Category Archives: Quantum Computer
New Materials Could Make Quantum Computers More Practical – Tom’s Hardware
A team of researchers from Stanford University has been investigating some new materials that they believe will bring us closer to building practical quantum computers.
One possible way to build quantum computers would be to use lasers to isolate spinning electrons inside a semiconductor material. When the laser hits the electron, it shows how the electron is spinning by emitting one or more light particles. The spin states can then be used as the most fundamental building blocks for quantum computing, the same way conventional computing uses 1s and 0s.
According to Stanford electrical engineering Professor Jelena Vuckovic, who has been investigating these new materials to build quantum computers, quantum computing would be ideal for studying biological systems, doing cryptography, or data mining, as well as for any other complex problem that cant be solved by conventional computers.
When people talk about finding a needle in a haystack, thats where quantum computing comes in, said Vuckovic.
The challenge in isolating spinning electrons is finding a material that can confine the electrons when the lasers hit them. Vuckovics team has identified three materials that can potentially do this: quantum dots, diamonds, and silicon carbide.
A quantum dot is a small amount of indium arsenide inside a crystal of gallium arsenide. The atomic properties of the two materials are known to trap spinning electrons.
In a recent paper, Kevin Fischer, a graduate student in the Vuckovic lab, described how the laser-electron processes can be used within a quantum dot system to control the input and output of light. For instance, by applying more power behind the lasers, two photons could be emitted instead of one. This could be used as an alternative to the 1s and 0s of conventional computers.
One issue is that the quantum dot system still requires cryogenic cooling, which doesnt make it a suitable candidate for general-purpose computing.
Vuckovics team has also been investigating modifying the crystalline lattice of a diamond to trap light in what is known as a color center. The team replaced some of the carbon atoms in the diamonds crystalline lattice with silicon atoms.
Like the quantum dots approach, doing quantum computing within diamond color centers requires cryogenic cooling.
Silicon carbide is a hard and transparent crystal that is used to make clutch plates, brake pads, and bulletproof vests, among other things. Prior research has shown that silicon carbide could be modified to create color centers at room temperature, but not in a way thats efficient enough to create a quantum chip.
Vuckovics team was able to eliminate some of the atoms in the silicon carbide lattice to create much more efficient color centers. The team also fabricated nanowires around the color centers to improve photon extraction.
Trapping electrons at room temperature could be a significant step forward for quantum computers, according to Vuckovich. However, she and her team are also not sure which method to create a practical quantum computer will work best in the end.
Some of the biggest technology companies in the world are working on building quantum computers right now, including Google, IBM, and Microsoft. Teams at many universities around the world are also experimenting with different approaches to building quantum computers.
Both Google and IBM believe well reach quantum supremacy--the point when quantum computers will be faster than conventional computers at solving a certain type of complex problems--when quantum computers have around 50 qubits (from the fewer than 10 qubits they do now). The two companies expect this goal to be reached in the next few years.
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New Materials Could Make Quantum Computers More Practical - Tom's Hardware
World’s First Quantum Computer Is Here – Wall Street Pit – Wall Street Pit
China has achieved another remarkable feat. Aside from having the worlds fastest supercomputer (TaihuLight), they have now developed a quantum computer that can supposedly work 24,000 times faster than any existing supercomputer, including their own.
While other tech companies like D-Wave and IBM have already managed to build their own quantum computers, what differentiates Chinas quantum computer is the use of multiple photons (the visible particles of light), specifically five of them, which is what gives its computing speed a super boost.
Prior to this, Pan Jianwei the leader of the research team from the University of Science and Technology of China who built the multi-photon quantum computer together with one of his colleagues, Lu Chaoyang, was credited with developing the worlds best semiconductor quantum dots-based single photon source. Using this photon source and an electronically programmable photonic circuit, made it possible for them to build their multi-photon quantum computing device.
According to Pan Jianwei, their machine can do calculations 10 to 100 times faster than ENIAC, the first electronic digital computer built in the 1940s. And while theres no practical use for it yet, its ability to predict the highly complex behavior and movement of photons (something that traditional computers are incapable of doing) is a clear testament to the potential of quantum computers.
Traditional computers store and process information in bits that can represent either 0 or 1. On the other hand, quantum computers use qubits (short for quantum bits) that can represent 0, or 1, or 0 and 1 simultaneously through the quantum concepts superpositioning (being able to exist in two states at once) and entanglement. This is what makes a quantum computer special it can process data and calculate outcomes simultaneously. And the more qubits that can be manipulated, the faster its computing ability becomes.
A common analogy used to explain the concept of quantum computing is reading books in a library. With traditional computing, its like reading one book at a time, finishing one before moving on to another. With quantum computing, its like reading all the books at the same time.
