In
quantum computing
Quantum computing is a type of computation whose operations can harness the phenomena of quantum mechanics, such as superposition, interference, and entanglement. Devices that perform quantum computations are known as quantum computers. Though ...
, and more specifically in
superconducting quantum computing
Superconducting quantum computing is a branch of solid state quantum computing which implements superconducting electronic circuits using superconducting qubits as artificial atoms, or quantum dots. For superconducting qubits, the two logic sta ...
, a transmon is a type of
superconducting
Superconductivity is a set of physical properties observed in certain materials where electrical resistance vanishes and magnetic flux fields are expelled from the material. Any material exhibiting these properties is a superconductor. Unlike ...
charge qubit that was designed to have reduced sensitivity to charge noise. The transmon was developed by
Robert J. Schoelkopf
Robert J. Schoelkopf III (born January 24, 1964) is an American physicist, most noted for his work on quantum computing as one of the inventors of superconducting qubits. Schoelkopf's main research areas are quantum transport, single-electron d ...
,
Michel Devoret
Michel Devoret is a French physicist and F. W. Beinecke Professor of Applied Physics at Yale University. He also holds a position as the Director of the Applied Physics Nanofabrication Lab at Yale. He is known for his pioneering work on macroscopi ...
,
Steven M. Girvin, and their colleagues at
Yale University
Yale University is a private research university in New Haven, Connecticut. Established in 1701 as the Collegiate School, it is the third-oldest institution of higher education in the United States and among the most prestigious in the wo ...
in 2007.
Its name is an abbreviation of the term ''
transmission line
In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmis ...
shunted
plasma oscillation Plasma oscillations, also known as Langmuir waves (after Irving Langmuir), are rapid oscillations of the electron density in conducting media such as plasmas or metals in the ultraviolet region. The oscillations can be described as an instability ...
qubit
In quantum computing, a qubit () or quantum bit is a basic unit of quantum information—the quantum version of the classic binary bit physically realized with a two-state device. A qubit is a two-state (or two-level) quantum-mechanical system, ...
''; one which consists of a
Cooper-pair box "where the two superconductors are also capacitatively shunted in order to decrease the sensitivity to charge noise, while maintaining a sufficient anharmonicity for selective qubit control".
![4 Qubit, 4 Bus, 4 Resonator IBM Device (Jay M](https://upload.wikimedia.org/wikipedia/commons/a/a4/4_Qubit%2C_4_Bus%2C_4_Resonator_IBM_Device_%28Jay_M._Gambetta%2C_Jerry_M._Chow%2C_and_Matthias_Steffen%2C_2017%29.png)
The transmon achieves its reduced sensitivity to charge noise by significantly increasing the ratio of the
Josephson energy
In physics, the Josephson effect is a phenomenon that occurs when two superconductors are placed in proximity, with some barrier or restriction between them. It is an example of a macroscopic quantum phenomenon, where the effects of quantum mech ...
to the charging energy. This is accomplished through the use of a large shunting capacitor. The result is energy level spacings that are approximately independent of offset charge. Planar on-chip transmon qubits have
''T''1 coherence times ~ 30 μs to 40 μs.
By replacing the superconducting
transmission line
In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmis ...
cavity with a three-dimensional superconducting cavity, recent work on transmon qubits has shown significantly improved ''T''
1 times, as long as 95 μs.
These results demonstrate that previous ''T''
1 times were not limited by
Josephson junction
In physics, the Josephson effect is a phenomenon that occurs when two superconductors are placed in proximity, with some barrier or restriction between them. It is an example of a macroscopic quantum phenomenon, where the effects of quantum mec ...
losses. Understanding the fundamental limits on the coherence time in
superconducting qubits such as the transmon is an active area of research.
