Quantum machine
Encyclopedia
A quantum machine is a human-made device whose collective motion follows the laws of quantum mechanics
. The idea that macroscopic
objects may follow the laws of quantum mechanics dates back to the advent of quantum mechanics in the early 20th century. However, as highlighted by the Schrödinger's cat
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 quantum decoherence
, although currently there is insufficient evidence to support this claim. Researchers created the first quantum machine in 2009, and the achievement was named the "Breakthrough of the Year" by Science
in 2010.
while pursuing his Ph.D. under the direction of Andrew N. Cleland and John M. Martinis at the University of California, Santa Barbara
. O'Connell and his colleagues coupled
together a mechanical resonator
, similar to a tiny springboard, and a qubit
, a device that can be in a 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 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.
The groundbreaking work was subsequently published in the journal Nature
in March 2010. The journal Science
declared the creation of the first quantum machine to be the "Breakthrough of the Year
" of 2010.
(the state with the lowest possible energy
). Specifically, a temperature T « hf/k was required, where h is the Planck Constant
, f is the frequency
of the resonator and k is the Boltzmann constant. Previous teams of researchers had struggled with this stage, as a 1 MHz resonator, for example, would need to be cooled to the extremely low temperature of 50 μK. O'Connell's team constructed a different type of resonator, a film bulk acoustic resonator
, with a much higher resonant frequency (6 GHz) which would hence reach its ground state at a (relatively) higher temperature (~0.1 K): this temperature could then be easily reached with a dilution refrigerator
. In the experiment, the resonator was cooled to 25 mK.
with a superconducting phase qubit
, a device used in 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 superposition
state, where the qubit was in a superposition of two states at the same time, onto the mechanical resonator. 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
. The vibrations lasted just a few nanoseconds before being broken down by disruptive outside influences. 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."
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
. The idea that macroscopic
Macroscopic
The macroscopic scale is the length scale on which objects or processes are of a size which is measurable and observable by the naked eye.When applied to phenomena and abstract objects, the macroscopic scale describes existence in the world as we perceive it, often in contrast to experiences or...
objects may follow the laws of quantum mechanics dates back to the advent of quantum mechanics in the early 20th century. However, as highlighted by the Schrödinger's cat
Schrödinger's cat
Schrödinger's cat is a thought experiment, usually described as a paradox, devised by Austrian physicist Erwin Schrödinger in 1935. It illustrates what he saw as the problem of the Copenhagen interpretation of quantum mechanics applied to everyday objects. The scenario presents a cat that might be...
thought experiment
Thought experiment
A thought experiment or Gedankenexperiment considers some hypothesis, theory, or principle for the purpose of thinking through its consequences...
, 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 quantum decoherence
Quantum decoherence
In quantum mechanics, quantum decoherence is the loss of coherence or ordering of the phase angles between the components of a system in a quantum superposition. A consequence of this dephasing leads to classical or probabilistically additive behavior...
, although currently there is insufficient evidence to support this claim. Researchers created the first quantum machine in 2009, and the achievement was named the "Breakthrough of the Year" by Science
Science (journal)
Science is the academic journal of the American Association for the Advancement of Science and is one of the world's top scientific journals....
in 2010.
The first quantum machine
The first quantum machine was created on August 4, 2009 by Aaron D. O'ConnellAaron D. O'Connell
Aaron Douglas O'Connell is an American experimental quantum physicist. While working under Andrew N. Cleland and John M. Martinis at the University of California, Santa Barbara, he created the world's first quantum machine...
while pursuing his Ph.D. under the direction of Andrew N. Cleland and John M. Martinis at the University of California, Santa Barbara
University of California, Santa Barbara
The University of California, Santa Barbara, commonly known as UCSB or UC Santa Barbara, is a public research university and one of the 10 general campuses of the University of California system. The main campus is located on a site in Goleta, California, from Santa Barbara and northwest of Los...
. O'Connell and his colleagues coupled
Coupling (physics)
In physics, two systems are coupled if they are interacting with each other. Of special interest is the coupling of two vibratory systems by means of springs or magnetic fields, etc...
together a mechanical resonator
Resonator
A resonator is a device or system that exhibits resonance or resonant behavior, that is, it naturally oscillates at some frequencies, called its resonant frequencies, with greater amplitude than at others. The oscillations in a resonator can be either electromagnetic or mechanical...
, similar to a tiny springboard, and a qubit
Qubit
In quantum computing, a qubit or quantum bit is a unit of quantum information—the quantum analogue of the classical bit—with additional dimensions associated to the quantum properties of a physical atom....
