# Category Archives: Quantum Computer

## What Can We Do with a Quantum Computer? | Institute for …

When I was in middle school, I read a popular book about programming in BASIC (which was the most popular programming language for beginners at that time). But it was 1986, and we did not have computers at home or school yet. So, I could only write computer programs on paper, without being able to try them on an actual computer.

Surprisingly, I am now doing something similarI am studying how to solve problems on a quantum computer. We do not yet have a fully functional quantum computer. But I am trying to figure out what quantum computers will be able to do when we build them.

The story of quantum computers begins in 1981 with Richard Feynman, probablythe most famous physicist ofhis time. At a conference on physics and computation atthe Massachusetts Institute of Technology, Feynman asked the question: Can we simulate physics on a computer?

The answer wasnot exactly. Or, more preciselynot all of physics. One of the branches of physics is quantum mechanics, which studiesthe laws of nature on the scale of individual atoms and particles. If we try to simulate quantum mechanics on a computer, we run into a fundamental problem. The full description of quantum physics has so many variables that we cannot keep track of all of them on a computer.

If one particle can be described by two variables, then to describe the most general state of n particles, we need 2n variables. If we have 100 particles, we need 2100 variables, which is roughly 1 with 30 zeros. This number is so big that computers will never have so much memory.

By itself, this problem was nothing newmany physicists already knew that. But Feynman took it one step further. He asked whether we could turn this problem into something positive: If we cannot simulate quantum physics on a computer, maybe we can build a quantum mechanical computerwhich would be better than the ordinary computers?

This question was asked by the most famous physicist of the time. Yet, over the next few years, almost nothing happened. The idea of quantum computers was so new and so unusual that nobody knew how to start thinking about it.

But Feynman kept telling his ideas to others, again and again. He managed to inspire a small number of people who started thinking: what would a quantum computer look like? And what would it be able to do?

Quantum mechanics, the basis for quantum computers, emerged from attempts to understand the nature of matter and light. At the end of the nineteenth century, one of the big puzzles of physics was color.

The color of an object is determined by the color of the light that it absorbs and the color of the light that it reflects. On an atomic level, we have electrons rotating around the nucleus of an atom. An electron can absorb a particle of light (photon), and this causes the electron to jump to a different orbit around the nucleus.

In the nineteenth century, experiments with heated gasses showed that each type of atom only absorbs and emits light of some specific frequencies. For example,visible light emitted by hydrogen atoms only consists of four specific colors. The big question was: how can we explain that?

Physicists spent decades looking for formulas that would predict the color of the light emitted by various atoms and models that would explain it. Eventually, this puzzle was solved by Danish physicist Niels Bohr in 1913 when he postulated that atoms and particles behave according to physical laws that are quite different from what we see on a macroscopic scale. (In 1922, Bohr, who would become a frequent Member at the Institute, was awarded a Nobel Prize for this discovery.)

To understand the difference, we can contrast Earth (which is orbiting around the Sun) and an electron (which is rotating around the nucleus of an atom). Earth can be at any distance from the Sun. Physical laws do not prohibit the orbit of Earth to be a hundred meters closer to the Sun or a hundred meters further. In contrast, Bohrs model only allows electrons to be in certain orbits and not between those orbits. Because of this, electrons can only absorb the light of colors that correspond to a difference between two valid orbits.

Around the same time, other puzzles about matter and light were solved bypostulating that atoms and particles behave differently from macroscopic objects. Eventually, this led to the theory of quantum mechanics, which explains all of those differences, using a small number of basic principles.

Quantum mechanics has been an object of much debate. Bohr himself said, Anyone not shocked by quantum mechanics has not yet understood it. Albert Einstein believed that quantum mechanics should not be correct. And, even today, popular lectures on quantum mechanics often emphasize the strangeness of quantum mechanics as one of the main points.

But I have a different opinion. The path of how quantum mechanics was discovered was very twisted and complicated. But the end result of this path, the basicprinciples of quantum mechanics, is quite simple. There are a few things that are different from classical physics and one has to accept those. But, once you accept them, quantum mechanics is simple and natural. Essentially, one can think of quantum mechanics as a generalization of probability theory in which probabilities can be negative.

In the last decades, research in quantum mechanics has been moving into a new stage. Earlier, the goal of researchers was to understand the laws of nature according to how quantum systems function. In many situations, this has been successfully achieved. The new goal is to manipulate and control quantum systems so that they behave in a prescribed way.

This brings the spirit of research closer to computer science. Alan Key, a distinguished computer scientist, once characterized the difference between natural sciences and computer science in the following way. In natural sciences, Nature has given us the world, and we just discovered its laws. In computers, we can stuff the laws into it and create the world. Experiments in quantum physics are now creating artificial physical systems that obey the laws of quantum mechanics but do not exist in nature under normal conditions.

An example of such an artificial quantum system is a quantum computer. A quantum computer encodes information into quantum states and computes by performing quantum operations on it.

There are several tasks for which a quantum computer will be useful. The one that is mentioned most frequently is that quantum computers will be able to read secret messages communicated over the internet using the current technologies (such as RSA, Diffie-Hellman, and other cryptographic protocols that are based on the hardness of number-theoretic problems like factoring and discrete logarithm). But there are many other fascinating applications.

First of all, if we have a quantum computer, it will be useful for scientists for conducting virtual experiments. Quantum computing started with Feynmans observation that quantum systems are hard to model on a conventional computer. If we had a quantum computer, we could use it to model quantum systems. (This is known as quantum simulation.) For example, we could model the behavior of atoms and particles at unusual conditions (for example, very high energies that can be only created in the Large Hadron Collider) without actually creating those unusual conditions. Or we could model chemical reactionsbecause interactions among atoms in a chemical reaction is a quantum process.

Another use of quantum computers is searching huge amounts of data. Lets say that we have a large phone book, ordered alphabetically by individual names (and not by phone numbers). If we wanted to find the person who has the phone number 6097348000, we would have to go through the whole phone book and look at every entry. For a phone book with one million phone numbers, it could take one million steps. In 1996, Lov Grover from Bell Labs discovered that a quantum computer would be able to do the same task with one thousand steps instead of one million.

More generally, quantum computers would be useful whenever we have to find something in a large amount of data: a needle in a haystackwhether this is the right phone number or something completely different.

