Atoms Coupled to SQUIDs Saurabh Paul C. P. Vlahacos, Jonathan Hoffman, Jeffrey Grover, Daniel Hemmer, Z.Kim, B.Palmer, A. Dragt, J. Taylor, C.J. Lobb, J.R. Anderson, L. Orozco, S. Rolston, F.C. Wellstood Physics Frontier Center -NSF Joint Quantum Institute Department of Physics University of Maryland, College Park, Maryland Funding by NSF-PFC, JQI, and CNAM

Outline Motivation Basic idea Main challenge- Trapping atoms next to cold circuits Basics of atom trapping Designing the Proof-of-Principle Experiment Future work Conclusion

Motivation Construct a hybrid qubit system by coupling neutral atoms to superconducting qubits In this case, we will be using the hyperfine splitting of the Rb(87) atoms (6.83 GHz), coupled to flux qubit at same frequency. The long coherence time of atomic qubits combined with the scalability in superconducting qubits opens the possibility to create devices that can perform at levels unachievable by either technology alone. Also, learn how to manipulate individual atoms at cryogenic temperatures and couple them to extremely sensitive solid state devices.

Basic Idea Coupling Neutral Atoms to Superconducting qubits Cool flux qubit (SQUID) to 20 mK, manipulate and read out electrically flux qubit 2 mm Trap and manipulate atoms with light cloud of Rb87 atoms Move atoms close to SQUID loop to couple to magnetic moment of the atoms Demonstrate proof of principle expect typically about 100 Hz to 1 kHz for a Rb87 atom Improve SQUID coherence and coupling to atoms Push to few atom limit

Main Challenge - trapping atoms next to cold circuits We need to trap atoms just a few microns above superconducting circuits. Trap atoms just a few microns above superconducting circuits…… without significantly heating the superconducting device. Trapping method may involve large magnetic fields, or time varying fields, which may interfere with the superconducting devices.

Outline Motivation Basic idea Main challenge- Trapping atoms next to cold circuits Basics of atom trapping Designing the Proof-of-Principle Experiment Conclusion Future work

Trapping atoms in dipole potentials resulting from interaction with Optical Dipole Trap Trapping atoms in dipole potentials resulting from interaction with red-detuned light atoms Laser Dipole moment induced by electric field, Where is the polarizability The interaction potential is given by is called the detuning (i.e., red-detuned) we have an attractive trapping potential If worked out fully, we will get

Focused-beam traps Use a red-detuned focused Gaussian laser beam to create dipole traps lens atoms U r

Large Power incident on edge of chip Disadvantages: Large optical power (100mW) required. Difficult to get focal point close to surface (few mm) without introducing significant heating Atoms not very tightly confined. zo atoms Superconducting device

Magneto-Optic Trap (MOT) 1 Dimensional MOT J=0 J=1 z E B(z)=A.z -1 1 Atoms are in The ground state Moving randomly In +z and –z directions

Two coils in anti-helmholtz configuration Disadvantages of using MOT Two coils in anti-helmholtz configuration 3D view of a MOT Requires application of relatively large and time-varying magnetic fields which may couple to superconducting devices The problem of large amount of Laser power and heating effect is also significant. Atoms not tightly confined (big trap)

Atoms can be trapped in this evanescent wave, but how? Trapping atoms with a tapered optical fiber What is a tapered optical fiber? Why is it useful?.....Evanescent Wave 125 microns 500 nm Normal optical fiber Hydrogen flame Evanescent wave Atoms can be trapped in this evanescent wave, but how?

Trapping atoms with a tapered optical fiber (continued) Red-detuned light atoms We already know that And for a red-detuned light, it looks like Distance(nm) E So, if we only have a red-detuned light The atoms will stick to the fiber The blue-detuned light provides the Repulsive force to trap the atoms a few hundred nm from the surface

Used Experimental Setup (Arno Rauschenbeutel, Univ. of Mainz) Beam splitter Red light Blue light Potential wells atoms Tapered Optical fiber Once the atoms are trapped in the wells, it is also possible to move them along the length of the tapered region

Potential well along the tapered fiber (Arno Rauschenbeutel, Univ. of Mainz)

Trapping atoms using a tapered optical fiber 2 mm tapered optical fiber 3 cm linearly polaized red+blue light Advantages: Relatively low power is required 99% of the light can be confined to very near the fiber, strong coupling to atom. Buildable with a large tapered part Disadvantages: Still requires mW of power. Maybe 1% of light lost as heat or scattering out of fiber. Not used much previously at low temperature Need to figure out how to load atoms

Outline Motivation Basic idea Main challenge- Trapping atoms next to cold circuits Basics of atom trapping Designing the Proof-of-Principle Experiment Conclusion Future work

