Currently, the most advanced and promising (actively developed by big companies such as Google, IBM, Intel, etc.) platform for future quantum computing and simulations is superconducting circuits. These typically consist of thin film layers of superconducting materials and Josephson junctions comprising altogether a network of resonators and qubits. They interact precisely by means of microwave photons, and the quantum state of the qubits is read out by measuring these photons. Quantum information processing occurs inside a fridge at milikelvin temperatures, since higher temperatures imply additional thermal photons which destroy coherence.
Proposals for novel quantum information processing techniques often rely on a quantum network, linking together multiple qubits or groups of qubits to enable quantum‐secure communication, novel metrology techniques, or distributed quantum computing. However, microwave frequency photons are difficult to transmit over long distances—typical attenuation in low‐loss microwave cables at 10 GHz is more than 1 dB m−1, which compares very poorly with optical fibres with losses below 0.2 dB km−1 at telecom wavelengths ( 𝜆≈1550nm, 𝑓≈193THz). The advantages of transmitting quantum information over fibers is immediately apparent.
By transmitting information in the optical telecom band, ﬁber-based quantum networks over tens or even hundreds of kilometers can be envisaged. In the optical regime, impressive technological advances have been reached in the last years, such as the first quantum communication between ground and satellites, as well as the first proof-of-principle experiments in quantum sensing. The experimental progress in this area with optical photons has been astonishing, including a 143 km quantum communication between the Spanish islands of Tenerife and La Palma, a 96 km connection between Sicily and Malta through a submarine cable, or the recent quantum communication and quantum key distribution using satellites, among others.
“In order to connect several quantum computing nodes over large distances into a quantum internet, it is therefore vital to be able to convert quantum information from the microwave to the optical domain, and back,” says Prof. Simon Groeblacher of Delft University of Technology. “This will not only be extremely interesting for quantum applications, but also for highly efficient, low-noise conversion between classical optical and electrical signals.”
However, the advances in the use of microwaves in the quantum regime for technological applications were more gradual than with optical photons. There are several technological challenges which make the control of microwave photons much subtler than optical photons. Conversion between signals in the microwave and optical domains is of great interest, particularly for connecting future superconducting quantum computers into a global quantum network. Many leading eﬀorts in quantum technologies, including superconducting qubits and quantum dots, share quantum information through photons in the microwave regime. While this allows for an impressive degree of quantum control, it also limits the distance the information can realistically travel before being lost to a mere few centimeters.
Advantages of Microwaves in Quantum radars
Quantum sensing is defined as the use of quantum systems and properties, especially entanglement, as an extra resource to perform a measurement of a physical quantity with higher accuracy or smaller number of classical resources than any classical protocol. In quantum illumination, entangled radiation could be employed to enhance the detection accuracy of a radar or to reduce the amount of photons demanded to detect the presence of a low-reflectivity object in a noisy environment (detect it without being detected).
Ordinary radar fails at low power levels that involve small numbers of microwave photons. That’s because hot objects in the environment emit microwaves of their own. In a room temperature environment, this amounts to a background of around 1,000 microwave photons at any instant, and these overwhelm the returning echo. This is why radar systems use powerful transmitters.
Entangled photons overcome this problem. The signal and idler photons are so similar that it is easy to filter out the effects of other photons. So it becomes straightforward to detect the signal photon when it returns. The quantum radar, also allows one to detect an object without being detected oneself, by making use of the additional asset provided by quantum entanglement to reduce the intensity of the signal.
In general, the idea consists in preparing a pair of entangled optical or microwave beams and irradiating the target with one of them, while preserving the other one in the lab. In comparison to classical light, the existence of quantum correlations between the two beams allows us to declare the presence or absence of the object with either a higher accuracy or less resources, theoretically achieving up to 6 dB of quantum advantage in terms of signal-to-noise ratio.
A quantum radar, generating a large number of entangled photon pairs and shooting one twin into the air, would also be capable of receiving critical information about a target, including its shape, location, speed, temperature and even the chemical composition of its paint, from returning photons.
