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建造大型量子计算机存在许多技术挑战。物理学家 David DiVincenzo 列出了实用量子计算机的下列要求:

建造大型量子计算机存在许多技术挑战。物理学家 David DiVincenzo 列出了实用量子计算机的下列要求:

== Obstacles ==

== Obstacles 阻碍==

There are a number of technical challenges in building a large-scale quantum computer.<ref>{{cite journal |last=Dyakonov |first=Mikhail |url=https://spectrum.ieee.org/computing/hardware/the-case-against-quantum-computing |title=The Case Against Quantum Computing |journal=[[IEEE Spectrum]] |date=2018-11-15}}</ref> Physicist [[David P. DiVincenzo|David DiVincenzo]] has listed the following [[DiVincenzo's criteria|requirements]] for a practical quantum computer:<ref>{{cite journal| arxiv=quant-ph/0002077|title=The Physical Implementation of Quantum Computation|last=DiVincenzo |first=David P.|date=2000-04-13|doi=10.1002/1521-3978(200009)48:9/11<771::AID-PROP771>3.0.CO;2-E|volume=48|issue=9–11|journal=Fortschritte der Physik|pages=771–783|bibcode=2000ForPh..48..771D}}</ref>

There are a number of technical challenges in building a large-scale quantum computer.<ref>{{cite journal |last=Dyakonov |first=Mikhail |url=https://spectrum.ieee.org/computing/hardware/the-case-against-quantum-computing |title=The Case Against Quantum Computing |journal=[[IEEE Spectrum]] |date=2018-11-15}}</ref> Physicist [[David P. DiVincenzo|David DiVincenzo]] has listed the following [[DiVincenzo's criteria|requirements]] for a practical quantum computer:<ref>{{cite journal| arxiv=quant-ph/0002077|title=The Physical Implementation of Quantum Computation|last=DiVincenzo |first=David P.|date=2000-04-13|doi=10.1002/1521-3978(200009)48:9/11<771::AID-PROP771>3.0.CO;2-E|volume=48|issue=9–11|journal=Fortschritte der Physik|pages=771–783|bibcode=2000ForPh..48..771D}}</ref>

* Scalable physically to increase the number of qubits

* Scalable physically to increase the number of qubits

 

* Qubits that can be initialized to arbitrary values

* Qubits that can be initialized to arbitrary values

 

* Quantum gates that are faster than [[decoherence]] time

* Quantum gates that are faster than [[decoherence]] time

 

* Universal gate set

* Universal gate set

Sourcing parts for quantum computers is also very difficult. Many quantum computers, like those constructed by Google and IBM, need Helium-3, a nuclear research byproduct, and special superconducting cables that are only made by the Japanese company Coax Co.

Sourcing parts for quantum computers is also very difficult. Many quantum computers, like those constructed by Google and IBM, need Helium-3, a nuclear research byproduct, and special superconducting cables that are only made by the Japanese company Coax Co.

* Qubits that can be read easily

* Qubits that can be read easily

 

Sourcing parts for quantum computers is also very difficult. Many quantum computers, like those constructed by [[Google]] and [[IBM]], need [[Helium-3]], a [[Nuclear physics|nuclear]] research byproduct, and special [[superconducting]] cables that are only made by the Japanese company Coax Co.<ref>{{cite news |last1=Giles |first1=Martin |title=We'd have more quantum computers if it weren't so hard to find the damn cables |url=https://www.technologyreview.com/s/612760/quantum-computers-component-shortage/ |publisher=MIT Technology Review |date=17 January 2019}}</ref>

Sourcing parts for quantum computers is also very difficult. Many quantum computers, like those constructed by [[Google]] and [[IBM]], need [[Helium-3]], a [[Nuclear physics|nuclear]] research byproduct, and special [[superconducting]] cables that are only made by the Japanese company Coax Co.<ref>{{cite news |last1=Giles |first1=Martin |title=We'd have more quantum computers if it weren't so hard to find the damn cables |url=https://www.technologyreview.com/s/612760/quantum-computers-component-shortage/ |publisher=MIT Technology Review |date=17 January 2019}}</ref>

The control of multi qubit systems requires the generation and coordination of a large number of electrical signals with tight and deterministic timing resolution. This had led to the development of [[quantum controllers]] which enable interfacing the qubit.  Scaling these systems to support a growing number of qubits is an additional challenge in the scaling of quantum computers.{{Citation needed|date=August 2020}}   

