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Better quantum light sources are possible with new developments. Vienna and Innsbruck Multi-Photon Emission Control improvements for quantum technologies.

Recent discoveries by Vienna and Innsbruck research teams have improved the quest for non-classical light, which is needed for quantum technologies like secure communication and quantum computing. Despite the goal of producing pure single photons, resonant excitation in atoms, whether natural or artificial, always emits multi-photons. To better handle these complex light-matter interactions and generate more reliable and effective quantum light sources, two complementary research efforts have yielded crucial insights and feasible solutions.

Artificial Atom Multi-Photon Dynamics Revealed

L. Jehle, L. Carosini, L. M. Hansen, J. C. Loredo, and P. Walther along with F. Giorgino and P. Zahálka at the University of Vienna have studied multi-photon processes in silicon quantum dots. Because of their strong optical properties and solid-state synthesis, silicon quantum dots are becoming more popular platforms for quantum light source creation.

In “Multi-photon emission from a resonantly pumped quantum dot,” upper-order auto-correlation functions (g(2), g(3), and g(4)) and high-resolution temporal measurements quantify these emissions, providing a nuanced understanding of light-matter interactions. They proved that a single excitation pulse can emit four photons.

Conclusions from their detailed analysis include:

With single-photon emission, two, three, or four photons can be detected from a single stimulation. By showing that a bunched source (g(2)(0) > 1) does not always have a better probability of emitting two photons than one, the work showed the importance of the vacuum probability (likely to produce no photons). Increased multi-photon contributions occur in even pulse areas (Θ = 2nπ) due to quantum dot re-excitation during the excitation pulse duration. Closely resolved temporal studies confirmed that these multi-photon events are caused by successive spontaneous emissions, meaning the photons do not share the same temporal mode. This study showed a time-gating approach to purify single-photon sources. By rejecting early-arriving photons and catching photons during a specified time frame after excitation, researchers can prevent multi-photon events while maintaining detection efficiency. This works better than lowering excitation power for purity.

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Enhancing Multi-Photon Generation with Passive Demultiplexing

Gregor Weihs and Vikas Remesh from the University of Innsbruck led a worldwide research team that developed a sophisticated approach to overcome the disadvantages of traditional multi-photon state creation. Researchers develop multi-photon states from a single quantum dot using fast electro-optic modulators (EOMs) to multiplex emission into spatial and temporal modes. EOMs require specialised engineering, are expensive, and reduce efficiency.

Using stimulated two-photon excitation (sTPE), the Innsbruck team produces photon streams in multiple polarisation states from a quantum dot without active switching elements. This method limits multi-photon rate by the quantum dot's intrinsic lifetime rather than an EOM's switching speed.

The procedure:

By carefully timing laser pulses to excite the quantum dot, a biexciton state is generated. After that, polarization-controlled stimulation pulses that deterministically emit photons in the horizontal (H) and vertical (V) polarisation states follow. When used with active demultiplexing technologies, passive demultiplexing is cost-effective and doubles multi-photon generation. With g(2)(0) values of 0.022(2) for V-polarized photons and 0.028(2) for H-polarized photons, the group produced high-quality two-photon states with excellent single-photon characteristics. They validated high indistinguishability with adjusted Hong-Ou-Mandel (HOM) visibilities of 90(1)% for V-polarized photons and 93.7(3)% for H-polarized photons.

This innovation enables safe multi-party communication and fits directly into quantum key distribution systems. It also shows potential for multi-photon interference studies, which test quantum mechanical ideas.

New quantum control: subharmonic driving transforms fluxonium qubit coherence

Researchers discovered a new control mechanism for superconducting fluxonium qubits, which could increase coherence and enable more dependable quantum processors. Subharmonic control overcomes the fundamental quantum computing problem of balancing rapid, high-fidelity control with decoherence caused by required links to the quantum system.

Due to the reciprocal relationship between information loss and control speed, quicker operations require tighter coupling to control elements, which accelerates quantum decohesion. Using fluxonium qubits' strong nonlinearity, researchers have broken this reciprocity and enabled control via multiple photons at a fraction of the resonance frequency.

Fluxonium: A Unique Qubit Benefits

Transmon qubits have advanced scalable, error-corrected quantum processors employing superconducting circuits, a leading platform. The biggest impediment to increasing gate fidelities is still decoherence. This decoherence can be caused by external sources like qubit control signal lines or internal causes like material faults (TLSs).

Fluxonium qubits have record-high superconducting qubit coherence periods due to their lower coupling to TLSs at low transition frequencies. Due to its significant nonlinearity, the new subharmonic control technique is ideal for multiphoton control operations. Fluxonium qubits' resonant flux control provides stronger single-qubit-gate fidelities than charge-based control, making it a good multiphoton gate solution.

How Subharmonic Control Works

Qubit resonance frequency is usually controlled via single-photon interactions. The photons used in subharmonic control operate at an integer fraction of the qubit's transition frequency, but their total energy equals it. This allows the attachment of a low-pass filter to the flux line, which flux-biases the qubit at its optimal working position that is flux noise-resistant.

Filtering the control channel below the fundamental qubit frequency eliminates qubit decay through the control channel by suppressing the ambient density of states at the qubit's transition frequency. Highly accurate transversal control and DC flux biassing are implemented using this Purcell-protected channel, reducing control-induced decoherence.

Massive Coherence and Fidelity Gains

The experimental results of this novel control system are promising:

T2-echo time and T1 (energy relaxation time) increased 10-fold and 5-fold, respectively, in the filtered arrangement. The low-pass filter isolates the qubit from outside noise while idle, confining it to internal losses and the readout-resonator population. In Action: Strong Nonlinearity To illustrate the fluxonium potential's extraordinary nonlinearity, the researchers used subharmonic drives with up to 11 photons to coherently control the qubit. By achieving gate fidelities above 99.94%, a 3-photon subharmonic drive was shown to be an on-resonance drive. This performance approaches the device's coherence limit. Predictive Modelling: The excellent agreement between measured Rabi frequencies and drive-induced frequency shifts and numerical and analytical models verified the theoretical understanding of this complex interaction. Due to matrix-element asymmetries, fluxonium qubits have a naturally greater subharmonic Rabi frequency than weakly anharmonic systems like transmons, making this control especially helpful for them. Read Chinese Team Discovers Quantum Friction In Folded Graphene.

Prospects: Scalable Quantum Processors

This paper proposes a possible bosonic quantum computation route. The fluxonium's extended coherence lengths and circuit design and in situ tuning flexibility make it a promising high-performance control qubit with little side effects.

Importantly, this technology simplifies wiring like fixed-frequency transmons and provides a scalable and hardware-efficient fluxonium-based processor control architecture. Due to multiphoton transitions' substantial amplitude dependency, drive frequencies can be chosen more freely, reducing off-resonant driving of neighbouring qubits and crosstalk.

Even though resonator decay limited the planar prototype device's fidelities, simulations suggest that adding reflecting low-pass filters on-chip could lessen flux-line segment decay. Adding closed-loop systems and predistorted pulses should make subharmonic gates as good as on-resonance gates.