Active noise reduction in gated quantum devices

Solid-state quantum devices are highly sensitive to low-frequency noise that passive filters cannot remove without degrading signals. We suppress 50 Hz interference in a sub-Kelvin Si/SiGe quantum dot by injecting phase-coherent anti-noise. Automated feedback adaptively tunes phase and amplitude, lowering the noise floor and stabilizing transport measurements. This method can be generalized to noise-sensitive solid-state quantum platforms.

For more details: doi.org/10.1063/5.0303853

A strategy for realizing van der Waals Josephson junction arrays

Noise, beyond a few kHz, can be effectively filtered by an array of low-pass filters operated at various temperature stages. Minimizing noise in the lower frequency bands presents a significant challenge, particularly in experiment-specific voltage sweeps, where the sweep duration is susceptible to low-frequency periodic noise or 1/f noise having long-term correlations. This paper addresses this gap by introducing a predictive noise cancellation strategy for improved noise mitigation in short-term voltage sweeps. Our system consists of a home-built microvolt precision, dual digital-to-analog converter (DAC). The dual-DAC strategy allows control and compensation in the milli- and microvolt range with better noise isolation independently.

For more details: DOI: 10.1039/d5nr03782d

Low-Frequency Noise Mitigation in Short-Term Voltage Sweeps Using Predictive Active Noise Cancellation in a Dual-DAC System

Noise, beyond a few kHz, can be effectively filtered by an array of low-pass filters operated at various temperature stages. Minimizing noise in the lower frequency bands presents a significant challenge, particularly in experiment-specific voltage sweeps, where the sweep duration is susceptible to low-frequency periodic noise or 1/f noise having long-term correlations. This paper addresses this gap by introducing a predictive noise cancellation strategy for improved noise mitigation in short-term voltage sweeps. Our system consists of a home-built microvolt precision, dual digital-to-analog converter (DAC). The dual-DAC strategy allows control and compensation in the milli- and microvolt range with better noise isolation independently.

For more details: 10.1109/TIM.2025.3589709

Shot-Noise-Driven Macroscopic Vibrations and Displacement Transduction in Quantum Tunnel Junctions

Back-action is an inevitable result of quantum measurement. Although the microscopic impacts of shot-noise back-action have been explored, macroscopic evidence is seldom documented, especially in the field of electrical transport. In this study, shot-noise back-action is examined at much higher operational frequencies and wider bandwidths by employing a GaAs QPC integrated into a planar superconducting cavity within the circuit-QED framework, leading to the observation of the excitation of multiple mechanical modes.

For more details: https://doi.org/10.1088/1361-648x/ade21b

Refining Au/Sb alloyed ohmic contacts in undoped Si/SiGe strained quantum wells

This work addresses the challenges of implementing Au/Sb as an ohmic contact to an undoped Si/SiGe system and presents a simple recipe to establish high-efficiency contacts to realize shallow enhancement mode devices. The electrical characteristics of the alloyed contacts are similar to those realized by phosphorus ion-implantation, suggesting that this method can serve as an alternative method for the realization of quantum devices on shallow undoped Si/SiGe heterostructures.

For more details: https://doi.org/10.1088/1361-648x/ade21b

Overdamped van der Waals Josephson junctions by area engineering

Josephson junctions (JJs) are the building blocks of superconducting quantum devices. The current technology exploits defective systems and interfaces - conventional superconducting and insulating thin films - resulting in compromised coherence times. JJs realized by stacking few-layered NbSe2 flakes, a highly crystalline layered system that exploits the van der Waals gap between the flakes as the tunnel barrier, resulting in defect-free JJs and high-quality superconducting devices. we demonstrate both underdamped and overdamped NbSe2-NbSe2 JJs by controlling the junction area. We devise a minimally invasive microfabrication procedure, post-junction formation, to control the junction area precisely.
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For more details : https://doi.org/10.1002/pssr.202500090

Qtran Lab participated in the IInvenTiv - 2025, a mega R&D fair for top higher education institutions of India

Congratulations to Anusha S for sucessfully defending her thesis on "Configuring Quantum Point Contacts for Broadband Electrical Amplification"

Congratulations to Prasad M for securing the Best Poster Award in the "Frontier Symposium 2025 - IISER Thiruvananthapuram"