This theory has been around for a while, but researchers have yet to figure out the best approach that can transform this quantum computing dream into a reality.
This is what makes Chinas accomplishment so significant. Their latest quantum device which they are calling a Boson sampling machine isnt just able to perform calculations for five photons; its able to do so at a speed thats 24,000 times faster than what previous proof-of-concept experiments showed. Even better, the team says that scaling up their architecture to a higher number of photons is feasible, adding up to its potential to compete and defeat the performance of classical computers.
According to Shanghai Daily, the team is aiming to be able to manipulate 20 entangled photons by the end of the year. Their research was recently published on Nature Photonics.
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World's First Quantum Computer Is Here - Wall Street Pit - Wall Street Pit
China adds a quantum computer to high-performance computing arsenal – PCWorld
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China already has the world's fastest supercomputer and has now built a crude quantum computer that could outpace today's PCs and servers.
Quantum computers have already been built by companies like IBM and D-Wave, but Chinese researchers have taken a different approach. They are introducing quantum computing using multiple photons, which could provide a superior way to calculate compared to today's computers.
The Chinese quantum computing architecture allows forfive-photonsampling and entanglement. It's an improvement over previous experiments involving single-photon sourcing, up to 24,000 times faster, the researchers claimed.
The Chinese researchers have built components required for Boson sampling, which has been theorized for a long time and is considered an easy way to build a quantum computer. The architecture built by the Chinese can include a large number of photons, which increases the speed and scale of computing.
China is strengthening its technology arsenal in an effort to be self-sufficient. China's homegrown chip powers TaihuLight, the world's fastest computer.
In 2014, China said it would spend US$150 billion on semiconductor development so that PCs and mobile devices would convert to homegrown chips. Afraid that low-cost Chinese chips will flood the market, the U.S. earlier this year accused China of rigging the semiconductor market to its advantage.
It's not clear yet if a quantum computer is on China's national agenda. But China's rapid progress of technology is worrying countries like the U.S. A superfast quantum computer could enhance the country's progress in areas like weapons development, in which high-performance computers are key.
But there's a long way to go before China builds its first full-fledged quantum computer. The prototype quantum computer is good for specific uses but is not designed to be a universal quantum computer that can run any task.
The research behind quantum computers is gaining steam as PCs and servers reach their limit. It's becoming difficult to shrink chips to smaller geometries, which could upset the cycle of reducing costs of computers while boosting speeds.
If they deliver on their promise, quantum computers will drive computing into the future. They are fundamentally different from computers used today.
Bits on todays computers are stored as ones or zeros, while quantum computers rely on qubits, also called quantum bits. Qubits can achieve various states, including holding a one and zero simultaneously, and those states can multiply.
The parallelism allows qubits to do more calculations simultaneously. However, qubits are considered fragile and highly unstable, and can easily breakdown during entanglement, a technical term for when qubits interact. A breakdown could bring instability to a computing process.
The Chinese quantum computer has a photon device based on quantum dots, demultiplexers, photonic circuits, and detectors.
There are multiple ways to build a quantum computer, including via superconducting qubits, which is the building block for D-Wave Systems' systems. Like the Chinese system, D-Wave's quantum annealing method is another easy way to build a quantum computer but is not considered ideal for a universal quantum computer.
IBM already has a 5-qubit quantum computer that is available via the cloud. It is now chasing a universal quantum computer using superconducting qubitsbut has a different gating model to stabilize systems. Microsoft is trying to chase a new quantum computer based on a new topography and a yet-undiscovered particle called non-abelian anyons.
In a bid to build computers of the future, China has also built a neuromorphic chip called Darwin.
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China adds a quantum computer to high-performance computing arsenal - PCWorld
The Quantum Computer Revolution Is Closer Than You May Think – National Review
Lets make no mistake: The race for a quantum computer is the new arms race.
As Arthur Herman wrote in a recent NRO article, Quantum Cryptography: A Boon for Security, the competition to create the first quantum computer is heating up. The country that develops one first will have the ability to cripple militaries and topple the global economy. To deter such activity, and to ensure our security, the United States must win this new race to the quantum-computer revolution.
Classical computers operate in bits, with each bit being either a 0 or 1. Quantum computers, by contrast, operate in quantum bits, or qubits, which can be both 0 and 1 simultaneously. Therefore, quantum computers can do nearly infinite calculations at once, rather than sequentially. Because of these properties, a single quantum computer could be the master key to hijack our country.
The danger of a quantum computer is its ability to tear through the encryption protecting most of our online data, which means it could wipe out the global financial system or locate weapons of mass destruction. Quantum computers operate much differently from todays classical computers and could crack encryption in less time than it takes to snap ones fingers.
In 2016, 4.2 billion computerized records in the United States were compromised, a staggering 421 percent increase from the prior year. Whats more, foreign countries are stealing encrypted U.S. data and storing it because they know that in roughly a decade, quantum computers will be able to get around the encryption.