Another example of that is if we want to find two equal numbers in a large amount of data. Again, if we have one million numbers, a classical computer might have to look at all of them and take one million steps. We discovered that a quantum computer could do it in a substantially smaller amount of time.

All of these achievements of quantum computing are based on the same effects of quantum mechanics. On a high level, these are known as quantum parallelism and quantum interference.

A conventional computer processes information by encoding it into 0s and 1s. If we have a sequence of thirty 0s and 1s, it has about one billion of possible values. However, a classical computer can only be in one of these one billion states at the same time. A quantum computer can be in a quantum combination of all of those states, called superposition. This allows it to perform one billion or more copies of a computation at the same time. In a way, this is similar to a parallel computer withone billion processors performing different computations at the same timewith one crucial difference. For a parallel computer, we need to have one billion different processors. In a quantum computer, all one billion computations will be running on the same hardware. This is known as quantum parallelism.

The result of this process is a quantum state that encodes the results of onebillion computations. The challenge for a person who designs algorithms for a quantum computer (such as myself) is: how do we access these billion results? If we measured this quantum state, we would get just one of the results. All of the other 999,999,999 results would disappear.

To solve this problem, one uses the second effect, quantum interference. Consider a process that can arrive at the same outcome in several different ways. In the non-quantum world, if there are two possible paths toward one result and each path is taken with a probability , the overall probability of obtaining this result is += . Quantumly, the two paths can interfere, increasing the probability of success to 1.

Quantum algorithms combine these two effects. Quantum parallelism is used to perform a large number of computations at the same time, and quantum interference is used to combine their results into something that is both meaningful and can be measured according to the laws of quantum mechanics.

The biggest challenge is building a large-scale quantum computer. There are several ways one could do it. So far, the best results have been achieved using trapped ions. An ion is an atom that has lost one or more of its electrons. An ion trap is a system consisting of electric and magnetic fields, which can capture ions and keep them at locations. Using an ion trap, one can arrange several ions in a line, at regular intervals.

One can encode 0 into the lowest energy state of an ion and 1 into a higher energy state. Then, the computation is performed using light to manipulate the states of ions. In an experiment by Rainer Blatts group at the University of Innsbruck, Austria, this has been successfully performed for up to fourteen ions. The next step is to scale the technology up to a bigger number of trapped ions.

There are many other paths toward building a quantum computer. Instead of trapped ions, one can use electrons or particles of lightphotons. One can even use more complicated objects, for example, the electric current in a superconductor. A very recent experiment by a group led by John Martinis of the University of California, Santa Barbara, has shown how to perform quantum operations on one or two quantum bits with very high precision from 99.4% to 99.92% using the superconductor technology.

The fascinating thing is that all of these physical systems, from atoms to electric current in a superconductor, behave according to the same physical laws. And they all can perform quantum computation. Moving forward with any of these technologies relates to a fundamental problem in experimental physics: isolating quantum systems from environment and controlling them with high precision. This is a very difficult and, at the same time, a very fundamental task and being able to control quantum systems will be useful for many other purposes.

Besides building quantum computers, we can use the ideas of information to think about physical laws in terms of information, in terms of 0s and 1s. This is the way I learned quantum mechanicsI started as a computer scientist, and I learned quantum mechanics by learning quantum computing first. And I think this is the best way to learn quantum mechanics.

Quantum mechanics can be used to describe many physical systems, and in each case, there are many technical details that are specific to the particular physicalsystem. At the same time, there is a common set of core principles that all of those physical systems obey.

Quantum information abstracts away from the details that are specific to a particular physical system and focuses on the principles that are common to all quantum systems. Because of that, studying quantum information illuminates the basic concepts of quantum mechanics better than anything else. And, one day, this could become the standard way of learning quantum mechanics.

For myself, the main question still is: how will quantum computers be useful? We know that they will be faster for many computational tasks, from modeling nature to searching large amounts of data. I think there are many more applications and, perhaps, the most important ones are still waiting to be discovered.

Originally posted here:

What Can We Do with a Quantum Computer? | Institute for ...

## Qubit – Wikipedia

In quantum computing, a qubit()or quantum bit(sometimes qbit) is the basic unit of quantum informationthe quantum version of the classical binary bit physically realized with a two-state device. A qubit is a two-state (or two-level) quantum-mechanical system, one of the simplest quantum systems displaying the peculiarity of quantum mechanics. Examples include: the spin of the electron in which the two levels can be taken as spin up and spin down; or the polarization of a single photon in which the two states can be taken to be the vertical polarization and the horizontal polarization. In a classical system, a bit would have to be in one state or the other. However, quantum mechanics allows the qubit to be in a coherent superpositionof both states/levels simultaneously, a property which is fundamental to quantum mechanics and quantum computing.

The coining of the term qubit is attributed to Benjamin Schumacher.[1] In the acknowledgments of his 1995 paper, Schumacher states that the term qubit was created in jest during a conversation with William Wootters. The paper describes a way of compressing states emitted by a quantum source of information so that they require fewer physical resources to store. This procedure is now known as Schumacher compression.

A binary digit, characterized as 0 and 1, is used to represent information in classical computers. A binary digit can represent up to one bit of Shannon information, where a bit is the basic unit of information.However, in this article, the word bit is synonymous with binary digit.

In classical computer technologies, a processed bit is implemented by one of two levels of low DC voltage, and whilst switching from one of these two levels to the other, a so-called forbidden zone must be passed as fast as possible, as electrical voltage cannot change from one level to another instantaneously.

There are two possible outcomes for the measurement of a qubitusually taken to have the value "0" and "1", like a bit or binary digit. However, whereas the state of a bit can only be either 0 or 1, the general state of a qubit according to quantum mechanics can be a coherent superpositionof both.[2] Moreover, whereas a measurement of a classical bit would not disturb its state, a measurement of a qubit would destroy its coherence and irrevocably disturb the superposition state. It is possible to fully encode one bit in one qubit. However, a qubit can hold more information, e.g. up to two bits using superdense coding.

