Control theory deals with the optimization of performance of fixed systems through regulation by a controller. Following standard engineering terminology, the designated system is called the plant, and the system used to alter the plant dynamics is the controller. The system (or “plant”): the device we want to control, having inputs and outputs; The input(s) to the system (or “control”): the entity that we have freedom to choose to affect the system; The output(s) of the system (or “yield”): this includes the quantity we want to control, and any quantities we can measure to obtain information about the plant.
Classical physics is not capable of correctly describing physical behavior at the nano scale and below. Quantum Control is defined as Control of physical systems whose behaviour is dominated by the laws of quantum mechanics. Quantum technology takes into account and/or exploits quantum behavior. Design of complex systems require some form of control. There is emerging a need a control theory that can cope with quantum models and potentially exploit quantum resources. Quantum control likely to play a fundamental role in the development of new quantum technologies.
The control is open loop if we do not use feedback; otherwise, it is closed-loop control. In Open loop – control actions are predetermined, no feedback is involved. In classical control theory, one seeks to control the output of a system by making making some measurement of the output and then using the results of the measurement to determine changes to the input that will effect the desired result.
However, in quantum mechanics, it’s not that simple, because a measurement of a quantum system will collapse it into an eigenstate of the measurement apparatus. So the measurement itself can strongly affect the quantum system, which is not the case in classical systems. There are a certain class of measurement, known as weak measurements, that do not collapse the quantum state and can be used as a feedback for affecting quantum evolution.
In the closed-loop situation, the feedback may be entirely dynamical (i.e. the coupled plant and controller form a single dynamical system). We refer to this as coherent-feedback control. Alternatively, the feedback may be entirely information theoretic insofar as the controller gains information about the plant owing to measurement of the plant. This is measurement-based control. In classical closed-loop control, the form of the information fed back in is not important and we do not make the distinction between coherent-feedback and measurement-based control. This reflects the universal nature of classical information: it does not matter if we use digital or analogue.
In quantum feedback, coherent-feedback control and measurement-based control are fundamentally different schemes owing to the non-trivial effect of the measurement process. In Wiseman’s seminal work on feedback control, he showed that feedback mediated by continuous measurements can in fact be implemented without measurements. To see how this works, let us consider two parallel mirrors between which a single mode of the electromagnetic field is trapped (the two mirrors are referred to as an “optical cavity”). The light that leaks out through one of the mirrors can be detected, and the information is used to manipulate the optical mode. Alternatively, the output light can be directed to a mirror of another optical cavity, and thus forms an input for this cavity. If we then connect an output from the second cavity back to the first we have a loop, and light can be made to travel only one way around the loop by the use of optical circulators
One example of the effect of making measurements on a quantum system is known as the quantum Zeno effect. According to quantum electrodynamics, an excited atom will decay at some random point in time. The average behaviour is described by a characteristic decay time. However, if the state of the atom is rapidly and repeatedly measured before it decays, the excited state can be maintained (ie. the atom prevented from decaying simply by measuring it). This effect has been observed experimentally and really is just another variant of Schrodinger’s cat.
Over the last few years, quantum optimal control theory (QOCT) has been applied to different aspects of quantum information processing, in particular to the implementation of scalable quantum gates with real physical systems. Quantum error correction enables fault-tolerant quantum computation to be performed, provided that each elementary operation meets a certain fidelity threshold but unfortunately, this puts extremely demanding constraints on the allowable errors. Threshold estimates vary between 0.01% to fractions of a percent, but none of the candidate physical implementations available to date has met such requirements yet. The central problem facing the construction of practical quantum computers is how do we make them robust! This is also a question of quantum control engineering.