Many experts agree that the U.S. still has the advantage in the nascent world of quantum computing, thanks to heavy investment by giants such as Microsoft, Intel, IBM, D-Wave, and Google. Yet with China graduating 4.7 million of its students per year with STEM degrees while the U.S. graduates a little over half a million, how long can the U.S. maintain its lead?
Maybe not for long. Half of the global landmark scientific achievements of 2014 were led by a European consortium and the other half by China, according to a 2015 MIT study. The European Union has made quantum research a flagship project over the next ten years and is committed to investing nearly $1 billion. While the U.S. government allocates about $200 million per year to quantum research, a recent congressional report noted that inconsistent funding has slowed progress.
According to Dr. Chad Rigetti, a former member of IBMs quantum-computing group and now the CEO of Rigetti Computing, computing superiority is fundamental to long-term economic superiority, safety, and security. Our strategy, he continues, has to be viewing quantum computing as a way to regain American superiority in high-performance computing.
Additionally, cyber-policy advisor Tim Polk stated publicly that our edge in quantum technologies is under siege. In fact, China leads in unhackable quantum-enabled satellites and owns the worlds fastest supercomputers.
While quantum computers will lead to astounding breakthroughs in medicine, manufacturing, artificial intelligence, defense, and more, rogue states or actors could use quantum computers for fiercely destructive purposes. Recall the hack of Sony by North Korea, Russian spies hacking Yahoo accounts, and the exposure of 22 million federal Office of Personnel Management records by Chinese hackers.
How can the United States win this race? We must take a multi-pronged approach to guard against the dangers of quantum computers while reaping their benefits. The near-term priority is to implement quantum-cybersecurity solutions, which fully protect against quantum-computer attacks. Solutions can soon be built directly into devices, accessed via the cloud, integrated with online browsers, or implemented alongside existing fiber-optic infrastructure.
Second, the U.S. needs to consider increasing federal research and development and boost incentives for industry and academia to develop technologies that align private interests with national-security interests, since quantum technology will lead to advances in defense and forge deterrent capabilities.
Third, as private companies advance quicker than government agencies, Washington should engage regularly with industry. Not only will policies evolve in a timely manner, but government agencies could become valuable early adopters.
Fourth, translating breakthroughs in the lab to commercial development will require training quantum engineers. Dr. Robert Schoelkopf, director of the Yale Quantum Institute, launched Quantum Circuits, Inc., to bridge this gap and to perform the commercial development of a quantum computer.
The United States achieved the unthinkable when it put a man on the Moon. Creating the first quantum computer will be easier but the consequences if we dont will be far greater.
Idalia Friedson is a research assistant at the Hudson Institute.
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The Quantum Computer Revolution Is Closer Than You May Think - National Review
China builds five qubit quantum computer sampling and will scale to 20 qubits by end of this year and could any beat … – Next Big Future
Chinese researchers have built a 10 qubit quantum computer.
China builds ten qubit quantum computer, They will scale to 20 qubits by end of this year and could beat the performance of any regular computer next year with a 30 qubit system.
A chinese research team led by Pan Jianwei is exploring three technical routes to quantum computers: 1. systems based on single photons, 2. ultra-cold atoms and 3. superconducting circuits.
Experimental set-up for multiphoton boson-sampling. The set-up includes four key parts: the single-photon device, demultiplexers, ultra-low-loss photonic circuit and detectors. The single-photon device is a single InAs/GaAs quantum dot coupled to a 2-m-diameter micropillar cavity
Pan Jianwei and his colleagues Lu Chaoyang and Zhu Xiaobo, of the University of Science and Technology of China, and Wang Haohua, of Zhejiang University set two international records in quantum control of the maximal numbers of entangled photonic quantum bits And entangled superconducting quantum bits.
Pan doubling that manipulation of multi-particle entanglement is the core of quantum computing technology and has been the focus of international competition in quantum computing research.
In the photonic system, his team has made the first 5, 6, 8 and 10 entangled photons in the world and is at the forefront of global developments.
Last year, Pan and Lu Chaoyang developed the worlds best single photon source based on semiconductor quantum dots. Now, they are using the high-performance single photon source and electronically programmable photonic circuit to build a multi-photon quantum computing prototype to run the Boson Sampling task.
The Chinese photonic computer is 10 to 100 times faster than the first electronic computer, ENIAC, and the first transistor computer, TRADIC, in running the classical algorithm.
The Hefei reporter quantum device, called a boson sampling machine, can now carry out calculations for five photons, but at a speed 24,000 times than previous experiments.
