JP2010109697A - Microwave photon detector and detecting method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a microwave photon detector that can detect an individual photon of microwave. <P>SOLUTION: Superconducting quantum bits are coupled with a superconducting resonator for optically receiving microwaves and a superconducting resonator for reading quantum bits. Microwave photons resonantly-incident on the resonator for optically receiving excites a coupling mode of the quantum bits and the resonator. A nonlinear resonator for reading quantum bits, having a different resonance frequency, is continuously driven by an external microwave source, and its responding amplitude or phase is continuously measured. Once the quantum bits are excited by incidence of signal photons, response of the resonator for reading has noncontinuous changes, and single photon detection is obtained. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a microwave photon detector and a microwave photon detection method used in a quantum information processing apparatus and a low noise high sensitivity measuring instrument.

  With the development of microwave technology in recent years, the role of measurement technology of micro microwave signals is increasing in the fields of communication technology using microwaves, high frequency circuit technology, microwave astronomy, quantum information processing technology, etc. It is getting bigger.

  Technologies related to micro microwave signal measurement are mainly signal amplification using a low noise amplifier, homodyne measurement / heterodyne measurement using mixing with a coherent microwave output from a local oscillator, and power measurement using a power meter. Used for. In addition, in order to overcome the quantum noise limit in a linear amplifier, a parametric amplifier using a nonlinear circuit has also been realized.

  On the other hand, single photon detectors such as photomultiplier tubes (photomultipliers) and avalanche photodiodes are frequently used in the measurement of electromagnetic waves having large energy above infrared. Using such a detector that responds to the input of individual photons, photon number measurement, photon number correlation between different channels, temporal statistical measurement of the number of photons, etc. are performed to evaluate the state of the input signal light In addition to being useful, it is also used for applications such as quantum cryptography.

  However, since it is difficult to apply a photomultiplier tube or the like for measuring electromagnetic waves having large energy to the measurement of micro microwave photons, a microwave photon detector for measuring micro microwave photons has not been realized. There is no actual situation.

  In recent years, with the development of quantum information processing technology, microwave engineering, which has been mainly targeted at coherent microwaves, is also interested in non-classical states such as squeezed states and photon number states. Is held. In particular, in superconducting quantum circuits, as shown in Non-Patent Documents 1 to 3, research has been conducted to generate such a state using a superconducting resonator in the microwave band or a superconducting qubit. .

  The photon number detection technique described in Non-Patent Documents 1 to 3 is expected to play an important role in evaluating and applying the state, but a microwave photon detection circuit and a detection method for accurately measuring microwave photons As mentioned above, it has not been proposed yet.

  On the other hand, as a technology relating to a quantum circuit constituting a quantum computer, Patent Document 1 discloses that a plurality of superconducting particle bit devices are coupled to each other using a superconducting resonator and a microwave pulse, and the implementation and quantum A quantum circuit that facilitates operations is disclosed.

  Patent Document 2 describes a qubit coupling method that can increase the coupling force by electrostatically controlling coupling between qubits with an electron gas.

Amplification and squeezing of quantum noise with a tunable Josephson metamaterial (arXiv: cond-mat / 0806.0659, 4 Jun 2008) Generating single microwave photons in a circuit (Nature (London) / Vol. 449/328, 17 Jul 2007) Generation of Fock states in a superconducting quantum circuit (Nature (London) / Vol. 454/310, 17 Jul 2008) JP 2006-165812 A JP 2007-287933 A

  When the conventional measurement technique is applied to micro microwave photons, there is a problem that single photon detection cannot be performed. For this reason, photon count measurement and photon count statistical measurement cannot be performed, and there is a limit to information obtained when measuring and evaluating an unknown microwave signal.

  On the other hand, Patent Documents 1 and 2 only disclose controlling coupling between qubits, and do not suggest any technique for detecting photons, particularly micro-microwave photons.

  The present invention is intended to improve at least a part of the problems described above.

  A microwave photon detector according to one aspect of the present invention includes a superconducting qubit coupled to a superconducting resonator for receiving microwaves and a superconducting resonator for reading qubits.

  In another aspect of the present invention, there is provided a microwave photon detection method characterized by detecting a microwave photon by exciting a qubit in response to an input photon and reading the excited state of the qubit. It is done.

  According to the present invention, a microwave non-cochlear detector and a detection method capable of detecting a single photon can be obtained by exciting a qubit with input photons and reading out this state with a readout resonator.

  Before describing the embodiment of the present invention, the principle of the present invention will be described.

