Weakly Flux-Tunable Superconducting Qubit

  1. José M. Chávez-Garcia,
  2. Firat Solgun,
  3. Jared B. Hertzberg,
  4. Oblesh Jinka,
  5. Markus Brink,
  6. and Baleegh Abdo
Flux-tunable qubits are a useful resource for superconducting quantum processors. They can be used to perform cPhase gates, facilitate fast reset protocols, avoid qubit-frequency collisions
in large processors, and enable certain fast readout schemes. However, flux-tunable qubits suffer from a trade-off between their tunability range and sensitivity to flux noise. Optimizing this trade-off is particularly important for enabling fast, high-fidelity, all-microwave cross-resonance gates in large, high-coherence processors. This is mainly because cross-resonance gates set stringent conditions on the frequency landscape of neighboring qubits, which are difficult to satisfy with non-tunable transmons due to their relatively large fabrication imprecision. To solve this problem, we realize a coherent, flux-tunable, transmon-like qubit, which exhibits a frequency tunability range as small as 43 MHz, and whose frequency, anharmonicity and tunability range are set by a few experimentally achievable design parameters. Such a weakly tunable qubit is useful for avoiding frequency collisions in a large lattice while limiting its susceptibility to flux noise.

Weakly Flux-Tunable Superconducting Qubit

  1. José M. Chávez-Garcia,
  2. Firat Solgun,
  3. Jared B. Hertzberg,
  4. Oblesh Jinka,
  5. Markus Brink,
  6. and Baleegh Abdo
Flux-tunable qubits are a useful resource for superconducting quantum processors. They can be used to perform cPhase gates, facilitate fast reset protocols, avoid qubit-frequency collisions
in large processors, and enable certain fast readout schemes. However, flux-tunable qubits suffer from a trade-off between their tunability range and sensitivity to flux noise. Optimizing this trade-off is particularly important for enabling fast, high-fidelity, all-microwave cross-resonance gates in large, high-coherence processors. This is mainly because cross-resonance gates set stringent conditions on the frequency landscape of neighboring qubits, which are difficult to satisfy with non-tunable transmons due to their relatively large fabrication imprecision. To solve this problem, we realize a coherent, flux-tunable, transmon-like qubit, which exhibits a frequency tunability range as small as 43 MHz, and whose frequency, anharmonicity and tunability range are set by a few experimentally achievable design parameters. Such a weakly tunable qubit is useful for avoiding frequency collisions in a large lattice while limiting its susceptibility to flux noise.

On-chip single-pump interferometric Josephson isolator for quantum measurements

  1. Baleegh Abdo,
  2. Oblesh Jinka,
  3. Nicholas T. Bronn,
  4. Salvatore Olivadese,
  5. and Markus Brink
Nonreciprocal microwave devices, such as circulators and isolators, are critical in high-fidelity qubit readout schemes. They unidirectionally route the readout signals and protect
the qubits against noise coming from the output chain. However, cryogenic circulators and isolators are prohibitive in scalable superconducting architectures because they rely on magneto-optical effects. Here, we realize an on-chip, single-microwave-pump Josephson ISolator (JIS), formed by coupling two nondegenerate Josephson mixers in an interferometric scheme. We unravel the interplay between the orientation parity of the magnetic fluxes, biasing the mixers, and the JIS directionality. Furthermore, we build a motherboard, which integrates the JIS and other superconducting components, including a Josephson directional amplifier, into a printed circuit and use it to read out a qubit with 92% fidelity, while maintaining 75% of its T2E. Improved versions of this motherboard could replace magnetic circulators and isolators in large superconducting quantum processors.