ENIAC contained 17,468 vacuum tubes, 7200 crystal diodes, 1500 relays, 70,000 resistors, 10,000 capacitors and approximately 5,000,000 hand-soldered joints. It could perform 5000 simple addition or subtraction operations per second. ENIAC could perform 500 floating point operations per second.
The Chinese team led by Pan, Zhu Xiaobo and Wang Haohua have broken that record. They dependent developed a superconducting quantum circuit containing 10 superconducting quantum bits and successfully entangled the 10 quantum bits through a global quantum operation.
Nature Photonics High-efficiency multiphoton boson sampling
They will try to design and manipulate 20 superconducting quantum bits by the end of the year. They also plan to launch a quantum cloud computing platform by the end of this year.
Our architecture is feasible to be scaled up to a larger number of photons and with a higher rate to race against increasingly advanced computers, they said in the research paper.
Professor Scott Aaronson, who is based at the University of Texas at Austin and proposed the idea of the boson sampling machine, questioned whether it was useful to compare the latest results with technology developed over 60 years ago, but he said the research had shown Exciting experimental progress .
Its a step towards boson sampling with say 30 photons or some number thats large enough that no one will have to squint or argue about whether a quantum advantage has been attained, he said.
Araronson said one of the main purposes of making boson sampling machines was to prove that quantum devices could be shown to have an advantage in one area of complex calculations over existing types of computer.
Doing so would answer the quantum computing sceptics and help pave the way towards universal quantum computation, he said.
Abstract
Boson sampling is considered as a strong candidate to demonstrate quantum computational supremacy over classical computers. However, previous proof-of-principle experiments suffered from small photon number and low sampling rates owing to the inefficiencies of the single-photon sources and multiport optical interferometers. Here, we develop two central components for high-performance boson sampling: robust multiphoton interferometers with 99% transmission rate and actively demultiplexed single-photon sources based on a quantum dotmicropillar with simultaneously high efficiency, purity and indistinguishability. We implement and validate three-, four- and five-photon boson sampling, and achieve sampling rates of 4.96kHz, 151Hz and 4Hz, respectively, which are over 24,000 times faster than previous experiments. Our architecture can be scaled up for a larger number of photons and with higher sampling rates to compete with classical computers, and might provide experimental evidence against the extended ChurchTuring thesis.
18 pages of supplemental material
Quantum Computing | D-Wave Systems
Quantum Computation
Rather than store information using bits represented by 0s or 1s as conventional digital computers do, quantum computers use quantum bits, or qubits, to encode information as 0s, 1s, or both at the same time. This superposition of statesalong with the other quantum mechanical phenomena of entanglement and tunnelingenables quantum computers to manipulate enormous combinations of states at once.
In nature, physical systems tend to evolve toward their lowest energy state: objects slide down hills, hot things cool down, and so on. This behavior also applies to quantum systems. To imagine this, think of a traveler looking for the best solution by finding the lowest valley in the energy landscape that represents the problem.
Classical algorithms seek the lowest valley by placing the traveler at some point in the landscape and allowing that traveler to move based on local variations. While it is generally most efficient to move downhill and avoid climbing hills that are too high, such classical algorithms are prone to leading the traveler into nearby valleys that may not be the global minimum. Numerous trials are typically required, with many travelers beginning their journeys from different points.
In contrast, quantum annealing begins with the traveler simultaneously occupying many coordinates thanks to the quantum phenomenon of superposition. The probability of being at any given coordinate smoothly evolves as annealing progresses, with the probability increasing around the coordinates of deep valleys. Quantum tunneling allows the traveller to pass through hillsrather than be forced to climb themreducing the chance of becoming trapped in valleys that are not the global minimum. Quantum entanglement further improves the outcome by allowing the traveler to discover correlations between the coordinates that lead to deep valleys.
The D-Wave system has a web API with client libraries available for C/C++, Python, and MATLAB. This allows users to access the computer easily as a cloud resource over a network.
To program the system, a user maps a problem into a search for the lowest point in a vast landscape, corresponding to the best possible outcome. The quantum processing unitconsiders all the possibilities simultaneously to determine the lowest energy required to form those relationships. The solutions are values that correspond to the optimal configurations of qubits found, or the lowest points in the energy landscape. These values are returned to the user program over the network.
Because a quantum computer is probabilistic rather than deterministic, the computer returns many very good answers in a short amount of timethousands of samples in one second. This provides not only the best solution found but also other very good alternatives from which to choose.
D-Wave systems are intended to be used to complement classical computers. There are many examples of problems where a quantum computer can complement an HPC (high-performance computing) system. While the quantum computer is well suited to discrete optimization, for example,the HPC system is better at large-scale numerical simulations.
Download this whitepaper to learn more about programming a D-Wave quantum computer.
