The world's lowest noise laser

Squeezed light source: a crystal that is illuminated with green light places photons of an infrared laser beam (not visible) in a specific order, thereby reducing the photon noise in that infrared laser. Image: Roman Schnabel / MPI for Gravitational Physics


The world's lowest noise laser: Researchers outsmart quantum physics.
Researchers at the Max Planck Institute for Gravitational Physics and Leibniz University of Hanover have produced a laser beam of especially high quality. In doing so, they have achieved a new world record in the control of photons by precisely placing the photons in a specific order.
This results in a reduction in the quantum mechanical intensity fluctuations, known as photon noise, of 90 percent. Using this extremely quite light in gravitational wave detectors can drastically increase their sensitivity. This so-called squeezed light can also be used in quantum key distribution, where a message is encrypted using a key whose security is guaranteed by quantum mechanics.

Light is not equal to light. There is the everyday light of a light bulb, laser light and squeezed laser light. The latter is particularly valuable since the intensity of the light, the number of photons, is essentially held constant over a certain amount of time. In the everyday light of a light bulb or even in standard laser beams these photons are randomly distributed. This is stipulated by the statistical nature of quantum physics. Similar to a rain shower, where many or just a few rain drops hit the ground, sometimes a bunch of photons arrive and sometimes just one. This fluctuation of the intensity, known as photon noise, perturbs especially sensitive measurements.

Physicists at the Planck Institute for Gravitational Physics (Albert Einstein Institute) and the Leibniz University of Hanover have reduced the photon noise by a factor of 10 (by 90 percent). In the technical parlance, this effect is called squeezing. With that they have set a new world record. Just a few months before, they set another world record when they moved individual photons by up to half a second in a laser beam, so as to achieve a more regular photon distribution. "The statistical nature of the quantum physics is not violated", according to Roman Schnabel, Junior Professor at the Leibniz University of Hanover and the Max Planck Institute for Gravitational Physics. "The appearance of photons remains probabilistic, however, we can connect the photons pair wise so that they arrive within regular intervals". This effect is known as entanglement.

More sensitive detectors for gravitational waves

"Using our techniques we can now increase the sensitivity of the gravitational wave detector GEO600", according to Prof. Schnabel. These methods are even being implemented in the larger US-American LIGO detector. With these detectors scientists are hunting down gravitational waves whose detection requires extremely precise measurements. Squeezed light can also be applied to the task of optical data transmission. It could be used to securely transport a secret key because any outside interference with the transmission results in the degradation of the highly ordered sequence of photons. "We are only at the beginning of investigating the applications to quantum cryptography", according to Prof. Schnabel.

Normally the intensity fluctuations in a laser beam are not apparent because each photon carries a tiny amount of energy. Physicists must look really closely in order to notice the fluctuations. This is exactly what they have done at the Albert Einstein Institute. They inject a laser beam into the gravitational wave detector GEO600 in order to measure extremely small changes in the distance between two mirrors. Their goal is to directly observe gravitational waves from the cosmos. These measurements are so sensitive, that the photon noise of the laser beam is clearly visible.
Crystal as hard drive for light

All current gravitational wave detectors use infrared laser light. Using the extremely uniform laser beams that Roman Schnabel and workers are able to produce, the detectors are greatly improved. "We reorganise the photons in the beam so that they are more uniformly distributed", explains Roman Schnabel. To this end, they enlist the help of double refraction crystals. Green light, which has half the wavelength of the infrared beam, is sent through these crystals. "The green laser prepares the crystal so that the infrared laser can be squeezed", according to Roman Schnabel. The green light polarises the crystal, causing the electron cloud of the crystal's atoms to oscillate with the frequency of the green light. In this state, the crystal can store photons from the infrared beam. This is exactly what the crystal does when the infrared beam sends a lot of photons into the crystal. The stored photons are replaced into the infrared laser beam when the photon flux becomes less. This way a more regular photon distribution is achieved.

"Using the squeezed light that we can generate, we can extend the reach of gravitational wave detectors by a factor of three", according to Roman Schnabel. This would enable the observation of black hole collisions at the edge of the universe. That is not possible with current detectors. A gravitational wave detector, such as GEO600, is composed of two perpendicular tunnels through which a laser beam is reflected back and forth. The beams are rejoined resulting in a so called interference pattern. If a gravitational wave impinges on the detector, one of the interferometer arms will be elongated while the other arm will be shortened. This changes the path length of the laser beams, thereby changing the interference pattern from the original. Sensitive measuring devices detect such small changes in the interference pattern and allow the researchers to identify a passing gravitational wave, at least in theory.

