U.S. patent application number 14/632505 was filed with the patent office on 2016-09-01 for systems and methods for suppressing magnetically active surface defects in superconducting circuits.
The applicant listed for this patent is Pradeep Kumar, Robert Francis McDermott III. Invention is credited to Pradeep Kumar, Robert Francis McDermott III.
Application Number | 20160254434 14/632505 |
Document ID | / |
Family ID | 56799170 |
Filed Date | 2016-09-01 |
United States Patent
Application |
20160254434 |
Kind Code |
A1 |
McDermott III; Robert Francis ;
et al. |
September 1, 2016 |
Systems and Methods for Suppressing Magnetically Active Surface
Defects in Superconducting Circuits
Abstract
Systems and methods for suppressing magnetically active surface
defects in superconducting quantum circuits are provided. A method
includes providing one or more superconducting quantum circuits,
and arranging the one or more superconducting quantum circuits in a
hermetic enclosure capable of isolating the one or more
superconducting circuits from ambient surroundings. The method also
includes controlling an environment in the hermetic enclosure to
suppress magnetically active surface defects associated with the
one or more superconducting quantum circuits. In some aspects, the
method further includes introducing an inert gas into the hermetic
enclosure to passivate a surface of the one or more superconducting
quantum circuits. In other aspects, the method further includes
coating a surface of the one or more superconducting circuits with
a non-magnetic encapsulation layer. In yet other aspects, the
method further includes irradiating the one or more superconducting
circuits using ultraviolet light.
Inventors: |
McDermott III; Robert Francis;
(Madison, WI) ; Kumar; Pradeep; (Madison,
WI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
McDermott III; Robert Francis
Kumar; Pradeep |
Madison
Madison |
WI
WI |
US
US |
|
|
Family ID: |
56799170 |
Appl. No.: |
14/632505 |
Filed: |
February 26, 2015 |
Current U.S.
Class: |
505/170 |
Current CPC
Class: |
H01L 39/045 20130101;
H01L 39/2493 20130101; G06N 10/00 20190101; H01L 27/18
20130101 |
International
Class: |
H01L 39/04 20060101
H01L039/04; H01L 39/24 20060101 H01L039/24; H01L 27/18 20060101
H01L027/18 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0001] This invention was made with government support under
W911NF-10-1-0494 and W911NF-09-1-0375 awarded by the ARMY/ARO. The
government has certain rights in the invention.
Claims
1. A method for reducing magnetic noise in qubit circuits, the
method comprising: providing one or more qubit circuits; arranging
the one or more qubit circuits in a hermetic enclosure capable of
isolating the one or more qubit circuits from ambient surroundings;
and controlling a background pressure of one or more magnetically
active species in the hermetic enclosure to suppress magnetic
active surface defects associated with the one or more qubit
circuits.
2. The method of claim 1, wherein the one or more magnetically
active species comprises oxygen.
3. The method of claim 1, the method further comprising elevating a
temperature of the hermetic enclosure.
4. The method of claim 1, the method further comprising irradiating
the one or more qubit circuits using ultraviolet light.
5. The method of claim 1, the method further comprising passivating
a surface of the one or more qubit circuits.
6. The method of claim 1, the method further comprising coating a
surface of the one or more qubit circuits with a non-magnetic
encapsulation layer.
7. The method of claim 1, the method further comprising introducing
an inert gas into the hermetic enclosure.
8. The method of claim 7, wherein the inert gas includes an ammonia
gas.
9. The method of claim 1, wherein the background pressure is less
than about 110.sup.-6 Torr.
10. A system for suppressing magnetically active surface defects in
superconducting quantum circuits, the system comprising: a hermetic
enclosure configured to accommodate therein one or more
superconducting quantum circuits, and capable of isolating the one
or more superconducting circuits from ambient surroundings; and a
vacuum system removably coupled to the hermetic enclosure, and
configured to control an environment in the hermetic enclosure such
that magnetic active surface defects associated with the one or
more superconducting quantum circuits are suppressed.
11. The system of claim 10, wherein the one or more superconducting
quantum circuits includes at least one qubit circuit.