D-Waves flagship product, the 2000qubit D-Wave 2000Q quantum computer, is the most advanced quantum computer in the world. It is based on a novel type of superconducting processor that uses quantum mechanics to massively accelerate computation. It is best suited to tackling complex optimization problems that exist across many domains such as:
Download the Technology Overview
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Quantum Computing | D-Wave Systems
quantum computer – WIRED
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quantum computer - WIRED
Beyond classical computing without fault-tolerance: Looking for the … – Phys.Org
April 27, 2017 Quantum Circuits that form part of the new theoretical framework published in Quantum journal. Credit: Michael Bremner/cqc2t.org
A team of researchers from Australia and the UK have developed a new theoretical framework to identify computations that occupy the 'quantum frontier'the boundary at which problems become impossible for today's computers and can only be solved by a quantum computer. Importantly, they demonstrate that these computations can be performed with near-term, intermediate, quantum computers.
"Until recently it has been difficult to say definitively when quantum computers can outperform classical computers," said Professor Michael Bremner, Chief Investigator at the Centre for Quantum Computation and Communication Technology and founding member of the UTS Centre for Quantum Software and Information (UTS:QSI).
"The big challenge for quantum complexity theorists over the last decade has been to find stronger evidence for the existence of the quantum frontier, and then to identify where it lives. We're now getting a sense of this, and beginning to understand the resources required to cross the frontier to solve problems that today's computers can't."
The team has identified quantum computations that require the least known physical resources required to go beyond the capabilities of classical computers, significant because of the technological challenges associated with scaling up quantum computers.
Prof Bremner said that the result also indicates that full fault-tolerance may not be required to outperform classical computers. "To date, it has been widely accepted that error correction would be a necessary component of future quantum computers, but no one has yet been able to achieve this at a meaningful scale," said Bremner.
"Our work shows that while some level of error mitigation is needed to cross the quantum frontier, we may be able to outperform classical computers without the added design complexity of full fault tolerance," he said.
Dr Ashley Montanaro of the University of Bristol collaborated with Bremner to develop the framework.
"We started out with the goal of defining the minimum resources required to build a post-classical quantum computer, but then found that our model could be classically simulated with a small amount of noise, or physical imperfection," said Montanaro.
"The hope among scientists had always been that if the amount of noise in a quantum system was small enough then it would still be superior to a classical computer, however we have now shown that this probably isn't the case, at least for this particular class of computations," he said.
"We then realised that it is possible to use a classical encoding on a quantum circuit to overcome 'noise' in a much simpler way to mitigate these errors. The effectiveness of this approach was surprising. What it suggests is that we could use such structures to develop new quantum algorithms in a way that can directly avoid certain types of errors."
"This is a result that could lead to useful 'intermediate' quantum computers in the medium term, while we continue to pursue the goal of a full-scale universal quantum computer."
Explore further: Construction of practical quantum computers radically simplified
More information: Michael J. Bremner et al, Achieving quantum supremacy with sparse and noisy commuting quantum computations, Quantum (2017). DOI: 10.22331/q-2017-04-25-8
Provided by: Centre for Quantum Computation & Communication Technology
Scientists at the University of Sussex have invented a ground-breaking new method that puts the construction of large-scale quantum computers within reach of current technology.
When future users of quantum computers need to analyze their data or run quantum algorithms, they will often have to send encrypted information to the computer.
Scientists and engineers from the Universities of Bristol and Western Australia have developed how to efficiently simulate a "quantum walk" on a new design for a primitive quantum computer.
What does the future hold for computing? Experts at the Networked Quantum Information Technologies Hub (NQIT), based at Oxford University, believe our next great technological leap lies in the development of quantum computing.
A research team from the University of Bristol's Centre for Quantum Photonics (CQP) have brought the reality of a quantum computer one step closer by experimentally demonstrating a technique for significantly reducing the ...
A team of scientists led by Tim Taminiau of QuTech, the quantum institute of TU Delft and TNO, has now experimentally demonstrated that errors in quantum computations can be suppressed by repeated observations of quantum ...
Australian and German researchers have collaborated to develop a genetic algorithm to confirm the rejection of classical notions of causality.
By precisely controlling the quantum behavior of an ultracold atomic gas, Rice University physicists have created a model system for studying the wave phenomenon that may bring about rogue waves in Earth's oceans.
New research from North Carolina State University has found that combining digital and analog components in nonlinear, chaos-based integrated circuits can improve their computational power by enabling processing of a larger ...
New research could make lasers emitting a wide range of colors more accessible and open new applications from communications and sensing to displays.
NIST has been granted a patent for technology that may hasten the advent of a long-awaited new generation of high-performance, low-energy computers.
Epilepsy is a complex neurological disorder that afflicts approximately 50 million people worldwide. Although this disease has been known to exist for centuries, the exact mechanism of its cardinal symptom, the epileptic ...
