Quantum Computing in Cryogenic Systems

Background

Quantum computing promises to deliver major advances in a wide variety of fields including simulations of the natural world, virtual quantum experiments, quantum cryptography, data communication systems, and new pharmaceutical drug search and design. These exciting research frontiers in quantum computing rely on two hallmarks of quantum physics, namely, the superposition of states and quantum interference. Classical computers process all computations using a two-state system known as a bit. The classical bit takes on the value of zero or one. A bit is used in conjunction with universal logic operations (or gates) to perform arithmetic that forms the basis of modern computing systems. In quantum computing, the fundamental unit of information is a quantum bit or “qubit.” Like a classical bit, the qubit can take the values of zero or one, but the differentiating factor of the qubit is that it can take on both the value of zero and one simultaneously, a property known as superposition. Importantly, the qubit also has a phase which is responsible for the interference effects that enable quantum algorithms such as the Shor and Grover algorithms used widely in quantum cryptography.

There are several active modalities under consideration for the realization of a scalable quantum computer. Two of the most promising candidates for realizing a quantum computer are the ion trap and superconducting circuit designs. Below we discuss the experimental setups and the challenges that must be addressed.

Montana Instruments has developed a cryogenic platform to meet the demanding needs of the quantum computing research community and has helped alleviate the barrier to entry for ion trap and superconducting circuit research. Below you will find common experimental challenges faced by the research community and how Montana Instruments has helped researchers overcome these barriers.

Experimental Considerations

Common Challenges
  • Disruptions to the local sample environment such as mechanical vibrations can impart energy to the qubit states and destroy the quantum environment
  • Trapped ion experiments require high vacuum conditions to reduce the number of molecular collisions with trapped ions
  • Cryogenic conditions are required because thermal energy can excite vibrational motion that disrupts the quantum computing operations
  • Thermal radiation can drive undesirable internal RF transitions in trapped ions or can raise a superconducting circuit above its critical temperature
  • Power fluctuations in laser sources as well as RF power source instabilities perturb the QC system
  • Fluctuating external magnetic fields can alter atomic transitions (Zeeman effect)
Keys for Optimizing a Quantum Computing Experiment

A high vacuum, ultra-stable mechanical and thermal sample environment is required to prevent any unwanted excitation of the qubit state. Superior optical access (low working distance and high numerical aperture) for spatially resolved laser excitation and high collection efficiency fluorescent readout are also necessary.

Focus Area
Why It's Important
The Cryostation Difference
Low Vibration
Mechanical stability is required to prevent energy transfer to the qubits and distortion of the system quantum state.
Minimal system vibrations (<5nm; with options for <1nm available) provide an ultra-stable environment for preserving the quantum states of qubits.
Low Temperature (<4K)
A cryogenic environment (<4K) prevents thermal excitation of the qubits. The cryogenic environment also provides cryopumping action to achieve vacuum levels better than 1x10-7 torr to prevent molecular/atomic collisions.
Cryogenic temperatures <4K are accessed seamlessly. Dial in your experimental temperature using the automated software and your sample will be there shortly.
Low Working Distance
A low working distance (WD) objective lens with a high numerical aperture (NA) provides a narrow excitation spot to focus on individual trapped ions, and also provides high collection efficiency for fluorescent readout of qubits.
Objective lenses with 1mm WD and options for 0.31mm WD (0.9 NA) objectives are quickly and easily integrated into the experimental setup. The objective lens is temperature controlled to prevent drift to ensure maximum data collection time and minimal experimental setup/alignment.
Optical Access
In addition to low WD and high NA optics, several additional window ports will be required to laser ablate (generate the ions), laser cool (prepare the quantum states), and repump the ion to avoid transitions into optically inactive states.
The Cryostation can be configured for 4 or 7 side windows with an additional top window. The large sample space makes it easy to address the sample from multiple incident angles.
Electrical Access
Many electrical feedthroughs may be required to either generate the RF trapping potential or operate the superconducting circuit.
The base panels of the Cryostation can be used to add many low frequency/DC wires in addition to coaxial wires for low loss, higher frequency signal (up to ~20 GHz). The sample space is kept uncluttered through the use of specially designed low thermal heat load cryogenic ribbon cables.
High Vacuum
Molecular and atomic collisions can excite qubits out of their quantum state or completely knock an ion out of the trap, destroying the quantum crystal.
The sample chamber of the Cryostation reaches a base pressure of better than 1x10-7 torr at base temperature due to cryopumping on the 1st stage of the cryocooler. An optional turbo pump can be added to lower the base pressure further. The combination of cryogenics and high vacuum provides a stable environment for trapped ions to be studied for weeks or months at a time.
Magnetic Shielding
Fluctuating magnetic fields can alter the electronic energy states of the trapped ion which changes the laser frequency required for gating operations.
Our engineering team can help design magnetic shielding to reduce the influence of stray electromagnetic radiation while still maintaining intimate optical access to the sample space.