12. The system of claim 10, wherein the vacuum system is further
configured to control a background pressure of one or more
magnetically active species in the hermetic enclosure.
13. The system of claim 12, wherein the background pressure is less
than about 110.sup.-6 Torr.
14. The system of claim 12, wherein the magnetically active species
comprises oxygen.
15. The system of claim 10, wherein the vacuum system is further
configured to introduce an inert gas in the hermetic enclosure to
passivate a surface of the one or more superconducting quantum
circuits.
16. The system of claim 15, wherein the inert gas includes an
ammonia gas.
17. The system of claim 10, the hermetic enclosure further
comprising a light source capable of irradiating the one or more
superconducting circuits using ultraviolet light.
18. The system of claim 10, the system further comprising a heat
source for elevating a temperature of the hermetic enclosure.
19. A method for suppressing magnetically active surface defects in
superconducting quantum circuits, the method comprising: providing
one or more superconducting quantum circuits; arranging the one or
more superconducting quantum circuits in a hermetic enclosure; and
controlling an environment in the hermetic enclosure to suppress
magnetic active surface defects associated with the one or more
superconducting quantum circuits.
20. The method of claim 19, wherein the one or more superconducting
quantum circuits includes at least one qubit circuit.
21. The method of claim 19, wherein controlling the environment
includes reducing a background pressure of one or more magnetically
active species in the hermetic enclosure using a vacuum system
coupled to the hermetic enclosure.
22. The method of claim 21, wherein the magnetically active species
comprises oxygen.
23. The method of claim 19, the method further comprising elevating
a temperature of the hermetic enclosure.
24. The method of claim 19, the method further comprising
irradiating the one or more superconducting circuits using
ultraviolet light.
25. The method of claim 19, the method further comprising
passivating a surface of the one or more superconducting quantum
circuits.
26. The method of claim 19, the method further comprising coating a
surface of the one or more superconducting quantum circuits with a
non-magnetic encapsulation layer.
27. The method of claim 19, the method further comprising
introducing an inert gas into the hermetic enclosure to passivate a
surface of the one or more superconducting quantum circuits.
28. The method of claim 27, wherein the inert gas includes an
ammonia gas.
Description
BACKGROUND OF THE INVENTION
[0002] The field of the disclosure is directed to superconducting
quantum circuits and devices. More particularly, the disclosure is
directed to systems and methods related to quantum information
processing and quantum computation.
[0003] Superconducting integrated circuits are finding increased
use in a variety of applications. For instance, in the field of
quantum computation, the performance of superconducting quantum
bits ("qubits") has advanced rapidly in recent years, with
preliminary multi-qubit implementations leading toward scalable,
surface code architectures. In contrast to classical computational
methods that rely on binary data stored in the form of definite
on/off states, or bits, methods in quantum computation take
advantage of the quantum mechanical nature of superconducting
quantum systems, which may be represented using a superposition of
multiple quantum states.
[0004] However, maintaining a superposition state is challenging
for practical implementations. This is because various sources of
noise induce a loss of quantum ordering, or coherence in the phase
angles between the different components of the system in quantum
superposition. Such dephasing makes the realization of quantum
computers difficult, since sufficient preservation of coherent
quantum states is required in order to perform useful computation.
For superconducting qubits, low-frequency magnetic flux noise is a
dominant source of dephasing, resulting in appreciable errors when
implemented in large-scale circuits. In addition, the magnitude of
flux noise is roughly universal across various different device
materials and fabrication processes. Despite thirty years of
research, there has been no successful demonstration of reducing
this noise, placing severe limitations on progress in quantum
information processing and quantum computation.
[0005] In general, during the fabrication process, superconducting
devices are exposed to ambient atmospheric surroundings for
extended periods of time. Subsequently, in operation, the
superconducting devices are cooled to low temperatures, typically
using vacuum cryostats that maintain poor background pressure,
allowing the adsorption of a high density of magnetically active
defects. Such defects can produce low-frequency magnetic flux noise
that leads to strong dephasing. In the case of qubit devices, some
efforts to avoid magnetic flux noise have been made by operating
the devices at fixed frequencies where the qubit is insensitive to
first order to magnetic flux fluctuations. However, such
implementations severely constrain the architectures of multi-qubit
circuits and make scaling to larger systems a major challenge.