  The microwave photon detector according to the present invention includes a light receiving resonator and a reading resonator, and includes a light receiving resonator and a qubit coupled to the reading resonator. According to this configuration, the microwave photons that have entered the light receiving resonator in a resonant manner excite the coupled mode of the qubit and the resonator. A qubit readout resonator having another resonance frequency is continuously driven by an external microwave source, and the amplitude or phase of the response is continuously measured. When a qubit is excited by the incidence of a signal photon, a discontinuous change occurs in the response of the readout resonator, and single photon detection is realized.

  Next, embodiments of the present invention will be described in detail with reference to the drawings.

  Referring to FIG. 1, a circuit diagram of a microwave photon detector according to the first embodiment of the present invention is shown. A micro microwave photon that is an input signal to be measured is input from an input signal port 101 to a light receiving resonator 102 having a resonance angular frequency ω1 constituted by a superconducting LC circuit. Of the incident microwave photons, the photons transmitted through the light receiving resonator 102 are absorbed by the 50Ω termination 103.

  On the other hand, the light receiving resonator 102 is inductively coupled to a superconducting flux qubit 105 constituted by three Josephson elements and two self-inductances L1 and L2. In the illustrated example, a light receiving inductive coupling 104 including a mutual inductance determined by the superconducting inductance L of the light receiving resonator 102 and the inductance L1 of the superconducting flux qubit 105 is formed.

  The coupling energy g in the inductive coupling 104 for receiving light is much larger than the attenuation rate κ of the resonator and the energy relaxation rate γ of the qubit, and is in the so-called strong coupling limit.

  Further, the illustrated microwave photon detector includes a readout resonator 106, which is inductively coupled to the superconducting flux qubit 105. The illustrated readout resonator is constituted by a readout nonlinear resonator 106, and the readout nonlinear resonator 106 is constituted by a single Josephson junction 107, an inductance L3, and a capacitance. The illustrated light receiving resonator 102 and readout nonlinear resonator 106 are both superconducting resonators.

  As described above, the readout nonlinear resonator 106 is a resonator to which the Josephson junction 107 is added as a nonlinear circuit element, and has a resonance angular frequency ω 2 that is far away from the resonance angular frequency ω 1 of the light receiving resonator 102.

  Also, the superconducting flux qubit 105 and the readout nonlinear resonator 106 are provided by the readout inductive coupling 108 including a mutual inductance determined by the self-inductance L3 of the readout nonlinear resonator 106 and the self-inductance L2 of the superconducting flux qubit 105. Are combined.

  In this state, the readout nonlinear resonator 106 is continuously driven by the readout microwave source 109. The response of the readout nonlinear resonator 106 is detected from the readout port 110.

  The effective band of the illustrated microwave photon detector is determined by the attenuation rate κ of the light receiving resonator 102. The input microwave band needs to be about κ, that is, the time spread of individual photons needs to be about 1 / κ. The center frequency of the input light is set to the lowest energy (ω1-g) among the excited quantum levels formed by the coupled system of the resonator and the superconducting flux qubit 105 in the light receiving resonator 102.

  In this state, when a photon arrives in the light receiving resonator 102, the lowest level among the above-described excited quantum levels is excited. On the other hand, the read-out nonlinear resonator 106 is driven at such a high power that a dynamic bistable state is generated, and only when the superconducting flux qubit 105 is excited, the change in threshold value causes a change between the bistable states. Switching occurs and this discontinuous response is detected from the read port 110.

  When this signal is detected, the power of the readout microwave source 109 is temporarily increased or decreased to reset the dynamic stable state.

  Referring to FIG. 2, the characteristics of the readout nonlinear resonator 106 shown in FIG. 1 are shown. Here, the angular frequency and the amplitude indicate the nonlinear characteristics. More specifically, when the resonance angular frequency is ω1, the amplitude changes from p1 to p2, and when each resonance frequency decreases from ω1 to ω2, it decreases to p4 without stopping at the amplitude p3.

  When the superconducting flux qubit 105 is excited by incidence on the input photon, the nonlinear resonator 106 inductively coupled to the superconducting flux qubit 105 has a bistable characteristic characterized by hysteresis characteristics as shown in FIG. Indicates the state.

  More specifically, as shown in FIG. 3, when a single photon P is incident and the qubit 105 is excited, as a result, the output amplitude of the nonlinear resonator 106 increases according to the curve c1 and reaches the point d1. The amplitude of the nonlinear resonator 106 increases rapidly to the amplitude a1. When the input photon is not incident, the amplitude of the nonlinear resonator 106 decreases according to the curve c2, and when reaching the point d2, it decreases to the amplitude a2 and stabilizes in this state.

  By using the resonator 106 having such characteristics, single photons can be individually measured.