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Beyond classical computing without fault-tolerance: Looking for the ... - Phys.Org
Computing on the boundary between conventional and quantum – Electronics Weekly
Along the way, they have shown that even simple error-prone quantum computers could have the edge over classical computers for some problems.
Until recently it has been difficult to say definitively when quantum computers can out-perform classical computers, said Professor Michael Bremner, chief investigator at the Australian Centre for Quantum Computation and Communication Technology. The big challenge for quantum complexity theorists over the last decade has been to find stronger evidence for the existence of the quantum frontier, and then to identify where it lives. Were now getting a sense of this, and beginning to understand the resources required to cross the frontier to solve problems that todays computers cant.
The team has identified quantum computations that require the least known physical resources required to go beyond the capabilities of classical computers and, according to Bremner, result indicate that full fault-tolerance may not be required.
To date, it has been widely accepted that error correction would be a necessary component of future quantum computers, but no one has yet been able to achieve this at a meaningful scale, said Bremner. Our work shows that while some level of error mitigation is needed to cross the quantum frontier, we may be able to outperform classical computers without the added design complexity of full fault tolerance.
Dr Ashley Montanaro of the University of Bristol works on the team.
The hope among scientists had always been that if the amount of noise in a quantum system was small enough then it would still be superior to a classical computer, however we have now shown that this probably isnt the case, at least for this particular class of computations, said Montanaro. We then realised that it is possible to use a classical encoding on a quantum circuit to overcome noise in a much simpler way to mitigate these errors. The effectiveness of this approach was surprising. What it suggests is that we could use such structures to develop new quantum algorithms in a way that can directly avoid certain types of errors.
Such intermediate quantum computers could be used in the medium term, while routes to full-scale universal quantum computers are sought.
Image credit Michael Bremner Australian Centre for Quantum Computation and Communication Technology
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Computing on the boundary between conventional and quantum - Electronics Weekly
Quantum cryptography – Wikipedia
Quantum cryptography is the science of exploiting quantum mechanical properties to perform cryptographic tasks. The best known example of quantum cryptography is quantum key distribution which offers an information-theoretically secure solution to the key exchange problem. Currently used popular public-key encryption and signature schemes (e.g., RSA and ElGamal) can be broken by quantum adversaries. The advantage of quantum cryptography lies in the fact that it allows the completion of various cryptographic tasks that are proven or conjectured to be impossible using only classical (i.e. non-quantum) communication (see below for examples). For example, it is impossible to copy data encoded in a quantum state and the very act of reading data encoded in a quantum state changes the state. This is used to detect eavesdropping in quantum key distribution.
Quantum cryptography uses Heisenberg's uncertainty principle[1] formulated in 1927, and the No-cloning theorem[2] first articulated by Wootters and Zurek and Dieks in 1982. Werner Heisenberg discovered one of the fundamental principles of quantum mechanics: "At the instant at which the position of the electron is known, its momentum therefore can be known only up to magnitudes which correspond to that discontinuous change; thus, the more precisely the position is determined, the less precisely the momentum is known, and conversely[3] (Heisenberg, 1927: 1745). This simply means that observation of quanta changes its behavior. By measuring the velocity of quanta we would affect it, and thereby change its position; if we want to find a quant's position, we are forced to change its velocity. Therefore, we cannot measure a quantum system's characteristics without changing it[4] (Clark, n.d.) and we cannot record all characteristics of a quantum system before those characteristics are measured. The No-cloning theorem demonstrates that it is impossible to create a copy of an arbitrary unknown quantum state. This makes unobserved eavesdropping impossible because it will be quickly detected, thus greatly improving assurance that the communicated data remains private.
Quantum cryptography was proposed first by Stephen Wiesner, then at Columbia University in New York, who, in the early 1970s, introduced the concept of quantum conjugate coding. His seminal paper titled "Conjugate Coding" was rejected by IEEE Information Theory Society, but was eventually published in 1983 in SIGACT News (15:1 pp.7888, 1983). In this paper he showed how to store or transmit two messages by encoding them in two "conjugate observables", such as linear and circular polarization of light, so that either, but not both, of which may be received and decoded. He illustrated his idea with a design of unforgeable bank notes. In 1984, building upon this work, Charles H. Bennett, of the IBM's Thomas J. Watson Research Center, and Gilles Brassard, of the Universit de Montral, proposed a method for secure communication based on Wiesner's "conjugate observables", which is now called BB84.[5] In 1991 Artur Ekert developed a different approach to quantum key distribution based on peculiar quantum correlations known as quantum entanglement.[6]
Random rotations of the polarization by both parties (usually called Alice and Bob) have been proposed in Kak's three-stage quantum cryptography protocol.[7] In principle, this method can be used for continuous, unbreakable encryption of data if single photons are used.[8] The basic polarization rotation scheme has been implemented.[9]
The BB84 method is at the basis of quantum key distribution methods. Companies that manufacture quantum cryptography systems include MagiQ Technologies, Inc. (Boston, Massachusetts, United States), ID Quantique (Geneva, Switzerland), QuintessenceLabs (Canberra, Australia) and SeQureNet (Paris, France).