[0006] In light of the above, there remains a need for novel
approaches that address noise sources affecting superconducting
integrated circuits.
SUMMARY OF THE INVENTION
[0007] The present disclosure introduces a novel approach for
controlling noise in superconducting quantum circuits that
overcomes the drawbacks of previous technologies. Specifically, the
present disclosure recognizes that dominant sources of noise can
arrive via molecular species found in ambient surroundings, rather
than inherently from materials and geometries utilized therein. For
instance, molecular oxygen is a magnetically active species that
exhibits long range magnetic order at low temperatures and
pressures. Adsorption of molecular oxygen can lead to appreciable
magnetic noise in superconducting quantum circuits, such as
superconducting qubits. Therefore, in accordance with the present
invention, provided systems and methods are directed to controlling
the proximate environment of superconducting quantum circuits. By
suppressing surface effects, such as magnetically active defects,
sources of noise can be appreciably reduced or eliminated.
[0008] In accordance with one aspect of the present disclosure, a
method for reducing magnetic noise in qubit circuits is provided.
The method includes providing one or more qubit circuits, and
arranging the one or more qubit circuits in a hermetic enclosure
capable of isolating the one or more qubit circuits from ambient
surroundings. The method also includes controlling a background
pressure of one or more magnetically active species in the hermetic
enclosure to suppress magnetically active surface defects
associated with the one or more qubit circuits.
[0009] In accordance with another aspect of the present disclosure,
a system for suppressing magnetically active surface defects in
superconducting quantum circuits is provided. The system includes a
hermetic enclosure configured to accommodate therein one or more
superconducting quantum circuits, and capable of isolating the one
or more superconducting circuits from ambient surroundings. The
system also includes a vacuum system removably coupled to the
hermetic enclosure, and configured to control an environment in the
hermetic enclosure such that magnetically active surface defects
associated with the one or more superconducting quantum circuits
are suppressed.
[0010] In accordance with yet another aspect of the present
disclosure, a method for suppressing magnetically active surface
defects in superconducting quantum circuits is provided. The method
includes providing one or more superconducting quantum circuits,
and arranging the one or more superconducting quantum circuits in a
hermetic enclosure. The method also includes controlling an
environment in the hermetic enclosure to suppress magnetically
active surface defects associated with the one or more
superconducting quantum circuits.
[0011] The foregoing and other aspects and advantages of the
invention will appear from the following description. In the
description, reference is made to the accompanying drawings that
form a part hereof, and in which there is shown by way of
illustration a preferred embodiment of the invention. Such
embodiment does not necessarily represent the full scope of the
invention, however, and reference is made therefore to the claims
and herein for interpreting the scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 shows a flowchart setting forth steps of a method in
accordance with the present disclosure.
[0013] FIG. 2 shows another flowchart setting forth steps of a
method in accordance with the present disclosure.
[0014] FIG. 3A shows an example hermetic enclosure in accordance
with aspects of the present disclosure.
[0015] FIG. 3B shows an example of vacuum enclosure for use in
coating the surfaces of superconducting circuits with non-magnetic
encapsulation layers to prevent subsequent adsorption of
magnetically active defects, in accordance with aspects of the
present disclosure.
[0016] FIG. 4 is a graph showing oxygen x-ray magnetic circular
dichroism ("XMCD") signal for thin film air-dosed aluminum as a
function of magnetic field at 10 Kelvin.
[0017] FIG. 5 is a graph of X-ray absorption spectra for thin film
air-dosed aluminum indicating the presence of adsorbed molecular
oxygen for temperatures below 50 Kelvin.
[0018] FIG. 6 is another graph of X-ray absorption spectra for thin
film air-dosed aluminum indicating the presence of adsorbed
molecular oxygen for temperatures below 50 Kelvin.
[0019] FIG. 7 is a graph illustrating the effect of ammonia
exposure on a superconducting quantum interference device
("SQUID").
[0020] FIG. 8 is a graph illustrating the effect of ultraviolet
light exposure on a SQUID.
