View source for Physics:Quantum machine

{{Short description|Quantum mechanical macroscopic object}}

{{Quantum book backlink|Quantum information and computing}}

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{{distinguish|quantum computer}}

. The mechanical resonator is located to the lower left of the coupling capacitor (small white square).  The [[Qubit|qubit]] is connected to upper right of the coupling capacitor.]]
A '''quantum machine''' is a human-made device whose collective motion follows the laws of [[Physics:Quantum mechanics|quantum mechanics]].  The idea that [[Physics:Macroscopic|macroscopic]] objects may follow the laws of quantum mechanics dates back to the advent of quantum mechanics in the early 20th century.<ref>{{Cite journal |first=E. |last=Schrödinger |title=The present situation in quantum mechanics |journal=Naturwissenschaften |volume=23 |issue= 48|pages=807–812; 823–828; 844–849 |year=1935 |doi=  10.1007/BF01491891|bibcode = 1935NW.....23..807S |s2cid=206795705 }}</ref><ref>{{Cite journal |first=A. J. |last=Leggett |title=Testing the limits of quantum mechanics: motivation, state of play, prospects |journal=J. Phys.: Condens. Matter |volume=14 |issue=15 |pages=R415–R451 |year=2002 |doi=10.1088/0953-8984/14/15/201 |bibcode = 2002JPCM...14R.415L |citeseerx=10.1.1.205.4849 |s2cid=250911999 }}.</ref>  However, as highlighted by the [[Physics:Schrödinger's cat|Schrödinger's cat]] [[Thought experiment|thought experiment]], quantum effects are not readily observable in large-scale objects. Consequently, quantum states of motion have only been observed in special circumstances at extremely low temperatures.  The fragility of quantum effects in macroscopic objects may arise from rapid [[Physics:Quantum decoherence|quantum decoherence]].<ref>{{Cite journal |first=W. H. |last=Zurek |title=Decoherence, einselection, and the quantum origins of the classical |journal=Reviews of Modern Physics |volume=75 |issue=3 |pages=715–765 |doi=10.1103/RevModPhys.75.715 |year=2003 |bibcode=2003RvMP...75..715Z|arxiv = quant-ph/0105127 |title-link=einselection |s2cid=14759237 }}</ref> Researchers created the first quantum machine in 2009, and the achievement was named the "Breakthrough of the Year" by ''Science'' in 2010.
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[[File:QubitMechanicalResonator.jpg|thumb|280px|Aaron D. O'Connell]]
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==History==
[[File:QuantumMachine SEM MechanicalResonator.jpg|thumb|[[Scanning electron micrograph]] of the film bulk acoustic resonator. The mechanically active part of the resonator is supported to the left by two metal leads which act as electrical connections.]]
The first quantum machine was created on August 4, 2009, by [[Biography:Aaron D. O'Connell|Aaron D. O'Connell]] while pursuing his Ph.D. under the direction of Andrew N. Cleland and [[Biography:John M. Martinis|John M. Martinis]] at the [[Organization:University of California, Santa Barbara|University of California, Santa Barbara]].  O'Connell and his colleagues [[Engineering:Coupling|coupled]] together a mechanical [[Physics:Resonator|resonator]], similar to a tiny springboard, and a [[Qubit|qubit]], a device that can be in a [[Physics:Quantum superposition|superposition]] of two quantum states at the same time. They were able to make the resonator vibrate a small amount and a large amount simultaneously—an effect which would be impossible in [[Physics:Classical physics|classical physics]]. The mechanical resonator was just large enough to see with the naked eye—about as long as the width of a human hair.<ref>{{Cite news |first=Alan |last=Boyle |url=http://cosmiclog.msnbc.msn.com/_news/2010/12/16/5660752-the-year-in-science-a-quantum-leap |archive-url=https://web.archive.org/web/20101219143542/http://cosmiclog.msnbc.msn.com/_news/2010/12/16/5660752-the-year-in-science-a-quantum-leap |url-status=dead |archive-date=2010-12-19 |title=The year in science: a quantum leap |publisher=MSNBC |access-date=2010-12-23 }}</ref> The work was subsequently published in the journal ''Nature'' in March 2010.<ref name="nature">{{Cite journal |first1=A. D. |last1=O’Connell |first2=M. |last2=Hofheinz |first3=M. |last3=Ansmann |first4=R. C. |last4=Bialczak |first5=M. |last5=Lenander |first6=E. |last6=Lucero |first7=M. |last7=Neeley |first8=D. |last8=Sank |first9=H. |last9=Wang |display-authors=8|name-list-style=amp |title=Quantum ground state and single-phonon control of a mechanical resonator |journal=Nature |volume=464 |issue= 7289|pages=697–703 |year=2010 |doi=10.1038/nature08967 |bibcode = 2010Natur.464..697O |pmid=20237473 |s2cid=4412475 }}</ref>  The journal ''Science'' declared the creation of the first quantum machine to be the "Breakthrough of the Year" of 2010.<ref>{{cite journal |last=Cho|first=Adrian |title=Breakthrough of the Year: The First Quantum Machine |journal=Science |volume=330 |issue=6011 |pages=1604 |year=2010 |doi=10.1126/science.330.6011.1604|bibcode = 2010Sci...330.1604C |pmid=21163978}}</ref>

===Cooling to the ground state===
In order to demonstrate the quantum mechanical behavior, the team first needed to cool the mechanical resonator until it was in its quantum [[Physics:Ground state|ground state]], the state with the [[Physics:Zero-point energy|lowest possible energy]]. 