The most well known and developed application of quantum cryptography is quantum key distribution, which is the process of using quantum communication to establish a shared key between two parties (Alice and Bob, for example) without a third party (Eve) learning anything about that key, even if Eve can eavesdrop on all communication between Alice and Bob. If Eve tries to learn information about the key being established, key establishment will fail causing Alice and Bob to notice. Once the key is established, it is then typically used for encrypted communication using classical techniques. For instance, the exchanged key could be used as for symmetric cryptography.
The security of quantum key distribution can be proven mathematically without imposing any restrictions on the abilities of an eavesdropper, something not possible with classical key distribution. This is usually described as "unconditional security", although there are some minimal assumptions required, including that the laws of quantum mechanics apply and that Alice and Bob are able to authenticate each other, i.e. Eve should not be able to impersonate Alice or Bob as otherwise a man-in-the-middle attack would be possible.
One aspect of quantum key distribution is that it is secure against quantum computers. Its strength does not depend on mathematical complexity, like post-quantum cryptography, but on physical principles.
Unlike quantum key distribution, quantum coin flipping is a protocol that is used between two participants who do not trust each other.[10] The participants communicate via a quantum channel and exchange information through the transmission of qubits.[11] Alice will determine a random basis and sequence of qubits and then transmit them to Bob. Bob then detects and records the qubits. Once Bob has recorded the qubits sent by Alice, he makes a guess to Alice on what basis she chose. Alice reports whether he won or lost to Bob and then sends Bob her entire original qubit sequence. Since the two parties do not trust each other, cheating is likely to occur at any step in the process. [12]
Quantum coin flipping is theoretically a secure means of communicating through two distrustful parties, but it is difficult to physically accomplish. [10]
Following the discovery of quantum key distribution and its unconditional security, researchers tried to achieve other cryptographic tasks with unconditional security. One such task was commitment. A commitment scheme allows a party Alice to fix a certain value (to "commit") in such a way that Alice cannot change that value while at the same time ensuring that the recipient Bob cannot learn anything about that value until Alice reveal it. Such commitment schemes are commonly in cryptographic protocols. In the quantum setting, they would be particularly useful: Crpeau and Kilian showed that from a commitment and a quantum channel, one can construct an unconditionally secure protocol for performing so-called oblivious transfer.[13]Oblivious transfer, on the other hand, had been shown by Kilian to allow implementation of almost any distributed computation in a secure way (so-called secure multi-party computation).[14] (Notice that here we are a bit imprecise: The results by Crpeau and Kilian[13][14] together do not directly imply that given a commitment and a quantum channel one can perform secure multi-party computation. This is because the results do not guarantee "composability", that is, when plugging them together, one might lose security. Later works showed, however, how composability can be ensured in this setting.[citation needed])
Unfortunately, early quantum commitment protocols[15] were shown to be flawed. In fact, Mayers showed that (unconditionally secure) quantum commitment is impossible: a computationally unlimited attacker can break any quantum commitment protocol.[16]
Yet, the result by Mayers does not preclude the possibility of constructing quantum commitment protocols (and thus secure multi-party computation protocols) under assumptions that they are much weaker than the assumptions needed for commitment protocols that do not use quantum communication. The bounded quantum storage model described below is an example for a setting in which quantum communication can be used to construct commitment protocols. A breakthrough in November 2013 offers "unconditional" security of information by harnessing quantum theory and relativity, which has been successfully demonstrated on a global scale for the first time.[17]
One possibility to construct unconditionally secure quantum commitment and quantum oblivious transfer (OT) protocols is to use the bounded quantum storage model (BQSM). In this model, we assume that the amount of quantum data that an adversary can store is limited by some known constant Q. We do not, however, impose any limit on the amount of classical (i.e., non-quantum) data the adversary may store.
In the BQSM, one can construct commitment and oblivious transfer protocols.[18] The underlying idea is the following: The protocol parties exchange more than Q quantum bits (qubits). Since even a dishonest party cannot store all that information (the quantum memory of the adversary is limited to Q qubits), a large part of the data will have to be either measured or discarded. Forcing dishonest parties to measure a large part of the data allows to circumvent the impossibility result by Mayers;[16] commitment and oblivious transfer protocols can now be implemented.
The protocols in the BQSM presented by Damgrd, Fehr, Salvail, and Schaffner[18] do not assume that honest protocol participants store any quantum information; the technical requirements are similar to those in QKD protocols. These protocols can thus, at least in principle, be realized with today's technology. The communication complexity is only a constant factor larger than the bound Q on the adversary's quantum memory.
