since the passage inverts the field, the system goes from the ground to the excited state. From this fluorescent state the molecule can emit only a single photon. The molecular resonance is swept by Stark effect at fixed laser frequency. This is done easily by applying a radio frequency electric field to the molecule. The results showed photon states with statistics that differ radically from those of coherent states. But this method requires cryogenic temperatures (2K) since narrow zero phonon lines are needed for the adiabatic following. At room temperature a source of triggered single photons based on the concept of pulsed optical excitation of a single highly fluorescent molecule has been demonstrated [10]. A short pulse of green laser light pumps the molecule from the ground state to a vibronically excited level of the first electronic excited state. After fast (ps) relaxation to the lowest electronic excited state, the molecule subsequently emits a single photon. A frequency-doubled mode-locked Nd-YAG laser is used to pump highly stable single terrylene molecules contained in a p-terphenyl molecular crystal where they are protected from exposure to diffusing quenchers (such as oxygen). The result shows that the photons are emitted after the laser pulses at times separated by the laser repetition time within the lifetime of terrylene in p-terphenyl. Since the pump pulse width is very short compared to the fluorescence lifetime, the probability of two (or more) photon emission per pulse is always zero. At the maximum laser power, 90% of the laser pulses lead to a single photon emission. The parameters of this source (repetition rate and single photon generation probability) were limited only by the laser system used; nevertheless, this performance already surpasses that of previous work. The high performance combined with the vanishing two photon emission probability and the simplicity of this source suggests that it can be considered for a variety of quantum optical experiments and for secure quantum cryptography [11]. Semiconductor quantum dots (QD) are other potential systems for single photon generation [12]. QDs are often referred to as “artificial atoms” because many of their optical properties, such as ultranarrow transitions, arise from discrete, atomic-like energy levels [13, 14]. To reinforce this analogy a study of the photon antibunching in this system was needed. QDs differ from atoms and molecules since the creation of multiple excitons is possible in QDs. Consequently, if two electron-hole pairs can radiatively recombine, the simultaneous emission of two photons from a QD is then possible. Thus different regimes of photon statistics were expected because the probability of multiple exciton creation depends on excitation intensity. The fluorescence intensity correlation function of a single CdSe quantum dot has been investigated [15] using a coincidence setup (start-stop experiment). A strong photon antibunching has been observed over a large range of intensities (0.1-100 kW/cm2). The lack of coincidence at zero time delay indicates a highly efficient Auger ionization process [16], which suppresses multi-photon emission in these colloidal quantum dots. This result shows that room temperature single photon sources can be achieved with colloidal nanoparticles.
The concept of the photon , central to Einstein's explanation of the photoelectric effect, is exactly 100 years old. Yet, while photons have been detected individually for more than 50 years, devices producing individual photons on demand have only appeared in the last few years. New concepts for single-photon sources, or 'photon guns', have originated from recent progress in the optical detection, characterization and manipulation of single quantum objects. Single emitters usually deliver photons one at a time. This so-called antibunching of emitted photons can arise from various mechanisms, but ensures that the probability of obtaining two or more photons at the same time remains negligible. We briefly recall basic concepts in quantum optics and discuss potential applications of single-photon states to optical processing of quantum information: cryptography, computing and communication. A photon gun's properties are significantly improved by coupling it to a resonant cavity mode, either in the Purcell or strong-coupling regimes. We briefly recall early production of single photons with atomic beams, and the operation principles of macroscopic parametric sources, which are used in an overwhelming majority of quantum-optical experiments. We then review the photophysical and spectroscopic properties and compare the advantages and weaknesses of various single nanometre-scale objects used as single-photon sources: atoms or ions in the gas phase and, in condensed matter, organic molecules, defect centres, semiconductor nanocrystals and heterostructures. As new generations of sources are developed, coupling to cavities and nano-fabrication techniques lead to improved characteristics, delivery rates and spectral ranges. Judging from the brisk pace of recent progress, we expect single photons to soon proceed from demonstrations to applications and to bring with them the first practical uses of quantum information.
Summary form only given. On-demand single-photon sources that produce a single photon as a response to a deterministic input are widely regarded as critical light sources for quantum key distribution, quantum repeaters, and quantum information science generally [1]. The wavepackets from such a source must have low probability to contain more than one photon, exist in a pure state so they may interfere with one another, and have a spatio-temporal profile that is appropriate for their application [2]. A very popular source for generating high quality single-photon wavepackets is the quantum two-level system, an artificial atom with one ground and one excited state, often implemented using solid-state quantum dots embedded within microcavities [3]. The two-level system is excited with a short resonant pulse of width τFWHM that prepares the system in its excited state, called a π-pulse, where it subsequently decays and emits a single photon with almost unity probability. Because the typical pulse lengths are orders of magnitude shorter than the lifetime of the system, it is often not appreciated that there is finite chance for an ideal two-level system to emit more than one photon at a time.
Summary form only given.It has previously been shown that a traveling-wave optical parametric amplifier can generate multimode squeezed states of light. Homodyne detection of multimode squeezed light, under certain physical conditions, results in sub-Poissonian statistics of photons not only in time but also in the transverse plane of the light beam. Of particular relevance to applications is a situation when the cross section of the light beam contains several nonoverlapping areas with sub-Poissonian statistics of photons within each of them. Under photodetection of such a beam of light, several sub-shot-noise photocurrents can be produced according to the number of independent areas with sub-Poissonian statistics. This is in drastic contrast to the case of a single-mode sub-Poissonian light beam, where any attempt to collect light only from a part of the beam deteriorates the degree of shot-noise reduction. This property of multimode squeezed light opens a range of new interesting applications in optical imaging, optical parallel processing of information, parallel computing, and many other phenomena where it is desirable to have the regular statistics of photons across the transverse plane of the electromagnetic wave. We describe the application of multimode squeezed light in sub-shot-noise microscopy, noiseless amplifications of optical images, and optical imaging. We also consider the possibility of improving the resolution in optical imaging and microscopy beyond the Rayleigh limit via multimode squeezed light.