For a system of n components, a complete description of its state in classical physics requires only n bits, whereas in quantum physics it requires 2n1 complex numbers.[3]

In quantum mechanics, the general quantum state of a qubit can be represented by a linear superposition of its two orthonormal basis states (or basis vectors). These vectors are usually denoted as | 0 = [ 1 0 ] {displaystyle |0rangle ={bigl [}{begin{smallmatrix}1\0end{smallmatrix}}{bigr ]}} and | 1 = [ 0 1 ] {displaystyle |1rangle ={bigl [}{begin{smallmatrix}0\1end{smallmatrix}}{bigr ]}} . They are written in the conventional Diracor "braket"notation; the | 0 {displaystyle |0rangle } and | 1 {displaystyle |1rangle } are pronounced "ket 0" and "ket 1", respectively. These two orthonormal basis states, { | 0 , | 1 } {displaystyle {|0rangle ,|1rangle }} , together called the computational basis, are said to span the two-dimensional linear vector (Hilbert) space of the qubit.

Qubit basis states can also be combined to form product basis states. For example, two qubits could be represented in a four-dimensional linear vector space spanned by the following product basis states: | 00 = [ 1 0 0 0 ] {displaystyle |00rangle ={biggl [}{begin{smallmatrix}1\0\0\0end{smallmatrix}}{biggr ]}} , | 01 = [ 0 1 0 0 ] {displaystyle |01rangle ={biggl [}{begin{smallmatrix}0\1\0\0end{smallmatrix}}{biggr ]}} , | 10 = [ 0 0 1 0 ] {displaystyle |10rangle ={biggl [}{begin{smallmatrix}0\0\1\0end{smallmatrix}}{biggr ]}} , and | 11 = [ 0 0 0 1 ] {displaystyle |11rangle ={biggl [}{begin{smallmatrix}0\0\0\1end{smallmatrix}}{biggr ]}} .

In general, n qubits are represented by a superposition state vector in 2n-dimensional Hilbert space.

A pure qubit state is a coherent superposition of the basis states. This means that a single qubit can be described by a linear combination of | 0 {displaystyle |0rangle } and | 1 {displaystyle |1rangle } :

where and are probability amplitudes and can in general both be complex numbers.When we measure this qubit in the standard basis, according to the Born rule, the probability of outcome | 0 {displaystyle |0rangle } with value "0" is | | 2 {displaystyle |alpha |^{2}} and the probability of outcome | 1 {displaystyle |1rangle } with value "1" is | | 2 {displaystyle |beta |^{2}} . Because the absolute squares of the amplitudes equate to probabilities, it follows that {displaystyle alpha } and {displaystyle beta } must be constrained by the equation

Note that a qubit in this superposition state does not have a value in between "0" and "1"; rather, when measured, the qubit has a probability | | 2 {displaystyle |alpha |^{2}} of the value 0 and a probability | | 2 {displaystyle |beta |^{2}} of the value "1". In other words, superposition means that there is no way, even in principle, to tell which of the two possible states forming the superposition state actually pertains. Furthermore, the probability amplitudes, {displaystyle alpha } and {displaystyle beta } , encode more than just the probabilities of the outcomes of a measurement; the relative phase of {displaystyle alpha } and {displaystyle beta } is responsible for quantum interference, e.g., as seen in the two-slit experiment.

It might, at first sight, seem that there should be four degrees of freedom in | = | 0 + | 1 {displaystyle |psi rangle =alpha |0rangle +beta |1rangle ,} , as {displaystyle alpha } and {displaystyle beta } are complex numbers with two degrees of freedom each. However, one degree of freedom is removed by the normalization constraint ||2 + ||2 = 1. This means, with a suitable change of coordinates, one can eliminate one of the degrees of freedom. One possible choice is that of Hopf coordinates:

Additionally, for a single qubit the overall phase of the state ei has no physically observable consequences, so we can arbitrarily choose to be real (or in the case that is zero), leaving just two degrees of freedom:

where e i {displaystyle e^{iphi }} is the physically significant relative phase.

The possible quantum states for a single qubit can be visualised using a Bloch sphere (see diagram). Represented on such a 2-sphere, a classical bit could only be at the "North Pole" or the "South Pole", in the locations where | 0 {displaystyle |0rangle } and | 1 {displaystyle |1rangle } are respectively. This particular choice of the polar axis is arbitrary, however. The rest of the surface of the Bloch sphere is inaccessible to a classical bit, but a pure qubit state can be represented by any point on the surface. For example, the pure qubit state ( ( | 0 + i | 1 ) / 2 ) {displaystyle ((|0rangle +i|1rangle )/{sqrt {2}})} would lie on the equator of the sphere at the positive y axis. In the classical limit, a qubit, which can have quantum states anywhere on the Bloch sphere, reduces to the classical bit, which can be found only at either poles.

The surface of the Bloch sphere is a two-dimensional space, which represents the state space of the pure qubit states. This state space has two local degrees of freedom, which can be represented by the two angles {displaystyle phi } and {displaystyle theta } .

A pure state is one fully specified by a single ket, | = | 0 + | 1 , {displaystyle |psi rangle =alpha |0rangle +beta |1rangle ,,} a coherent superposition as described above. Coherence is essential for a qubit to be in a superposition state. With interactions and decoherence, it is possible to put the qubit in a mixed state, a statistical combination or incoherent mixture of different pure states. Mixed states can be represented by points inside the Bloch sphere (or in the Bloch ball). A mixed qubit state has three degrees of freedom: the angles {displaystyle phi } and {displaystyle theta } , as well as the length r {displaystyle r} of the vector that represents the mixed state.

There are various kinds of physical operations that can be performed on pure qubit states.

An important distinguishing feature between qubits and classical bits is that multiple qubits can exhibit quantum entanglement. Quantum entanglement is a nonlocal property of two or more qubits that allows a set of qubits to express higher correlation than is possible in classical systems.

The simplest system to display quantum entanglement is the system of two qubits. Consider, for example, two entangled qubits in the | + {displaystyle |Phi ^{+}rangle } Bell state:

In this state, called an equal superposition, there are equal probabilities of measuring either product state | 00 {displaystyle |00rangle } or | 11 {displaystyle |11rangle } , as | 1 / 2 | 2 = 1 / 2 {displaystyle |1/{sqrt {2}}|^{2}=1/2} . In other words, there is no way to tell if the first qubit has value 0 or 1 and likewise for the second qubit.

