# Quantum Technology: An introduction

*This blog is part of a series on Quantum Technology and future land warfare, put out by the Robotic and Autonomous Systems Implementation and Coordination Office (RICO), a part of Future Land Warfare Branch tasked with coordination and de-confliction for Army's Robotics and Autonomous Systems (RAS)/Artificial Intelligence (AI) efforts in accordance with emerging disruptive technology.*

The trouble with quantum technology is that it seems impenetrable, improbable and impractical. In this post, I accordingly aim to demystify quantum technology for defence personnel by answering the key introductory questions: What are quantum technologies? How do they work? What do they do and why are they advantageous? In subsequent posts, I will expand on the characteristics (operating principles, performance metrics, requirements and limitations), capabilities and readiness of each type of quantum technology and provide practical examples of their use.

## What are quantum technologies?

A quantum technology is one whose functionality derives from engineering the states of quantum systems.[i] This distinguishes quantum technologies from the various 20th century technologies (e.g. lasers, magnetic resonance imaging, semiconductor electronics) that employ quantum phenomena (e.g. coherence, quantised energy, tunnelling), but do not directly initialise, manipulate and measure the states of individual quantum systems. For this definition and the following sections to make more sense, some basic quantum physics is needed.

Quantum physics is best summarised as the strange physical laws of the microscopic world of elementary particles (e.g. electrons, photons and nuclei), which ultimately govern everything.[ii] A *quantum system* is a system of these particles. Measurements of a quantum system have random values whose probabilities are determined by the system’s *state* at the time of measurement. Following measurement, a quantum system is *projected* into a state that matches the measurement mechanism and value. A quantum system’s state at a given moment can be described as a *superposition* of the states associated with a measurement mechanism—the simultaneous occupation of multiple states with definite relative amplitudes and phases. Some states exhibit *entanglement* of two or more sub-systems (i.e. sub-groups of particles) of the quantum system. Entanglement produces statistical correlations in the values of measurements of the individual sub-systems. Interactions between a quantum system and its environment can randomise its state. This process is called *decoherence* and it ultimately limits how precisely a quantum system’s state can be engineered. Each of these concepts will be illustrated in the following and future blog posts.

## How do they work?

The fundamental building block of quantum technology is the *qubit*—the simplest quantum system—which has two states |0> and |1>^{[iii]}.

Quantum technologies function by using normal ‘classical’ devices (e.g. lasers, microwave electronics and photodetectors) to initialise (e.g. by a laser pulse), manipulate (e.g. by a microwave pulse) and measure (e.g. by detecting emitted photons) the state of their qubit(s). A normal ‘classical’ computer is used to program and control these devices and record the measurement data. So, when you operate a quantum technology, you simply interface with a familiar classical computer. Despite the qubits being a relatively small component of the quantum technology, their different physical behaviour is what delivers advantage over classical technologies.

The performance of a quantum technology is determined by both its qubits and its classical control system and methods. Like other technologies, performance can be described in terms of precision, accuracy, speed and endurance. *Precision *quantifies how reproducibly the qubit state(s) can be engineered and measured, whereas *accuracy *pertains to how close the actual qubit state(s) and measurement(s) are to the ideal state(s) and measurement(s). Speed is the rate at which the different processes (i.e. initialisation, manipulation and measurement) can be performed, and *endurance *is the lifetime of the qubit state(s) before decoherence, which sets the maximum time over which manipulation can be performed. In subsequent blog posts, I will detail how precision, accuracy, speed and endurance combine to form the key performance metrics of different types of quantum technology.

Performance is improved by engineering higher quality qubit systems, classical control hardware and methods, and shielding the qubits from the environment. The first three are pushing the limits of material growth, microfabrication, electrical, optical and mechanical engineering, and optimal control design. Environmental shielding requirements depend on the type of technology and qubit system. Some require cryogenics to achieve ultra-low temperatures and/ or vacuum systems to achieve ultra-high mechanical isolation. Others don’t require either and can operate in ambient and extreme conditions. Nevertheless, high quality materials, fabrication and device engineering is the key to high performance quantum technology.

## What do they do and why are they advantageous?

Thus far, quantum technologies can be categorised into three main types: quantum sensing and imaging, communications, and computing. Each category has different characteristics, capabilities, scopes of application and readiness.