The control of multi qubit systems requires the generation and coordination of a large number of electrical signals with tight and deterministic timing resolution. This had led to the development of [[quantum controllers]] which enable interfacing the qubit.  Scaling these systems to support a growing number of qubits is an additional challenge in the scaling of quantum computers.{{Citation needed|date=August 2020}}   

 

=== Quantum decoherence 量子退相干===

=== Quantum decoherence ===

One of the greatest challenges involved with constructing quantum computers is controlling or removing quantum decoherence. This usually means isolating the system from its environment as interactions with the external world cause the system to decohere. However, other sources of decoherence also exist. Examples include the quantum gates, and the lattice vibrations and background thermonuclear spin of the physical system used to implement the qubits. Decoherence is irreversible, as it is effectively non-unitary, and is usually something that should be highly controlled, if not avoided. Decoherence times for candidate systems in particular, the transverse relaxation time T₂ (for NMR and MRI technology, also called the dephasing time), typically range between nanoseconds and seconds at low temperature. Currently, some quantum computers require their qubits to be cooled to 20 millikelvins in order to prevent significant decoherence. A 2020 study argues that ionizing radiation such as cosmic rays can nevertheless cause certain systems to decohere within millisections.

One of the greatest challenges involved with constructing quantum computers is controlling or removing quantum decoherence. This usually means isolating the system from its environment as interactions with the external world cause the system to decohere. However, other sources of decoherence also exist. Examples include the quantum gates, and the lattice vibrations and background thermonuclear spin of the physical system used to implement the qubits. Decoherence is irreversible, as it is effectively non-unitary, and is usually something that should be highly controlled, if not avoided. Decoherence times for candidate systems in particular, the transverse relaxation time T₂ (for NMR and MRI technology, also called the dephasing time), typically range between nanoseconds and seconds at low temperature. Currently, some quantum computers require their qubits to be cooled to 20 millikelvins in order to prevent significant decoherence. A 2020 study argues that ionizing radiation such as cosmic rays can nevertheless cause certain systems to decohere within millisections.

构建量子计算机的最大挑战之一是控制或移除量子退相干。这通常意味着将系统与其环境隔离开来，因为与外部世界的相互作用会导致系统解码。然而，退相干的其他来源也存在。例子包括量子门、晶格振动和用于实现量子位的物理系统的背景热核自旋。退相干是不可逆的，因为它实际上是非幺正的，而且通常应该受到高度控制，如果不能避免的话。特别是候选系统的退相干时间，横向弛豫时间 t < sub > 2 </sub > (对于 NMR 和 MRI 技术，也称为退相时间) ，通常在低温下纳秒到秒之间。目前，一些量子计算机需要将它们的量子比特冷却到20毫升以防止明显的退相干。2020年的一项研究认为，像宇宙射线这样的电离辐射可以引起某些系统在毫秒内退相干。

构建量子计算机的最大挑战之一是控制或移除'''<font color="#ff8000"> 量子退相干Quantum decoherence</font>'''。这通常意味着将系统与其环境隔离，因为与外部世界的交互会导致系统去中心化。然而，也存在其他的消相干源。例如量子门，晶格振动和用于实现量子比特的物理系统的背景热核自旋。退相干是不可逆的，因为它实际上是非幺正的，如果不能避免的话，通常是应该高度控制的。对于候选系统，尤其是横向弛豫时间T₂（对于核磁共振和核磁共振技术，也称为去相时间），在低温下通常在纳秒和秒之间。目前，一些量子计算机要求将量子比特冷却到20毫开尔文，以防止严重的退相干。2020年的一项研究认为，宇宙射线等电离辐射仍然可以导致某些系统在毫秒范围内发生衰变。

 