Imagine that these two entangled qubits are separated, with one each given to Alice and Bob. Alice makes a measurement of her qubit, obtainingwith equal probabilitieseither | 0 {displaystyle |0rangle } or | 1 {displaystyle |1rangle } , i.e., she can not tell if her qubit has value 0 or 1. Because of the qubits' entanglement, Bob must now get exactly the same measurement as Alice. For example, if she measures a | 0 {displaystyle |0rangle } , Bob must measure the same, as | 00 {displaystyle |00rangle } is the only state where Alice's qubit is a | 0 {displaystyle |0rangle } . In short, for these two entangled qubits, whatever Alice measures, so would Bob, with perfect correlation, in any basis, however far apart they may be and even though both can not tell if their qubit has value 0 or 1 a most surprising circumstance that can not be explained by classical physics.

Controlled gates act on 2 or more qubits, where one or more qubits act as a control for some specified operation. In particular, the controlled NOT gate (or CNOT or cX) acts on 2 qubits, and performs the NOT operation on the second qubit only when the first qubit is | 1 {displaystyle |1rangle } , and otherwise leaves it unchanged. With respect to the unentangled product basis { | 00 {displaystyle {|00rangle } , | 01 {displaystyle |01rangle } , | 10 {displaystyle |10rangle } , | 11 } {displaystyle |11rangle }} , it maps the basis states as follows:

A common application of the CNOT gate is to maximally entangle two qubits into the | + {displaystyle |Phi ^{+}rangle } Bell state. To construct | + {displaystyle |Phi ^{+}rangle } , the inputs A (control) and B (target) to the CNOT gate are:

1 2 ( | 0 + | 1 ) A {displaystyle {frac {1}{sqrt {2}}}(|0rangle +|1rangle )_{A}} and | 0 B {displaystyle |0rangle _{B}}

After applying CNOT, the output is the | + {displaystyle |Phi ^{+}rangle } Bell State: 1 2 ( | 00 + | 11 ) {displaystyle {frac {1}{sqrt {2}}}(|00rangle +|11rangle )} .

The | + {displaystyle |Phi ^{+}rangle } Bell state forms part of the setup of the superdense coding, quantum teleportation, and entangled quantum cryptography algorithms.

Quantum entanglement also allows multiple states (such as the Bell state mentioned above) to be acted on simultaneously, unlike classical bits that can only have one value at a time. Entanglement is a necessary ingredient of any quantum computation that cannot be done efficiently on a classical computer. Many of the successes of quantum computation and communication, such as quantum teleportation and superdense coding, make use of entanglement, suggesting that entanglement is a resource that is unique to quantum computation.[4] A major hurdle facing quantum computing, as of 2018, in its quest to surpass classical digital computing, is noise in quantum gates that limits the size of quantum circuits that can be executed reliably.[5]

A number of qubits taken together is a qubit register. Quantum computers perform calculations by manipulating qubits within a register. A qubyte (quantum byte) is a collection of eight qubits.[6]

Similar to the qubit, the qutrit is the unit of quantum information that can be realized in suitable 3-level quantum systems. This is analogous to the unit of classical information trit of ternary computers. Note, however, that not all 3-level quantum systems are qutrits.[7] The term "qu-d-it" (quantum d-git) denotes the unit of quantum information that can be realized in suitable d-level quantum systems.[8]

Any two-level quantum-mechanical system can be used as a qubit. Multilevel systems can be used as well, if they possess two states that can be effectively decoupled from the rest (e.g., ground state and first excited state of a nonlinear oscillator). There are various proposals. Several physical implementations that approximate two-level systems to various degrees were successfully realized. Similarly to a classical bit where the state of a transistor in a processor, the magnetization of a surface in a hard disk and the presence of current in a cable can all be used to represent bits in the same computer, an eventual quantum computer is likely to use various combinations of qubits in its design.

The following is an incomplete list of physical implementations of qubits, and the choices of basis are by convention only.

In a paper entitled "Solid-state quantum memory using the 31P nuclear spin", published in the October 23, 2008, issue of the journal Nature,[9] a team of scientists from the U.K. and U.S. reported the first relatively long (1.75 seconds) and coherent transfer of a superposition state in an electron spin "processing" qubit to a nuclear spin "memory" qubit. This event can be considered the first relatively consistent quantum data storage, a vital step towards the development of quantum computing. Recently, a modification of similar systems (using charged rather than neutral donors) has dramatically extended this time, to 3 hours at very low temperatures and 39 minutes at room temperature.[10] Room temperature preparation of a qubit based on electron spins instead of nuclear spin was also demonstrated by a team of scientists from Switzerland and Australia.[11]

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Qubit - Wikipedia

## Quantum computer | computer science | Britannica.com

Quantum computer, device that employs properties described by quantum mechanics to enhance computations.

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computer: Quantum computing

According to quantum mechanics, an electron has a binary (two-valued) property known as spin. This suggests another way of representing a bit of information. While single-particle information storage is attractive, it would be difficult to manipulate. The fundamental idea of quantum computing, however,

As early as 1959 the American physicist and Nobel laureate Richard Feynman noted that, as electronic components begin to reach microscopic scales, effects predicted by quantum mechanics occurwhich, he suggested, might be exploited in the design of more powerful computers. In particular, quantum researchers hope to harness a phenomenon known as superposition. In the quantum mechanical world, objects do not necessarily have clearly defined states, as demonstrated by the famous experiment in which a single photon of light passing through a screen with two small slits will produce a wavelike interference pattern, or superposition of all available paths. (See wave-particle duality.) However, when one slit is closedor a detector is used to determine which slit the photon passed throughthe interference pattern disappears. In consequence, a quantum system exists in all possible states before a measurement collapses the system into one state. Harnessing this phenomenon in a computer promises to expand computational power greatly. A traditional digital computer employs binary digits, or bits, that can be in one of two states, represented as 0 and 1; thus, for example, a 4-bit computer register can hold any one of 16 (24) possible numbers. In contrast, a quantum bit (qubit) exists in a wavelike superposition of values from 0 to 1; thus, for example, a 4-qubit computer register can hold 16 different numbers simultaneously. In theory, a quantum computer can therefore operate on a great many values in parallel, so that a 30-qubit quantum computer would be comparable to a digital computer capable of performing 10 trillion floating-point operations per second (TFLOPS)comparable to the speed of the fastest supercomputers.