One feature is common to all candidate QIP implementations: the need for an extremely accurate control of the quantum dynamics at the individual level, with much better precision than has been achieved before. Optimal control theory is a very powerful set of methods developed over the last decades to optimize the time evolution of a broad variety of complex systems, from aeronautics to economics. The basic underlying idea is to pick a specific path in parameter space to perform a specific task. This is expressed mathematically by a cost functional that depends on the state of the system and is minimized with respect to some control parameters. More recently, this approach is being successfully applied to quantum systems, e.g., in the context of ultra-fast laser pulses and light-assisted molecular reactions. A big advantage is that, in a quantum-mechanical situation, the goal can be reached via interference of many different paths in parameter space, rather than just one. This allows, for instance, to exploit faster non-adiabatic processes, allowing to perform more gate operations within the decoherence time, which is crucial to apply fault-tolerant error correction.
One application of Quantum technology, which harnesses quantum physics as a resource, is Quantum sensing where the greatest weakness of quantum systems – what makes them fragile – is turned into a strength in the detection of weak signals such as magnetic and gravitational fields. However, various signals interfere with a target to be detected, including ambient broadband electromagnetic noise and instabilities induced by mechanical vibrations in realistic environments. In the field of quantum sensing quantum control provides the key to extracting more – and more useful – information for a range of applications in standoff detection and precision navigation and timing.
ONR has developed novel techniques for Magnetic Anomaly Detection which permit the provably optimal suppression of out-of-band signals, allowing clutter rejection in complex environments. They cleverly tailored quantum control solutions to improve the performance of atom interferometers, providing a means to suppress vibration-induced performance limits, and improving precision inertial navigation systems. “Overall our work shows a pathway to dramatically enhancing the capabilities of standoff detection and precision navigation and timing in military settings through the use of quantum control.”
Scientists and engineers at University of Sydney and Microsoft Corporation develop controller chip for quantum computers
Scientists and engineers at EQUS, the University of Sydney and Microsoft Corporation have opened the next chapter in quantum technology with the invention of a single chip that can generate control signals for thousands of qubits, the building blocks of quantum computers. “To realise the potential of quantum computing, machines will need to operate thousands if not millions of qubits,” said EQUS Chief Investigator Professor David Reilly, a designer of the chip who holds a joint position with Microsoft and the University of Sydney. “The world’s biggest quantum computers currently operate with just 50 or so qubits,” he said. “This small scale is partly because of limits to the physical architecture that control the qubits.” “Our new chip puts an end to those limits.” The results are published in Nature Electronics.
Most quantum systems require quantum bits, or qubits, to operate at temperatures close to absolute zero (−273.15 degrees Celsius). This is to prevent them losing their ‘quantumness’, the character of matter or light that quantum computers need to perform their specialised computations. For quantum devices to do anything useful, they need instructions. That means sending and receiving electronic signals to and from the qubits. With current quantum architecture, that involves a lot of wires. “Current machines create a beautiful array of wires to control the signals; they look like an inverted gilded birds’ nest or chandelier. They’re pretty, but fundamentally impractical. It means we can’t scale the machines up to perform useful calculations. There is a real input–output bottleneck,” said Professor Reilly.
Microsoft Senior Hardware Engineer, Dr Kushal Das, a joint inventor of the chip, said: “Our device does away with all those cables. With just two wires carrying information as input, it can generate control signals for thousands of qubits. “This changes everything for quantum computing.” The control chip was developed at the Microsoft Quantum Laboratories at the University of Sydney, an industry–academic partnership that is changing the way scientists tackle engineering challenges. “Building a quantum computer is perhaps the most challenging engineering task of the 21st century. This can’t be achieved working with a small team in a university laboratory in a single country but needs the scale afforded by a global tech giant like Microsoft,” Professor Reilly said.
“Through our partnership with Microsoft, we haven’t just suggested a theoretical architecture to overcome the input–output bottleneck, we’ve built it. “We have demonstrated this by designing a custom silicon chip and coupling it to a quantum system. I’m confident to say this is the most advanced integrated circuit ever built to operate at deep cryogenic temperatures.” If realised, quantum computers promise to revolutionise information technology by solving problems beyond the scope of classical computers, in fields as diverse as cryptography, medicine, finance, artificial intelligence and logistics.