{{Main|Quantum decoherence}}

{{Main|Quantum decoherence}}

One of the greatest challenges involved with constructing quantum computers is controlling or removing [[quantum decoherence]]. This usually means isolating the system from its environment as interactions with the external world cause the system to decohere. However, other sources of decoherence also exist. Examples include the quantum gates, and the lattice vibrations and background thermonuclear spin of the physical system used to implement the qubits. Decoherence is irreversible, as it is effectively non-unitary, and is usually something that should be highly controlled, if not avoided. Decoherence times for candidate systems in particular, the transverse relaxation time ''T''₂ (for [[Nuclear magnetic resonance|NMR]] and [[MRI]] technology, also called the ''dephasing time''), typically range between nanoseconds and seconds at low temperature.<ref name="DiVincenzo 1995">{{cite journal |last=DiVincenzo |first=David P. |title=Quantum Computation |journal=Science |year=1995 |volume=270 |issue=5234 |pages=255–261 |doi= 10.1126/science.270.5234.255 |bibcode = 1995Sci...270..255D |citeseerx=10.1.1.242.2165 |s2cid=220110562 }} {{subscription required}}</ref> Currently, some quantum computers require their qubits to be cooled to 20 millikelvins in order to prevent significant decoherence.<ref>{{cite journal|last1=Jones|first1=Nicola|title=Computing: The quantum company|journal=Nature|date=19 June 2013|volume=498|issue=7454|pages=286–288|doi=10.1038/498286a|pmid=23783610|bibcode=2013Natur.498..286J|doi-access=free}}</ref> A 2020 study argues that [[ionizing radiation]] such as [[cosmic rays]] can nevertheless cause certain systems to decohere within millisections.<ref>{{cite journal |last1=Vepsäläinen |first1=Antti P. |last2=Karamlou |first2=Amir H. |last3=Orrell |first3=John L. |last4=Dogra |first4=Akshunna S. |last5=Loer |first5=Ben |last6=Vasconcelos |first6=Francisca |last7=Kim |first7=David K. |last8=Melville |first8=Alexander J. |last9=Niedzielski |first9=Bethany M. |last10=Yoder |first10=Jonilyn L. |last11=Gustavsson |first11=Simon |last12=Formaggio |first12=Joseph A. |last13=VanDevender |first13=Brent A. |last14=Oliver |first14=William D. |display-authors=5 |title=Impact of ionizing radiation on superconducting qubit coherence |journal=Nature |date=August 2020 |volume=584 |issue=7822 |pages=551–556 |doi=10.1038/s41586-020-2619-8 |pmid=32848227 |url=https://www.nature.com/articles/s41586-020-2619-8 |language=en |issn=1476-4687|arxiv=2001.09190 |s2cid=210920566 }}</ref>

One of the greatest challenges involved with constructing quantum computers is controlling or removing [[quantum decoherence]]. This usually means isolating the system from its environment as interactions with the external world cause the system to decohere. However, other sources of decoherence also exist. Examples include the quantum gates, and the lattice vibrations and background thermonuclear spin of the physical system used to implement the qubits. Decoherence is irreversible, as it is effectively non-unitary, and is usually something that should be highly controlled, if not avoided. Decoherence times for candidate systems in particular, the transverse relaxation time ''T''₂ (for [[Nuclear magnetic resonance|NMR]] and [[MRI]] technology, also called the ''dephasing time''), typically range between nanoseconds and seconds at low temperature.<ref name="DiVincenzo 1995">{{cite journal |last=DiVincenzo |first=David P. |title=Quantum Computation |journal=Science |year=1995 |volume=270 |issue=5234 |pages=255–261 |doi= 10.1126/science.270.5234.255 |bibcode = 1995Sci...270..255D |citeseerx=10.1.1.242.2165 |s2cid=220110562 }} {{subscription required}}</ref> Currently, some quantum computers require their qubits to be cooled to 20 millikelvins in order to prevent significant decoherence.<ref>{{cite journal|last1=Jones|first1=Nicola|title=Computing: The quantum company|journal=Nature|date=19 June 2013|volume=498|issue=7454|pages=286–288|doi=10.1038/498286a|pmid=23783610|bibcode=2013Natur.498..286J|doi-access=free}}</ref> A 2020 study argues that [[ionizing radiation]] such as [[cosmic rays]] can nevertheless cause certain systems to decohere within millisections.<ref>{{cite journal |last1=Vepsäläinen |first1=Antti P. |last2=Karamlou |first2=Amir H. |last3=Orrell |first3=John L. |last4=Dogra |first4=Akshunna S. |last5=Loer |first5=Ben |last6=Vasconcelos |first6=Francisca |last7=Kim |first7=David K. |last8=Melville |first8=Alexander J. |last9=Niedzielski |first9=Bethany M. |last10=Yoder |first10=Jonilyn L. |last11=Gustavsson |first11=Simon |last12=Formaggio |first12=Joseph A. |last13=VanDevender |first13=Brent A. |last14=Oliver |first14=William D. |display-authors=5 |title=Impact of ionizing radiation on superconducting qubit coherence |journal=Nature |date=August 2020 |volume=584 |issue=7822 |pages=551–556 |doi=10.1038/s41586-020-2619-8 |pmid=32848227 |url=https://www.nature.com/articles/s41586-020-2619-8 |language=en |issn=1476-4687|arxiv=2001.09190 |s2cid=210920566 }}</ref>

 

These issues are more difficult for optical approaches as the timescales are orders of magnitude shorter and an often-cited approach to overcoming them is optical pulse shaping. Error rates are typically proportional to the ratio of operating time to decoherence time, hence any operation must be completed much more quickly than the decoherence time.

