reveal
A rapid sample-exchange mechanism for cryogen-free dilution refrigerators compatible with multiple high-frequency signal
connections
G.Batey,S.Chappell,M.N.Cuthbert,M.Erfani,A.J.Matthews ⇑,G.Teleberg
官僚资本
Oxford Instruments Omicron NanoScience,Tubney Woods,Abingdon,Oxfordshire OX135QX,UK
a r t i c l e i n f o Article history:
Received 29October 2013
Received in revid form 13January 2014Accepted 15January 2014
Available online 24January 2014Keywords:
Dilution refrigerator Sample exchange Cryogen-free
a b s t r a c t
Rearchers attempting to study quantum effects in the solid-state have a need to characteri samples at very low-temperatures,and frequently in high magnetic fields.Often coupled with this extreme environ-ment is the requirement for high-frequency signalling to the sample for electrical control or measure-ments.Cryogen-free dilution refrigerators allow the necessary wiring to be installed to the sample more easily than their wet counterparts,but the limited cooling power of the clod cycle coolers ud in the systems means that the experimental turn-around time can be longer.Here we shall describe a sample loading arrangement that can be coupled with a cryogen-free refrigerator and that allows sam-ples to be loaded from room temperature in a matter of minutes.
The loaded sample is then cooled to temperatures $10mK in $7h.This apparatus is compatible with systems incorporating superconducting magnets and allows multiple high-frequency lines to be connected to the cold sample.
Ó2014The Authors.Published by Elvier Ltd.This is an open access article under the CC BY-NC-ND
licen (creativecommons/licens/by-nc-nd/3.0/).
1.Introduction
Over the past century studying condend matter systems at extremely low temperatures,and often in extremely high magnetic fields,has lead to the discovery of veral new states of matter,such as:superconductivity in mercury [1];superfluidity in 4He [2,3];superfluidity in 3He [4];the integer quantum Hall effect in silicon MOSFET devices [5];the fractional quantum Hall effect in GaAs–AlGaAs heterojunctions [6].
More recently there has been a drive to harness the quantum systems to reali devices that exploit their quantum nature,for example in the field of quantum information processing [7],with the r
ealisation of a general quantum computer [8]being the holy grail.Inevitably the development of the quantum devices re-quires temperatures <10mK,and possibly magnetic fields >10T,however in addition to the environmental constraints device characterisation and development also requires the necessary experimental rvices be installed at the sample position:most challengingly high-bandwidth,high-fidelity micro-wave cabling.In the following ctions we describe briefly a suitable experi-mental environment for quantum device development (or any other experiments requiring high-frequency measurements at
low-temperatures),then we show that device characterisation is more convenient with a sample loading mechanism,and describe its realisation,operation and performance,before providing a brief conclusion.
lit2.Experimental environment
Pul-tube precooled dilution refrigerators [9]are becoming increasingly popular.Initially this popularity stemmed from the fact that they were cryogen-free,meaning that they could be in-stalled at institutions without the associated low-temperature re-arch infrastructure,such as a helium liquefaction plant,or in remote locations.Additionally,there are benefits from an opera-tional point of vi
ew as such systems can be automated to a higher degree than their ‘‘wet’’counterparts.It has also been found that the cryogen-free systems have further benefits when compared to wet systems with regards to the installation of experimental r-vices,as will be discusd in the following ctions,and this has driven the recent ri in their uptake.
With the installation of high-frequency wiring the refrigera-tors have been developed into measurement systems for circuit quantum electrodynamics [10]and superconducting qubits [11].The integration of superconducting magnets [12],with the entire system able to be run from a single pul-tube cooler,has enabled
dx.doi/10.nics.2014.01.007
0011-2275/Ó2014The Authors.Published by Elvier Ltd.
This is an open access article under the CC BY-NC-ND licen (creativecommons/licens/by-nc-nd/3.0/).
⇑Corresponding author.Tel.:+441865393440;fax:+441865393333.
E-mail address:Anthony. (A.J.Matthews).
tnt是什么意思a wider range of experiments(tho requiring magneticfields)to be performed using this cryogen-free technology[13].
2.1.Low-temperatures and high magneticfields
Cryogenic systems using liquid helium are usually designed to minimi its consumption.This is becau liquid helium is expen-sive,refilling the system can be time consuming,and refilling the system may perturb the experiment to an unacceptable level. The central neck of a cryostat is often responsible for the biggest single heat load into the helium bath,and as a result the necks are usually made as long and as narrow as possible.Dilution refrig-erators designed to be inrted into such a cryostat have to inherit this aspect ratio,which has tended to limit the experimental real estate available for the installation of rvices.
