When it comes to measuring physical quantities, the more that quantum uncertainties can be squeezed the better. But when just one atom is involved, demonstrating less squeezing is the real challenge. See Letter p.623
Subtle is the atom, but malicious it is not. This paraphrase of Einstein's famous remark aptly summarizes the work reported by Ourjoumtsev et al.1 on page 623 of this issue. Subtle, because the atom is a quantum object that can, in principle, produce quantum light — the most immediate example being the emission of a single photon. There is a great deal beyond single photons, however: the work of the past 50 years has brought many quantum subtleties to our awareness, thus creating the field of quantum information and the quest for a quantum computer. As for malice, the atom bears none, because, as the authors demonstrate, even its most guarded subtlety, hidden for 30 years, can nevertheless be revealed by the resolute.
Early in the development of quantum mechanics, the uncertainty principle made its appearance, along with the inevitable noise that accompanies a measurement. Over the years, most physicists came to terms with the principle's pronouncement: a product of uncertainties must equal or exceed a number set by Planck's constant. The subtleties, however, begin to shine only under the scrutiny of a deeper question: the principle places no restriction on the uncertainties prior to the product2; shaping these, squeezing one and enlarging the other with the product intact, fulfils the uncertainty relationship while tempting us with the possibility of a noise-free measurement. The temptation came to be seen as a research challenge, the challenge of squeezing.
Among the many quantum objects subject to the uncertainty principle, electromagnetic radiation — light — is ubiquitous. We understand its fundamental properties and have excellent detectors with which to measure its intensity. Measurement of its oscillating field relies on interference; in this way even the fluctuations of the field can be measured. Interference brings with it the possibility of measuring the amplitude of the field at different angles, thus measuring different phases of the field. It then turns out that uncertainties in the field amplitude measured at any two perpendicular angles (in-quadrature phases) must satisfy the uncertainty relationship. For laser light, the uncertainties are equal and minimal — they correspond to perpendicular diameters of a circle. This circle of uncertainty may, in principle, be deformed into an ellipse, in any particular direction, by squeezing.
Reshaping of the uncertainty area — the product of the two uncertainties — happens only through a nonlinear process, if energy is not simply proportional to frequency, as it is for a free photon. Reshaping the circle into an ellipse produces a minor axis smaller than the circle and a major axis that is larger. Importantly, the circle of uncertainty is present even in a vacuum, and it sets a boundary between those electromagnetic fields whose fluctuations are smaller and those that are larger than the standard quantum limit.
Passing from the original realization of squeezing3 some 25 years ago to the current achievement of a squeeze of more than a factor of ten4,5, which can be used in gravitational-wave interferometers, the challenge has always been more and more squeezing. It has been a grand one, and we all look forward to further improvement. Surprisingly, the work of Ourjoumtsev et al.1 moves in the opposite direction. The authors show the atom to be subtle by demonstrating its delicate squeezing of light; by permitting the squeeze at all, after many years of anticipation6,7, the atom clearly is not malicious. This reveals that even subtle quantum-mechanical predictions are correct — and accessible to the resolute — lending confidence to the promise of quantum-information science.
The atom is subtle in terms of its size — even compared with the wavelength of the light that excites it, the atom is tiny. The demonstrated squeeze is also tiny. It is a mere 0.2% reduction of noise (fluctuation) power against the standard quantum limit — an ever-so-slight reshaping of the circle. Such a delicate squeeze calls for a clever method of detection and a degree of cooperation from the atom. All the requirements are carefully put in place in the beautiful experiment of Ourjoumtsev and colleagues1.
An atom too strongly driven by a resonant laser tends towards an interrupted emission of light — interrupted by spontaneous emission. These interruptions, or quantum jumps, create undesirable noise, so only when the laser power is weak does the atom behave in any way close to a simple antenna, radiating without interruption from a coherent dipole, a regularly oscillating distribution of positive and negative electric charge. In their study, Ourjoumtsev et al.1 used only 0.033 photons at a time to entice the atom to continuously, without interruption, produce squeezed light. They had to ensure that the atom was always set in place, just where they expected to find it, ready to receive photons, so they trapped and cooled it to prevent it from moving around too much. The atom was not trapped in empty space but between two mirrors, separated by about 1 millimetre (or, to use the technical jargon, it was trapped inside an optical cavity to which its coherent dipole, or polarization, would strongly couple).
This arrangement induced the necessary cooperation. Thus arranged, the atom and cavity formed a composite quantum entity, a 'molecule' or polariton, one-half atom and the other half photon (the light inside the cavity). This atom–light molecule is highly nonlinear and can readily produce squeezing — although being restricted to a mere fraction of a photon of excitation, only by a very little. It is remarkable that, after 30 years of waiting6, the little has been seen.
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