The advantage of the BQSM is that the assumption that the adversary's quantum memory is limited is quite realistic. With today's technology, storing even a single qubit reliably over a sufficiently long time is difficult. (What "sufficiently long" means depends on the protocol details. By introducing an artificial pause in the protocol, the amount of time over which the adversary needs to store quantum data can be made arbitrarily large.)
An extension of the BQSM is the noisy-storage model introduced by Wehner, Schaffner and Terhal.[19] Instead of considering an upper bound on the physical size of the adversary's quantum memory, an adversary is allowed to use imperfect quantum storage devices of arbitrary size. The level of imperfection is modelled by noisy quantum channels. For high enough noise levels, the same primitives as in the BQSM can be achieved[20] and the BQSM forms a special case of the noisy-storage model.
In the classical setting, similar results can be achieved when assuming a bound on the amount of classical (non-quantum) data that the adversary can store.[21] It was proven, however, that in this model also the honest parties have to use a large amount of memory (namely the square-root of the adversary's memory bound).[22] This makes these protocols impractical for realistic memory bounds. (Note that with today's technology such as hard disks, an adversary can cheaply store large amounts of classical data.)
The goal of position-based quantum cryptography is to use the geographical location of a player as its (only) credential. For example, one wants to send a message to a player at a specified position with the guarantee that it can only be read if the receiving party is located at that particular position. In the basic task of position-verification, a player, Alice, wants to convince the (honest) verifiers that she is located at a particular point. It has been shown by Chandran et al. that position-verification using classical protocols is impossible against colluding adversaries (who control all positions except the prover's claimed position).[23] Under various restrictions on the adversaries, schemes are possible.
Under the name of 'quantum tagging', the first position-based quantum schemes have been investigated in 2002 by Kent. A US-patent[24] was granted in 2006, but the results only appeared in the scientific literature in 2010.[25] After several other quantum protocols for position verification have been suggested in 2010,[26][27] Buhrman et al. were able to show a general impossibility result:[28] using an enormous amount of quantum entanglement (they use a doubly exponential number of EPR pairs, in the number of qubits the honest player operates on), colluding adversaries are always able to make it look to the verifiers as if they were at the claimed position. However, this result does not exclude the possibility of practical schemes in the bounded- or noisy-quantum-storage model (see above). Later Beigi and Knig improved the amount of EPR pairs needed in the general attack against position-verification protocols to exponential. They also showed that a particular protocol remains secure against adversaries who controls only a linear amount of EPR pairs.[29]
A quantum cryptographic protocol is device-independent if its security does not rely on trusting that the quantum devices used are truthful. Thus the security analysis of such a protocol needs to consider scenarios of imperfect or even malicious devices. Mayers and Yao[30] proposed the idea of designing quantum protocols using "self-testing" quantum apparatus, the internal operations of which can be uniquely determined by their input-output statistics. Subsequently, Roger Colbeck in his Thesis[31] proposed the use of Bell tests for checking the honesty of the devices. Since then, several problems have been shown to admit unconditional secure and device-independent protocols, even when the actual devices performing the Bell test are substantially "noisy," i.e., far from being ideal. These problems include quantum key distribution,[32][33]randomness expansion,[33][34] and randomness amplification.[35]
Quantum computers may become a technological reality; it is therefore important to study cryptographic schemes used against adversaries with access to a quantum computer. The study of such schemes is often referred to as post-quantum cryptography. The need for post-quantum cryptography arises from the fact that many popular encryption and signature schemes (such as RSA and its variants, and schemes based on elliptic curves) can be broken using Shor's algorithm for factoring and computing discrete logarithms on a quantum computer. Examples for schemes that are, as of today's knowledge, secure against quantum adversaries are McEliece and lattice-based schemes. Surveys of post-quantum cryptography are available.[36][37]
There is also research into how existing cryptographic techniques have to be modified to be able to cope with quantum adversaries. For example, when trying to develop zero-knowledge proof systems that are secure against quantum adversaries, new techniques need to be used: In a classical setting, the analysis of a zero-knowledge proof system usually involves "rewinding", a technique that makes it necessary to copy the internal state of the adversary. In a quantum setting, copying a state is not always possible (no-cloning theorem); a variant of the rewinding technique has to be used.[38]
Post quantum algorithms are also called "quantum resistant", because unlike QKD it is not known or provable that there will not be potential future quantum attacks against them. Even though they are not vulnerable to Shor's algorithm the NSA are announcing plans to transition to quantum resistant algorithms.[39] The National Institute of Security and Technology (NIST) believes that it is time to think of quantum-safe primitives.[40]
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Quantum cryptography - Wikipedia