On-chip cryogenic low-pass filters based on finite ground-plane coplanar waveguides for quantum measurements

Quantum technology exploits fragile quantum electronic phenomena whose energy scales demand ultra-low electron temperature operation. The lack of electron–phonon coupling at cryogenic temperatures makes cooling the electrons down to a few tens of millikelvin a non-trivial task, requiring extensive efforts on thermalization and filtering high-frequency noise. Existing techniques employ bulky and heavy cryogenic metal-powder filters, which prove ineffective at sub-GHz frequency regimes and unsuitable for high-density quantum circuits such as spin qubits. In this work, we realize ultra-compact and lightweight on-chip cryogenic filters based on the attenuation characteristics of finite ground-plane coplanar waveguides. These filters are made of aluminum on sapphire substrates using standard microfabrication techniques. The attenuation characteristics are measured down to a temperature of 500 mK in a dilution refrigerator in a wide frequency range of a few hundred kHz to 8.5 GHz. We find their performance is superior by many orders compared to the existing filtering schemes, especially in the sub-GHz regime, negating the use of any lumped-element low-pass filters. The compact and scalable nature makes these filters a suitable choice for high-density quantum circuits such as quantum processors based on quantum dot spin qubits.
For more details : https://doi.org/10.1063/5.0243614

Congratulations to Sreevidya N for sucessfully defending her thesis on "Voltage tunable phases on electric double layer gated MoS2"

Electric double layer (EDL) gating technique is employed in few layered MoS₂ to study gate induced 2D superconductivity in the system. Exotic vortex phases such as Bose metal phase and thermally activated flux flow are observed in the system. Vortex dynamics corresponding to each phase are investigated. The highly mobile vortices leave transient signatures in the magnetoresistance. By analysing the temporal behaviour of the resistance, we establish a one-to-one correlation between the vortex dynamics and the quantum phases realised on our system against the magnetic field, the driving current, and the temperature. The temperature and magnetic field dependence of the transient nature and the noise characteristics of the magnetoresistance confirm that quantum fluctuations are solely responsible for the Bose metal state and the fragility of the superconducting state. The device shows a re-entrant insulating phase at higher gating voltage.The application of EDL gating to selectively dope the contact region to establish low-resistance barrier-free contacts is studied. The contact regions are exposed to ionic liquid, while the active channel region is protected with a h-BN layer. The devices exhibit ohmic source and drain contacts, while the active region maintains excellent ON-OFF characteristics, making this technique establish ohmic contacts on these systems. Experiments on transport across EDL-gated metallic MoS₂-MoS₂ homojunctions to characterize the van der Waals gap (vW) is also studied. From the current-voltage and transconductance measurements down to a temperature of 4 K, we confirm the metallic nature of the constituent MoS₂ flakes, while the inter-flake transport shows a clear signature of the tunnel junction, confirming that the vW gap survives the electronic doing by the EDL gating technique.

Congratulations to Annu Anns Sunny for securing the Best Poster Award in the "Asia-Pacific Conference on Condensed Matter Physics 2024"

QTran lab participated in the Kerala Science Slam 2024

Annu Anns Sunny presented her work on 'Quantum technologies based on two-dimensional Josephson Junctions' and Parvathy Gireesan presented her work on 'Electron spin state based quantum computing' at the Kerala Science Slam'24 held by Kerala Shastra Sahithya Parishath.

Light-Induced Transformation of a Supramolecular Gel to a Stronger Covalent Polymeric Gel

A polymerizable diacetylene gelator, containing urea and urethane groups, that congeals various non-polar solvents was synthesized. The gelator molecules self-assemble forming non-covalent polymers through intermolecular hydrogen bonding, as evidenced from FT-IR and concentration-dependent ¹H NMR spectroscopy. The self-assembly positions the diyne units of adjacent molecules at proximity and in a geometry suitable for their topochemical polymerization. UV irradiation of the gel resulted in topochemical polymerization, transforming the non-covalent polymer to a covalent polymer, in situ, in the gel state. The polymerization was confirmed by characterizing the polydiacetylene (PDA) using UV-Vis and Raman spectroscopy. Time-dependent rheological studies revealed gradual strengthening of the gel with the duration of irradiation, suggesting that the degree of polymerization increases with the duration of irradiation. The PDA formed is a semiconductor, which might be useful for various applications.
For more details : https://doi.org/10.1002/cphc.202400861