These issues are more difficult for optical approaches as the timescales are orders of magnitude shorter and an often-cited approach to overcoming them is optical pulse shaping. Error rates are typically proportional to the ratio of operating time to decoherence time, hence any operation must be completed much more quickly than the decoherence time.

As described in the Quantum threshold theorem, if the error rate is small enough, it is thought to be possible to use quantum error correction to suppress errors and decoherence. This allows the total calculation time to be longer than the decoherence time if the error correction scheme can correct errors faster than decoherence introduces them. An often cited figure for the required error rate in each gate for fault-tolerant computation is 10⁻³, assuming the noise is depolarizing.

As described in the Quantum threshold theorem, if the error rate is small enough, it is thought to be possible to use quantum error correction to suppress errors and decoherence. This allows the total calculation time to be longer than the decoherence time if the error correction scheme can correct errors faster than decoherence introduces them. An often cited figure for the required error rate in each gate for fault-tolerant computation is 10⁻³, assuming the noise is depolarizing.

正如量子阈值定理所描述的那样，如果误差率足够小，人们认为可以利用量子误差修正来抑制误差和退相干。这使得总计算时间比消相干时间更长，如果纠错方案能够比消相干引入的误差更快地纠正误差。假设噪声是去极化的，容错计算中每个门所需的错误率经常被引用的数字是10 < sup >-3 </sup > 。

正如量子阈值定理所描述的那样，如果误差率足够小，人们认为可以利用量子误差修正来抑制误差和退相干。这使得总计算时间比消相干时间更长，如果纠错方案能够比消相干引入的误差更快地纠正误差。假设噪声是去极化的，容错计算中每个门所需的错误率经常被引用的数字是10⁻³。

These issues are more difficult for optical approaches as the timescales are orders of magnitude shorter and an often-cited approach to overcoming them is optical [[pulse shaping]]. Error rates are typically proportional to the ratio of operating time to decoherence time, hence any operation must be completed much more quickly than the decoherence time.

These issues are more difficult for optical approaches as the timescales are orders of magnitude shorter and an often-cited approach to overcoming them is optical [[pulse shaping]]. Error rates are typically proportional to the ratio of operating time to decoherence time, hence any operation must be completed much more quickly than the decoherence time.

Meeting this scalability condition is possible for a wide range of systems. However, the use of error correction brings with it the cost of a greatly increased number of required qubits. The number required to factor integers using Shor's algorithm is still polynomial, and thought to be between L and L², where L is the number of qubits in the number to be factored; error correction algorithms would inflate this figure by an additional factor of L. For a 1000-bit number, this implies a need for about 10⁴ bits without error correction. With error correction, the figure would rise to about 10⁷ bits. Computation time is about L² or about 10⁷ steps and at 1&nbsp;MHz, about 10 seconds.

满足这种可伸缩性条件对于各种系统都是可能的。然而，纠错的使用带来了大量增加所需量子比特的代价。使用Shor算法对整数进行因子运算所需的数量仍然是多项式的，并且被认为在L和L²之间，其中L是要被分解的数量中的量子位数；纠错算法将使这个数字膨胀一个额外的系数L。对于1000位的数字，这意味着需要大约10⁴位没有纠错。通过纠错，这个数字将上升到大约10⁷位。计算时间约为L² 或约 10⁷步，在1兆赫时，大约10秒。

As described in the [[Quantum threshold theorem]], if the error rate is small enough, it is thought to be possible to use quantum error correction to suppress errors and decoherence. This allows the total calculation time to be longer than the decoherence time if the error correction scheme can correct errors faster than decoherence introduces them. An often cited figure for the required error rate in each gate for fault-tolerant computation is 10⁻³, assuming the noise is depolarizing.

As described in the [[Quantum threshold theorem]], if the error rate is small enough, it is thought to be possible to use quantum error correction to suppress errors and decoherence. This allows the total calculation time to be longer than the decoherence time if the error correction scheme can correct errors faster than decoherence introduces them. An often cited figure for the required error rate in each gate for fault-tolerant computation is 10⁻³, assuming the noise is depolarizing.

 

A very different approach to the stability-decoherence problem is to create a topological quantum computer with anyons, quasi-particles used as threads and relying on braid theory to form stable logic gates.

A very different approach to the stability-decoherence problem is to create a topological quantum computer with anyons, quasi-particles used as threads and relying on braid theory to form stable logic gates.

