Expanding our view of the Universe into the 'quantum realm'
We can now ‘see’ the Universe in several ways. With visible light we can see in greater detail as we build ever more powerful telescopes, but we also have instruments that detect signals from radio waves, microwaves, and even gravitational waves.
These instruments have allowed researchers to discover some incredible things, including mysterious ‘fast radio bursts’ whose origins are still debated, the heat left over from the Big Bang, and the results of merging black holes.
Researchers at the Hong Kong University of Science and Technology (HKUST) are going a step further, taking light-based (optical) observation into the quantum realm. The new camera, called Single Photon Imager for Nanosecond Astrophysics (SPINA), is being built by Albert Wai Kit Lau under the supervision of Professor George Smoot at HKUST, along with collaborators from Nazarbayev University, the University of California, Berkeley, and the University of Paris.
Sensing each quantum of light
Light has characteristics of both particles and waves. Scientists call the smallest unit of light – each ‘quantum’ – a photon. Normal cameras, like the ones in our phones, usually only detect the intensity of light.
The SPINA camera will map the properties of single photons, including the time they arrived at the camera, their position when they strike the sensors, and the number of photons that come together. When combined with suitable optics and data analysis algorithms, SPINA will explore the quantum properties of photons, like the degree of coherence and spin angular momentum.
This information will vastly improve optical observation, allowing researchers to capture information about events happening on extremely fast timescales – from milliseconds down to the nanosecond range.
As well as detecting faster events, the SPINA system could reveal more about the source of each light emission, revealing more about the essential nature of phenomena including hot stars and magnetar flares to catastrophic neutron stars and the discs surrounding black holes.
Where to look
There are several targets for the camera at the top of Lau’s list, including the mysterious fast radio bursts (FRBs). So far only detected in the radio realm, these strange signals repeat pulses ranging in length from a fraction of a millisecond to three seconds. There has been no consensus on the source of these signals, though likely candidates include magnetars or other kinds of neutron stars, which are made of incredibly dense matter and rotate quickly. SPINA could reveal whether these signals have an optical component, providing more clues as to their origins.
This new camera would be helpful when we already know an object we want to study, but what about discovery, i.e., finding things we weren’t looking for? One way is to point the camera at a relatively noise-free part of the sky – away from the Moon or the centre of the galaxy – and collect a lot of data. It would take an exceptional stroke of luck to catch rare events though, like the death of a star.
An array of cameras then is one potential aim: a series of the instruments covering a wider field of sky. Proving the SPINA system works and ensuring the cost-effectiveness of each unit may lead to this future.
First tests
For the moment, Lau has been perfecting the camera’s operation. This summer, he took it to the Assy-Turgen Astrophysical Observatory in Kazakhstan and attached it to the Nazarbayev University Transient Telescope for a first test run.
This first step involves calibrating and focussing the camera. One issue with such a sensitive instrument is extracting signals from noise. For example, if you want to monitor a star and how its brightness changes, one thing that can get in the way is the Earth’s atmosphere. Lau captured a day’s worth of atmospheric data and checked whether his system could effectively remove this noise and capture simulated star brightness data.
The calibration and focus steps went well, but there are more improvements to be made before the camera can point at real astronomical objects, says Lau. These include improving the cooling and shielding, removing more noise in the measurements, and working out how to select for the data from the approximate 1gb/second the camera captures.
Ultra-cool technology
The SPINA system uses semiconductor technology, for which the noise reduces as the temperature falls. But cooling beyond a certain level with certain materials leads to a whole new phenomenon: superconductivity.
The noise-cancelling power of superconductivity is unrivalled, and it could produce far more sensitive photon detectors, though currently it is expensive to accomplish. But such systems could directly measure the energy of individual photons, which is inaccessible for semiconductor detectors.
Smoot says: “Progress in science – in materials and nanofabrication – has put all the elements in place for us to make use of quantum phenomena that appear in superconductors. Cooling a system was originally designed to get rid of noise, but now we can cool so far as to create transitions in quantum states. We have reached a new threshold.”