With no boil-off considerations,cryogen-free systems have evolved to be much wider than their wet counterparts with exper-imental plates(to which rvices can be mounted)typically veral hundred mm in diameter[12].This has enabled more and/or more complex rvices to be installed on dilution refrigerator systems,in particular bulky signal conditioning elements such as cryogenic amplifiers,microwave components(bias-tees,circulators, switches,etc.)andfiltering(such as metal powderfilters,for exam-ple[14]and the references therein).
Cryogen-free systems can also be designed without the need for a low-temperature,vacuum-tight vesl,the so called inner vac-uum chamber(IVC),which makes the routing and heat-sinking of the installed rvices much more straightforward,e Section2.2.2.
The range of magnets that are able to be produced for cryogen-free operation is also continually expanding with higherfields (>16T)and vector-rotation(>6–1–1T)available.
For the reasons cryogen-free dilution refrigerators with inte-grated magnets have become the workhor of quantum device development laboratories around the world.
2.2.High-frequency wiring
As was noted in Section1high-fidelity,high-bandwidth wiring is an experimental requirement for quantum device development applications.In addition to the quality of the signal transmission performance of the cables,they also need to be thermally an-chored adequately to ensure that they do not affect adverly the ba temperature performance of the system onto which they are installed.In this ction we shall:review various options for the coaxial lines and some of the materials available for the lines themlves,and discuss their relative merits;describe a conve-nient method for mounting multiple high-frequency lines onto a dilution refrigerator;quantify the frequency
dependence of signal transmission of installed lines with S12measurements made with a vector network analyr;comment on the heat load to the mix-ing chamber likely to result from the installation of the type of wir-ing described.
铁中铮铮2.2.1.Coaxial cables and materials
To date,most high-frequency cabling installed in dilution refrigerators have been of‘‘mi-rigid’’construction with the UT-85cable(having an outer diameter of85/1000of an inch,approx-imately2.16mm)being commonly ud.The optimal choice of coaxial cable,in terms of both size and material,depends on its in-tended application.Typically coaxial cables are ud to(1)improve noi immunity for‘‘small’’signals and/or(2)transmit high-fre-quency signals to/from the sample.
If using coaxial cables for either of the reasons one should en-sure that the cables themlves are suitable for the intended appli-cation.For dilution refrigerator bad experiments,this suitability is generally determined by two key parameters:the heat load to the experiment due to the thermal conductivity of the cable;and its(frequency dependent)attenuation.Both of the parameters are affected by the choice of the cable geometry(size)and conduc-tor materials.
The heat load conducted to the coldest parts of the dilution refrigerator is always to be minimid.For
a given choice of coaxial cable material and geometry there is a lower limit to this heat load determined by the bulk thermal conductivity of the cable materi-als.This limit is approached as the cable(both the inner and outer conductor)is perfectly thermally connected to every available temperature stage in the refrigerator,of cour the conducted heat load can be much higher than this limit if the thermal connections are inadequate.A convenient method of installing mi-rigid coax-ial cables into a dilution refrigerator that gives good thermal per-formance is discusd in Section2.2.4.The heat load can only be reduced further by using either cables with a smaller cross c-tional area and/or cables made from materials with a lower ther-mal conductivity,however such changes may well have implications for the cable attenuation.
The frequency dependent attenuation of a coaxial cable is deter-mined by the cable geometry(outer diameter of the inner conduc-tor and inner diameter out of the outer conductor),the (temperature and frequency dependent)resistivity of the conduc-tor materials and the dielectric loss[15].In general smaller diameter cables have higher attenuation at high frequencies than larger diameter ones,and cables manufactured from materials with higher bulk resistivity have higher attenuation(at a given fre-quency)than low resistance ones.Depending on the application, this increa in attenuation can be fortuitous or problematic.In applications where coaxial cables are ud for noi immunity for sma
ll,low-frequency signals,having incread attenuation at high frequencies is advantageous:in fact‘‘lossy’’coax cables have been ud as microwavefilters[16].