A ruler for a small fraction of the diameter of an atom

Such detection has not yet been achieved. The problem: the desired change in the interference pattern is far too small. An incident gravitational wave would only create a path length difference of one billion times smaller than the diameter of an atom. In order to detect such small changes in the path length, the laser beams that produce the interference pattern must have a very regular intensity. Any change in intensity of the interference pattern could be confused with a passing gravitational wave.

This is precisely the advantage that squeezed light offers: because of the exceedingly regular intensity of the beam, the interference pattern also remains extremely regular. As a result, the detector becomes more sensitive to weak waves and can probe deeper into space. Gravitational waves are an exciting possibility for astronomers. They are created when massive objects are accelerated, for instance when black holes coalesce or when neutron stars vibrate.

Until now there has been no direct detection of gravitational waves due to their weakness. There exist telescopes for these waves, including the German-British gravitational wave detector GEO600, the American detector LIGO and the French-Italian project, VIRGO. Their sensitivity should soon be improved by utilising, amongst other methods, squeezed light. The detection of gravitational waves would allow researchers to observe black holes, probe the innermost structure of neutron stars and unveil the riddle of dark matter. Such objects cannot be detected with normal telescopes.

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LOW-LIGHT-LEVEL DETECTORS: Silicon photomultipliers replace vacuum-tube technology.
For the past 75 years, the vacuum-tube-based photomultiplier tube (PMT) has served as the standard by which all low-light-level optical detectors are measured. Low-light sensing and the applications that have evolved from it would simply not exist without the technology behind the ubiquitous vacuum tube. In fact, many other items that are taken for granted today were made possible by the invention of the vacuum tube-the light bulb, the cathode-ray tube, the first diode, and the first computers all were made possible through the existence of the ability to evacuate a glass bulb and interact in that environment with an electric field. In each of these cases, semiconductors have either replaced or are making serious inroads to displacing their vacuum-tube equivalents.
The same migration from vacuum tubes is also occurring in the area of optical detection, as the PMT is facing competition from a silicon-based alternative known as the silicon photomultiplier (SPM). The replacement of the PMT with a more-robust and higher-performance SPM will allow the creation of more-sensitive, smaller, lower-power, and better-performing optical-detection equipment.
The silicon photomultiplier
The SPM is a novel high-gain low-light sensor created by exploiting the single-photon-counting ability of the silicon Geiger-mode avalanche photodiode as fabricated in large parallel arrays from approximately 1000 to 9000 pixels (or microcells) in size. Each of the microcells in an SPM contains a single-photon-counting Geiger-mode avalanche photodiode and a quenching resistor (see Fig. 1).
A typical single-photon-counting microcell includes a photon-counting diode and internal resistor (left). An SPM is a large array of single microcells, each of which responds to individual photons, placing a fixed charge on the output node in response to each photon (center). A 50 μm “aggressive” SPM has 302 microcells 50 μm in size in a 1 × 1 mm array, or 1930 in a 3 × 3 mm array (right).
The microcell contains a p-n diode with a specially designed amplification region generated between the anode and cathode that allows the low noise amplification of single photons into easily measurable output responses of approximately one million electrons per photon. Careful fabrication processes allow the diode to be biased above the breakdown voltage with no current flow until a photon initiates avalanche breakdown and a large current flow. By connecting the diode in series with a resistor, it is possible to create a reciprocating circuit in which single photons incident on the diode are amplified, then quenched by the voltage drop across the series resistor, and the diode recharged to its previous voltage above breakdown. This breakdown, quench, and reset cycle makes photon counting with silicon diodes possible and allows for highly accurate measurement of the quantity and arrival time of photons on the detector (see “Silicon photon-counting detectors enable next-generation imaging,” www.laserfocusworld.com/articles/245131).
The output of each microcell is connected in parallel with the outputs of the other microcells in the array. The output of an SPM is therefore the charge from each single-photon event incident on the individual microcells. This allows the SPM to have a quasi-analog output in response to the number of photons that are incident on the active area. Because of the high uniformity of the microcells, an exciting feature of the SPM is that it is possible to discriminate the number of photons incident on the detector as discrete levels on the output node. The ability to measure the single-photoelectron spectrum is a feature of the SPM that is not possible with PMT detectors, which have much more variability in the output response. What is truly novel in the SPM is that the high internal gain of the single microcells simplifies the external circuitry and the requirements on external amplification. This allows the fast (subnanosecond) timing response of the photon-counting microcell to be preserved.
There are several parameters that are important in assessing the performance of an SPM in comparison to a PMT. The most important of these are photon-detection efficiency, dynamic range, and signal-to-noise ratio (SNR).
Photon-detection efficiency
Because an SPM is made up of many individual photon-counting microcells, there is a trade-off between the dynamic range and the fill factor, and thus the photon-detection efficiency (PDE). Simply put, the more microcells packed into an SPM array, the higher the dynamic range, as there are more active microcells. However, because a quenching resistor is required for each microcell, there are limitations to the filling factor that can be achieved. The quenching resistor for each microcell takes up a small but significant fraction of space in the microcell and therefore reduces the overall photon-detection efficiency of the SPM array. The PDE is therefore a fundamental parameter of the SPM detector. The PDE represents the efficiency of the SPM at converting optical signals to a measurable electrical response. In an SPM, the PDE is equal to the quantum efficiency (QE) of the individual microcell (which approaches 50%) multiplied by the fill factor, or the ratio of active to nonactive silicon areas (see Fig. 1).
Typical PDE for 50 μm SPM detectors can be compared with those for a high-performance PMT typically used in life-sciences imaging (see Fig. 2). The SPM has a higher PDE in most of the visible spectrum; there is room in the silicon SPM structure to further optimize for blue and red wavelengths. Different active areas yield different fill factors and numbers of active microcells. A 20 μm circular microcell structure will allow an approximately 18% fill factor with 640 individual microcells, while a more-aggressive 20‑μm-square structure allows a fill factor of about 34% and 1144 microcells in a 1 × 1 mm SPM array. Moving to a 50 μm internal diameter for a microcell allows a fill factor of greater than 70% to be achieved with 302 individual microcells in a 1 × 1 mm SPM. In a 3 × 3 mm SPM, the number of cells increases with the area from 4496 for a 20 μm “standard” structure to 8640 for a 20 μm “aggressive” structure; 1930 microcells for a 50 μm aggressive structure are achievable in the larger SPM array size.