, a device that can be in a superposition
Superposition
Superposition can refer to:* The superposition principle in physics, mathematics, and engineering, describes the overlapping of waves. Particular applications include :** Quantum superposition, in quantum physics** Superposition theorem, in electronics....
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 classical physics
Classical physics
What "classical physics" refers to depends on the context. When discussing special relativity, it refers to the Newtonian physics which preceded relativity, i.e. the branches of physics based on principles developed before the rise of relativity and quantum mechanics...
. The mechanical resonator was just large enough to see with the naked eye—about as long as the width of a human hair.
The groundbreaking work was subsequently published in the journal Nature
Nature (journal)
Nature, first published on 4 November 1869, is ranked the world's most cited interdisciplinary scientific journal by the Science Edition of the 2010 Journal Citation Reports...
in March 2010. The journal Science
Science (journal)
Science is the academic journal of the American Association for the Advancement of Science and is one of the world's top scientific journals....
declared the creation of the first quantum machine to be the "Breakthrough of the Year
Breakthrough of the Year
The Breakthrough of the Year is an annual award made by the journal Science for the most significant development in scientific research. Originating in 1989 as the Molecule of the Year, inspired by Time's Man of the Year, it was renamed the Breakthrough of the Year in 1996...
" of 2010.
Cooling to the ground state
In order to demonstrate the quantum mechanical behaviour, the team first needed to cool the mechanical resonator until it was in its quantum ground stateGround state
The ground state of a quantum mechanical system is its lowest-energy state; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than the ground state...
(the state with the lowest possible energy
Zero-point energy
Zero-point energy is the lowest possible energy that a quantum mechanical physical system may have; it is the energy of its ground state. All quantum mechanical systems undergo fluctuations even in their ground state and have an associated zero-point energy, a consequence of their wave-like nature...
). Specifically, a temperature T « hf/k was required, where h is the Planck Constant
Planck constant
The Planck constant , also called Planck's constant, is a physical constant reflecting the sizes of energy quanta in quantum mechanics. It is named after Max Planck, one of the founders of quantum theory, who discovered it in 1899...
, f is the frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
of the resonator and k is the Boltzmann constant. Previous teams of researchers had struggled with this stage, as a 1 MHz resonator, for example, would need to be cooled to the extremely low temperature of 50 μK. O'Connell's team constructed a different type of resonator, a film bulk acoustic resonator
Thin film bulk acoustic resonator
Thin Film Bulk Acoustic Resonator is a device consisting of a piezoelectric material sandwiched between two electrodes and acoustically isolated from the surrounding medium. FBAR devices using piezoelectric films with thicknesses ranging from several micrometres down to tenth of micrometres...
, with a much higher resonant frequency (6 GHz) which would hence reach its ground state at a (relatively) higher temperature (~0.1 K): this temperature could then be easily reached with a dilution refrigerator
Dilution refrigerator
A dilution refrigerator is a cryogenic device first proposed by Heinz London. Its refrigeration process uses a mixture of two isotopes of helium: helium-3 and helium-4...
. In the experiment, the resonator was cooled to 25 mK.
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 coupledCoupling (physics)
In physics, two systems are coupled if they are interacting with each other. Of special interest is the coupling of two vibratory systems by means of springs or magnetic fields, etc...
with a superconducting phase qubit
Phase qubit
The phase qubit is a superconducting device based on the superconductor-insulator-superconductor Josephson junction, designed to operate as a quantum bit, or qubit. The phase qubit is closely related, yet distinct from, the flux qubit and the charge qubit, which are also quantum bits implemented...
, a device used in 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 superposition
Quantum superposition
Quantum superposition is a fundamental principle of quantum mechanics. It holds that a physical system exists in all its particular, theoretically possible states simultaneously; but, when measured, it gives a result corresponding to only one of the possible configurations.Mathematically, it...
state, where the qubit was in a superposition of two states at the same time, onto the mechanical resonator. 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
American Association for the Advancement of Science
The American Association for the Advancement of Science is an international non-profit organization with the stated goals of promoting cooperation among scientists, defending scientific freedom, encouraging scientific responsibility, and supporting scientific education and science outreach for the...
. The vibrations lasted just a few nanoseconds before being broken down by disruptive outside influences. 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."
External links
- Aaron D. O'Connell, December 2010, "A Macroscopic Mechanical Resonator Operated in the Quantum Limit" (PhD Thesis)