However,for high-bandwidth signals the change in attenuation, a,with frequency,f,is undesirable as it results in the‘‘shape’’of signals(in the time-domain)being modified as they propagate along the cable and this can cau problems with,for example, high-fidelity qubit control.Techniques borrowed from the NMR/ MRI world for pul preshaping using a posteriori knowledge of the cabling transfer function[17]can be applied to compensate for this effect,but it would still be advantageous to keep the fre-quency respon of the cable asflat as possible.Using(lots of) large-diameter low-resistance cables can be incompatible with experiments at dilution refrigerator temperatures,as the thermal and electrical conductivity of a normal metal are cloly related [18].However,superconducting cables made from Nb,or prefera-bly NbTi(due to its higher criticalfield and temperature,and lower thermal conductivity),can be ud.Below their superconducting transition temperature the cables provide very low attenuation and have a small thermal conductivity[15]so in many cas are the ideal solution to this problem.However,with cryogen free dilution refrigerators enabling experiments over extended temper-ature ranges[12]some care needs to be taken,as the electrical per-formance of the lines will change(attenuation will increa) dramatically above their transition temperature.
Onefinal point is that the desire to keep d a
df
%0is not the same as keeping a%0.Indeed,the types of cables described here are very good at transmitting‘‘thermal noi’’from warmer parts of the refrigerator to colder ones,equating h m%K B T gives a photon fre-quency of20GHz at1K and UT-85cables operational range can extend to>60GHz[19],and so having some attenuation in the line is desirable to reduce the thermal perturbations.Attenuators with aflat frequency respon,compatible with cryogenic temper-atures[20],can be ud to increa the attenuation of a line whilst avoiding the complications of distorting high-bandwidth signals. Details of measurements of such lines will be given in Section2.2.3.
G.Batey et al./Cryogenics60(2014)24–3225
2.2.2.High-frequency wiring cartridges
As described in Section2.2.1,for some experiments small diam-eter coaxial cables with high attenuation at high-frequencies can be appropriate.For example,UT-13cables have an outer diamete
r of approximately330l m and can be installed and thermally an-chored likeflexible‘‘DC’’wiring.In this ction we focus on mi-rigid cables and describe a convenient method of installing multiple,configurable,mi-rigid coaxial lines into a dilution refrigerator in a way that gives good electrical and thermal perfor-mance and allows for the cable asmblies to be rapidly demount-ed and modified if necessary.
Cryogen-free dilution refrigerators typically have veral large (40–100mm diameter)line-of-sight(LoS)ports that allow connec-tions between the room temperature top-plate and the mixing chamber plate.Whilst traditional wet dilution refrigerators also of-ten feature LoS ports they tend to be less numerous and of smaller diameter.Wet systems also require an IVC and so rvices need to be installed in vacuum tubes from room temperature to4K,mak-ing the thermal anchoring of the installed rvices more difficult (rvices can of cour be thermalid by bringing them through the main helium bath,but then cryogenically compatible,hermet-ically aled feed-throughs are required to bring the rvices into the IVC).
A typical cryogen-free refrigerator will have experimental plates that can be ud to thermally anchor wiring at temperatures of approximately50K,3K,0.8K,100mK and the mixing chamber at around10mK.The wiring cartridge shown in Fig.1has anchor-moved in one piece allows for bench te
sting of the microwave lines prior to installing them into the system.It also means,for example, that should there be a desire to change installed attenuators for ones with a different attenuation value the asmbly can be re-moved from the refrigerator by simply opening one room temper-ature o-ring al and looning the clamping bolts.With the asmbly removed,the microwave lines or attenuators,between the bulkhead connectors,can be reconfigured and tested before being refitted to the system.
2.2.
3.Transmission measurements
The microwave performance of installed coaxial cable asm-blies has been measured with an Antitsu model MS2028C/2vector network analyr[22]which recorded the scattering parameters at frequencies up to12GHz.Typical curves between5kHz and8GHz are shown in Fig.2.The S12parameter can be associated with the total attenuation in the line and the measured values agree well with cable manufactures’data for expected values of the frequency dependent attenuation(per unit length)of the cables they produce [19,23],in this ca the coaxial cable ctions themlves were sil-ver-plated stainless steel inner conductor,stainless steel outer con-ductor from r
oom temperature to the4K plate,and NbTi inner and outer from4K to the mixing chamber.Faults with the coaxial cables,such as loo connectors or cracked solder joints,can be identified from scattering parameter measurements[24],and for the cables installed on the systems typically result in additional attenuation(reflection)features at frequencies of a few GHz,Fig.2.