Quantum computers are at a similar stage that classical computers were in the 1940s. Machines like ENIAC, the world’s first electronic computer, required rooms of control systems to achieve any useful function. It has taken decades to overcome the scientific and engineering challenges that now allows for billions of transistors to fit into your mobile phone. “Our industry is facing perhaps even bigger challenges to take quantum computing beyond the ENIAC stage,” Professor Reilly said. “We need to engineer highly complex silicon chips that operate at 0.1 kelvin. That’s an environment 30 times colder than deep space.”
Dr Sebastian Pauka’s doctoral research at the University of Sydney encompassed much of the work to interface quantum devices with the chip. He said: “Operating at such cold temperatures means we have an incredibly low power budget. If we try to put more power into the system, we overheat the whole thing.” To achieve their result, the team built the most advanced integrated circuit to operate at cryogenic temperatures. “We have done this by engineering a system that operates in close proximity to the qubits without disturbing their operations,” Professor Reilly said. “Current control systems for qubits are removed metres away from the action, so to speak. They exist mostly at room temperature. “In our system we don’t have to come off the cryogenic platform. The chip is right there with the qubits. This means lower power and higher speeds. It’s a real control system for quantum technology.”
“Working out how to control these devices takes years of engineering development,” Professor Reilly said. “For this device we started four years ago when the University of Sydney started its partnership with Microsoft, which represents the single biggest investment in quantum technology in Australia. “We built lots of models and design libraries to capture the behaviour of transistors at deep cryogenic temperatures. Then we had to build devices, get them verified, characterised and finally connect them to qubits to see them work in practice.
Professor Reilly said the field has now fundamentally changed. “It’s not just about ‘here is my qubit’. It’s about how you build all the layers and all the tech to build a real machine. “Our partnership with Microsoft allows us to work with academic rigour, with the benefit of seeing our results quickly put into practice.”
Q-CTRL to Accelerate Quantum Technology Solutions for National Security Applications
Quantum startup Q-CTRL announced a strategic investment by In-Q-Tel (IQT), the not-for-profit strategic investor that identifies innovative technology solutions to support the national security communities of the U.S. and its allies. Q-CTRL is a pioneer in the field of quantum control engineering, delivering software products and professional services to help customers and partners harness the exotic properties of quantum physics for real-world benefit.
Q-CTRL focuses on building the foundational software that will lead to the advent of effective quantum technologies more quickly,” said Mike Ferrari, Managing Director and co-head of IQT’s Australian office. “We see the company as a key strategic partner to guide efforts towards suitable applications of effective quantum technologies to efficiently solve mission-critical and computationally demanding problems.”
The company’s practice in quantum computing solves the Achilles heel of this new technology – hardware error and instability – by delivering a set of techniques that allow quantum computations to be executed with greater success. More recently, Q-CTRL has begun applying its tools and expertise to quantum enhanced sensing, helping to improve the efficiency and performance of standoff detection as well as precision navigation and timing for defense and aerospace. In recognition of this work, Professor Michael J. Biercuk, founder and CEO of Q-CTRL, was invited to present the U.S. Office of Naval Research distinguished lecture in February 2020.
We have assembled the world’s leading collection of specialists in quantum control, and this investment by In-Q-Tel is a validation of the critical role our team will play in the development of quantum technologies for national security,” said Professor Biercuk. “We’re committed to leveraging our products and expertise to benefit national security, and we’re thrilled to participate in this cross-border collaboration between the U.S. and Australia.” Professor Biercuk and his academic team have previously received scientific research funding from U.S. defense and intelligence organizations, including the U.S. Army Research Office and the Intelligence Advanced Research Projects Activity (IARPA), part of the Office of the Director of National Intelligence.
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