[0021] FIG. 9 is a graph illustrating the effect of ultraviolet
light power on a SQUID.
DETAILED DESCRIPTION OF THE INVENTION
[0022] Surface effects, such as magnetically active defects, can
represent significant sources of noise that can impede or limit the
functionality of certain superconducting devices. For example,
recent investigations by the inventors demonstrated that the
dominant contribution to magnetic flux noise observable in
superconducting quantum bit ("qubit") devices originated from
oxygen-containing adsorbates that produced a high density of
magnetically active defects at the surface of superconducting
devices. Such low-frequency magnetic flux noise represents a
dominant source of dephasing, a key figure of merit for
superconducting qubit operation.
[0023] Therefore, the present disclosure describes systems and
methods directed to controlling the environment of superconducting
quantum circuits for purposes including mitigating potential
sources of noise, such as magnetic noise, found therein. For
instance, as will be described, the density of surface defects,
such as magnetically active surface defects, may reduced by
limiting or prevention exposure to, and/or inducing desorption of
active adsorbates, such as oxygen-containing adsorbates.
[0024] Turning to FIG. 1, a flowchart setting forth steps of a
process 100 in accordance with aspects of the present disclosure is
shown. The process 100 may begin at process block 102 where one or
more superconducting quantum circuit(s), such as qubit circuits,
may be provided. In some aspects, the one or more superconducting
quantum circuit(s) may be fabricated at process block 102 in
accordance with standard device practice. In some designs, the
surface of the superconducting quantum circuit(s) may be coated
with a non-magnetic encapsulation layer. This may be advantageous
particularly to devices that are sensitive to magnetically active
defects and magnetic noise. By way of example, candidate materials
can include waxes, similar to the etch resist Apiezon W, with layer
thicknesses on the order of millimeters, although other materials
and layer thickness may be possible.
[0025] At process block 104, the superconducting quantum circuit(s)
may then be arranged or positioned in a hermetic enclosure
configured to accommodate therein one or more superconducting
quantum circuits. As will be described, the sealable hermetic
enclosure can be configured in any manner, and capable of a range
of functionality, including isolating the superconducting quantum
circuit(s) from ambient surroundings.
[0026] Then, as indicated by process block 106, the environment in
the hermetic enclosure may be controlled, for instance, in a manner
such that magnetically active surface defects in the
superconducting quantum circuit(s) are suppressed. In some aspects,
this step can include generating a vacuum or near-vacuum
environment, for instance, by operating a vacuum system coupled to
the hermetic enclosure.
[0027] In some modes of operation, the vacuum system may be capable
of controlling the background pressure of the hermetic enclosure
such that high vacuum or ultrahigh vacuum conditions are achieved.
By way example, a high vacuum can be in a pressure range roughly
between 10.sup.-6 to 10.sup.-8 Torr, and ultrahigh vacuum can be in
a range of 10.sup.-8 Torr or lower, although other pressure values
may be possible. In some aspects, the temperature of the hermetic
enclosure may be elevated while reducing the pressure therein in
order to bake out, or desorb, and subsequently remove active
adsorbates or contaminants present in or about the enclosure walls.
In some aspects, the native surface of the superconducting circuits
can be passivated or modified at process block 106, for example, by
backfilling, or pressurizing, the hermetic enclosure after
evacuation with an inert or nonmagnetic gas, such as ammonia gas.
In addition, the surface of the superconducting circuits can also
be irradiated using light at process block 106, for instance, while
performing a device cool-down protocol in order to promote
photodesorption of active adsorbates. By way of example,
ultraviolet light may be used to irradiate the superconducting
circuit(s).
[0028] Performing any combination of the steps detailed with
respect to process block 106, the density of one or more active
species, such as magnetically active species, may be controlled
such that sources of noise can be appreciably reduced. In
particular, any such steps can be applied to the superconducting
quantum circuit(s) or portions thereof that are sensitive to noise
and dephasing, and are particularly relevant to large-scale
multi-qubit circuits for gate-based quantum computing or quantum
annealing.