Active protection of a superconducting qubit with an interferometric Josephson isolator

  1. Baleegh Abdo,
  2. Nicholas T. Bronn,
  3. Oblesh Jinka,
  4. Salvatore Olivadese,
  5. Antonio D. Corcoles,
  6. Vivekananda P. Adiga,
  7. Markus Brink,
  8. Russell E. Lake,
  9. Xian Wu,
  10. David P. Pappas,
  11. and Jerry M. Chow
Nonreciprocal microwave devices play several critical roles in high-fidelity, quantum-nondemolition (QND) measurement schemes. They separate input from output, impose unidirectional
routing of readout signals, and protect the quantum systems from unwanted noise originated by the output chain. However, state-of-the-art, cryogenic circulators and isolators are disadvantageous in scalable superconducting quantum processors because they use magnetic materials and strong magnetic fields. Here, we realize an active isolator formed by coupling two nondegenerate Josephson mixers in an interferometric scheme. Nonreciprocity is generated by applying a phase gradient between the same-frequency pumps feeding the Josephson mixers, which play the role of the magnetic field in a Faraday medium. To demonstrate the applicability of this Josephson-based isolator for quantum measurements, we incorporate it into the output line of a superconducting qubit, coupled to a fast resonator and a Purcell filter. We also utilize a wideband, superconducting directional coupler for coupling the readout signals into and out of the qubit-resonator system and a quantum-limited Josephson amplifier for boosting the readout fidelity. By using this novel quantum setup, we demonstrate fast, high-fidelity, QND measurements of the qubit while providing more than 20 dB of protection against amplified noise reflected off the Josephson amplifier.

Multi-Path Interferometric Josephson Directional Amplifier for Qubit Readout

  1. Baleegh Abdo,
  2. Nick Bronn,
  3. Oblesh Jinka,
  4. Salvatore Olivadese,
  5. Markus Brink,
  6. and Jerry M. Chow
We realize and characterize a quantum-limited, directional Josephson amplifier suitable for qubit readout. The device consists of two nondegenerate, three-wave-mixing amplifiers that
are coupled together in an interferometric scheme, embedded in a printed circuit board. Nonreciprocity is generated by applying a phase gradient between the same-frequency pumps feeding the device, which plays the role of the magnetic field in a Faraday medium. Directional amplification and reflection-gain elimination are induced via wave interference between multiple paths in the system. We measure and discuss the main figures of merit of the device and show that the experimental results are in good agreement with theory. An improved version of this directional amplifier is expected to eliminate the need for bulky, off-chip isolation stages that generally separate quantum systems and preamplifiers in high-fidelity, quantum-nondemolition measurement setups.

Demonstrating a gyrator operation using Josephson mixers

  1. Baleegh Abdo,
  2. Markus Brink,
  3. and Jerry M. Chow
Nonreciprocal microwave devices such as circulators are useful in routing quantum signals in quantum networks and protecting quantum systems against noise coming from the detection
chain. However, commercial, cryogenic circulators, used nowadays, are unsuitable for scalable superconducting quantum architectures due to their appreciable size, loss, and inherent magnetic field. In this work, we report on the measurement of a key nonreciprocal element, i.e., the gyrator, which can be used to realize a circulator. Unlike state-of-the-art gyrators, which use a magneto-optic effect to induce a phase shift of π between transmitted signals in opposite directions, our device uses the phase nonreciprocity of a Josephson-based three-wave mixing device. By coupling two of these mixers and operating them in noiseless frequency conversion mode, we show that the device acts as a nonreciprocal phase shifter, whose phase shift is controlled by the phase difference of the microwave tones driving the mixers. Such a device could be used to realize a lossless, on-chip, superconducting circulator suitable for quantum information processing applications.

Time-multiplexed amplification in a hybrid-less and coil-less Josephson parametric converter

  1. Baleegh Abdo,
  2. Jose M. Chavez-Garcia,
  3. Markus Brink,
  4. George Keefe,
  5. and Jerry M. Chow
Josephson parametric converters (JPCs) are superconducting devices capable of performing nondegenerate, three-wave mixing in the microwave domain without losses. One drawback limiting
their use in scalable quantum architectures is the large footprint of the auxiliary circuit needed for their operation, in particular, the use of off-chip, bulky, broadband hybrids and magnetic coils. Here, we realize a JPC which eliminates the need for these bulky components. The pump drive and flux bias are applied in the new device through an on-chip, lossless, three-port power divider and on-chip flux line, respectively. We show that the new design considerably simplifies the circuit and reduces the footprint of the device while maintaining a comparable performance to state-of-the-art JPCs. Furthermore, we exploit the tunable bandwidth property of the JPC and the added capability of applying alternating currents to the flux line in order to switch the resonance frequencies of the device, hence demonstrating time-multiplexed amplification of microwave tones that are separated by more than the dynamical bandwidth of the amplifier. Such a measurement technique can potentially serve to perform time-multiplexed, high-fidelity readout of superconducting qubits.