Photon-detection efficiency (left) of a 50 μm aggressive SPM detector (red circles) is compared to that for a high-performance PMT detector (green triangles). The signal-to-noise (SNR) ratio (right) of a PMT (dotted blue line) is compared to that of an SPM (solid red line) and a p-i-n photodiode (dashed green line). The SPM has a higher SNR across a wide dynamic range; the SNR can be improved with further development.


Dynamic range
Considerations for the dynamic range depend on whether the optical signal is pulsed or continuous-wave. In a pulsed measurement, the dynamic range is a pure function of the number of microcells that are in the array. For applications such as nuclear medicine, high-energy physics, and radiation detection, the SPM is typically coupled with a scintillator that converts the high-energy signal pulses to visible photons. For these applications, the dynamic range of the SPM must be optimized to allow enough microcells to be available in the array to detect the signal with adequate resolution. It is possible to trade off dynamic range and PDE to produce a detector optimal for each application. A subnanosecond timing response, combined with robustness, magnetic-field immunity, and high PDE make the SPM well-suited for these applications in comparison to PMTs.

In a continuous-wave application, the signal is typically read out over longer time periods ranging from a microsecond to seconds. Therefore the dynamic range is a function of the number of microcells that can be activated in the time period, allowing for the minimal dead time (typically less than 50 to 100 ns) for the microcell to recover from each photon event. For the most-aggressive 20 μm structures (with 8640 microcells), this represents dynamic ranges of up to 1.72 × 1011 photons per second with a 17% PDE, corresponding to light levels up to 360 nW.

Signal-to-noise ratio
Signal-to-noise ratios have been calculated for the SPM detector in continuous-wave mode, as well as for a standard p-i-n photodiode and a PMT (see Fig. 2). The analysis assumes a current high-performance PMT, the current SensL SPM detector performance, and a high-quality p‑i-n photodiode connected to a low-noise JFET (junction-gate field-effect-transistor) amplifier.1 Because of the intrinsic low noise amplification of the SPM detector (which has a low excess-noise factor), the SPM has a much higher SNR above 5 × 10-13 W (for a wavelength of 550 nm) than is possible with standard PMT detectors and p-i-n photodiodes. Below 5 × 10-13 W, the SPM has a slightly reduced SNR compared to a PMT; this is a function of the dark current inherent to the SPM detector. This dark current is being reduced through process improvements and, as the detector fabrication matures, will allow the SPM to fully replace PMT detectors in applications such as cell imaging, biomedical detection, nuclear medicine, high-energy physics, and radiation detection.