[0029] Turning to FIG. 2, another flowchart setting forth steps of
a process 200 in accordance with aspects of the present disclosure
is shown. The process 200 may begin at process block 202 where one
or more qubit circuit(s) or devices are arranged or positioned in a
hermetic enclosure.
[0030] At process block 204, the environment in the hermetic
enclosure may be controlled by reducing background pressure to
obtain a target coverage, or lack thereof, of magnetic, as well as
other undesirable adsorbates, on the surface of the qubit
circuit(s). As described, this can be achieved by evacuating the
hermetic enclosure to a high or ultrahigh vacuum, while optionally
baking out the enclosure. In some aspects, as indicated by process
block 206, evacuation may also be followed by backfilling the
enclosure with inert gases in order to occupy available adsorption
sites at the surface of the qubit circuit(s), thus preventing the
adsorption of residual magnetically active species, such as
molecular oxygen. By way of example, ammonia gas may be a suitable
candidate for passivating a device surface such that magnetically
active surface defects are suppressed, although other gases are
also possible.
[0031] At process block 208, the qubit circuit(s) may then be
operated with a suppressed density of surface defects. As
described, qubit circuit(s) may particularly benefit from a reduced
density of magnetically active surface defects that would reduce
sources of noise, decoherence and dephasing. In some aspects,
further control in the density of magnetically active adsorbed
defects can include irradiation of the qubit circuit(s) in the
hermetic enclosure with light, such as ultraviolet light, either
during the evacuation process at process block 204, and/or during a
cool down process associated with operation at process block
208.
[0032] In accordance with aspects of the present disclosure, a
system for suppressing magnetically active surface defects in
superconducting quantum circuits is provided. The system can
include a hermetic enclosure configured to accommodate therein at
least one or more superconducting quantum circuits, such as qubit
circuits, and a vacuum system removably coupled to the hermetic
enclosure, and configured to control an environment in the hermetic
enclosure such that surface defects, such as magnetically active
surface defects, associated with the superconducting quantum
circuits are suppressed.
[0033] The hermetic enclosure can be designed in any manner, and
include capabilities for controlling and operating devices,
circuits or circuit components, including superconducting quantum
circuits, arranged therein. Specifically, the hermetic enclosure
may be capable of isolating such devices, circuits or circuit
components from ambient surroundings. This may be implemented using
various features or elements suitable for achieving and sustaining
vacuum or near-vacuum conditions, pressurized conditions,
low-temperature conditions, and so forth. For instance, in some
implementations, the hermetic enclosure may contain all-metal
seals, such as conflat gaskets, and be constructed from welded
aluminum with aluminum-stainless steel bimetal flanges for the
vacuum seals, or may be constructed from an alloy of titanium
machined to form knife edges for use in producing vacuum seals.
However, the hermetic enclosure may be constructed in other ways as
well.
[0034] Other functionalities of the hermetic enclosure include, for
instance, configurations for mitigating, reducing or eliminating
sources of noise found in ambient surroundings, such as thermal,
electrical, and magnetic sources of noise, and other sources of
noise. Also, the hermetic enclosure may configured to include or
accommodate a heat source for elevating a temperature of the
hermetic enclosure, for example, during an evacuation process. The
hermetic enclosure may also include a light source, such as an LED
device, capable of irradiating devices, circuits or circuit
components therein using light, such as ultraviolet light. For
instance, the light source may be operated during a cooling
procedure, such that active species present on the surface of the
superconducting circuits are desorbed.
[0035] By way of example, FIG. 3A shows an example hermetic
enclosure 300 in accordance with aspects of the present disclosure.
As illustrated, the hermetic enclosure 300 may be constructed using
a first enclosing portion 302 and second enclosing portion 304,
which when coupled together via a metallic seal, or other seal, can
provide vacuum-tight enclosure. The hermetic enclosure 300 is shown
to include a number of electrical feedthroughs 306 connectable to
circuits arranged therein, although it may be appreciated that
other types of feedthroughs are possible. The hermetic enclosure
also includes a sealable evacuation port 308 configured to be
coupled to the vacuum system such that environment in the hermetic
enclosure 300 can be controlled.