Non-Poissonian Quantum Jumps of a Fluxonium Qubit due to Quasiparticle Excitations

  1. Uri Vool,
  2. Ioan M. Pop,
  3. Katrina Sliwa,
  4. Baleegh Abdo,
  5. Chen Wang,
  6. Teresa Brecht,
  7. Yvonne Y. Gao,
  8. Shyam Shankar,
  9. Michael Hatridge,
  10. Gianluigi Catelani,
  11. Mazyar Mirrahimi,
  12. Luigi Frunzio,
  13. Robert J. Schoelkopf,
  14. Leonid I. Glazman,
  15. and Michel H. Devoret
As the energy relaxation time of superconducting qubits steadily improves, non-equilibrium quasiparticle excitations above the superconducting gap emerge as an increasingly relevant
limit for qubit coherence. We measure fluctuations in the number of quasiparticle excitations by continuously monitoring the spontaneous quantum jumps between the states of a fluxonium qubit, in conditions where relaxation is dominated by quasiparticle loss. Resolution on the scale of a single quasiparticle is obtained by performing quantum non-demolition projective measurements within a time interval much shorter than T1, using a quantum limited amplifier (Josephson Parametric Converter). The quantum jumps statistics switches between the expected Poisson distribution and a non-Poissonian one, indicating large relative fluctuations in the quasiparticle population, on time scales varying from seconds to hours. This dynamics can be modified controllably by injecting quasiparticles or by seeding quasiparticle-trapping vortices by cooling down in magnetic field.

Josephson directional amplifier for quantum measurement of superconducting circuits

  1. Baleegh Abdo,
  2. Katrina Sliwa,
  3. S. Shankar,
  4. Michael Hatridge,
  5. Luigi Frunzio,
  6. Robert Schoelkopf,
  7. and Michel Devoret
We have realized a microwave quantum-limited amplifier that is directional and can therefore function without the front circulator needed in many quantum measurements. The amplification
takes place in only one direction between the input and output ports. Directionality is achieved by multi-pump parametric amplification combined with wave interference. We have verified the device noise performances by using it to readout a superconducting qubit and observed quantum jumps. With an improved version of this device, qubit and preamplifer could be integrated on the same chip.

Directional amplification with a Josephson circuit

  1. Baleegh Abdo,
  2. Katrina Sliwa,
  3. Luigi Frunzio,
  4. and Michel Devoret
Non-reciprocal devices, which have different transmission coefficients for propagating waves in opposite directions, are crucial components in many low noise quantum measurements. In
most schemes, magneto-optical effects provide the necessary non-reciprocity. In contrast, the proof-of-principle device presented here, consists of two on-chip coupled Josephson parametric converters (JPCs), which achieves directionality by exploiting the non-reciprocal phase response of the JPC in the trans-gain mode. The non-reciprocity of the device is controlled in-situ by varying the amplitude and phase difference of two independent microwave pump tones feeding the system. At the desired working point and for a signal frequency of 8.453 GHz, the device achieves a forward power gain of 15 dB within a dynamical bandwidth of 9 MHz, a reverse gain of -6 dB and suppression of the reflected signal by 8 dB. We also find that the amplifier adds a noise equivalent to less than one and a half photons at the signal frequency (referred to the input). It can process up to 3 photons at the signal frequency per inverse dynamical bandwidth. With a directional amplifier operating along the principles of this device, qubit and readout preamplifier could be integrated on the same chip.