Fig.1.A wiring cartridge for a cryogen-free dilution refrigerator:(a)Shows a fully asmbled cartridge with hermetic feed-throughs on the room temperature top plate and additional attenuators installed above and below some of the thermal stages.(b)Shows the detail of a split clamp ud to thermally anchor the cartridge to the refrigerator and the bulkhead connectors through the cartridge plate.(c) Shows how such a ction of such a cartridge could be installed through a line-of-sight port of a dilution refrigerator.
Scattering parameter measurements on coaxial cables installed The red and black traces show lines with no additional
The green and blue traces show lines with an additional28dB attenuation.The reduction in the attenuation between room temperature being cooled is due principally to ctions of superconducting coaxial
below their transition temperature.The dip in the attenuation
(circled)was due to a loo connector in the cartridge asmbly, prior to the cartridge being installed into the system and cooled. interpretation of the references to color in thisfigure legend,the reader version of this article.)
26G.Batey et al./Cryogenics60(2014)24–32
leak extracted from the difference in the temperatures.For the measurements three wiring cartridges,each containing eight UT-85coaxial lines(24lines in total)manufactured from cupronickel conductors,were installed onto a Triton200[25]dilution refriger-ator system,as show in Fig.3.After the addition of the wiring car-tridges the temperature of the plate mounted at the end of the continuous heat exchanger,colloquially know as the‘‘100mK plate’’,had incread from65mK to120mK as measured with a resistive temperature nsor[26].The ba temperature of the dilution refrigerator,measured using a nuclear orientation ther-mometer[27],was found to have rin to9.1mK,corresponding to an incread heat load of%600nW.Extrapolating available data for the thermal conductivity of cupronickel[28]to100mK and cal-culating the anticipated heat load conducted through24UT-85 coaxial lines with the geometry defined by manufactures[23]ac-counts for%200nW
rectonof this increa,with a further additional %300nW expected through the stainless steel refrigerator support structure,calculated using published values for the thermal con-ductivity[29],due to the increa in temperature of the100mK plate.
3.Rapid sample exchange
In Section2it was shown that cryogen-free dilution refrigera-tors integrated with superconducting magnets provide an ideal environment for quantum device development experiments due to their ea of u and the convenience of installing experimental rvices.The systems do,however,have one significant draw-back compared to their wet counterparts as the integrated super-conducting magnets become larger:the experimental turnaround time.High-field cryogen-free magnets can have mass well in ex-cess of50kg and require enthalpy changes of veral MJ to cool from room temperature to4K.The pul tube coolers ud in the systems typically have cooling powers at the cond stage of %140W at room temperature,falling to%1W at4K[30].This lim-ited cooling power means that the initial cool down from room 3.1.The sample exchange concept
Attaching a sample and experimental wiring directly to a probe and loading the entire asmbly into a dilution refrigerator has been attempted,but it was found that the resulting thermal perfor-mance an
d limited space is incompatible with multiple high-fre-quency lines and additional microwave components(amplifies,filters,etc.),e Section II A of[31].
An alternative approach to sample loading,as also implemented in[31],is to leave the experimental wiring on the refrigerator, where it can be efficiently thermally anchored,in this ca by using the wiring cartridge design discusd in Section2.2,and to load a ‘‘sample holder’’to connect to this installed wiring.Additionally, this means that the full sample space of the refrigerator can be uti-lid to install other components into the experimental wiring cir-cuits which may notfit onto a smaller diameter probe.Loading only a sample holder into the refrigerator introduces the complica-tion of requiring demountable microwave connectors,but in the following ctions we show that this requirement can be fulfilled. It is often also desirable to be able to bias or ground electrical con-nections to delicate samples during the cool down process to pre-vent,for example,electrostatic sample damage.This is accomplished with a‘‘make-before-break’’arrangement whereby all DC and microwave connections to the sample holder are indi-vidually connected to room temperature connectors on the loading probe.As will discusd in Section3.1.2the sample holder can also be made demountable,allowing the loading probe to be removed after the sample is attached to the refrigerator.
With a cryogen-free system without an IVC there is no preferred direction for sample loading.Samples can either be introduced from the top of the system using a top-loading load-lock(TLLL) or from below using a bottom-loading load-lock(BLLL).TLLLs re-quire a(central)LoS access port through which the sample can be introduced,and BLLLs require access through the vacuum and radiation shields through which the sample can pass.The distance from the refrigerator top-plate to the magneticfield centre-line is normally longer than that from the bottom of the system tofield
image showing how multiple coaxial cable cartridges can be installed onto a dilution refrigerator system.In-line attenuators are visible below
position of the dilution refrigerator still.(b)A plot of the typical cooling capacity available at the mixing chamber of such a dilution refrigerator.