During the 1980s and 90s the theory of quantum computers advanced considerably beyond Feynmans early speculations. In 1985 David Deutsch of the University of Oxford described the construction of quantum logic gates for a universal quantum computer, and in 1994 Peter Shor of AT&T devised an algorithm to factor numbers with a quantum computer that would require as few as six qubits (although many more qubits would be necessary for factoring large numbers in a reasonable time). When a practical quantum computer is built, it will break current encryption schemes based on multiplying two large primes; in compensation, quantum mechanical effects offer a new method of secure communication known as quantum encryption. However, actually building a useful quantum computer has proved difficult. Although the potential of quantum computers is enormous, the requirements are equally stringent. A quantum computer must maintain coherence between its qubits (known as quantum entanglement) long enough to perform an algorithm; because of nearly inevitable interactions with the environment (decoherence), practical methods of detecting and correcting errors need to be devised; and, finally, since measuring a quantum system disturbs its state, reliable methods of extracting information must be developed.

Plans for building quantum computers have been proposed; although several demonstrate the fundamental principles, none is beyond the experimental stage. Three of the most promising approaches are presented below: nuclear magnetic resonance (NMR), ion traps, and quantum dots.

In 1998 Isaac Chuang of the Los Alamos National Laboratory, Neil Gershenfeld of the Massachusetts Institute of Technology (MIT), and Mark Kubinec of the University of California at Berkeley created the first quantum computer (2-qubit) that could be loaded with data and output a solution. Although their system was coherent for only a few nanoseconds and trivial from the perspective of solving meaningful problems, it demonstrated the principles of quantum computation. Rather than trying to isolate a few subatomic particles, they dissolved a large number of chloroform molecules (CHCL3) in water at room temperature and applied a magnetic field to orient the spins of the carbon and hydrogen nuclei in the chloroform. (Because ordinary carbon has no magnetic spin, their solution used an isotope, carbon-13.) A spin parallel to the external magnetic field could then be interpreted as a 1 and an antiparallel spin as 0, and the hydrogen nuclei and carbon-13 nuclei could be treated collectively as a 2-qubit system. In addition to the external magnetic field, radio frequency pulses were applied to cause spin states to flip, thereby creating superimposed parallel and antiparallel states. Further pulses were applied to execute a simple algorithm and to examine the systems final state. This type of quantum computer can be extended by using molecules with more individually addressable nuclei. In fact, in March 2000 Emanuel Knill, Raymond Laflamme, and Rudy Martinez of Los Alamos and Ching-Hua Tseng of MIT announced that they had created a 7-qubit quantum computer using trans-crotonic acid. However, many researchers are skeptical about extending magnetic techniques much beyond 10 to 15 qubits because of diminishing coherence among the nuclei.

Just one week before the announcement of a 7-qubit quantum computer, physicist David Wineland and colleagues at the U.S. National Institute for Standards and Technology (NIST) announced that they had created a 4-qubit quantum computer by entangling four ionized beryllium atoms using an electromagnetic trap. After confining the ions in a linear arrangement, a laser cooled the particles almost to absolute zero and synchronized their spin states. Finally, a laser was used to entangle the particles, creating a superposition of both spin-up and spin-down states simultaneously for all four ions. Again, this approach demonstrated basic principles of quantum computing, but scaling up the technique to practical dimensions remains problematic.

Quantum computers based on semiconductor technology are yet another possibility. In a common approach a discrete number of free electrons (qubits) reside within extremely small regions, known as quantum dots, and in one of two spin states, interpreted as 0 and 1. Although prone to decoherence, such quantum computers build on well-established, solid-state techniques and offer the prospect of readily applying integrated circuit scaling technology. In addition, large ensembles of identical quantum dots could potentially be manufactured on a single silicon chip. The chip operates in an external magnetic field that controls electron spin states, while neighbouring electrons are weakly coupled (entangled) through quantum mechanical effects. An array of superimposed wire electrodes allows individual quantum dots to be addressed, algorithms executed, and results deduced. Such a system necessarily must be operated at temperatures near absolute zero to minimize environmental decoherence, but it has the potential to incorporate very large numbers of qubits.

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Quantum computer | computer science | Britannica.com

## IBMs new quantum computer is a symbol, not a breakthrough

In the grueling race to build a practical quantum computer, tech companies are keeping their spirits up by loudly cheering every milestone no matter how small. One of the most vocal competitors is IBM, which today at CES unveiled the IBM Q System One: a 20-qubit quantum computer thats built for stability, but with some very flashy design.

IBM is touting the Q System One as the worlds first fully integrated universal quantum computing system designed for scientific and commercial use. But thats a description that needs a lot of context. The Q System One may be designed for commercial use, but its not exactly ready for it. Not in the way you might think.

Quantum computers like the Q System One are still very much experimental devices. They cant outperform classical computers at useful tasks (in fact, your laptop is probably more powerful when it comes to real-life computation), but are instead supposed to be research tools; letting us work out, qubit by qubit, how quantum devices might work at all.

Its more like a stepping stone than a practical quantum computer, Winfried Hensinger, professor of quantum technologies at the UKs University of Sussex, told The Verge. Dont think of this as a quantum computer that can solve all of the problems quantum computing is known for. Think of it as a prototype machine that allows you to test and further develop some of the programming that might be useful in the future.

And even as an experimental device, its not like IBM is going to start selling the Q System One at Best Buy. The company wont say how much it costs to buy one of these machines or even how many its made. Like IBMs other quantum computers, its accessible only via the cloud, where companies and research institutes can buy time on the IBM Q Network. And today IBM announced two new customers on the network: energy giant ExxonMobil, and European research lab CERN, the organization that built the Large Hadron Collider.

So whats special about the Q System One? Well, IBM says the main achievement is turning an experimental quantum machine into something with reliability (and looks) closer to that of a mainframe computer. Quantum computing is an extremely delicate business. Chips need to be kept at freezing temperatures and can be disturbed by the tiniest electrical fluctuations or physical vibrations. The Q System One, says IBM, minimizes these problems.

This is something IBM brings to the market that no one else really does. We know how to do integrated systems, IBMs VP of quantum research, Bob Sutor, tells The Verge. The electronics for a quantum computer are not something you go buy off the shelf. You need a temperature controlled environment, you need to minimize the vibrations anything that might disrupt the quantum calculations.