  The resonance frequency f1 corresponding to the resonance angular frequency ω1 of the light receiving resonator 102 is 2-8 GHz. The superconducting magnetic flux qubit 105 uses a circular current of 100-500 nA, a magnetic flux that is half of the quantized magnetic flux as an external magnetic flux, and an energy difference between the 0 state and the 1 state close to that of the light receiving resonator 102. The magnetic flux qubit 105 and the light receiving resonator 102 are inductively coupled via inductances L and L1 so that g = about 100 MHz. The attenuation rate κ of the light receiving resonator 102 is about 1 MHz, and the energy relaxation rate of the superconducting magnetic flux qubit 105 is also 1 MHz.

  On the other hand, the readout nonlinear resonator 106 is designed so that the resonance angular frequency ω2 is about 10 GHz and the Q value is about 1000. The binding energy with the qubit is about 10 MHz. The critical current of the Josephson junction is about 1-3 μA, and a bistable state can be realized when the number of photons in the readout nonlinear resonator 106 is about 100-1000.

  The threshold at which switching between bistable states occurs depends on the state of the superconducting flux qubit 105. Since the operating temperature needs to be lower than the microwave frequency scale, it is about 10 mK.

  The superconducting flux qubit 105 is not limited to the one using three Josephson junctions, and may be one using a single or two Josephson junctions.

[Other Embodiments of the Invention]
As the superconducting resonator constituting the light receiving resonator 102, a distributed constant type such as a microstrip line or a coplanar transmission line can be used instead of a lumped constant type component. The resonator may be a transmission type having two ports or a reflection type having only one port. In the case of one port, it is necessary to separate the reflected output of the readout resonator 106 from its input using a circulator or the like. In addition to the magnetic flux qubit, a charge qubit can be used as the superconducting qubit 105. In the case of a charge qubit, electrostatic coupling is used between the qubit and the resonator. The same applies to the reading resonator 106.

  The microwave photon detection circuit according to the above-described embodiment observes a microwave single photon by coupling a superconducting qubit with a superconducting resonator for receiving microwaves and a superconducting resonator for reading qubits. It has the effect of being able to do it.

  In the above-described embodiment, an example in which a nonlinear resonator is used as a readout resonator has been described. However, even if a linear resonator is used, a single photon can be similarly detected.

  Examples of the application of the present invention include signal detection circuits used in microwave communication technology, high-frequency circuit technology, microwave astronomy, quantum information processing technology, and the like.

It is a circuit diagram which shows embodiment of the microwave photon detector of this invention. It is a figure which shows the characteristic of the read-out nonlinear resonator used for the microwave photon detector shown by FIG. It is a figure which shows the input output characteristic of the nonlinear resonator for a reading shown by FIG.

Explanation of symbols

101 signal input port 102 light receiving resonator 103 50Ω termination 104 light receiving inductive coupling 105 superconducting flux qubit 106 reading nonlinear resonator 107 Josephson junction 108 reading inductive coupling 109 reading microwave source 110 reading output port

Claims (14)

  A superconducting resonator for receiving microwaves, a superconducting resonator for reading qubits, and a superconducting qubit combining the superconducting resonator for receiving microwaves and the superconducting resonator for reading qubits A microwave photon detector, comprising:   2. The microwave photon detector according to claim 1, wherein the superconducting resonator for reading is a nonlinear resonator.   2. The microwave photon detector according to claim 1, wherein the superconducting resonator for reading is a linear resonator.   4. The microwave photon detector according to claim 1, wherein the superconducting qubit is a superconducting flux qubit.   5. The microwave photon detector according to claim 4, wherein the superconducting flux qubit has three Josephson junctions.   6. The superconducting flux qubit according to claim 4, wherein the superconducting flux qubit is inductively coupled to the light receiving superconducting resonator and the readout superconducting resonator via mutual inductances. Microwave photon detector.   4. The microwave photon detector according to claim 1, wherein the superconducting qubit is a superconducting charge qubit.   8. The microwave according to claim 7, wherein the superconducting flux qubit is electrostatically coupled to the light-receiving superconducting resonator and the readout superconducting resonator via capacitors. Photon detector.   A microwave photon detection method, wherein a microwave photon is detected by exciting a qubit in response to an input photon and reading an excitation state of the qubit.   10. The microwave photon detection method according to claim 9, wherein the excited state of the qubit is read using a superconducting resonator for reading.   11. The microwave photon detection method according to claim 10, wherein the qubit is excited through a superconducting resonator for receiving light that resonates in response to the input photon.   12. The microwave photon detection method according to claim 11, wherein a resonance frequency of the superconducting resonator for light reception and a resonance frequency of the superconducting resonator for reading are different from each other.   13. The readout superconducting resonator according to claim 12, wherein the readout superconducting resonator is continuously driven by an external microwave source, and the qubit is excited by the incidence of the input photon, the readout superconducting resonator is not activated. A microwave photon detection method, wherein the input photons are detected by causing a continuous change.   14. The microwave detection method according to claim 9, wherein single photons can be detected.

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