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tbc是什么意思G.Batey et al./Cryogenics60(2014)24–3227
normal operation.For TLLL systems the can be controlled with a drive rod mechanically connected
to the room temperature top plate.For BLLL systems it is more convenient to make the baffles spring-loaded as the baffles themlves are attached to demount-able radiation shields.
3.1.1.Connectors
The choice of connectors is critical to the microwave perfor-mance of a cable asmbly.With standard UT-85type cables the usual room temperate choices are SMA[32]connectors for opera-tion up to18GHz and SK[33]connectors for operation up to 40GHz.Both of the connectors are screw lock,so unsuitable for pushfit applications,the BMA[34]connector range is a blind-mate equivalent of the SMA connector,but suffers from being rated only to$20GHz and being rather bulky($10mm diameter)which limits the density of connections.
SMP[35]connectors have the advantage of being blind-mate,
Connectors for multiple DC lines were also trialled cryogenically and a nano d-type connector[36]was chon,principally due to its extremely small footprint.
3.1.2.The loading probe
The loading probe is esntially identical regardless of whether the system is top or bottom loading s
ave for the direction of inr-tion.The loading probe consists of a vacuum lock which is mounted onto a gate valve on the top/bottom of the main vacuum chamber and evacuated prior to introducing the sample holder into the system.Optionally,the loading probe vacuum lock itlf can be fitted with an additional gate valve to allow samples to be stored under vacuum prior to loading into the system,and after removal.圣诞前夜
The sample holder is mechanically connected to drive rods which enter the vacuum lock via piston als,and electrically con-nected to the biasing/grounding wiring on the probe.The drive
piece for SMP connectors.(b)The round-trip attenuation through pairs of the connectors measured at room temperature as a function number.The data for thefirst25cycles are for one pair of connectors,the last25cycles are for the cond pair.(c)The round-trip attenuation measured at room temperature as a function of the temperature of the connectors.
28G.Batey et al./Cryogenics60(2014)24–32
3.1.3.The docking station
The docking station provides the mating electrical and thermal connections for the sample holder.The cabling attached to the refrigerator is routed to the docking station.For TLLL systems the docking station is a ring around the(central)LoS port,for BLLL sys-tems it is a stand-off that brings th
argumentative
e connectionflange into the bore the magnet.Typically48DC lines and14microwave cables can connected to the docking station,however we note that it straightforward to scale up the number of connectors,if required, particularly on TLLL systems as this can be achieved without the need for a larger magnet bore.
A BLLL sample holder attached to its docking station is shown Fig.5.Microwave cable links arefitted between the wiring car-tridges,running through the refrigerator,and the docking station.3.1.4.The sample holder
Examples of BLLL sample holders are shown in Fig.6.In thefig-ure,panel(b)is a design for integration with high-field magnets with57mm cold bore diameter,giving a clear diameter sample space inside the sample holder of$25mm(reduced from the diameter of the sample holder by the drive rods internal to the holder required to make the bolted connections to the docking sta-tion,visible in thefigure)by90mm long,symmetric about the field centre line of the magnet.Also shown(c)is a larger diameter sample holder for magnets with a90mm cold bore giving a clear sample space diameter of$50mm.
Fig.6(a)shows the mating surface of the BLLL sample holder.In particular the SMP connector‘‘bullet’’幼片下载
adaptors can be en(the bullet has been removed from the lower left shroud).On the sam-ple holder,‘‘full detent’’shrouds are ud to retain the bullet.On the docking station smooth bore so called‘‘catcher’s mit’’shrouds are ud which allow for a certain amount of radial and axial mis-alignment between the shrouds during loading.
The sample holder features an integrated radiation shield, which also protects the sample mechanically during the loading and unloading process.
3.2.Sample cool-down
5.A bottom loading sample holder,with its integrated radiation shield,
connected to the mixing chamber docking station.The microwave cable links
between the wiring cartridges and the docking station are visible.
surface of a bottom loading sample holder showing the14SMP connectors and51-way nano d-connector.Two alignment
M4captive fasteners are visible at the top and bottom.(b)A bottom loading sample holder with the radiation shield
sample whilst loading can be en entering the sample holder from the bottom,and the experimental wiring entering
could be ud for mounting a sample into the holder.(c)A larger diameter bottom loading sample holder connected to the
load-lock.The four drive rods are visible at the ba of the sample holder.
60(2014)24–3229