A very different approach to the stability-decoherence problem is to create a [[topological quantum computer]] with [[anyon]]s, [[quasi-particle]]s used as threads and relying on [[braid theory]] to form stable logic gates.<ref>{{cite journal

A very different approach to the stability-decoherence problem is to create a [[topological quantum computer]] with [[anyon]]s, [[quasi-particle]]s used as threads and relying on [[braid theory]] to form stable logic gates.<ref>{{cite journal

"So the number of continuous parameters describing the state of such a useful quantum computer at any given moment must be... about 10³⁰⁰... Could we ever learn to control the more than 10³⁰⁰ continuously variable parameters defining the quantum state of such a system? My answer is simple. No, never."

"So the number of continuous parameters describing the state of such a useful quantum computer at any given moment must be... about 10³⁰⁰... Could we ever learn to control the more than 10³⁰⁰ continuously variable parameters defining the quantum state of such a system? My answer is simple. No, never."

“因此，描述这样一个有用的量子计算机在任何给定时刻的状态的连续参数的数量必须是... ... 大约10 < sup > 300 </sup > ... ... 我们能否学会控制定义这样一个系统的量子态的超过10 < sup > 300 </sup > 连续可变参数？我的答案很简单。不，永远不会。”

“因此，描述这样一个有用的量子计算机在任何给定时刻的状态的连续参数的数量必须是... ... 大约10³⁰⁰ ... ... 我们能否学会控制定义这样一个系统的量子态的超过10³⁰⁰ 连续可变参数？我的答案很简单。不，永远不会。”

  | last1 = Freedman | first1 = Michael H. | author1-link = Michael Freedman

  | last1 = Freedman | first1 = Michael H. | author1-link = Michael Freedman

For physically implementing a quantum computer, many different candidates are being pursued, among them (distinguished by the physical system used to realize the qubits):

For physically implementing a quantum computer, many different candidates are being pursued, among them (distinguished by the physical system used to realize the qubits):

  | year = 2003}}</ref><ref>{{cite journal |last=Monroe |first=Don |url=https://www.newscientist.com/channel/fundamentals/mg20026761.700-anyons-the-breakthrough-quantum-computing-needs.html |title=Anyons: The breakthrough quantum computing needs? |journal=[[New Scientist]] |date=2008-10-01}}</ref>

  | year = 2003}}</ref><ref>{{cite journal |last=Monroe |first=Don |url=https://www.newscientist.com/channel/fundamentals/mg20026761.700-anyons-the-breakthrough-quantum-computing-needs.html |title=Anyons: The breakthrough quantum computing needs? |journal=[[New Scientist]] |date=2008-10-01}}</ref>

Physicist [[Mikhail Dyakonov]] has expressed skepticism of quantum computing as follows:

Physicist [[Mikhail Dyakonov]] has expressed skepticism of quantum computing as follows:

:"So the number of continuous parameters describing the state of such a useful quantum computer at any given moment must be... about 10³⁰⁰... Could we ever learn to control the more than 10³⁰⁰ continuously variable parameters defining the quantum state of such a system? My answer is simple. ''No, never.''"<ref>{{cite web |last1=Dyakonov |first1=Mikhail |title=The Case Against Quantum Computing |url=https://spectrum.ieee.org/computing/hardware/the-case-against-quantum-computing |website=IEEE Spectrum |accessdate=3 December 2019}}</ref><ref>{{cite book |last1=Dyakonov |first1=Mikhail |title=Will We Ever Have a Quantum Computer? |date=24 March 2020 |url=https://www.springer.com/gp/book/9783030420185 |publisher=Springer |isbn=9783030420185 |accessdate=22 May 2020}}{{page needed|date=May 2020}}</ref>

:"So the number of continuous parameters describing the state of such a useful quantum computer at any given moment must be... about 10³⁰⁰... Could we ever learn to control the more than 10³⁰⁰ continuously variable parameters defining the quantum state of such a system? My answer is simple. ''No, never.''"<ref>{{cite web |last1=Dyakonov |first1=Mikhail |title=The Case Against Quantum Computing |url=https://spectrum.ieee.org/computing/hardware/the-case-against-quantum-computing |website=IEEE Spectrum |accessdate=3 December 2019}}</ref><ref>{{cite book |last1=Dyakonov |first1=Mikhail |title=Will We Ever Have a Quantum Computer? |date=24 March 2020 |url=https://www.springer.com/gp/book/9783030420185 |publisher=Springer |isbn=9783030420185 |accessdate=22 May 2020}}{{page needed|date=May 2020}}</ref>

 

== Developments ==

== Developments ==