Proof-of-Principle Experiment Couple atoms to superconducting LC resonator Couple atoms to SQUIDs load atoms red/blue light tapered optical fiber move to within 1 to 10 mm of chip - High-Q, LC resonator - Cool to 20 mK - Needs to be tunable to 6.83 GHz pump resonator at 6.83 GHz, monitor absorbed power 6.83 GHz Pin Pout Advantages for proof of principle: relatively simple to build LC resonator, "simple" to measure, robust, very sensitive

How do we know when the atoms couple to the resonator ? Monitor the microwave power absorbed, and look for sharp absorption drop due to the atoms at 6.83 GHz….i.e. NMR Look for any possible shifts in the resonant frequency From the optics side, we can detect the state of the atomic spins by fluorescence detection For larger atomic samples, Faraday rotation may also be a possible detection technique

Superconducting LC-Resonator – thin film Al on sapphire (Z. Kim and B. Palmer, LPS) Al on Sapphire 1 mm 100 mm T = 350 mK fo = 5.5773 GHz Qtot = 25,000 Qintrinsic ~ 75,000 Qintrinsic up to 106 at 20 mK Capacitor 380 fF

Tuning arm and inductor/chip Mechanical Tuning Tuning arm Z= distance between Tuning arm and inductor/chip We will use a mechanical tuning arm, made of Al. The effective inductance of the inductor is By varying “z”, we can vary and L, and in turn the resonant frequency of the oscillator. Inductor with self Inductance L0 a

Basic Idea of Tuning assembly  bottom of dilution fridge copper axial shield  Mechanical z-stage Piezo z-stage (Attocube)  metallized fabric copper frame SMA connectors sample tuning arm

Basic Idea of Tuning assembly bottom of dilution fridge     Mechanical z-stage copper axial shield Piezo z-stage (Attocube) copper fabric frame sample metallized fabric tuning arm SMA connectors

Basic Idea of Tuning assembly bottom of dilution fridge     Mechanical z-stage copper axial shield Piezo z-stage (Attocube) copper fabric frame sample metallized fabric tuning arm SMA connectors

Tuning assembly bolted to bottom of DR dilution fridge copper base plate axial shield copper retaining ring z-axis stage metalized fabric z-Attocube Al tuning arm Nb resonator sample box

Fiber-Resonator spacing and alignment adjustment ~ 200 mW Pin~ 20 mW load atoms fiber support frame Metalized fabric ANR101 ANRv101 f q ANPx101 ANPz101

Fiber-Resonator spacing and alignment adjustment load atoms ~ 200 mW Pin~ 20 mW fiber fiber support frame ANPz101 Metalized fabric ANPx101 ANPz101 q ANRv101 ANR101 f

Fiber-Resonator spacing and alignment adjustment load atoms ~ 200 mW Pin~ 20 mW fiber fiber support frame ANPz101 Cu diaphragm ANPx101 ANRv101 q ANR101 f

Fiber-Resonator spacing and alignment adjustment load atoms ~ 200 mW Pin~ 20 mW fiber fiber support frame ANPz101 Cu diaphragm ANPx101 q ANRv101 ANR101 f

Fiber-Resonator spacing and alignment adjustment ~ 200 mW Pin~ 20 mW load atoms fiber support frame Metalized fabric ANR101 ANRv101 f q ANPx101 ANPz101

The entire assembly for the sample box Base of DR Tuning assembly  Mechanical z-stage Piezo z-stage (Attocube) c fiber support frame ANR101 ANRv101 f q ANPx101 ANPz101 fiber Pin load atoms Metalized fabric

Dilution Refrigerator Inside the Fridge Dilution Refrigerator Sample box 100mK plate Mixing chamber plate 20 cm c

Future Work Demonstrate mechanical tunability and stability of the superconducting resonator while maintaining very high Q. Build and operate a mechanism for moving chip very close to fiber at mK temperatures. Fabricate tapered optical fibers and test their robustness at cryogenic temperatures. Study the light scattered/lost from the fibers, and check the heat load at the sample. Load and trap atoms on the fiber at mK. Do the proof-of-principle experiment by trapping atoms a few microns above the superconducting resonator and measure their magnetic coupling to the resonator.

Conclusion We are trying to make a hybrid quantum system by coupling neutral atoms to superconducting devices. As a proof-of-principle, we are working on an experiment which involves coupling Rb atoms to a superconducting LC resonator at mK. The resonant frequency of the resonator will be tuned in situ mechanically using piezo stages (attocubes). We propose to trap atoms along a tapered optical fiber, and bring it to within a few microns of the resonator surface using piezo stages. We will measure the coupling between the atoms and the resonator by microwave absorption and optically.

Thanks