Sutor says that a practical advantage of engineering a machine like the Q System One is that it reduces research downtime. Resetting a quantum computer after an upset caused by a power surge or a disgruntled look from a technician is much, much quicker with a device like the Q System One. What used to take days and weeks now takes hour or days, says Sutor.

And while these might sound like marginal gains, if were ever going to have quantum computers that do change the world in all the ways we dream of (by discovering new drugs, for example, and unlocking fusion energy) reliable research will absolutely be key.

And perhaps just as importantly, the Q System One looks the part. The machine was designed by Map Project Office, an industrial design consultancy thats worked with companies like Sonos, Honda, and Graphcore. The Q System One is contained in a nine-foot borosilicate glass cube, with its delicate internals sheathed by a shiny, rounded black case. Its reminiscent of both Apples dustbin-like 2013 Mac Pro and the Monolith from 2001: A Space Odyssey. It looks like a computer from the future.

For IBM this is not simply a side benefit its part of the plan. The 107-year-old company may still rake in billions in revenue each quarter (mostly from legacy enterprise deals), but its facing what some analysts have called irreversible structural decline. Its failed to come out ahead in the tech industrys most recent growth areas, mobile and cloud computing, and it needs new revenue streams to carry it through its second century of existence. AI is one bet, quantum computing another.

Sutor doesnt mention these problems, but he does note that the Q System One is supposed to inspire confidence both in quantum computing and in IBM itself.

People, when they see quantum computing systems, their eyes just glow, he tells The Verge. And its because they understand that these things that were just rumored about, or that were just too futuristic, are now starting to be produced. They can look at these things and say, Ah, IBM sees the path forward!

And machines like the Q System One are still useful on these terms, giving people a glimpse of the future. But we need to remember, says Hensinger, that theres lots of work yet to be done. I wouldnt call this a breakthrough, he says. But its a productive step towards commercial realization of quantum computing.

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IBMs new quantum computer is a symbol, not a breakthrough

## IBM unveils the world’s first quantum computer that …

For many years, quantum computers have been within only the confines of the research lab.

On Tuesday, though, IBM unveiled the IBM Q System One, billed as the first-ever quantum computer designed for businesses to put to their own use though the company is clear that this is only the first step toward a broader revolution.

Quantum computing is considered one of the most promising early-stage technologies out there today. That's because quantum computers can process exponentially more data and have the potential to completely transform entire industries. For example, they could streamline aerospace and military systems, calculate risk factors to make better investments, or, perhaps, find a cure for cancer and other diseases.

"Data will be the world's most valuable natural resource," IBM CEO Ginni Rometty said on stage at the Consumer Electronics Show in Las Vegas, where the IBM Q System One was unveiled.

Don't expect to install one in your office any time soon, though. While the computer is open to paying customers, developers will access its power from the comfort of their own homes or offices via the IBM Cloud. IBM Q System One. IBM

Average computers store data in binary, as either zeroes or ones strings of ones and zeroes represent numbers or letters. However, quantum computers are much more powerful. That's because they store data using "qubits," which have a special property that allows zeroes and ones to exist simultaneously. This seemingly small thing gives quantum computers the ability to do exponentially more calculations at once, making them powerful enough for incredibly complicated tasks like drug discovery, intensive data analysis, and even creating unbreakable codes.

Enclosed in a 9-foot-tall, 9-foot-wide glass case that forms an air-tight environment, this sleek computer is IBM's first effort to bring quantum computing to businesses. The casing is important: Qubits lose their quantum-computing properties outside of very specific conditions. A quantum computer has to be kept well below freezing in an environment that is mostly free of vibration and electromagnetic radiation.

IBM's new system aims to address this challenge with an integrated quantum computer that solves all of that on behalf of its customers hence the casing, which keeps everything in shipshape. However, this relative fragility is why you won't be installing an IBM Q System One in your own office while it's definitely a major step forward, it's far away from being something you can order and have delivered.

"The IBM Q System One is a major step forward in the commercialization of quantum computing," Arvind Krishna, IBM's senior vice president of hybrid cloud and director of research, said in a statement. "This new system is critical in expanding quantum computing beyond the walls of the research lab as we work to develop practical quantum applications for business and science."

Read more: Here's why we should be really excited about quantum computers

Later this year, IBM will also open its first IBM Q Quantum Computation Center for commercial customers in Poughkeepsie, New York. At this lab, clients can use IBM's cloud-based quantum computing systems, as well as other high-performance computing systems.

IBM isn't the only company that's been working on quantum computing, as the technology is still far from ready for mass deployment.

Google is researching how to make quantum computers more stable and better able to find and fix errors, and it has also created and tested qubit processors as it pursues the technology. Microsoft is working on creating hybrid quantum computers, which combine the new technology with more conventional processors. Intel, too, has been working on quantum computing chips.

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IBM unveils the world's first quantum computer that ...

## Were Close to a Universal Quantum Computer, Heres Where We’re At

Quantum computers are just on the horizon as both tech giants and startups are working to kickstart the next computing revolution.

U.S. Nuclear Missiles Are Still Controlled By Floppy Disks - https://youtu.be/Y8OOp5_G-R4

Read More:Quantum Computing and the New Space Racehttp://nationalinterest.org/feature/q...In January 2017, Chinese scientists officially began experiments using the worlds first quantum-enabled satellite, which will carry out a series of tests aimed at investigating space-based quantum communications over the course of the next two years.

Quantum Leap in Computer Simulationhttps://pursuit.unimelb.edu.au/articl...Ultimately it will help us understand and test the sorts of problems an eventually scaled-up quantum computer will be used for, as the quantum hardware is developed over the next decade or so.

How Quantum Computing Will Change Your Lifehttps://www.seeker.com/quantum-comput... The Perimeter Institute of Theoretical Physics kicked off a new season of live-streamed public lectures featuring quantum information expert Michele Mosca.