### Quantum sensing and imaging: new limits in precision.

Quantum sensors measure time, dynamics (i.e. forces, acceleration and rotation), and fields (i.e. gravitational, electromagnetic and mechanical) with unprecedented precision and stability.^{[iv]} They primarily achieve this by exploiting quantum *superposition *to apply interferometry techniques to detect small changes in a qubit’s state by the passage of time, dynamics or interactions with fields. Quantum *entanglement *between multiple qubits may be exploited to further enhance precision. Stability is achieved through the qubits having fixed and universal susceptibilities (e.g. electron gyromagnetic ratio and atomic mass). Imaging is an extension of quantum sensing where quantum sensors are combined with an imaging apparatus (e.g. that scans the position of the qubit).

### Quantum communications: networking quantum devices and physically-assured communication security.

Quantum communications can be used to network quantum sensors to correlate and enhance sensitivity over large areas (e.g synchronise clocks within a communication network) and networking quantum computers to efficiently exchange data and amass computational power.^{[v]} Quantum communications may also be used to securely transmit data between classical devices (e.g. distribute encryption keys) or securely access remote quantum computers.^{[vi]} Quantum communications implement these different functions by sending *superimposed *or *entangled *qubits between the devices. Security is assured by the *projective *nature of quantum measurements, which means that it is not physically possible to copy the qubit encoded information without modifying it. Thus, security is assured in the sense that interference by eavesdroppers can be detected and quantified.

### Quantum computing: a leap in computational power.

Quantum computers dramatically speed-up the solution of particular computational problems.^{[vii]} Whilst the full range of such problems is still being discovered, established examples are related to signal processing, optimisation, simulation, searching and factoring. They achieve this by exploiting quantum *superposition *and *entanglement *to represent and manipulate information in a fundamentally more dense and efficient way than classical computers. Thus, quantum computers require fewer physical resources and operations to solve the same problem as a classical computer.

Having introduced the key fundamental concepts and principles, the next three blog posts will delve into each category of quantum technology. I will outline their key characteristics, capabilities and readiness, as well as provide example applications and assess their implications for defence.

[i] Jonathan P Dowling and Gerard J Milburn, “Quantum technology: the second quantum revolution”, Philosophical Transactions of the Royal Society of London A, 361, (2003): 1655.

[ii] For a popular introduction to quantum physics and qubits, see Art Friedman and Leonard Susskind, Quantum Mechanics: The Theoretical Minimum (New York: Basic Books, 2015). For an engaging web resource, see https://www.edx.org/course/quantum-mechanics-and-quantum-computation or similar offerings on edx.

[iii] For a more advanced understanding of qubits and quantum technologies, see Michael A Nielsen and Isaac L Chuang, Quantum Computation and Quantum Information: 10th Anniversary Edition, (Cambridge: Cambridge University Press, 2010).

[iv] For an introduction to quantum sensing, see Edwin Cartlidge, “Quantum Sensors: A Revolution in the Offing?”, Optics & Photonics News, September 2019: 24. A comprehensive technical review of quantum sensing is Christian L Degen, Friedemann Reinhard and Paola Cappellaro, “Quantum sensing”, Reviews of Modern Physics, 89, (2017): 035002.

[v] A useful overview of the current state and future directions in quantum communications is Stephanie Wehner, David Elkouss, Ronald Hanson, “Quantum internet: A vision for the road ahead”, Science, 362 (2018): 303. For a technical understanding, see Nielsen and Chuang above.

[vi] Ibid.

[vii] For an introduction to quantum computing, see the online course offered by Microsoft https://www.linkedin.com/learning/introduction-to-quantum-computing/a-new-era-of-computing-is-emerging or this overview from New Scientist https://www.newscientist.com/round-up/quantum-information/. For a critical discussion of the current state of quantum computing see John Preskill, “Quantum Computing in the NISQ era and beyond”, Quantum 2, (2018): 79. For a technical understanding, see Nielsen and Chuang above.

The views expressed in this article and subsequent comments are those of the author(s) and do not necessarily reflect the official policy or position of the Australian Army, the Department of Defence or the Australian Government.

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