Measurement connects the world of quantum phenomena to the world of classical events. It has both a passive role—in observing quantum systems—and an active one, in preparing quantum states and controlling them. In quantum physics, a measurement is the testing or manipulation of a physical system in order to yield a numerical result. The predictions that quantum physics makes are in general probabilistic. Quantum measurements are widely used in characterising new materials and devices for emerging quantum technology applications such as quantum information processing (QIP), quantum computing (QC) and quantum sensing. Such devices hold the potential to revolutionise future technology in high-performance computing and sensing in the same way that semiconductors and the transistor did over half a century ago.
Quantum metrology, meanwhile, would benefit from a true single-photon source because its signal-to-noise ratio would not be restricted by lasers’ “shot-noise” limit (which is equal to the square root of the laser intensity). They would greatly support the emergence of applications of quantum technology such as entanglement assisted measurement techniques, i.e. sub-shot noise metrology, microscopy and spectroscopy.
Any quantum computer requires an integrated hardware approach using significant conventional hardware to enable qubits to be controlled, programmed, and read out. Therefore Quantum hardware can be conceptualized and modelled in four abstract layers: the “quantum data plane,” where the qubits reside; the “control and measurement plane,” responsible for carrying out operations and measurements on the qubits as required; the “control processor plane,” which determines the sequence of operations and measurements that the algorithm requires, potentially using measurement outcomes to inform subsequent quantum operations; and the “host processor,” a classical computer that handles access to networks, large storage arrays, and user interfaces. This host processor runs a conventional operating system/user interface, which facilitates user interactions, and has a high bandwidth connection to the control processor.
The control and measurement plane converts the control processor’s digital signals, which indicates what quantum operations are to be performed, to the analog control signals needed to perform the operations on the qubits in the quantum data plane. It also converts the analog output of measurements of qubits in the data plane to classical binary data that the control processor can handle.
Whilst quantum effects are typically prevalent only at extremely small scales and dominate the interactions between individual atoms, physicists are now working towards enabling these effects at larger scale, working towards mesoscale devices. Ultra-low temperatures close to absolute zero are required in these devices to reduce thermal noise and reveal the hidden quantum states. Quantum measurements themselves are used to characterise the properties of these devices and cover a wide range of techniques, from spectroscopy to electrical properties.
Quantum transport measurements such as the quantum Hall effect (QHE) and fractional quantum Hall effect (FQHE) in two-dimensional electron gases (2DEG) and topological insulators – along with a range of other more complex measurements – inform researchers on material properties with ultimate precision leading to primary standards. Oxford Instruments offer a wide range of low-temperature measurement solutions to enable these complex measurements. This not only includes robust cold environment solutions but also enable a wide variety of electronic optical and magnetic field measurements through our long history of technology expertise in cryogenics, superconducting magnets and complex quantum measurements.
Other applications to exploit sources of single photons could include quantum computing, where photons would play the role of quantum bits. It has also been shown that the availability of a single-photon source enables implementation of quantum computation using only linear optical elements and photodetectors.
Sensing and information analysis breakthroughs, Tay Fitzgerald, Acting Vice President of ACT, Raytheon Intelligence & Space explained, are often equally if not more impactful than other innovations by virtue of the scale and scope of their impact across platforms. For instance, she referred to how, working with academia, scientists at ACT developed a new microwave radiation detector 100,000 times more sensitive than existing technologies. The sensor technology, Raytheon ACT data explains, brings large implications for satellites, radar, and laser systems. Called a bolometer, the sensor achieves its margin of difference through the use of a material called graphene and a device which “acts as a semiconductor switch to detect infrared radiation at very high speeds with very high sensitivity,” a Raytheon essay states.
Quantum photon detectors
The first detector was a commercial single-photon counting module based on a silicon avalanche photodiode (APD). It has two detection outcomes, either outputting an electronic pulse (1 click) or not (0 clicks). Past evaluation of the detector has shown that the 1-click outcome is mainly associated with the arrival of one or more photons, although dark counts and afterpulsing can also create this outcome. The 0-click event is mainly associated with vacuum at the input or photons lost owing to non-unit efficiency of the photodiode. Having only two outcomes, this detector cannot directly measure the incoming photon number if it is above one.