Now, Shabir Barzanjeh at the Institute of Science and Technology Austria and a few colleagues has used entangled microwaves to create the world’s first quantum radar. Their device, which can detect objects at a distance using only a few photons, raises the prospect of stealthy radar systems that emit little detectable electromagnetic radiation.
There are important advantages of using microwaves in quantum radars. There are radars using lower frequencies, but microwaves are convenient for two main reasons: 1) Firstly, objects whose size is comparable to microwave wavelength, such as vehicles, ships, and airplanes, produce large reflections in this frequency range. The reason is that the range resolution, i.e. the accuracy ascertaining the distance of the target, is determined by the bandwidth of the signal. In this consideration, from the point of view of circuit design, a lower bandwidth is better; 2) Secondly, a narrow beam is usually required, which is afterwards used to scan around to locate the target, but the width of the beam is inversely proportional to the frequency of a given antenna.
Quantum Microwave Technology Challenges
The most important challenge when employing microwaves for quantum technologies when compared with optical photons is the requirement of cryogenics. Indeed, the thermal isolation required for photons in the gigahertz regime is much higher than in the terahertz regime. Therefore, as the energy is much smaller, many thermal microwave photons are created, which is not favorable for quantum applications. This is the fundamental reason why superconducting circuits, which typically operates in the 2 to 7 GHz regime use cryogenics at 30 mK.
Furthermore, the thermal occupancy of optical frequency channels is close to zero at room temperature, in contrast to microwave modes; a mode with a frequency of 10 GHz must be cooled below 100 mK to reduce the average photon occupancy below 1%. The availability of telecom band single photon detectors, quantum memories, and other technologies common in quantum optics experiments also suggests the need for the development of techniques for the bi‐directional transfer of quantum information between microwave and optical photons.
The use of microwave-optics-microwave transduction has been suggested but, as aforementioned, it is still not sufficiently efficient to date. Consequently, a direct quantum communication with microwaves constitutes a desirable goal. The smaller energy associated with microwave photons, which is a downside in photodetection, turns into an important advantage when energy consumption is taken into consideration. Atmosphere frequency-dependent losses contain two low-opacity windows, one in the visible spectrum, and one with even lower attenuation in the frequency range of 100 MHz-10 GHz. Especially under rainy conditions, microwaves are consequently a suitable frequency range for quantum radar and quantum communication applications.
The aforementioned fundamental difficulty due to thermal photons seems to limit possible applications of quantum microwaves to intra-fridge environments. This is sufficient for quantum computing applications with superconducting circuits, but only allows for proof-of-principle experiments in quantum communication and sensing.
Photodetectors are devices which transform photons into an electric current, usually by means of a p-n junction or the photoelectric effect. These effects very well fit with optical frequencies, what allows for the construction of photodetectors and photocounters, for flying photons, i.e. for photons which are not trapped inside a cavity. However, this approach cannot be directly applied to propagating quantum microwaves and only photodetectors with limited efficiency (or photodetectors for trapped photons) have been constructed so far.
The lack of single-photon photodetectors for propagating microwaves is one of the main challenges for any possible application of quantum microwaves in quantum communication and sensing. Physically, the reason of the difficulty in developing efficient microwave is that a microwave photon energy is four orders of magnitude smaller than the energy of its optical counterpart. Consequently, triggering out a photocurrent is obviously much more difficult in the microwave regime. Additionally, traditional applications of propagating quantum microwaves do not make use of photodetection. The lack of efficient photodetectors reduces the measurements achievable for propagating quantum microwaves to the quantification of electromagnetic field quadratures.
Such a transducer must have a high fidelity, or quantum capacity. Although error‐correcting quantum algorithms exist, they typically require an error rate less than 1% with the precise figure depending on both the scheme to be implemented and the nature of the errors. A transducer must also have a high quantum efficiency—close to one output photon must be produced for every input photon. Quantum capacity is finite only if the conversion efficiency is greater than 50%, although indirect schemes involving heralded entanglement of photons may avoid this limit.