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Were Close to a Universal Quantum Computer, Heres Where We're At

## Schrdinger’s Killer App: Race to Build the World’s First …

"This is a beautifully written book that presents the carefully researched facts in an engaging style. The historical narrative, everywhere, is spiced up with entertaining anecdotes and sprinkled with references. The math and physics are presented through simple examples illustrated by drawing on analogies, while avoiding the use of any equations."Contemporary Physics, 2014

" explains the difficult concepts of quantum mechanics to laypersons, using analogies that require no background in physics or advanced mathematics. These concepts include quantum entanglement, Schrodinger's cat, and quantum computational complexity. Dowling (Louisiana State) has worked with the US Department of Defense (DoD) in their development of quantum information sciences for the last 20 years. The book's title refers to the fact that all the encrypted communication of the Internet could easily be unveiled by a quantum computer, thus leading to competition to develop such a machine. The work begins with a discussion of Einstein, who fought against many notions of quantum physics, such as quantum entanglement and quantum computational complexity. The second chapter explains Bell's theorem, proving that the entanglement 'action at a distance' idea actually takes place. Later chapters address how the public-key encryption system used by the Internet can be broken by a quantum computer; one-time pad encryption together with the unbreakable quantum key distribution technique and the DoD's efforts to build a quantum computer; and the idea of building a quantum computer using entangled particles as the underlying building blocks. Recommended." C. Tappert, Pace University, CHOICE Magazine

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Schrdinger's Killer App: Race to Build the World's First ...

## How Quantum Computers Work

A quantum computer is a computer design which uses the principles of quantum physics to increase the computational power beyond what is attainable by a traditional computer. Quantum computers have been built on the small scale and work continues to upgrade them to more practical models.

Computers function by storing data in a binary number format, which result in a series of 1s & 0s retained in electronic components such as transistors.

Each component of computer memory is called a bit and can be manipulated through the steps of Boolean logic so that the bits change, based upon the algorithms applied by the computer program, between the 1 and 0 modes (sometimes referred to as "on" and "off").

A quantum computer, on the other hand, would store information as either a 1, 0, or a quantum superposition of the two states. Such a "quantum bit" allows for far greater flexibility than the binary system.

Specifically, a quantum computer would be able to perform calculations on a far greater order of magnitude than traditional computers ... a concept which has serious concerns and applications in the realm of cryptography & encryption. Some fear that a successful & practical quantum computer would devastate the world's financial system by ripping through their computer security encryptions, which are based on factoring large numbers that literally cannot be cracked by traditional computers within the lifespan of the universe.

A quantum computer, on the other hand, could factor the numbers in a reasonable period of time.

To understand how this speeds things up, consider this example. If the qubit is in a superposition of the 1 state and the 0 state, and it performed a calculation with another qubit in the same superposition, then one calculation actually obtains 4 results: a 1/1 result, a 1/0 result, a 0/1 result, and a 0/0 result.

This is a result of the mathematics applied to a quantum system when in a state of decoherence, which lasts while it is in a superposition of states until it collapses down into one state. The ability of a quantum computer to perform multiple computations simultaneously (or in parallel, in computer terms) is called quantum parallelism).

The exact physical mechanism at work within the quantum computer is somewhat theoretically complex and intuitively disturbing. Generally, it is explained in terms of the multi-world interpretation of quantum physics, wherein the computer performs calculations not only in our universe but also in other universes simultaneously, while the various qubits are in a state of quantum decoherence. (While this sounds far-fetched, the multi-world interpretation has been shown to make predictions which match experimental results. Other physicists have )

Quantum computing tends to trace its roots back to a 1959 speech by Richard P. Feynman in which he spoke about the effects of miniaturization, including the idea of exploiting quantum effects to create more powerful computers. (This speech is also generally considered the starting point of nanotechnology.)

Of course, before the quantum effects of computing could be realized, scientists and engineers had to more fully develop the technology of traditional computers. This is why, for many years, there was little direct progress, nor even interest, in the idea of making Feynman's suggestions into reality.

In 1985, the idea of "quantum logic gates" was put forth by University of Oxford's David Deutsch, as a means of harnessing the quantum realm inside a computer. In fact, Deutsch's paper on the subject showed that any physical process could be modeled by a quantum computer.

Nearly a decade later, in 1994, AT&T's Peter Shor devised an algorithm that could use only 6 qubits to perform some basic factorizations ... more cubits the more complex the numbers requiring factorization became, of course.

A handful of quantum computers has been built.

The first, a 2-qubit quantum computer in 1998, could perform trivial calculations before losing decoherence after a few nanoseconds. In 2000, teams successfully built both a 4-qubit and a 7-qubit quantum computer. Research on the subject is still very active, although some physicists and engineers express concerns over the difficulties involved in upscaling these experiments to full-scale computing systems. Still, the success of these initial steps does show that the fundamental theory is sound.

The quantum computer's main drawback is the same as its strength: quantum decoherence. The qubit calculations are performed while the quantum wave function is in a state of superposition between states, which is what allows it to perform the calculations using both 1 & 0 states simultaneously.

However, when a measurement of any type is made to a quantum system, decoherence breaks down and the wave function collapses into a single state. Therefore, the computer has to somehow continue making these calculations without having any measurements made until the proper time, when it can then drop out of the quantum state, have a measurement taken to read its result, which then gets passed on to the rest of the system.

The physical requirements of manipulating a system on this scale are considerable, touching on the realms of superconductors, nanotechnology, and quantum electronics, as well as others. Each of these is itself a sophisticated field which is still being fully developed, so trying to merge them all together into a functional quantum computer is a task which I don't particularly envy anyone ...

except for the person who finally succeeds.

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How Quantum Computers Work

## This is what a 50-qubit quantum computer looks like

That's where the pumps would normally come in. From top to bottom, the system gradually cools from four Kelvin -- liquid-helium temperatures -- to 800 milliKelvin, 100 milliKelvin and, finally, 10 milliKelvin. Inside the canister, that's 10 thousandths of a degree above absolute zero. The wires, meanwhile, carry RF-frequency signals down to the chip. These are then mapped onto the qubits, executing whatever program the research team wishes to run. The wiring is also designed in a way to ensure that no extraneous noise -- including heat -- is transported to the quantum computer chip at the bottom.

Many in the industry have suggested that a 50-qubit system could achieve "quantum supremacy." The term refers to the moment when a quantum computer is able to outperform a traditional system or accomplish a task otherwise thought impossible. The problem, though, is that quantum computers are only compatible with certain algorithms. They're well-suited to quantum chemistry, for instance, and material simulations. But it's unlikely you'll ever use a quantum computer to complete a PowerPoint presentation. "The world is not classical, it's quantum, so if you want to simulate it you need a quantum computer," Welser said.