[0036] The hermetic enclosure 300 may be manufactured using any
materials suitable for controlling an environment therein. By way
of example, the hermetic enclosure may be fabricated from grade 5
titanium alloy (Ti--6Al--4V), with the following advantageous
properties: 1) the material is hard enough to form an ultrahigh
vacuum conflat seal; 2) the material is known for its low
outgassing and is compatible with the desired ultrahigh vacuum
environment; 3) there are commercially available weld-in hermetic
wiring feedthroughs, for example of the SMA type, enabling
high-bandwidth electrical connections into an ultrahigh vacuum
environment; 4) Grade 5 titanium is a nonmagnetic material that is
superconducting at low temperatures. This provides magnetic
shielding for circuits or devices assembled in the hermetic
enclosure 300 that are sensitive to external magnetic field
fluctuations.
[0037] The vacuum system (not shown in FIG. 3A) may be configured
to control a background pressure of one or more active species in
the hermetic enclosure 300, such as magnetically active species
like molecular oxygen. As described, this may be achieved by
evacuating the hermetic enclosure 300 to a high or ultrahigh
vacuum, and optionally baking out the hermetic enclosure 300 using
a heat source.
[0038] In some aspects, the vacuum system may be configured to
introduce inert gases into the hermetic enclosure 300 in order to
passivate active surface defects of the superconducting circuits
therein, the inert gas occupying available adsorption sites. For
example, ammonia gas may be utilized, although other gases may also
be possible. In the case of qubit circuits, this would prevent
surface adsorption of residual magnetically active species, such as
molecular oxygen, and hence further suppress sources of qubit
decoherence and dephasing.
[0039] Turning to FIG. 3B, an example of vacuum enclosure 350, for
use in coating the surfaces of superconducting circuits with
non-magnetic encapsulation layers, is shown. Such encapsulation
layers would prevent adsorption of magnetically active defects
found in close proximity to the superconducting circuits, where
coupling to the surface defects is strong. By way of example,
non-magnetic encapsulation materials for use in the vacuum
enclosure 350 may include etch resist waxes, such as Apiezon W, or
UHV-compatible epoxies, such as Torr Seal or Epo-tek, but other
encapsulation materials are possible.
[0040] The vacuum enclosure 350 may include a broad range of
functionality, including capabilities for controlling an
environment therein, for instance, by reducing ambient pressure to
achieve vacuum or near vacuum conditions, or a targeted background
pressure. In addition, the vacuum enclosure 350 may be configured
with capabilities to dispense or deposit non-magnetic encapsulation
layers upon surfaces of superconducting circuits therein. As shown
in the example of FIG. 3B, the vacuum enclosure 350 can include an
inlet 352 configured to dispense non-magnetic encapsulation layers
using a dispensing tube 354. However, it may be appreciated,
however, that other methods for coating the surface of a device
inside the vacuum enclosure 350, using a non-magnetic encapsulation
layer, may be possible. Following vacuum encapsulation, the device
can be exposed to atmosphere, as well as cooled to low temperatures
in a non-hermetic enclosure, without appreciably deleterious
consequences, since any magnetically active defects would be
prevented from adsorbing in close proximity to the device.
[0041] Low-frequency 1/f flux noise is a dominant source of
dephasing in superconducting Josephson qubits. While it is possible
to avoid flux noise by replacing SQUID loops with single junctions
or by operating the qubit at a so-called flux "sweet spot," where
the device is insensitive to first order to magnetic flux
fluctuations, such strategies severely constrain qubit gates, and
hence overall architectures, since such limited qubits would no
longer be tunable. In previous investigations by the inventors, it
was demonstrated that there exists a high density of unpaired
magnetic defect states in the surfaces of superconducting thin
films, and it is believed that such defects are the source of the
ubiquitous 1/f flux noise. Therefore, systems and methods, in
accordance with aspects of the present disclosure, can be used to
reduce 1/f flux noise by controlling the environment proximate to
qubit devices, such that magnetically active surface defects are
suppressed.