related techniques & CONFIGURATIONS

The Cryostation Base Platforms offer multiple solutions for quantum computing research. A robust family of configurable options and accessories can be combined to meet the needs of various experimental techniques, including:

  • Ion Trap
  • Superconducting Circuits
  • Quantum Computing
  • Fluorescence with High NA Optics
Technique
Recommended Configuration
Research Spotlight
RF Ion Trap
Fusion with X-Plane and XYZ piezos for optical alignment
Kim Lab (Ref. 2)

Experimental Configurations for Cryogenic Quantum Computing Research

RF Ion Trap

High NA Fluorescence for Readout, Multiple Optical Access for Laser Cooling, RF + DC Electrical Feedthroughs for Generating the Ion Trap Confinement Potential 

At its core, an ion trap QC consists of N trapped ions. Each trapped ion has two stable or metastable states. Ions can be trapped in a potential well such as a Pauli (RF) trap or a Penning (magnetic field) trap. Experimentally, a Pauli ion trap is formed by patterning a set of RF electrodes on a sample in a specific geometry that creates a confining potential. Ions are loaded into the trap by first laser ablating a target substrate to release ions (137Yb+ has proven to be a good candidate). The highly excited ions are laser cooled into the quantum regime using Doppler cooling and Sisyphus cooling. Ions are then guided into the trap using a potential well that is carefully designed and applied with RF electrodes.

Once the ions are in the trap, they are spatially separated by several microns. Each ion represents one qubit. The qubits are coupled together through the Coulomb interaction which affects the collective oscillation of the N qubits. Each individual qubit encodes its local state into the collective motion through its alignment (or anti-alignment) with the Coulomb potential. In this way, each qubit on the 1D chain of ions is coupled to every other qubit.

The universal gating operations of a quantum computer (CROT, SWAP, and arbitrary rotations of the internal qubit state) can be applied to the qubits using laser excitation. In the 137YB+ ion chain, a laser wavelength of 355nm is ideal. Laser source stability is extremely important so that the excitation is matched closely (typically 10kHz or better) to the electronic resonances in the atomic ion to prevent other close lying states from being excited. UV lasers having an appropriate wavelength and excellent frequency stability have been well developed for applications in photolithography of semiconductors and are a convenient choice for quantum computing research.

The final state of the qubits is read out after completing a series of gating operations for the quantum algorithm of interest. The quantum state of the trapped ions (qubits) can be read out using state dependent fluorescence measurements. Most current research utilizes high numerical-aperture collection optics to achieve about 10% collection efficiency. Future quantum computing devices will likely make use of integrated optical cavities to improve fluorescent photon collection efficiency to >50%. Such integrated solutions would also further the development of scalable and reconfigurable quantum computing circuits.

To summarize, the keys to building and operating a reliable ion trap quantum computer are (1) stable laser sources with tight frequency control, (2) active and well controlled RF potentials for positioning and controlling the trapped ions, (3) spatially resolved laser pulses that are generated by a digital system to prepare, measure, and manipulate qubits, and (4) reliable detection and readout of the final quantum state.

References

  1. Johnson, K. G. et al. Active Stabilization of Ion Trap Radiofrequency Potentials. Review of Scientific Instruments 87, 53110 (2016).
  2. Brown, K. R., Kim, J. & Monroe, C. Co-Designing a Scalable Quantum Computer with Trapped Atomic Ions. arXiv:1602.02840 [quant-ph] (2016).
  3. Debnath, S. et al. Demonstration of a small programmable quantum computer with atomic qubits. Nature 536, 63–66 (2016).
  4. Steane, A. M. The Ion Trap Quantum Information Processor. Applied Physics B: Lasers and Optics 64, 623–643 (1997).