Researchers have already conducted experiments with quantum computers. Scientists at IBM were able to simulate beryllium hydride (BeH2) on a seven-qubit quantum processor last September, for example. But critics want to see a quantum computer accomplish something more tangible, which is more meaningful for the everyday consumer. That day, unfortunately, could still be a long way off.

"Somewhere between 50 and 100 qubits, we'll reach the point where we can at least say very clearly, 'I've just simulated a molecule here in a few minutes time that would have taken this giant system five days to do.' That level we'll be at fairly rapidly. When it gets to something that the public will understand in terms of an application they would use themselves, I can't really speculate at this point," Welser said.

Click here to catch up on the latest news from CES 2018.

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This is what a 50-qubit quantum computer looks like

## Inside Microsofts quantum computing world | InfoWorld

Quantum computers are the future, says Microsoft CEO Satya Nadella. And he has put Microsofts money where his mouth is, making quantum computing one of the three pillars of Microsofts strategy going forward. Along with AI and mixed/augmented reality, its an area where Nadella believes that Microsoft can make a significant impact, and where it can differentiate itself from its competition.

But building a quantum computer is hard. Microsofts current progress is the result of more than 20 years of research investment, working with universities around the world, mixing pure physics with computer science, and turning experimental ideas into products. Theres a lot of ambition here, with the eventual aim of building scalable quantum computers that anyone can use.

Microsofts approach to quantum computing differs from the technologies used by companies like DWave, taking a new approach to creating the qubits, the quantum bits at the heart of the process. Working with university researchers, Microsoft has been exploring use of a new type of particle, the Majorana fermion. Initially proposed in the late 1930s, Marjorana particles have only recently been detected in semiconductor nanowires at very low temperatures.

Compared to other qubit approaches, the Majorana particles used by Microsofts quantum computers are more stable and have lower error rates, spreading out the electron state across a topological knot thats less likely to evaporate when its state is read. This topological approach to quantum computing is something that Nadella calls a transistor moment for quantum computers. It might not be the quantum processor, but its the first step on that road.

Working with a quantum computer is very different from the machines we use today. A bits 1s and 0s are replaced by a qubit with a statistical blur of fractionalized electrons that needs interpretation. With qubits temperatures at near absolute zero, another specialised low-temperature (cryogenic) computer is used to program the qubits and read results, working with quantum algorithms to solve complex problemsand promising nearly instantaneous answers to problems that could take thousands, or even millions, of years with a modern supercomputer.

You can think of the relationship between the cryogenic controller and programs running on the ultralow-temperature quantum computer as something akin to how deep-sea divers work on underwater oil rigs. The quantum computer is the well head, isolated from the rest of the world by temperature. That makes the cryogenic control computer the equivalent of a divers pressurized diving bell, giving the programs a stepping stone between the normal temperatures of the outside world and the extreme cold of the quantum refrigerator, much like how a diving bell prepares divers for working at extreme depths.

Microsofts quantum computers are unlikely to run in your own datacenters. They require specialized refrigerators to chill the qubits, which are built from carefully grown nanowires. Microsofts consortium of universities can manufacture each part separately, bringing them together to deliver the current generation of test systems.

Microsoft intends to embed its quantum hardware in Azure, running a quantum simulator to help test quantum code before its deployed to actual quantum computers. Microsoft is also working on a new language to help developers write quantum code in Visual Studio.

Microsoft Research has already delivered a first cut at a quantum programming environment in Liqui|> (usually referred to as Liquid), a set of tools to simulate a 30-qubit environment on a PC with 32GB of memory. Microsoft says youll be able to deploy large quantum simulators with more than 40 qubits in 16TB on Azure, though solving problems of that size will take a long time without the acceleration of a real quantum computer.

Still, with Liquid, you can experiment with key quantum computing concepts using F#, seeing how youll build algorithms to handle complex mathematical concepts, as well as understanding how to work with low-level error-correction algorithms.

Microsofts new quantum computing language will build on lessons learned with Liquid, but it wont be based on F#. The languages name hasnt been revealed yet, but amusingly some early screenshots of quantum code being edited in Visual Studio appeared to use the same file extension as the classic Quick Basic.

I recently spoke with Krysta Svore, the lead of Microsoft Research s Redmond Quantum computing group, which works on building the software side of Microsofts planned scalable quantum computer. Its a fascinating side of the project, taking the low-level quantum algorithms needed to work with experimental hardware and finding ways of generating them from familiar high-level languages. If Svores team is successful, you wont need to know about the quantum computer youre programming; instead, youll write code, publish it to Azure, and run it.

The goal is that youll be able to concentrate on your code, not think about the underlying quantum circuitry. For example, instead of building the connections needed to construct a quantum Fourier transform, youll call a QFT library, writing additional code to prepare, load, and read data. As Svore notes, many quantum algorithms are hybrids, mixing preprocessing and postprocessing with quantum actions, often using them as part of loops run in a classical supercomputer.

Theres also a role for AI techniques, using machine learning to identify elements of code, understanding where and how they work best.

Developers who experiment with Liquid will be able to bring their applications to the new platform, with migration tools to help with the transition. Using the Azure-based quantum simulator should help, because it supports many more qubits than a PC does. Itll also let you explore working with execution-based parallelism, where you run multiple passes over the same data, rather than the more familiar GPGPU data parallelism model.

You can get a feel for what this means for computing when you consider an 80-qubit operation. Svore notes that a single operation in a quantum computer takes 100ns, no matter how many qubits you have. The same operation in a classical computer would require more particles than in the visible universe, taking longer than the lifetime of the universe. Solving that type of problem in 100ns is a huge leap forward, one that opens new directions for scientific computing.

Microsofts quantum computing work is a big bet on the future of computing. Today, its a long way from every day use, still in the domain of pure research, even if that research is coming up with promising results.

Where Microsofts quantum-computing work really will make a difference is if it can deliver a programming environment that will let us take hard problems and turn them into quantum algorithms quickly and repeatedly, without having to go beyond the familiar world of IDEs and parallel programming constructs. Getting that right will really change the world, in ways we cant yet imagine.

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Inside Microsofts quantum computing world | InfoWorld

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