The second detector was time-multiplexed detector (TMD) that circumvents this by splitting the incoming pulse into many spatially or temporally separate bins, making unlikely the presence of more than one photon per bin. Subsequently all of the bins are detected with two APDs. Photon-number resolution results by summing the number of 1-click outcomes from all of the bins.
Measuring the energy of qubits is at the heart of how quantum computers operate. Most quantum computers currently measure a qubit’s energy state by measuring the voltage induced by the qubit. However, there are three problems with voltage measurements: firstly, measuring the voltage requires extensive amplification circuitry, which may limit the scalability of the quantum computer; secondly, this circuitry consumes a lot of power; and thirdly, the voltage measurements carry quantum noise which introduces errors in the qubit readout.
A bolometer measures the energy of incoming radiation by determining how much the radiation heats up a material. Bolometers capable of detecting single microwave photons would be very useful in creating quantum computers and other technologies that use superconducting quantum bits (qubits). This is because superconducting qubits interact via microwaves and single photons provide a very efficient way of transferring quantum information between qubits.
Two graphene-based bolometers that are sensitive to detect single microwave photons have been built by independent teams of physicists. The devices could find a range of applications in quantum technologies, radio astronomy and even in the search for dark matter. Quantum computer researchers hope that by using bolometers to measure qubit energy, they can overcome all of these complications, and now Professor Möttönen’s team have developed one that is fast enough and sensitive enough for the job.
One bolometer was created in Finland by Mikko Möttönen and colleagues at Aalto University and VTT Technical Research Centre of Finland, while the other was created by an international team led by Kin Chung Fong at Raytheon BBN Technologies in the US.
Physicists at Aalto University and VTT Technical Research Centre of Finland have developed graphene bolometer detector to measure qubit energy which they hope can overcome all of these complications, and now Professor Möttönen’s team have developed one that is fast enough and sensitive enough for the job.
A new article shows potential for graphene bolometers to become a game-changer for quantum technology
Professor Mikko Möttönen’s Quantum Computing and Devices group at Aalto has been developing their expertise in bolometers for quantum computing over the past decade, and have now developed a device that can match current state-of-the-art detectors used in quantum computers.
So far, however, creating single-photon microwave detectors has been difficult because of the relatively low energies of microwave photons. The Finnish team had addressed the low-energy problem by creating a bolometer that used a gold-palladium alloy to absorb photons. While this device operates at very low noise levels, it is not fast enough to be useful when it comes to measuring the state of a superconducting qubit.
The breakthrough in this new work was achieved by swapping from making the bolometer out of gold-palladium alloys to making them out of graphene. To do this, they collaborated with Professor Pertti Hakonen’s NANO group — also at Aalto University — who have expertise in fabricating graphene-based devices. Graphene has a very low heat capacity which is a measure of the energy required to raise the temperature of a material by one degree. It is possible to detect very small changes in its energy quickly, and this speed in detecting the energy differences that makes it perfect for a bolometer with applications in measuring qubits and other experimental quantum systems. By swapping to graphene, the researchers have produced a bolometer that can make measurements in well below a microsecond, as fast as the technology currently used to measure qubits.
“Changing to graphene increased the detector speed by 100 times, while the noise level remained the same,” explains team member Pertti Hakonen. Indeed, the new device can make a measurement in less than microsecond, which is on par with technology that is currently used to measure the state of qubits. Hakonen adds, “After these initial results, there is still a lot of optimization we can do to make the device even better”.
Physicists at Aalto University and VTT Technical Research Centre of Finland have developed a new detector for measuring energy quanta at unprecedented resolution. This discovery could help bring quantum computing out of the laboratory and into real-world applications. The results have been published today in Nature.
‘It is amazing how we have been able to improve the specs of our bolometer year after year, and now we embark on an exciting journey into the world of quantum devices,’ says Möttönen. ‘Bolometers are now entering the field of quantum technology and perhaps their first application could be in reading out the quantum information from qubits. The bolometer speed and accuracy seems now right for it,’ says Professor Möttönen.