Finally, quantum systems rapidly lose information to their environment due to decoherence. The device must therefore have enough bandwidth for sufficient information to be transmitted before it is lost—the best decoherence times for superconducting qubits approach 0.1 ms, corresponding to a bandwidth of 10 kHz.
These requirements combine to make the task a challenging one. Efficient frequency mixing cannot occur unless a significant non‐linearity is introduced. This can come from the susceptibility of a transparent material such as lithium niobate (LiNbO3), leading to an electro‐optic non‐linearity. Alternatively, more extreme non‐linearities are found near the resonances of three (or more) level systems, such as rare earth ions in crystals, or rubidium vapors. The non‐linearity can also emerge due to indirect coupling mediated by another mode, such as mechanical or piezoelectric vibrations or magnetostatic modes. The effect of non‐linearities can be increased by placing the material in resonant cavities, where they experience both an enhanced photon interaction time, and a modified density of optical states.
Quantum radar technology
One of the approach avoids the problem of the microwave photodetector by using a microwave-to-optics transducer and employing optical photodetectors. Unfortunately, such a transducer has turned out to be as technologically demanding as photodetection in microwaves, and the efficiency of current implementations is not at all sufficient for practical applications. Other theory proposals in the microwave regime aim at the detection of cloaked objects or use quantum estimation techniques to obtain the optimal observables to measure.
To make a quantum radar, the particles needed are entangled photons in the microwave band. Researchers led by Stefano Pirandola have proposed that this could be achieved by interconverting microwave and optical signals using a device called an electro-optomechanical converter.
At the core of the proposed method is a means of interconverting microwave and optical signals using a so-called electro-optomechanical converter. This device would consist of optical and microwave cavities for storing each kind of radiation, with a nanoscale vibrating object (such as a piezoelectric crystal or a metallic membrane) serving as the connection between the two. The oscillator can couple electromagnetic vibrations in the two cavities, despite their different frequencies.
In the first step of their proposed experiment, a converter entangles the radiation in the two cavities, and the microwaves are then released as the probe beam, while the visible photons become the idler beam. If the probe bounces off the target and returns to the device, a second electro-optomechanical converter converts it to a visible-light beam, which is then allowed to interfere with the idler in a detector. The researchers call their scheme “quantum radar.”
Such a technique extends the powerful protocol of quantum illumination to its more natural spectral domain, namely microwave wavelengths. For any given transmission energy, this kind of quantum illumination setup would be more efficient than a classical microwave radar. The authors claim their system would be ideal for low-reflectivity objects embedded in a bright thermal background. A radar like this would come in handy for detecting stealthy targets that barely stand out against the background of the sky.
That would open up new opportunities for non-invasive detection, for example using magnetic-resonance methods such as NMR and MRI for looking at delicate biological samples where absorption of radiation must be minimized. Because the quantum radar operates at much lower energies than conventional systems, it has long-term potential for a range of applications in biomedicine. “Low energy means noninvasive. And noninvasiveness is the key point for this biomedical application,” Pirandola says.
Researchers led by Delft University of Technology advance microwave to optics converter
Researchers led by Delft University of Technology personnel have made two steps in the conversion of quantum states between signals in the microwave and optical domains. This is of great interest for connecting future superconducting quantum computers into a global quantum network. In Oct 2019, they reported on their findings in Nature Physics and in Physical Review Letters.
Several promising approaches have been taken to realize a microwave to optics converter, for instance by trying to couple the signals through a mechanical system (oscillator). But they have so far all operated with a substantial thermal noise background. “We have overcome this limitation and demonstrated coherent conversion between GHz microwave signals and the optical telecom band with minimal thermal background noise,” Moritz Forsch, one of the two lead authors on the publications, explains.
