Atom Quantum Computing & Simulation

To prepare neutral atoms for quantum computing and quantum simulation applications, they are laser-cooled and optically trapped (LINK: zu atom cooling & optical trapping page ) in so-called optical lattices, which are formed by standing laser light waves, or in optical tweezers and optical tweezer arrays, which are formed by tightly focused laser beams. All this is done within ultra high-vacuum systems, sometimes even in cryogenic environment, and requires specialized laser systems.

The most commonly used atomic species for atomic quantum computing are:

• Rubidium or Cesium with two states of the hyperfine groundstate manifold used as the qubit called "hyperfine qubit".
• Strontium or Ytterbium with various realizations of the qubit:
  • the two states of the optical clock transition (ground state and one long-lived metastable state), called "optical qubit" (or "clock qubit")
  • two metastable states (typ. in the manifold of the upper clock stateed), called "finestructure qubit" (or "metastable qubit")
  • two magnetic substates of the ground state, called "nuclear qubit" (or "Zeeman qubit")

Optical pumping and coherent transfer with lasers is used to prepare the qubits in the desired starting state (initialization). Again, high-quality specialized laser systems are a key ingredient for all operations where lasers are required.

• Single-qubit gates are realized by coherently driving a transition between the two qubit states |0> and |1> of one atom for a well defined time with well defined phase. The methods depend on the involved qubit states:
  • For optical quits, one usually uses a laser directly addressing the optical clock transition. Typically, one needs high-power low-noise lasers with linewidths on the order of few Hz that are frequency-stabilized to the optical clock transition with accuracy on the order of few Hz. 
  • Hyperfine and nuclear qubits are addressed in two ways. Either one applies
    • coherent radiofrequency or microwave fields, or
    • two lasers with optical frequencies that differ by the energy difference of the involved qubit states, that have a well-defined relative phase, and that are each close to a common upper quantum state for a well defined time with well defined phase.

• Two-qubit gates are usually realized by means of qubit-state-selective laser excitation of the atoms to a Rydberg state |r> and utilizing the so-called Rydberg blockade.
  A simplified picture is the following: The Rydberg energy levels of atoms that are close (~ few µm) to an atom that was excited into the Rydberg state are shifted by the large dipole moment associated with Ryberg states such that they are no longer in resonance with the Rydberg excitation laser. Consequently, shining in a laser that is resonant to the transition of one qubit state, e.g. |0>, to a defined Rydberg state |r> on atom 1 one of two close-by atoms 1 and 2 would lead to an excitation of atom 1 to |r> if it was in |0>. The second atom 2, if locally addressed subsequently with the same laser, would do the same, but only if atom 1 was not in state |0>. Because if atom 1 had been in state |0> it would now be in |r> shifting the transition of atom 2 to the Rydberg state |r> out of resonance with the laser due to the Rydberg blockade. Consequently, atom 2 would change its state depending on the initial state of atom 1. This is a two-qubit gate.

To read out the result of the computation performed by gate sequences, one reads out the final quantum state by shining in quantum-state-selective lasers, which are resonant to transitions that scatter many photons onto photodectors and/or cameras. The detection step might require additional repumping and prior optical pumping or coherent transfer.

Instead of applying gate sequences like in (digtial) quantum computing, for quantum simulation (analog quantum computing) one applies

• designed artificial potentials (optical, electric or magnetic fields)
• inertial forces or gravity
• interactions between atoms

to realize certain "Hamiltonians" withing the system. One then observes the time dependent evolution of the atoms' quantum states or the finally realized quantum states under the influence of this Hamiltonian. Detection is again achieved via fluorescence detection and imaging.

Atoms can be prepared in large numbers of hundreds of thousands in optical lattices and many thousands in optical tweezers. The atoms can be moved around or spatially arranged and re-arranged nearly arbitrarily, thus allowing for many different configurations for quantum computing and simulation. Two-qubit gates can be switched comparatively fast due to the strength (=speed) of the Rydberg blockade. 

Up to date, qubit fidelities that quantify how well or exact the gates can be and hence quantifying the performance of the quantum computing steps, are very high (0.99x) but still lower than the fidelities achieved with ion quantum computing. But in the last years significant progress has been achieved!

Laser play a key role in atom quantum computing and simulation. All of our products designed for quantum technologies are not only suited for this application but also integrated in scientific and commercial solutions. TOPSELLERs and customized versions comprise:

• Tunable diode laser systems
• Clock Laser Systems
• Cw fiber amplifiers and Raman fiber amplifiers
• Optical Frequency Combs
• Wavelength meters
• Frequency and phase stabilization electronic modules
• Complete laser rack systems containing the above mentioned products

Please contact our sales and applications experts for consulation of the best suited solutions.