What is quantum computing?

Over 50 years of advances in mathematics, materials science, and computer science have transformed quantum computing from theory to reality. Today, real quantum computers can be accessed through the cloud, and many thousands of people have used them to learn, conduct research, and tackle new problems.

Quantum computers could one day provide breakthroughs in many disciplines, including materials and drug discovery, the optimization of complex systems, and artificial intelligence. But to realize those breakthroughs, and to make quantum computers widely useable and accessible, we need to reimagine information processing and the machines that do it.

 

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Introduction to quantum computing

 

Introduction to quantum computing

If you'd like to hear quantum computing explained from the very beginning, check out this video from WIRED with Dr. Talia Gershon, Senior Manager of Quantum Experiences at IBM Research. In it, she explains quantum computing to a child, a teenager, a college student and a graduate student, and then discusses quantum computing myths and challenges with Professor Steve Girvin from Yale University.


Quantum computing basics

Why we need quantum computing

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Why we need quantum computing

We experience the benefits of classical computing every day. Today’s computers help and entertain us, connect us with people all over the world, and allow us to process huge amounts of data to solve problems and manage complex systems.

However, there are problems that today’s systems will never be able to solve. For challenges above a certain size and complexity, we don’t have enough computational power on Earth to tackle them. To stand a chance at solving some of these complex problems, we need a new kind of computing: one whose computational power also scales exponentially as the system size grows.

Here, watch Dr. Jerry Chow, manager of experimental quantum computing for IBM Research, explain how quantum computing can be applied to molecular simulation, which could lead to the discovery of new materials and medicines.

What makes it ‘quantum’?

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What makes it ‘quantum’?

what makes computing quantum illustration

All computing systems rely on a fundamental ability to store and manipulate information. Current computers manipulate individual bits, which store information as binary 0 and 1 states. Millions of bits work together to process and display information. Quantum computers leverage different physical phenomena — superposition, entanglement, and interference — to manipulate information. To do this, we rely on different physical devices: quantum bits, or qubits.

Superposition refers to a combination of states we would ordinarily describe independently. To make a classical analogy, if you play two musical notes at once, what you will hear is a superposition of the two notes.

Entanglement is a famously counter-intuitive quantum phenomenon describing behavior we never see in the classical world. Entangled particles behave together as a system in ways that cannot be explained using classical logic.

Finally, quantum states have a phase, and so can undergo interference. Quantum interference can be understood similarly to wave interference; when two waves are in phase, their amplitudes add, and when they are out of phase, their amplitudes cancel.


 

Quantum computation

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Quantum computation

how quantum computers make calculations

How do quantum systems use quantum properties to compute? This question can be split into two parts:

  1. How are algorithms developed for the quantum computers available today?
  2. How do people create algorithms for future “fault tolerant” quantum systems that are still theoretical?

Many algorithms for near-term quantum hardware require you to find the “best” solution from among many possible solutions, such as finding the lowest energy state of a molecule among various possible molecular bond lengths. For each possible bond length, you represent parts of the energy on a quantum processor, and then measure the energy directly. Trying various solutions eventually leads to the bond length with to the lowest energy state, which represents the equilibrium molecular configuration. More information can be found here.

Algorithms for the kinds of quantum systems we hope to have in the future, called fault tolerant universal quantum computers, were among the first to be developed in the field. This includes some of the most well known examples: Shor’s factorization algorithm and Grover’s algorithm for unstructured search. While near-term algorithms involve relatively few qubits and operations, and errors can be compensated by running the algorithm many times to get an average value, fault-tolerant algorithms require extremely long gate sequences and high gate fidelities to perform well. The quantum hardware needed for fault-tolerant algorithms is still being developed; however, they demonstrate that certain quantum algorithms can outperform their classical counterparts, given the right quantum system.


Scaling quantum systems

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Scaling quantum systems

how quantum computing will scale illustration

In order to increase the computational power of quantum computing systems, improvements are needed along two dimensions. One is qubit count; the more qubits you have, the more states can in principle be manipulated and stored. The second is to achieve lower error rates. We need to be able to manipulate the qubit states accurately and perform sequential operations that provide answers, not noise.

Combining these two concepts, we can create a single measure of a quantum computer’s power called quantum volume. Quantum volume measures the relationship between number and quality of qubits, circuit connectivity, and error rates of operations. Building larger systems with lower error rates will lead to discovering the first instances of quantum advantage, or applications where quantum computers can offer a computational advantage for solving real problems.


 

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Brief history of quantum computing

 

Brief history of quantum computing

the history of quantum computing

While quantum physics has been investigated since the early 20th century, quantum computing as a discipline only emerged in the 1970s and 1980s. In the 1990s, the first examples of algorithms that could run faster on a quantum computer were developed, leading to increased interest in the field. Throughout that decade, additional discoveries eventually led to a better understanding of how to build real systems that could implement quantum algorithms and correct for errors as they arose. Progress toward building real experimental systems has accelerated, including the invention of the transmon qubit, which is among the most promising qubits in use today. Tremendous progress has also been made in coherence times and other parameters required for robust quantum systems.

In May 2016, the IBM Q Experience was created, leading to the first demonstration that robust and reliable quantum computing prototypes could be kept online and stable. It provided the first cloud-based platform for quantum computation research and development. In 2017, IBM Q was born, along with the IBM Q Network, as a way of establishing partnerships to accelerate progress toward discovering the first applications.

 

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Programming a quantum computer

 

Programming a quantum computer

why we need quantum computing diagram showing how quantum computing works with the cloud

Wondering how to actually use a quantum computer? This infographic explains the process. A quantum experiment is defined on a regular computer and translated by electronics into a series of microwave pulses, which travel to the bottom of the dilution refrigerator, that houses the quantum chip. These microwaves can be controlled to change the state on the quantum processor. Relevant measurements specified by the code are taken and then returned as output, along with information on how the qubits and dilution refrigerator were performing at the time of the experiment.

If you are interested in creating quantum programs yourself, check out the Composer, a graphical web tool on the IBM Q Experience, and the Qiskit software developer kit, an open source, modular framework for writing quantum code.


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Run a program on a real quantum computer

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Play a quantum game

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Quantum computing at IBM

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