To achieve this, it was necessary to cool the mechanical oscillator into the quantum ground state of motion. The low thermal occupation forms the basis for quantum control over mechanical states. Rob Stockill, the other lead author, continues: “We use an integrated, on-chip electro-opto-mechanical device that couples surface acoustic waves driven by a resonant microwave signal to an optomechanical crystal. We initialize the mechanical mode in its quantum ground state, which allows us to perform the transduction process with minimal added thermal noise, while maintaining that microwave photons mapped into the mechanical resonator are eﬀectively upconverted to the optical domain.”
Groeblacher’s team has recently made another step forward in this field, by focusing on the use of novel piezoelectric materials. These materials, in which electrical fields are produced due to mechanical stress, could be of great interest for the transduction of quantum information between different carriers. The electromechanical coupling in principle allows for transduction of a quantum state between the microwave and optical frequency domains in this material. A promising approach is therefore to build integrated piezoelectric opto-mechanical devices, that are then coupled to microwave circuits.
“We have designed and characterized such a piezoelectric optomechanical device fabricated from gallium phosphide, in which a 2.9 GHz mechanical mode is coupled to a high quality factor optical resonator in the telecom band. The large electronic bandgap and the resulting low optical absorption of this new material, on par with devices fabricated from silicon, allows us to demonstrate quantum behavior of the structure,” says Prof. Groeblacher.
The device fabricated from gallium phosphide (GaP) far surpasses current achievements in GaAs or other piezoelectric materials typically used in similar approaches. The next step for the researchers is to build upon the successful operation of the GaP device in this parameter regime and further investigate the use of this exciting material. Given the wide electronic bandgap and piezoelectric properties of GaP, these research results open the door for novel quantum experiments as well as the potential for using such devices for microwave-to-optics conversion of single photons. The publication in Nature Physics was a collaboration between Delft University of Technology, the University of Vienna, Eindhoven University of Technology and NIST.
Researchers Demonstrate Microwave-Optical Entanglement via Mechanical Interface
Using lasers, researchers at the Niels Bohr Institute at the University of Copenhagen have developed a way to entangle electromagnetic fields from microwave radiation and optical beams. Creating entanglement between microwave and optical fields could help scientists solve the challenge of sharing entanglement between two distant quantum computers operating in the microwave regime. According to the researchers, one of today’s most advanced quantum systems is based on superconducting circuits, which work in the microwave regime. However, connecting these computers to provide a quantum network is challenging because microwaves can’t propagate far without loss, which is harmful to quantum computing tasks.
One way to alleviate this problem, the researchers said, is to first entangle microwaves with optical fields, then use optical links, which have far lower loss, for long-distance communication. However, due to the differences in wavelengths (mm for microwaves and μm for light), this conversion is not easy. When an electromagnetic field such as a laser beam is reflected off a vibrating object, it can read out the vibration. An electromagnetic field is composed of photons, which bombard the object as light is bounced off it, leading to additional vibration known as quantum backaction. Reflection of two electromagnetic fields upon the same mechanical object provides an effective interaction between the fields.
The researchers entangled two laser beams by bouncing them off the same mechanical resonator (a tensile membrane). They used a 3.6-mm × 3.6-mm × 20-nm membrane made of silicon nitride, and pierced it with a pattern of holes that isolated the motion of the central pad, making the device sensitive enough to show quantum backaction. They shined two lasers on the membrane simultaneously, so that each laser was able to “see” the quantum backaction of the other. This generated strong correlations, leading to entanglement between the two lasers. “You could say that the two lasers ‘talk’ through the motion of the membrane,” researcher Junxin Chen said.
“The membrane oscillator functions as an interaction media, because the lasers don’t talk to each other directly — the photons don’t interact themselves, only through the oscillator,” Chen said. “The interaction between photons and the membrane is wavelength-independent, allowing in principle microwave-optical entanglement.” Entanglement is preserved from the cryogenic mechanical mediator all the way to the laser beams analyzed in room-temperature homodyne detectors. This could make it possible for a class of hybrid quantum systems based on mechanical interfaces to harness entanglement between solid-state quantum systems, typically operating at low temperatures, and itinerant optical fields.