[0042] In experiments involving an X-ray Magnetic Circular
Dichroism ("XMCD") technique, native superconducting thin film
samples were irradiated with left and right circularly polarized
x-rays, and the differences in absorption spectra at various x-ray
edges were examined. By way of example, FIG. 4 shows a graph of
oxygen XMCD signal for thin film air-dosed aluminum as a function
of magnetic field at 10 Kelvin. Differences in x-ray absorption for
the opposite x-ray helicities reveal the orbital, and in some cases
spin, polarization of the hole states to which the photoelectrons
are promoted. Oxygen and aluminum K-edges of native aluminum films,
and the oxygen K-edge and niobium L-edge of native niobium films
were investigated (both the aluminum and niobium films were covered
with amorphous thermal oxide due to exposure to atmosphere). When
the samples were cooled down to 10 K in ultrahigh vacuum, no
evidence of magnetism at any of the absorption edges was observed.
However, when 10.sup.-5 Torr of air was introduced into the sample
chamber for one minute while the samples were cold, it was found
that the oxygen K-edge spectrum changed dramatically, and a large
XMCD signal appeared, as illustrated in FIGS. 5 and 6.
Specifically, FIG. 5 shows the appearance of an oxygen K-edge
signal in the absorption spectra of an air-dosed aluminum thin film
obtained using a total electron yield ("TEY") mode. A peak around
531 eV develops when the sample is cooled below 50 K. At 10 K, a
strong signal at the oxygen K-edge can be observed indicating the
presence of an adsorbed layer of oxygen on the thin film surface.
Similar results may be observed in the X-ray absorption spectra
using a total fluorescence yield ("TFY") mode (FIG. 6).
[0043] Density functional theory calculations assigned the measured
XMCD signal to molecular oxygen, which is known to be magnetically
active and exhibit long-range magnetic order in the
low-temperature, low-pressure regime relevant to superconducting
qubit applications. Moreover, the data support an early speculation
that reduced levels of flux noise seen in nitride-encapsulated
SQUIDs were due to the fact that the magnetic moment of oxygen has
a much higher energy barrier to reorientation on a nitride surface
than on an oxide surface, so that adsorbed oxygen would remain
magnetically active on conventional oxide-encapsulated devices, but
not on nitride-encapsulated devices. In other experiments, the
inventors showed that the magnetic signature of adsorbed air is
identical to that of pure oxygen. This may be understood as a
consequence of the extremely low solubility of nitrogen in solid
molecular oxygen. Significant adsorption of oxygen is observed only
below 50 K and only when the background pressure in the cryostat is
worse than a few times 10.sup.-8 Torr.
[0044] In recognizing that dominant sources of noise in
superconducting quantum circuits or devices need not be intrinsic
to the materials and geometries utilized, but rather originating
from active species present under ambient conditions, the present
disclosure provides a novel approach to control the proximate
environment of the devices prior to and/or during operation of such
circuits or devices.
[0045] As described, this can include generating vacuum or
near-vacuum conditions in a hermetic enclosure housing the circuits
or devices, as well as pressurizing the enclosure with an inert
gas. By way of example, FIG. 7 shows the temperature dependence of
flux in a SQUID device before and after exposure to ammonia gas for
a cooling field of .+-.256 microTesla. The data shows about a three
times reduction in the surface spin density after ammonia
exposure.
[0046] In addition, suppressing active surface defects, in
accordance with the present disclosure, can be achieved by exposure
to light, such as ultraviolet light. By way of example, FIG. 8
shows the temperature dependence of flux in a SQUID device
subjected to various ultraviolet exposure conditions compared to
air exposure. The device was irradiated with ultraviolet light at
different wavelengths, namely 275 nm, and 365 nm, while cooling
down from room temperature to 3 K. About 30% decrease in spin
density is observed after ultraviolet exposure, suggesting that
ultraviolet light provides energy, which is unfavorable for the
surface adsorption process. Similarly, FIG. 9 shows flux versus
temperature curves for a device using three different ultraviolet
light powers. No significant change in spin density was observed
when the power level was varied from 11 mW to 450 mW.
[0047] The present invention has been described in terms of one or
more preferred embodiments, and it should be appreciated that many
equivalents, alternatives, variations, and modifications, aside
from those expressly stated, are possible and within the scope of
the invention.
* * * * *