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Physical Review Letters

Detecting Magnetic Resonance through Quantum Jumps of Single Molecules

A. C. J. Brouwer, E. J. J. Groenen, Jan Schmidt

Published May 4, 1998• 44 citations
Publisher

Abstract

The fluorescence of a single molecule exhibits light and dark periods which reveal the jumping back and forth between the singlet and triplet manifold. In this paper we demonstrate how microwave-induced changes to the distribution of the triplet-residence times can be used to detect magnetic-resonance transitions between triplet sublevels and to determine their kinetics. By synchronizing resonant microwave pulses with the quantum jumps of a terrylene molecule it proved possible to record transient-nutation signals with a high contrast.

DOI
10.1103/physrevlett.80.3944

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The fluorescence of a single molecule exhibits light and dark periods which reveal the jumping back and forth between the singlet and triplet manifold. In this paper we demonstrate how microwave-induced changes to the distribution of the triplet-residence times can be used to detect magnetic-resonance transitions between triplet sublevels and to determine their kinetics. By synchronizing resonant microwave pulses with the quantum jumps of a terrylene molecule it proved possible to record transient-nutation signals with a high contrast.
Quantum emitters respond to resonant illumination by radiating electromagnetic fields. A component of these fields is phase-coherent with the driving tone, while another one is incoherent, consisting of spontaneously emitted photons and forming the fluorescence signal. Atoms and molecules are routinely detected by their fluorescence at optical frequencies, with important applications in quantum technology and microscopy. Spins, on the other hand, are usually detected by {their coherent response} at radio- or microwave frequencies, either in continuous-wave or pulsed magnetic resonance. Indeed, fluorescence detection of spins is hampered {by their low spontaneous emission rate} and by the lack of single-photon detectors in this frequency range. Here, using superconducting quantum devices, we demonstrate the detection of a small ensemble of donor spins in silicon by their fluorescence at microwave frequency and millikelvin temperatures. We enhance the spin radiative decay rate by coupling them to a high-quality-factor and small-mode-volume superconducting resonator, and we connect the device output to a newly-developed microwave single-photon counter based on a superconducting qubit. We discuss the potential of fluorescence detection as a novel method for magnetic resonance spectroscopy of small numbers of spins.
The effect of microwave radiation on the resonance fluorescence of a cloud of cold 85 Rb atoms in a magnetooptical trap is studied. The radiation frequency was tuned near the hyperfine splitting frequency of rubidium atoms in the 5S ground state. The microwave field induced magnetic dipole transitions between the magnetic sublevels of the 5S ( F =2) and 5S ( F =3) states, resulting in a change in the fluorescence signal. The resonance fluorescence spectra were recorded by tuning the microwave radiation frequency. The observed spectra were found to be substantially dependent on the transition under study and the frequency of a repump laser used in the cooling scheme.
Summary form only given. A microscopic system under continuous observation exhibits at random times sudden jumps between its states. Quantum jumps of trapped massive particles (electrons, ions or molecules) have already been observed, which is not the case of the jumps of light quanta. Here we report on the first observation of photon number quantum jumps. Microwave photons are stored in a superconducting cavity for times in the second range. They are repeatedly probed by a stream of non-absorbing atoms. An atom interferometer measures the atomic dipole phase shift induced by the non-resonant cavity field, so that the final atom state reveals directly the presence of a single photon in the cavity. Sequences of hundreds of atoms highly correlated in the same state, are interrupted by sudden state-switchings. These telegraphic signals record, for the first time, the birth, life and death of individual photons. Applying a similar QND procedure to mesoscopic fields with tens of photons opens new perspectives for the exploration of the quantum to classical boundary.
One of the experiments made possible by the trapping and cooling of a single atom is to observe the quantum jumps to a metastable state in the atom by monitoring the resonance fluorescence of a strong transition in which at least one of the states is coupled to the metastable state. When the atomic electron moves to the metastable state, the fluorescence from the strongly driven transition disappears. When the electron drops back into the strongly driven transition, the fluorescence abruptly returns. Statistical analysis of these random interruptions to the fluorescence signal implies photon antibunching in the emission of the weak photon.
A method is described whereby the time development of the coherent components within the spin sublevels of an excited triplet state can be monitored, using an observable which can measure only the total population of the sublevel manifold, such as absorbance to higher triplet states or singlet fluorescence. The approach is very similar to the ’’probe’’ pulse method recently developed for the optical detection of electron spin coherence in phosphorescent triplet states, but the procedure for extracting the value of the coherent component is different. In the present approach it is shown that the recovery of the total population of the sublevel manifold to steady state, after the pulse sequence has been applied, is sensitive to the coherence. By measuring the amplitude of the recovery curve as a function of pulse timing the lifetime of the coherent component can be obtained. Application of the method to some common coherence experiments is discussed.
We consider a single three-level atom placed inside a resonant cavity and simultaneously probed in a double-resonance ladder configuration by a relatively strong laser beam. It is shown that photocounting statistics of a fluorescence signal induced by the probe, in certain circumstances, exhibits dark periods (i.e., quantum jumps are present), reflecting the statistical properties of the field inside the cavity. Comparisons are made between the number, thermal, coherent, and squeezed states. Specific features of the latter are pointed out.