Congratulations to the graduating members of QTran Lab

One-electron gated quantum dot array with integrated readout scheme on GaAs/AlGaAs heterostructure for spin-qubit realization

Physical vapor deposition-free scalable high-efficiency electrical contacts to MoS₂

Fermi-level pinning caused by the kinetic damage during metallization has been recognized as one of the majorreasons for the non-ideal behavior of electrical contacts, forbidding reaching the Schottky-Mott limit. In thismanuscript, we present a scalable technique wherein Indium, a low-work-function metal, is diffused to contact afew-layered MoS2 flake. The technique exploits a smooth outflow of Indium over gold electrodes to make edgecontacts to pre-transferred MoS2 flakes. We compare the performance of three pairs of contacts made onto thesame MoS2 flake, the bottom-gold, top-gold, and Indium contacts, and find that the Indium contacts are superiorto other contacts. The Indium contacts maintain linear I-V characteristics down to cryogenic temperatures with anextracted Schottky barrier height of ~ 2.1 meV. First-principle calculations show the induced in-gap states closeto the Fermi level, and the damage-free contact interface could be the reason for the nearly Ohmic behavior of theIndium/MoS2 interface.
For more details : https://doi.org/10.1088/1361-6528/ad12e4

Polymorphism-driven Distinct Nanomechanical, Optical, Photophysical, and Conducting Properties in a Benzothiophene-quinoline

Polymorphic forms of organic conjugated small molecules, with  their  unique  molecular  shapes,  packing  arrangements,  and interaction patterns, provide an excellent opportunity to uncover how their microstructures influence their observable properties. Ethyl-2‐(1‐benzothiophene‐2‐yl)quinoline‐4‐carboxylate (BZQ) exists as dimorphs  with  distinct  crystal  habits―blocks(BZB)  and  needles (BZN). The crystal forms differ in their molecular arrangements―BZBhas a slip-stacked column-like structure in contrast to a zig-zag crystal packing  with  limited  π–overlap   in BZN.   The BZBcrystals characterized  by  extended  π-stacking  along  [100]  demonstrated semiconductor  behavior,  whereas  the BZN,  with  its  zig-zag  crystal packing  and  limited  stacking characteristics,  was  reckoned  as  an insulator.  Monotropically related  crystal  forms  also differ  in  their nanomechanical properties,  with BZBcrystals  being  considerably softer  than BZNcrystals.
For more details : https://doi.org/10.1002/chem.202303558

Transient vortex dynamics and evolution of Bose metal from a 2D superconductor on MoS₂

A metallic ground state in 2D is beyond the consensus of both Bosonic and Fermionic systems, and itsorigin and nature warrant a comprehensive theoretical understanding supplemented by in-depth experiments. A real-time observation of the influence of vortex dynamics on transport properties sofar has been elusive. We explore the nature and fate of a low-viscous, clean,  2D superconducting stateformed on an ionic-liquid gated few-layered MoS₂ sample. The vortex-core being dissipative, the elasticdepinning, intervortex interaction, and the subsequent dynamics of the vortex-lattice leave transientsignatures in the transport characteristics.  The temperature and magnetic field dependence of thetransient nature and  the noise characteristics of the magnetoresistance confirm that quantum fluctuationsare solely responsible for the Bose metal state and the fragility of the superconducting state. .
For more details : https://doi.org/10.1088/2053-1583/ad0b87

GHz operation of a quantum point contact using stub-impedance matching circuit

In this work, we couple a QPC galvanically to a superconducting stub tuner impedance matching circuit realised in a coplanar waveguide architecture to enhance the operation frequency into the GHz regime and investigate the electrical amplification and complex admittance characteristics. The device, operating at ~ 1.96 GHz, exhibits a conductance sensitivity of 2.92×10⁵(e²/h)/H^1/2 with a bandwidth of 13 MHz.
For more details : https://doi.org/10.1016/j.physo.2023.100181

First Semiconducting Quantum Dot measured at QTran Lab

After many sleepless months, patience and teamwork, We at QTran Lab have finally developed India's First Semiconducting Quantum Dot. Quantum Dots are fundamental blocks of Spin Qubits and that would be our next step.