Further experimental work will be necessary — in particular, operation of the membrane at a temperature close to absolute zero, the temperature at which superconducting quantum computers work today. Mechanically mediated microwave-optical entanglement, based on membrane electro-optomechanical systems, could deliver a much-needed resource for networks of quantum computers based on superconducting qubits.
Longest microwave quantum link
Physicists at ETH Zurich have demonstrated a five-meter-long microwave quantum link, the longest of its kind to date. It can be used both for future quantum computer networks and for experiments in basic quantum physics research.
Collaboration is everything—also in the quantum world. To build powerful quantum computers in the future, it will be necessary to connect several smaller computers to form a kind of cluster or local network (LAN). Since those computers work with quantum mechanical superposition states, which contain the logical values “0” and “1” at the same time, the links between them should also be “quantum links.”
The longest such link to date based on microwaves, at five meters long, was recently built in the laboratory of Andreas Wallraff, professor at the Quantum Device Lab at ETH Zurich. “That’s really a milestone for us,” Wallraff explains, “since now we can show that quantum-LANs are possible in principle. In the next 10 to 20 years, quantum computers will probably increasingly rely on them.” Currently there are computers with a few dozen quantum bits or qubits, but several hundreds of thousands of them are almost impossible to accommodate in existing devices. One reason for this is that qubits based on superconducting electrical oscillators, such as those used in the quantum chips in Wallraff’s lab (and also by IBM and Google), need to be cooled down to temperatures close to the absolute zero of -273,15 degrees Celsius. This supresses thermal perturbations that would cause the quantum states to lose their superposition property—this is known as decoherence—and hence errors in the quantum calculations to occur.
“The challenge was to connect two of those superconducting quantum chips in such a way as to be able to exchange superposition states between them with minimal decoherence,” says Philipp Kurpiers, a former Ph.D. student in Wallraff’s group. This happens by means of microwave photons that are emitted by one superconducting oscillator and received by another. In between, they fly through a waveguide, which is a metal cavity a few centimeters in width, which also needs to be strongly cooled so that the quantum states of the photons are not influenced.
Each of the quantum chips is cooled down over several days in a cryostat (an extremely powerful refrigerator), using compressed and also liquid helium, to a few hundredths of a degree above absolute zero. To that end, the five-meter waveguide that creates the quantum link was equipped with a shell consisting of several layers of copper sheet. Each of those sheets acts as a heat shield for the different temperature stages of the cryostat: -223 degrees, -269 degrees, -272 degrees and finally -273,1 degrees. Altogether, those heat shields alone weigh around a quarter of a tonne.
“So, this is definitely not a “table-top” experiment anymore that one can put together on a small workbench,” Wallraff says. “A lot of development work has gone into this, and ETH is an ideal place for building such an ambitious apparatus. It’s a kind of mini-CERN that we first had to build over several years in order to be able to do interesting things with it now.” Apart from the three Ph.D. students who carried out the experiments, several engineers and technicians, also in the workshops at ETH and at the Paul Scherrer Institute (PSI), were involved in producing and constructing the quantum link.
The physicists at ETH not only showed that the quantum link can be sufficiently cooled down, but also that it can actually be used to reliably transmit quantum information between two quantum chips. To demonstrate this, they created an entangled state between the two chips via the quantum link. Such entangled states, in which measuring one qubit instantaneously influences the result of a measurement on the other qubit, can also be used for tests in basic quantum research. In those “Bell tests,” the qubits must be far enough apart from each other, so that any information transfer at the speed of light can be ruled out.
While Wallraff and his collaborators are performing experiments with the new link, they have already started working on even longer quantum links. Already a year ago they were able to sufficiently cool down a ten-meter link, but without doing any quantum experiments with it. Now they are working on a 30-meter quantum link, for which a room at ETH has been specially prepared.