Now that the new bolometers can compete when it comes to speed, the hope is to utilise the other advantages bolometers have in quantum technology. While the bolometers reported in the current work performs on par with the current state-of-the-art voltage measurements, future bolometers have the potential to outperform them. Current technology is limited by Heisenberg’s uncertainty principle: voltage measurements will always have quantum noise, but bolometers do not. This higher theoretical accuracy, combined with the lower energy demands and smaller size — the graphene flake could fit comfortably inside a single bacterium — means that bolometers are an exciting new device concept for quantum computing.
The next steps for their research is to resolve the smallest energy packets ever observed using bolometers in real-time and to use the bolometer to measure the quantum properties of microwave photons, which not only have exciting applications in quantum technologies such as computing and communications, but also in fundamental understanding of quantum physics.
Many of the scientists involved in the researchers also work at IQM, a spin-out of Aalto University developing technology for quantum computers. “IQM is constantly looking for new ways to enhance its quantum-computer technology and this new bolometer certainly fits the bill,” explains Dr Kuan Yen Tan, Co-Founder of IQM who was also involved in the research.
Bolometers are devices that measure the power of incident electromagnetic radiation thru the heating of materials, which exhibit a temperature-electric resistance dependence. These instruments are among the most sensitive detectors so far used for infrared radiation detection and are key tools for applications that range from advanced thermal imaging, night vision, infrared spectroscopy to observational astronomy, to name a few. Even though they have proven to be excellent sensors for this specific range of radiation, the challenge lies in attaining high sensitivity, fast response time and strong light absorption, which not always are accomplished all together. Many studies have been conducted to obtain these higher-sensitivity bolometers by searching to reduce the size of the detector and thus increase the thermal response, and in doing so, they have found that graphene seems to be an excellent candidate for this.
Fong and colleagues created a bolometer in which graphene is integrated within a superconducting device called a Josephson junction. When the graphene warms up by absorbing a microwave photon, it affects the electrical current flowing through the Josephson junction – thus creating the detection signal. This device is a whopping 100,000 times faster than microwave bolometers based on other materials.
If we focus on the infrared range, several experiments have demonstrated that if you take a sheet of graphene and place it in between two layers of superconducting material to create a Josephson junction, you can obtain a single photon detector device. At low temperatures, and in the absence of photons, a superconducting current flows through the device. When a single infrared photon passes through the detector, the heat it generates is enough to warm up the graphene, which alters the Josephson junction such that no superconducting current can flow. So you can actually detect the photons that are passing through the device by measuring the current. This can be done basically because graphene has an almost negligible electronic heat capacity. This means that, contrary to materials that retain heat like water, in the case of graphene a single low-energy photon can heat the detector enough to block the superconducting current, and then dissipate quickly, allowing the detector to rapidly reset, and thus achieving very fast time responses and high sensitivities.
Team of scientists which includes ICFO researcher Dmitri Efetov, together with colleagues from Harvard University, Raytheon BBN Technologies, MIT, and the National Institute for Material Sciences, has been able to develop a graphene-based bolometer that can detect microwave photons at extremely high sensitivities and with fast time responses. Just like with the infrared range, the team took a sheet of graphene and placed it in between two layers of superconducting material to create a Josephson junction. This time, they went an entirely new route and attached a microwave resonator to generate the microwave photons and by passing these photons through the device, were able to reach an unprecedented detection levels. In particular, they were able to detect single photons with a much lower energy resolution, equivalent to that of a single 32 Ghz photon, and achieve detection readouts 100.000 times faster than the fastest nanowire bolometers constructed so far.
The results achieved in this study mean a major breakthrough in the field of bolometers. Not only has graphene proven to be an ideal material for infrared sensing and imaging, but it has also proven to span to higher wavelengths, reaching the microwave, where it has also shown to attain extremely high sensitivities and ultra-fast read out times. As Dmitri Efetov comments “such achievements were thought impossible with traditional materials, and graphene did the trick again. This open entirely new avenues for quantum sensors for quantum computation and quantum communication”.
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