A temperature {{nowrap|<math>T \ll \frac{hf}{k}</math>}} was required, where <math>h</math> is the [[Planck constant]], <math>f</math> is the [[Physics:Frequency|frequency]] of the resonator, and <math>k</math> is the [[Physics:Boltzmann constant|Boltzmann constant]].{{cref|a}} 

Previous teams of researchers had struggled with this stage, as a 1&nbsp;MHz resonator, for example, would need to be cooled to the extremely low temperature of 50&nbsp;μK.<ref>Steven Girvin, http://www.condmatjournalclub.org/wp-content/uploads/2010/04/jccm_april2010_013.pdf {{Webarchive|url=https://web.archive.org/web/20160512161850/http://www.condmatjournalclub.org/wp-content/uploads/2010/04/jccm_april2010_013.pdf |date=2016-05-12 }}</ref> O'Connell's team constructed a different type of resonator, a film bulk acoustic resonator,<ref name=nature/> with a much higher resonant frequency (6&nbsp;GHz) which would hence reach its ground state at a (relatively) higher temperature (~0.1&nbsp;K); this temperature could then be easily reached with a [[Physics:Dilution refrigerator|dilution refrigerator]].<ref name=nature/> In the experiment, the resonator was cooled to 25&nbsp;mK.<ref name=nature/>

===Controlling the quantum state===
The film bulk acoustic resonator was made of piezoelectric material, so that as it oscillated its changing shape created a changing electric signal, and conversely an electric signal could affect its oscillations. This property enabled the resonator to be [[Engineering:Coupling|coupled]] with a superconducting [[Phase qubit|phase qubit]], a device used in [[Quantum computing|quantum computing]] whose quantum state can be accurately controlled.

In quantum mechanics, vibrations are made up of elementary vibrations called phonons. Cooling the resonator to its ground state can be seen as equivalent to removing all of the phonons. The team was then able to transfer individual phonons from the qubit to the resonator. The team was also able to transfer a [[Physics:Quantum superposition|superposition]] state, where the qubit was in a superposition of two states at the same time, onto the mechanical resonator.<ref>Markus Aspelmeyer, "Quantum mechanics: the surf is up", ''Nature'' 464, 685–686 (1 April 2010)</ref> This means the resonator "literally vibrated a little and a lot at the same time", according to the ''American Association for the Advancement of Science''.<ref>Brandon Bryn, [http://www.aaas.org/news/releases/2010/1216sp_boy.shtml "Science: The breakthroughs of 2010 and insights of the decade"], American Association for the Advancement of Science, December 16, 2010</ref> The vibrations lasted just a few nanoseconds before being broken down by disruptive outside influences.<ref>Richard Webb, [https://www.newscientist.com/article/dn18669-first-quantum-effects-seen-in-visible-object.html "First quantum effects seen in visible object"], New Scientist, March 17, 2010</ref> In the ''Nature'' paper, the team concluded "This demonstration provides strong evidence that quantum mechanics applies to a mechanical object large enough to be seen with the naked eye."<ref name=nature/>

==Notes==
{{cnote|a|The ground state energy of an oscillator is proportional to its frequency: see [[Physics:Quantum harmonic oscillator|quantum harmonic oscillator]].}}

==References==
{{reflist}}

==External links==
* {{cite journal |last=Cho|first=Adrian |title=Breakthrough of the Year: The First Quantum Machine |journal=Science |volume=330 |issue=6011 |pages=1604  |date=2010-12-17 |url=https://www.science.org/doi/full/10.1126/science.330.6011.1604 |doi=10.1126/science.330.6011.1604|bibcode = 2010Sci...330.1604C |pmid=21163978|url-access=subscription }}
* {{cite journal |last=Brumfiel|first=Geoff |title=Scientists supersize quantum mechanics |journal=Nature|date=2010-03-17 |url=https://www.nature.com/news/2010/100317/full/news.2010.130.html|doi=10.1038/news.2010.130|doi-access=free|url-access=subscription}}
* Aaron D. O'Connell, December 2010, [http://www.physics.ucsb.edu/~martinisgroup/theses/OConnell2010.pdf "A Macroscopic Mechanical Resonator Operated in the Quantum Limit"] {{Webarchive|url=https://web.archive.org/web/20110725093755/http://www.physics.ucsb.edu/~martinisgroup/theses/OConnell2010.pdf |date=2011-07-25 }} (Ph.D. thesis)

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[[Category:Quantum mechanics]]

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