Demonstration of quantum-enhanced noise radar
Researchers at the Institute for Quantum Computing (IQC) performed the first demonstration of quantum-enhanced noise radar, opening the door to promising advancements in radar technology. The researchers showed how the quantum process can outperform a classical version of the radar by a factor of 10, enabling the detection of objects that are faster, smaller, or further away – all while making the radar less detectable to targets.
“We are applying technology developed for quantum computing to immediate, practical situations,” said Christopher Wilson, a professor in the Department of Electrical and Computer Engineering at the University of Waterloo and principal investigator of the Engineered Quantum Systems Lab (EQSL) at IQC. “Our results show a promising improvement for radar, an important real-world application, using quantum illumination.”
In the lab, Wilson’s team performed a proof-of-principle radar detection experiment to directly compare the performance of a quantum protocol to a classical protocol. The researchers generated entangled photons using a device they designed to produce multiphoton entanglement of microwave light at frequencies near 5 GHz—the same frequency band as wireless communications used by cellphones and Wi-Fi connections.
Next, they created a classical source of photons that, on the surface, replicated the signals produced by the quantum device, but without the entanglement. When the photons from each source were sent through the detection scheme, in a head-to-head comparison between the quantum and classical protocols, the researchers found that the quantum source outperformed the classical source by a factor of 10.
They found that the improved performance occurred only when the signal levels were around the single-photon level, which is much weaker than what a typical radar system uses. While there are clear technical paths to improve the signal power, Wilson notes, “There is an enhancement when your signal power is inherently small, so this has potential applications in situations where the user doesn’t want the subject to know they are being tracked.”
The experiment marked a milestone as the first demonstration of quantum illumination in the microwave regime. “This is exciting because it is the same frequency that most radar systems operate at, meaning there could be more immediate, practical applications for current radar technology,” explained Wilson.
It also shows the potential for quantum microwaves to have real-world applications outside of the cryostat at room temperature, an exciting prospect for Wilson: “Understanding why this actually works could be a really important step in unlocking more applications for quantum microwaves.”
Quantum-Enhanced Noise Radar, in collaboration with the Université de Sherbrooke and Defence Research and Development Canada (DRDC), appeared as the cover article of Applied Physics Letters on March 18. This research has been undertaken in part thanks to the Canada First Excellence Research Fund (CFREF).
Quantum radar has been demonstrated for the first time
The device is simple in essence. The researchers of Institute of Science and Technology Austria created pairs of entangled microwave photons using a superconducting device called a Josephson parametric converter. They beam the first photon, called the signal photon, toward the object of interest and listen for the reflection. In the meantime, they store the second photon, called the idler photon. When the reflection arrives, it interferes with this idler photon, creating a signature that reveals how far the signal photon has traveled.
Of course, entanglement is a fragile property of the quantum world, and the process of reflection destroys it. Nevertheless, the correlation between the signal and idler photons is still strong enough to distinguish them from background noise.
This allows Barzanjeh and co to detect a room temperature object in a room temperature environment with just a handful of photons, in a way that is impossible to do with ordinary photons. “We generate entangled fields using a Josephson parametric converter at millikelvin temperatures to illuminate a room-temperature object at a distance of 1 meter in a proof of principle radar setup,” they say.
The researchers go on to compare their quantum radar with conventional systems operating with similarly low numbers of photons and say it significantly outperforms them, albeit only over relatively short distances.That’s interesting work revealing the significant potential of quantum radar and a first application of microwave-based entanglement. But it also shows the potential application of quantum illumination more generally.
A big advantage is the low levels of electromagnetic radiation required. “Our experiment shows the potential as a non-invasive scanning method for biomedical applications, e.g., for imaging of human tissues or non-destructive rotational spectroscopy of proteins,” say Barzanjeh and co.
Then there is the obvious application as a stealthy radar that is difficult for adversaries to detect over background noise. The researchers say it could be useful for short-range low-power radar for security applications in closed and populated environments.
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