Comparison to Cooper-pair box
![Charge2transmon](https://upload.wikimedia.org/wikipedia/commons/d/d8/Charge2transmon.svg)
The transmon design is similar to the first design of the
charge qubit known as a "Cooper-pair box", both are described by the same Hamiltonian, with the only difference being the increase in the
ratio, achieved by shunting the
Josephson junction
In physics, the Josephson effect is a phenomenon that occurs when two superconductors are placed in proximity, with some barrier or restriction between them. It is an example of a macroscopic quantum phenomenon, where the effects of quantum mec ...
with an additional large
capacitor
A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals.
The effect of ...
. Here
is the
Josephson energy
In physics, the Josephson effect is a phenomenon that occurs when two superconductors are placed in proximity, with some barrier or restriction between them. It is an example of a macroscopic quantum phenomenon, where the effects of quantum mech ...
of the junction, and
is the charging energy inversely proportional to the total capacitance of the qubit circuit. The benefit of increasing the
ratio is insensitivity to charge noise - the energy levels become independent of electrical charge across the junction, thus the coherence times of the qubit are prolonged. The disadvantage is decrease in the anharmonicity
, where
is the energy of the state
. Reduced anharmonicity complicates the device operation as a two level system, e.g. exciting the device from the ground state to the first excited state by a resonant pulse also populates the second excited state. This complication is overcome by complex microwave pulse design, that takes into account the higher energy levels, and prohibits their excitation by destructive interference.
Measurement, control and coupling of the transmons is performed by means of microwave resonators with techniques of
circuit quantum electrodynamics
Circuit quantum electrodynamics (circuit QED) provides a means of studying the fundamental interaction between light and matter (quantum optics). As in the field of cavity quantum electrodynamics, a single photon within a single mode cavity coher ...
, also applicable to
other superconducting qubits. The coupling to the resonators is done by putting a capacitor between the qubit and the resonator, at a point where the resonator
electromagnetic field
An electromagnetic field (also EM field or EMF) is a classical (i.e. non-quantum) field produced by (stationary or moving) electric charges. It is the field described by classical electrodynamics (a classical field theory) and is the classical c ...
is biggest. For example, in
IBM Quantum Experience
The IBM Quantum Composer and the IBM Quantum Lab (previously known collectively as the IBM Quantum Experience) form an online platform allowing public and premium access to cloud-based quantum computing services provided by IBM Quantum. This includ ...
devices, the resonators are implemented with "
quarter wave"
coplanar waveguide
Coplanar waveguide is a type of electrical planar transmission line which can be fabricated using printed circuit board technology, and is used to convey microwave-frequency signals. On a smaller scale, coplanar waveguide transmission lines are ...
with maximal field at the signal-ground short at the waveguide end, thus every IBM transmon qubit has a long resonator "tail". The initial proposal included similar
transmission line
In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmis ...
resonators coupled to every transmon, becoming a part of the name. However, charge qubits operated at a similar
regime, coupled to different kinds of microwave cavities are referred to as transmons as well.
See also
*
Charge qubit
*
Anharmonicity
In classical mechanics, anharmonicity is the deviation of a system from being a harmonic oscillator. An oscillator that is not oscillating in harmonic motion is known as an anharmonic oscillator where the system can be approximated to a harmo ...
*
Circuit quantum electrodynamics
Circuit quantum electrodynamics (circuit QED) provides a means of studying the fundamental interaction between light and matter (quantum optics). As in the field of cavity quantum electrodynamics, a single photon within a single mode cavity coher ...
(CQED)
*
Dilution refrigerator
A 3He/4He dilution refrigerator is a cryogenics, cryogenic device that provides continuous cooling to temperatures as low as 2 Kelvin, mK, with no moving parts in the low-temperature region. The cooling power is provided by the heat o ...
*
List of quantum processors
This list contains quantum processors, also known as quantum processing units (QPUs). Some devices listed below have only been announced at press conferences so far, with no actual demonstrations or scientific publications characterizing the per ...
*
*
Superconducting quantum computing
Superconducting quantum computing is a branch of solid state quantum computing which implements superconducting electronic circuits using superconducting qubits as artificial atoms, or quantum dots. For superconducting qubits, the two logic sta ...
References
{{quantum information
Quantum information science
Quantum electronics
Superconductivity